Journal of Building Engineering 46 (2022) 103729

Available online 22 November 20212352-7102/© 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Alkylsiloxane/alkoxysilane sols as hydrophobic treatments for concrete: A comparative study of bulk vs surface application

Jorge Gonzalez-Coneo, Rafael Zarzuela *, Farid Elhaddad, Luis M. Carrascosa, M. L. Almoraima Gil, María J. Mosquera **

TEP-243 Nanomaterials Group, Department of Physical-Chemistry, Faculty of Sciences, University of Cadiz, 11510, Puerto Real, Spain

A R T I C L E I N F O

Keywords: Hydrophobic Waterproofing Concrete Admixtures Silanes Surface treatment

A B S T R A C T

Water and waterborne decay agents (e.g. salts, microorganisms) are commonly associated with undesired alterations and damages on concrete elements. A strategy to mitigate their impact is to decrease water retention in the material either by surface treatments or admixtures. In this work, hydrophobic concretes were developed by the addition of a hydrophobic sol containing TEOS and PDMS oligomers, synthetized by a surfactant-assisted sol-gel route, either as an admixture or as a surface treatment. The hydrophobic performance was similar for both application modes (>70% capillary absorption reduction) and higher than concrete containing a commercial (calcium stearate) admixture or a nanosilica-based hydrophobic coating. Addition as an admixture led to a higher durability in the rain and abrasion tests.

The hydrophobic sol as an admixture promotes an increase of surface roughness and porosity, as well as the formation of C–S–H like reaction products with the cement matrix components, as evidenced by AFM, MIP, SEM and FTIR. Despite the higher porosity, impact resistance and ma-terial cohesion were not negatively affected respect to the plain concrete. Application as a surface treatment decreased porosity and led to a higher amorphous SiO2 content. The material cohesion and impact resistance was increased by this application, although penetration was limited to the first 4 mm and the hydrophobic properties were more susceptible to mechanical damages to the surface.

1. Introduction

Concrete is ranked among the most commonly used and versatile building materials starting from the late 19th century. Ordinary concrete, composed by Portland cement, water, aggregates and, depending on the application, reinforcement materials (fibers, fly ash …) and admixtures (plasticizers, curing retardants …) [1] provides a satisfactory balance between production costs, mechanical performance, easy production/handling and durability. The last factor, however, heavily depends on the work conditions of the structure or building element (climate, water access …) and on factors which determine the final properties of the material [1,2], including but not limited to: quality and properties of the prime matters (size, shape, physical/chemical properties), the mix design, fabrication methodology and curing conditions. Water is probably the main “enemy” of concrete structures, since it can act as a vehicle for multiple decay agents (e.g. sulfates, chlorides, microorganisms, soluble CO2 …) or even act directly by physical mechanisms (e.g.

* Corresponding author. ** Corresponding author.

E-mail addresses: rafael.zarzuela@uca.es (R. Zarzuela), mariajesus.mosquera@uca.es (M.J. Mosquera).

Contents lists available at ScienceDirect

Journal of Building Engineering

journal homepage: www.elsevier.com/locate/jobe

https://doi.org/10.1016/j.jobe.2021.103729 Received 12 July 2021; Received in revised form 3 September 2021; Accepted 20 November 2021

Journal of Building Engineering 46 (2022) 103729

2

freeze-thaw cycles) or reacting with concrete components, leading to the formation of less resistant phases and/or volumetric changed that cause cracking, decohesion and loss of adherence, eventually compromising the structural integrity [3].

Water may ingress in the concrete porous structure, which include the matrix and the interfaces with the aggregate and rein-forcement materials, by different mechanisms: percolation, diffusion (in vapor phase) and capillarity [4]. The latter mechanism generally leads to its accumulation and is usually linked to decay phenomena. Concrete porosity is created during its curing either by formation of capillary pores, cavities/voids due to entrapment of air bubbles in the mixing stage or pores associated to the calcium silicate hydrate (C–S–H gel) [5,6], which is the predominant phase in a cement paste and main responsible of the engineering properties. A common strategy to limit capillary water absorption and, subsequently, durability of concrete is to decrease its porosity and pore size (of the capillary pores) during fabrication. To this end, effective strategies include employing low water/cement ratios, the use of superplasticizers to improve workability of the fresh mix and vibration of the paste in order to facilitate the release of entrapped air bubbles [1]. This strategy, however, can only go so far, as concrete is a highly hydrophilic material and high cement ratios increase the production cost while also requiring the use of more admixtures (superplasticizers) to maintain an acceptable workability.

A common and simple strategy for the protection of concrete against water consists on the application of surface treatments, classified as coatings and hydrophobic impregnations according to UNE-EN 1502 standard. The first category (e.g. epoxy resins, mortars …) creates a continuous film over the substrate, providing a physical barrier between the material and the environment, whereas impregnations (e.g. silanes, siloxanes …) penetrate into the pore structure and react with the substrate, creating a hydro-phobic material which decreases the surface energy and modifies roughness [7]. Unlike typical coatings, the produced hydrophobic material is porous and does not fully block the original structure, allowing the exchange of water vapor with the exterior and pre-venting stagnation.

The main factor affecting the wettability, correlated with water absorption through Washburn equation [8] of the material treated with a hydrophobic impregnation is the reduction of the solid-air surface energy in the solid/air/water interface. On an ideal flat surface, the water-surface contact angle depends exclusively on the phase composition. On real surfaces, however, roughness plays a relevant role in the wetting behavior. For a hydrophobic surface with irregular roughness, the contact angle increases with the micro- and nano-roughness, according to the Wenzel wetting model [9]. A special situation occurs on surfaces with regularly spaced micro- and/or nanometric features, where air pockets may get trapped between the surface and water, decreasing the contact area and promoting water repellence phenomena aside from high contact angles. These surfaces are described by the Cassie-Baxter wetting model [10], which has been used to explain the superhydrophobic behavior of materials as a function of their roughness and surface energy [11].

Even though surface treatments can be highly effective, their durability may be limited under exposure to very aggressive envi-ronments, depending on the material physical/chemical resistance, penetration depth (≤20 mm for impregnation treatments), interaction with the substrate, etc. [7]. Physical damages by abrasion or by other decay agents (e.g. salts, thermal expansion cycles, mechanical loads …), ultra-violet radiation, volumetric changes by reaction of the cementitious phases or capillary pressure during the treatment drying, may generate cracks or defects which facilitate water ingress up to the material core and reduce the treatment effectiveness over time [12]. Thus, an interesting alternative is to obtain the hydrophobic effect during the fabrication process itself via the incorporation of admixtures (e.g. calcium stearate, polymers, silicone oil …), so that the effect is extended through the bulk material, potentially reducing the need for interventions and re-treatments.

Several studies report the incorporation of admixtures to obtain hydrophobic concrete with positive results, although some authors have found non uniform behaviors for such concretes [13]. Other authors, such as Muzensky et al. [14], Tittarelli et al. [15] and Ferrara et al. [16] have studied the fabrication of hydrophobic concretes using alkylsilanes/siloxanes and derivates as admixtures, which are a popular choice for Surface treatments. Most studies are focused on evaluating the effect of the admixtures over water absorption, rebar protection and the compressive or flexural strength, though fewer information is available about the chemical in-teractions, microstructural changes, permeability, color/aesthetical changes and other mechanical properties relevant for non-structural concretes (abrasion resistance, impact resistance …). Huajun et al. [17] observed that functionalized silanes can retard cement hydration and interact with the anhydrous phases depending on the nature of functional groups, affecting porosity and me-chanical performance, although the effect over water absorption was not studied. On the other hand, several authors have reported the capacity of alkoxysilanes (e.g. TEOS or its oligomeric forms) to interact with the hydration phases in the cement matrix [18–22], including the reaction with portlandite to yield C–S–H gel, integration of the tetrahedral Si–O units in the C–S–H silica chains, and decomposition of Al-containing phases (ettringite, katoite, carboaluminate) to form amorphous aluminosilicate gels.

The objective of the present work is to compare the performance and durability of a alkoxysilane/alkylsiloxane based hydrophobic product, synthetized through a surfactant-assisted sol-gel route [23], on a non-structural concrete (typically used for claddings, floor tiles, windowsills …) used in two different forms: (1) as a surface hydrophobic impregnation treatment, (2) added as an admixture during fabrication. Mechanical properties, structural changes, chemical interactions and water absorption reduction were considered, as well as the effect over surface properties (roughness, contact angles, morphology). For comparison purposes, the treatment was compared with two commercially available treatments: a functionalized nanosilica-based superhydrophobic coating and a calcium stearate-based admixture.

2. Materials and methods

2.1. Synthesis of the hydrophobic sol

The sol used as a hydrophobizing admixture and surface treatment for the concrete specimens was synthetized via a surfactant-

J. Gonzalez-Coneo et al.

Journal of Building Engineering 46 (2022) 103729

3

assisted sol-gel route [23]. The starting sol was produced by mixing an oligomeric tetraethyl orthosilicate precursor (TES40 WN®, from Wacker), Si–OH terminated polydimethylsiloxane with has a polymerization degree of 12 (PDMS 4–6% –OH, from ABCR) and a previously prepared aqueous n-octylamine (98%, from Sigma-Aldrich) solution (1.57 M). The components were mixed under ultra-sonic stirring with a high power ultrasound probe (74% Amplitude) for 10 min, in a v/v proportion of 89.40%, 10.45% and 0.15% respectively. The interaction of these precursors allows to obtain a reticulated structure (due to the TEOS units) with the hydrophobic character of PDMS [24,25]. The n-octylamine acts as a catalyst and surfactant, decreasing surface tension and favoring the formation of nano-structured gel with a uniform mesoporous structure [26]. Co-polymerization of the precursors and their interaction with the cementing phases during or after hydration produces a hydrophobic concrete.

2.2. Concrete specimens fabrication

Three sets of concrete specimens were prepared for comparative purposes: a control one without admixtures (OC), and two hy-drophobic formulations. For the hydrophobic concretes, two different mix designs were tested: (1) HE-B: containing a commercial calcium stearate-based powder as an admixture, which is routinely used to reduce water absorption in concrete [27]. (2) SP-B: which contains the hydrophobic sol described in the previous section.

The concrete was prepared from type I portland cement and dolomitic aggregate, following a mix design recommended for the fabrication of claddings panels and floor tiles. White cement was selected for aesthetical requirements of some applications. De-ionized water was used in order to facilitate interpretation of the chemical interactions with the alkoxysilane, as fluctuations in the Ca content of tap water may affect their reactivity (i.e. different Ca/Si ratios in the pore solution favor either the reaction to form calcium silicate hydrate or auto-condensation to form SiO2 [18–22]). The mix proportions of the control specimens are presented in Table 1. All the solid components were mixed and homogenized prior to addition of water and the superplasticizer. For the hydrophobic concrete specimens, the HE-B admixture was mixed with the solid components in a 2% w/w proportion respect to the cement, whereas in SP-B the hydrophobic sol was added after the water in a 2% w/w proportion (respect to cement). The mixtures were manually stirred until obtaining a uniform paste, and poured afterwards in closed plastic molds. The paste was manually compacted with a metallic rod and vibrated for 30 s to set the mixture and facilitate the elimination of trapped air bubbles. After 24 h, the specimens were demolded and cured at laboratory conditions (22 ◦C ± 2 ◦C and RH 40% ± 5%). The curing process was monitored by measuring the ultrasound pulse transmission velocity with an Ultrasonic Tester BP-7 Series (UltraTest GmbH).

The surface-treated concrete specimens were prepared by application of the products 24 h after demolding. The products were applied by spraying 5 layers (until apparent saturation), letting the product absorb for 2 min between applications. Two different surface treated sets were prepared for comparison purposes: (1) SP-S: treated with the sol (hydrophobic impregnation) described in the previous section. (2) AQ-S: treated with a commercial hydrophobic coating (AquaShield® from Tecnan) based on a dispersion of hydrophobic nanosilica in isopropanol.

The maximum hydrophobic effect of the surface treated sets (SP–S and AQ-S) was reached in days, whereas it took up to 90 days for the bulk sets (SP–B and HE-B). For consistency sake, all the tests described in the following sections were carried out after 90 days.

2.3. Characterization of the concrete specimens

2.3.1. Chemical and structural characterization In order to evaluate possible alterations, the interaction between the concrete components and the treatments/admixtures, as well

as the presence of the hydrophobic components through the bulk different techniques were applied: The aesthetical alterations caused by the surface treatments or the addition of the components in the mix design were evaluated by measuring the Total Color Difference (Δ E*) [28], respect to the reference concrete, in the CIE L*a*b* color space by means of a Colorflex spectrophotometer colorimeter from Hunterlab. The measurements were registered using as a reference illuminant D65 and observer CIE 10◦ (1964).

Fourier Transformed Infrared Spectroscopy was employed for the chemical analysis of the components and the identification of new reaction products or modifications of the cement matrix. The measurements were carried out on powder samples in Attenuated Total Reflection (ATR-FTIR) mode in the 650-4000 cm− 1 range (with a 4 cm− 1 spectral resolution) using a IRAffinity-1S from Shimadzu equipped with a MIRacle10 ATR module from Pike Technologies.

Morphological study of the concrete structure and phases was carried out by Scanning Electron Microscopy (SEM) using a Nova NanoSem 450 model from FEI, working on secondary electrons mode. Samples from both the surface in contact with the molds (~20 ×20 mm) and the cross-section (~15 mm depth) were analyzed. Prior to observation, the samples were sputtered with a 5 nm Au layer. The topographical features and roughness of the concrete surfaces were studied by Atomic Force Microscopy (AFM), using a Dulcinea

Table 1 Mix design of the reference concretes.

Component % weight

Type I- 52.5-R white Portland cement 13.95 Coarse dolomitic aggregate (2–4 mm) 43.35 Coarse dolomitic aggregate (0–1 mm) 25.50 Fine dolomitic aggregate (<0.1 mm) 4.00 Siliceous sand (0–1 mm) 4.34 Water (de-ionized) 8.80 Superplasticizera 0.06 a Dynamon SP1 (Mapei S.p.A.).

J. Gonzalez-Coneo et al.

Journal of Building Engineering 46 (2022) 103729

4

model from Nanotec Electronica, operating in amplitude modulated tapping mode. The roughness mean square values (RMS) were calculated from 2.5 μm × 2.5 μm topographical maps, using at least five replicates from areas corresponding to the cementitious matrix (i.e. to avoid interference from the coarse aggregate).

Open porosity and pore size distributions were obtained by Mercury Intrusion Porosimetry (MIP), using a PoreMaster-60 apparatus from Quantachrome. The samples were cut from the concrete specimens to a size of c.a. 5.5 × 5.5 × 10 mm (volume of ~300 mm3) and the average values were calculated from at least five replicates. The water vapor permeability of the concrete samples was measured via the standard wet cup method with an automated equipment described in previous works [29], according to the specifications of ASTM E96M standard. The specimens were cut in 40 × 40 × 10 mm prisms.

2.3.2. Mechanical properties The effect of the admixtures and the surface treatments over the concrete properties was studied through measurements of the

ultrasound pulse transmission, drilling resistance and impact resistance. Cohesion of the material, presence of air bubbles and mechanical properties were studied through Ultrasound Pulse Velocity (UPV)

and Drilling Resistance (DRMS) measurements. The UPV measurements were performed on Ø30 mm cylindrical specimens cut to 40 mm height, using an “Ultrasonic Tester BP-7 Series” from Cullman. Drilling resistance was measured with a DRMS Cordless equipment from SYNT Technology, equipped with Ø5mm flat tip drill bits. The measurements were registered with a rotation speed of 900 rpm advancing at a 5 mm min− 1 rate.

The impact resistance of the concrete was measured using a variation of EN 14158–2004 standard. For the tests, a spherical steel weight (~0.5 kg) was dropped on the specimens (40 × 40 × 10 mm) at increasing 5 cm height intervals until the concrete specimen fractured. The impact resistance (J) was calculated as the energy of the impact at the fracture point (J = m⋅g⋅h), where “m” is the sphere weigh in kg, “h” the drop height, and “g” is the gravity acceleration in m/s− 2. In order to account for variations in specimen dimensions, the values were normalized by dividing J by sample thickness.

2.3.3. Evaluation of the hydrophobic properties The hydrophobic character of the concrete surfaces was evaluated through measurements of the static contact angle (SCA) by the

sessile drop method, depositing 10 μl water droplets on the horizontal surface. Water repellent properties were determined by measuring the contact angle hysteresis (HCA), calculated as the difference between the advancing and receding contact angles measured by adding and retrieving 5 μl volumes at a 0.2 μl/s rate (3 cycles). All contact angle measurements were carried out using a commercial software-controlled video-based equipment (OCA 15 plus w/SCA-22 software, DataPhysics instruments).

The hydrophobic performance of the concrete specimens was evaluated through the capillary water absorption test. Prior the analysis, the samples were oven-dried at 50 ◦C for 3 days and, afterwards, stored in desiccator at room temperature (20 ◦C) until reaching constant weight (difference <0.1% after 24 h). A single face of the specimens (the treated one for SP-S and AQ-S) was put in contact with de-ionized water, and water absorption was monitored through weight measurements. After the capillary absorption test, the contact angles were measured again, in order to check the stability of the hydrophobic components.

2.3.4. Durability evaluation The durability of the hydrophobic properties on the surface was evaluated through two different tests: Mechanic stability was determined by an Abrasion Resistance Test, which accounts for the gradual wear some elements such as floor

tiles experience during their service life. For this purpose, the concrete specimens were dragged over P180 abrasive paper, applying a pressure of 5 KPa controlled by placing a weight over the specimen. Static and dynamic contact angles were monitored after different abrasion distances to evaluate the hydrophobic performance. Physical damages to the surface caused by the abrasion were studied by SEM. Before carrying out the contact angle measurements, the abrasion debris were removed from specimens with pressurized air (3 bars).

In order to evaluate the stability of the treatments under rainy conditions, a Simulated Rain Test was performed [30]. For this purpose, water was dropped over the samples, placed in horizontal position from a deposit. The bottom of the deposit was pierced ~2 mm holes separated by ~8 mm. The deposit was placed at a 50 cm height (from the surface) and water was dropped at a 4.9 mL s− 1

flow rate. Stability of the hydrophobic surface was monitored by measuring the static contact angles at regular intervals (expressed in terms of cumulative water volume dropped over the surface). Prior to the contact angle measurements, the specimens were dried at 60 ◦C for 4 h and equilibrated at laboratory conditions (20 ◦C, 40% RH) for 1 h.

Schematic representations of the experimental setups are presented in supplementary material (Fig. S2).

3. Results and discussion

3.1. Chemical and structural characterization of concrete specimens

3.1.1. Color change The chemical interaction between the concrete components and the products might cause visible color changes that, despite not

affecting their permeability of mechanical properties, could limit their use as protective treatments due to the aesthetical differences respect other elements of the building. Thus, the Total Color Difference (ΔE*) of the surface treated and the (bulk) hydrophobic con-cretes respect to the conventional one was calculated from the CIE L*a*b* color coordinates (see Table 2). In general, all of the treatments or admixtures caused a color change below 5, which is generally considered the acceptable limit for applications where preserving the aesthetics is required [28], and alterations of the chromatic components, Δa* and Δb*, are minor (below 3) in all cases. In the case of the concretes containing hydrophobic admixture in bulk (SP–B and HE-B), the color change was even lower, falling below

J. Gonzalez-Coneo et al.

Journal of Building Engineering 46 (2022) 103729

5

the human eye perception threshold (ΔE* < 3). This small difference may be attributed to the more homogeneous distribution compared to the accumulation of the products near the surface on the SP-S and AQ-S mortars. For SP-S, the color changes are mainly attributed to the luminosity (ΔL*), as the polymerized product in the surface produces a “wet effect”. In the case of HE-B and AQ-S, the most relevant variation corresponds to an increase in the b* coordinate, indicating a contribution of the yellow color.

3.1.2. FTIR spectroscopy As noted in works by several authors [18,19,21,22], TEOS and other alkoxysilanes can undergo different reactions with compo-

nents of the cementitious matrix (e.g. Portlandite), yielding C–S–H like reaction products in addition the SiO2 xerogels formed by their auto-condensation. In order to study the chemical interactions of the hydrophobic sol with the cementitious matrix components (in SP-S) and its effects over the cement hydration process (in SP-B), the FTIR spectra were analyzed (Fig. 1). The spectrum of the concrete without admixtures (OC) presents predominantly the characteristic C–O bands (1400, 880 and 712 cm-1), corresponding to the dolomitic aggregate and calcite produced by carbonation of the cementitious matrix, and a wide band group at 1200-900 cm-1, resulting from different contributions: (1) S–O bands from ettringite at ~1150 cm-1. (2) Si–O–Si vibrations at ~1080 cm-1 from reticulated SiO2 networks (siliceous aggregate and highly polymerized C–S–H gels after carbonation). (3) Si–O bands at ~1020-1040 cm-1, which can be attributed to the Si–O chains in C–S–H gels with low Ca/Si ratios.

In both the surface treated (SP–S) and the bulk modified (SP–B) concretes, the increased relative intensity of the Si–O signals (respect to C–O bands) confirms the presence of the polymerization products of the sol. Furthermore, the position of the Si–O bands showed evidences of the reaction between the sol and the cementitious phases, as noted by the apparition of new signals at different wavenumbers from the main Si–O bands of the xerogels (~1080 cm− 1) formed by auto-condensation (Fig. 1b). The spectrum of SP-B is characterized by a major contribution of the components at ~1040 cm− 1 which can be attributed to Si-rich C–S–H gels, and at 966 cm- 1, corresponding to its mid-chain (Q2) Si–O units. Along with the increase of the H–O bands corresponding to hydration H2O at ~3500 and ~1600 cm− 1, this confirms that the alkoxysilane precursor is able to react with the cement hydration products (portlandite and C–S–H gel), as they are formed during the curing process, in line with prior studies [21,22]. It should be noted that the bands at 1260 cm-1 (corresponding to C–H in Si–CH3) and 798 cm− 1 (Si–C bonds) are still detected in the spectra, indicating that the hydrophobic component (PDMS) is not degraded under the conditions of the paste (high pH and presence of H2O).

On the other hand, when the sol is applied as a surface treatment (SP–S), the FTIR spectrum indicated the coexistence of the auto- condensation and C–S–H formation reactions. Specifically, the Si–O band group presents a remarkable contribution of the reticulated SiO2 structures at 1080 cm− 1, along with the signals at 1020-1040 cm− 1 and a minor contribution of the Q2 SI–O band at 960 cm− 1. As opposed to the conditions in the SP-B paste, in SP-S the hydration reactions of the anhydrous cement phases (C3S, C2S …) had partially occurred when the sol was applied and water availability was therefore more limited, thus favoring the auto-condensation of the alkoxysilane respect to the reaction with the cementitious phases. Similar interactions have been observed by the authors in previous works where an alkoxysilane-based treatment with a similar base composition was mixed with hydrated OPC pastes [21].

3.1.3. Scanning Electron Microscopy Changes in surface morphology and internal structure caused by the surface treatments and the admixtures in bulk are evidenced

by the SEM images (Fig. 2). The reference concrete surface shows a generally compact aspect, where micro- and sub-micrometric pores are observed at higher magnifications, in a similar vein to the cross-section micrographs. The surface treated concrete specimens showed a more compact aspect in both cases (SP–S and AQ-S), confirming the presence of the products. On a closer inspection, the AQ- S concrete surface showed the formation of a heterogeneous SiO2 nanoparticle (~10 nm) coating, and the structure of the cross-section is barely altered, since the particles aggregate and have a low penetration in the pore structure. On the other hand, the surface of the SP-S concrete is more uniform and presents different features which match the bands detected in the FTIR spectra Fig. 1: (1) amor-phous/globular structures characteristic of SiO2 xerogels. (2) Foil-like structures attributable to C–S–H gel. Similarly, the cross-section micrographs showed a more cohesive structure compared to the untreated concrete and, in this case, the presence of fibrous structures typical of type I C–S–H, in line with previous reports which showed that the oligomeric alkoxysilane precursor forms these structures when reacting with the hydrated cement matrix [21].

In contrast, incorporation of the hydrophobic sol in bulk (SP–B) promotes the formation of a less compact structure with abundant micron-sized pores, both on the surface and inside the material, and larger cavities near the surface. This can be attributed to different effects: (1) The hydrolysis of the silica precursors competes with the hydration reactions of the cement phases, leading to a deficit of H2O and altering the concrete setting process as observed by other authors [31]. (2) PDMS decreases the surface tension of the cement paste, favoring the formation of larger air bubbles which can become trapped after the concrete sets, meaning the manual vibration may have been insufficient to release them. The typical C–S–H morphologies in this case are less defined than in the surface treated concrete (SP–S), which may be indicate the formation of the “inner product” C–S–H [21,32] instead.

The HE-B concrete, containing calcium stearate in bulk, presented a more compact aspect on the surface, despite the presumably

Table 2 Total color difference (ΔE*) and CIE L*a*b* color coordinates differences respect to the reference concrete (ΔL*: luminosity, Δa*, Δb* chromatic components).

Type of concrete ΔE* Δa* ΔL* Δb*

SP-S 3.90 − 0.23 − 3.88 0.25 SP-B 1.26 0.43 − 0.98 0.60 AQ-S 3.06 1.06 − 1.65 2.32 HE-B 2.87 0.47 1.54 2.37

J. Gonzalez-Coneo et al.

JournalofBuildingEngineering46(2022)103729

6

Fig. 1. ATR-FTIR spectra of the (a) reference concrete and treated with the alkoxysilane/alkyl siloxane sol. (b) Alkoxysilane/alkyl siloxanes sol and xerogel formed by auto-condensation. (c–e) Zoom-in of the Si–O and C––O

band groups.

J. Gonzalez-Coneo et al.

JournalofBuildingEngineering46(2022)103729

7

Fig. 2. Representative SEM micrographs of the concrete specimens surface, before and after the abrasion test, and their cross-section.

J. Gonzalez-Coneo et al.

JournalofBuildingEngineering46(2022)103729

8

Fig. 3. a) Representative topographical maps, b) Typical roughness profiles and c) Root Mean Square of roughness of the concrete surfaces obtained by Atomic Force Microscopy.

J. Gonzalez-Coneo et al.

Journal of Building Engineering 46 (2022) 103729

9

low interaction of the component with the cement matrix. In this case, the changes may be directly attributed to the different workability of the paste resulting from the hydrophobic nature of the admixture [33], which will affect its setting on the mold. The cross-sectional micrographs showed a generally more compact structure of the matrix, but also the presence of Ca stearate fibers barely interacting with the matrix and some micro-cracks in their surroundings, which may be related with the (localized) lower water availability.

3.1.4. Atomic force microscopy The effect of the treatments and admixtures over the surface roughness was studied can be observed in the topographical maps and

roughness profiles obtained by atomic force microscopy (see Fig. 3). The plain concrete presents an irregular roughness characterized by the presence of peaks separated by wide valleys, in line with

the observations made from the SEM micrographs. After the surface treatment with the hydrophobic sol (SP–S), the product partially penetrates in the pore structure while simultaneously leaving a more compact layer near the surface, as manifested by the slightly lower RMS values. The separation between roughness peaks becomes tighter and more regular, which can be associated with the densification of the matrix by the C–S–H like structures observed by SEM. In the case of the nanoparticle-based coating (AQ-S), the RMS also experiences a slight decrease respect to the reference (OC) and the profiles show irregular peak-valley distances (similar to the untreated concrete), though a secondary nano-roughness can be discerned. This can be attributed to the deposition of the nano-particles on the surface which partially cover the surface pores and roughness valleys (see Fig. 3).

The incorporation of the hydrophobic sol in bulk (SP–B) promoted an increase in the overall roughness and leads to a surface characterized by taller and wider peaks separated by large valleys, which is consistent with the presence of pores/cavities detected by SEM. Aside from the micro-roughness, secondary nanometric features can be observed, likely corresponding to the structure of the hydration products. The addition of calcium stearate (HE-B) led to a surface with similar features to the plain concrete (i.e. irregular profile, wide valleys) but a higher roughness, with a RMS value similar to the SP-B concrete. This difference is likely attributed to modifications in the hydration process.

3.1.5. Mercury intrusion porosimetry The changes in porosity and pore size distribution hinted by the SEM micrographs are corroborated by the results of the MIP

analysis (Fig. 4). When applied as a surface treatment (SP–S), the hydrophobic sol promoted a decrease in the concrete porosity, as the sol penetrates in the concrete structure, up to c.a. 4 mm as estimated by the drilling resistance profiles (Fig. 5b), and the reaction products partially fill the original pores, especially those in the 0.1–1 μm range (Fig. 4a and b), which are more susceptible of being

Fig. 4. MIP analysis of the concrete specimens: (a) representative pore size distribution profiles. (b) Normalized pore volume % (respect total porosity) vs pore size ranges. (c)Total open porosity.

J. Gonzalez-Coneo et al.

Journal of Building Engineering 46 (2022) 103729

10

blocked than the larger ones. The pores larger than 1 μm are barely affected by the treatment. A slight increase in the smaller pores (<10 nm) was detected, which can be attributed to the porosity of the reaction products detected by other techniques: C–S–H gel and the mesoporous SiO2 xerogel formed by auto-condensation [34]. In contrast, the porosity of the SP-B concrete was significantly higher than the plain one practically through whole size range. This increase becomes especially evident in the 1–10 μm pores, whose diameter corresponds to the cavities observed in the SEM images (Fig. 2). In a similar vein to the SP-S specimens, the porous structure of the reaction products (mainly C–S–H gel in this case) led to a higher pore volume in the <10 nm range.

The concrete containing calcium stearate in bulk (HE-B) presented a higher porosity compared to the reference. The distribution showed a similar shape except for a noticeable increase in the 1–2 μm and 8–10 μm pores. These pores may result from two contri-butions: (1) the voids left by the low interaction between the Ca stearate fibers and the matrix or the cracks observed in the SEM micrographs. (2) The lower workability of the fresh paste, which may promote the entrapment of air bubbles. As expected due to the presumably low penetration of coatings, total porosity of AQ-S was practically the same as the untreated mortar. The shape of the pore size distribution is similar to the untreated concrete, although a decrease in the relative contribution of the 0.1–1 μm pores is observed, likely due to the accumulation of the SiO2NPs in the pores near the surface.

3.2. Evaluation of the hydrophobic properties

The hydrophobic performance of the treatments/admixtures was determined by the static and dynamic contact angle mea-surements of water droplets deposited on the surface, and their ability to decrease capillary water absorption. In addition, the contact angles after the capillary absorption test were measured as an indicator of the chemical stability.

As presented in Table 3 all of the treatments/admixtures promoted a decrease in the water absorption rate (WAC) and total water absorption with respect to the plain concrete, albeit their effectiveness is remarkably different (Table 3). The highest decrease (~75% total absorption and >85% for WAC) was observed for the hydrophobic sol, both as a surface treatment or as an admixture in bulk. The methyl functional groups present in the PDMS, as observed by FTIR, promote a marked decrease in the surface energy, which severely limits water penetration through the pores, as predicted by Washburn’s equation for the capillary flow in a porous material [8]. It is worth mentioning that the effect of the surface energy was enough to offset the influence of the porosity increase of SP-B in the 1–10 μm pores, which have the highest contribution towards capillary absorption [35]. In the case of SP-S, the reduction in capillary absorption was practically the same as SP-B, despite having a significantly lower porosity. This result suggests that, under the experiment con-ditions, the changes in porosity (lower than the reference concrete for SP-S) has a lower influence over capillary water transport than the surface energy-related aspects.

Calcium stearate addition (HE-B) decreased initial absorption rate but showed a lower effectiveness at reducing total absorption compared to the hydrophobic sol, with a decrease around 35%, which may be attributed to different factors: (1) the hydrophobic component is not distributed homogeneously (see Fig. 2) and it has a poor interaction with the cement matrix, leading to a lower overall reduction of the surface energy. (2) As analyzed by MIP (Fig. 4), HE-B presents a higher porosity in the capillary range (1–10 μm) compared to the plain concrete and, unlike SP-B, the surface energy decrease only compensates partially for this effect. On the

Fig. 5. Mechanical properties of the concrete specimens. (a) Ultrasound pulse velocity over curing time of the reference concrete and the concretes with hydrophobic admixtures in bulk; (b) Averaged drilling resistance profiles (5 replicates) of the concrete specimens through the first 15 mm; (c) Impact resistance of the concrete specimens, normalized by sample thickness.

J. Gonzalez-Coneo et al.

Journal of Building Engineering 46 (2022) 103729

11

other hand, the hydrophobic coating (AQ-S) is effective at decreasing the initial absorption rate (WAC decrease ~85%) but it is severely limited regarding total absorption, showing a final decrease of ~10%. Although its surface hydrophobicity is comparable to SP-S or SP-B and the coating offers a barrier effect during the first 2–3 h, as evidenced by the absorption curves (Fig. S3), its irregular coverage (Fig. 3) and low penetration limit the effectiveness after longer exposure to water. This behavior is consistent with previous works [36] where the AQ-S coating was applied on a concrete with similar composition and its penetration was estimated as ≤1 mm by observing the hydrophobic behavior of cross-section cuts. In a similar vein, the drilling resistance profiles (Fig. 5b) do not evidence the presence of the coating beyond the surface.

According to some sources such as the German Committee for Reinforced Concrete, a waterproofing treatment can be considered effective as long as it promotes a reduction in water absorption equal or above 50% respect to the unprotected concrete. More restrictive recommendations, such as The NHRP Report 244 of the ‘‘National Cooperative Highway Research Program’’ from USA, establish this threshold as 75% [4]. For both criteria, only the SP-S and SP-B concretes fulfilled the requirements under the studied conditions.

The hydrophobic character of the surfaces, related with surface energy, and water repellence of the materials were evaluated respectively through the static contact angle and the hysteresis between advancing and receding contact angles. Table 4 shows the average values before and after the capillary water absorption test. The plain concrete is a hydrophilic porous material that quickly absorbs the water droplets before measurement is possible. The static contact angle on all the treated or modified concrete surfaces was higher than 90◦, which is considered the threshold that defines a hydrophobic surface [12,37] and explains the lower capillary water absorption. On the other hand, hysteresis values lower than 10◦ (only observed for the SP-B surface) indicate water repellence (i.e. the water droplets easily slide over the surface). The lowest static angle and higher hysteresis corresponds to HE-B, where the hydrophobic component was heterogeneously distributed through the material. The rest of the mortars present high SCA values (>120◦), which can be attributed to the higher interaction of the hydrophobic component with the substrate and its homogeneous distribution. In order to explain the differences in their contact angles (SP–B > AQ-S > SP-S), the surface roughness must be taken into account (Fig. 3).

According to the Wenzel wetting model [9], applicable to surfaces with an irregular topography where water impregnates the wide roughness valleys, the contact angle increases respect to the theoretical value for an ideal flat surface (defined by Young’s model [38]) in a factor proportional to the surface roughness, which matches the trend of the RMS values obtained by AFM. This model, however, does not explain the lower hysteresis values observed for SP-B (and AQ-S after the capillary test). In this case, the measurements point towards a Cassie-Baxter wetting regime, which is typically observed on surfaces where regular sub-micrometric and/or nanometric roughness features are present (in line with the changes observed in SEM and AFM profiles) [10]. According to this model, air pockets become trapped between the roughness peaks, decreasing the effective contact area of the solid with the water droplet, which both increases the contact angle and promotes repellence phenomena.

After the capillary absorption test and drying the excess water at 60 ◦C, the contact angle of SP-S and SP-B did not experience significant changes, confirming the stability and the chemical interaction of the hydrophobic agent to the concrete components. The SCA of HE-B slightly decreased, which may indicate a partial loss of the calcium stearate due its poor attachment to the matrix (see Fig. 2). In the case of AQ-S, a decrease in the hysteresis was measured, which can be attributed to the elimination of remnant solvent (isopropanol) trapped in the coating.

The water vapor permeability of the concrete follows a similar order to the porosity values, as seen in Table 5. Surface treatment with the hydrophobic sol (SP–S) promoted a decrease in the diffusivity coefficient of 28%, which is related to the porosity decrease (7.8%) and the lower surface energy of the reaction products filling the pores (i.e. due to the –CH3 groups in PDMS). In a similar vein, a decrease can be observed for the concrete treated with the hydrophobic coating (AQ-S), although its magnitude is significantly lower due to the poor penetration of the treatment compared to the hydrophobic sol (~4–5 mm). In contrast, permeability of the SP-B concrete increased by 7%, which can be attributed to its higher porosity and pore size. It should be noted that the permeability in-crease is low compared to the porosity, which suggests that changes in the surface energy of the material (i.e. hydrophobic character) may be competing with this factor. This competitive effect would also explain the highest permeability increase of the HE-B concrete despite having a lower porosity compared to SP-B, considering the higher surface energy (i.e. lower contact angles) of HE-B.

Table 3 Capillary water absorption of the concrete specimens.

TYPE OF CONCRETE OC SP-S SP-B AQ-S HE-B

% Water absorption by capillarity (24H) 5.5 ± 0.6 1.3 ± 0.4 1.3 ± 0.3 5.0 ± 0.5 3.6 ± 0.3 % Decreasea – 76.5 ± 5.4 76.0 ± 4.1 9.2 ± 0.9 36.0 ± 2.2 WAC (Kg.m− 2.h− 0.5) 0.429 0.033 0.065 0.101 0.052 a Calculated respect to the reference concrete (OC).

Table 4 Static contact angle and contact angle hysteresis of the concrete surfaces before and after the capillary absorption test.

TYPE OF CONCRETE SP-S SP-B AQ-S HE-B

BEFORE AFTER BEFORE AFTER BEFORE AFTER BEFORE AFTER

Static contact angle 120◦ ± 6.0◦ 120◦ ± 8.0◦ 130◦ ± 6.0◦ 138◦ ± 9.0◦ 129◦ ± 4.0◦ 129◦ ± 2.0◦ 95◦ ± 3.0◦ 85.9◦ ± 6.0◦

Contact angle hysteresis 16.0◦ ± 0.3◦ 13◦ ± 0.8◦ 9.0◦ ± 1.8◦ 9.0◦ ± 0.2◦ 18.9◦ ± 0.9◦ 8.9◦ ± 0.7◦ 22.2◦ ± 1.3◦ 22.1◦ ± 0.3◦

J. Gonzalez-Coneo et al.

Journal of Building Engineering 46 (2022) 103729

12

It is worth mentioning that, in contrast with the transport of liquid water by capillarity (Table 3), the differences in porosity have a higher influence over water vapor permeability in the case of the hydrophobic concretes, since the vapor molecules have a higher mobility and their interactions with the pore walls are weaker.

In general, a decrease in permeability can increase the resistance of concrete against carbonation processes [39] which compromise the durability of reinforced concrete structures. However, an excessive reduction of the permeability has also been linked to a high susceptibility towards water-related damages (delamination, moisture accumulation, hygroscopic swelling …) due to the inability of the material to evacuate water once trapped inside its structure [40,41]. Thus, the suitability of an admixture or surface treatment should always be decided in a case-by-case basis, considering the specific application and the environmental conditions where the material is expected to be exposed. For non-structural concrete, the material used in this work, carbonation is generally considered non-problematic, and it may even increase the surface compaction.

3.3. Evaluation of mechanical properties

The ultrasound pulse velocity measurements, presented in Fig. 5a, can be related with differences in the concrete porosity (presence of voids, cracks, pores …), its internal cohesion and changes during the curing process induced by the presence of the hydrophobic admixture. The measurements after complete curing (28 days) showed a higher transmission speed for the plain concrete compared to SP-B and HE-B (~5% difference), which is consistent with their comparatively higher porosity values obtained by MIP (Fig. 4). However, the SP-B and HE-B concrete specimens presented virtually identical values despite the higher porosity of the former. Even though total porosity plays an important role on the ultrasound pulse transmission, two factors must be considered to understand this apparent discrepancy: (1) In the HE-B concrete, the increased porosity more influenced by cracks and voids between the hydrophobic component and the matrix than by the porosity of the components themselves. (2) As shown by the FTIR spectra, the hydrophobic sol added to SP-B promotes the formation of additional C–S–H gel, which can partially compensate the loss of cohesion induced by the pores.

Regarding the differences during the curing process, in all three cases the pulse velocity measurements stabilized around 8–10 days, indicating that the admixtures have a lower influence over the final curing time. The increase from 1 to 5 days, however, was relatively lower for the concrete with admixtures, which may be related to a delayed hydration rate caused by a lower water availability in the presence of the hydrophobic components and/or, in the case of SP-B, the competing reactions/interactions of the alkoxysilane with the anhydrous phases [17]. This factor did not affect the day 1 measurements (which are indeed higher) because, under the experimental conditions, water availability during that interval was not a limitation (i.e. the molds were closed).

Drilling resistance was measured as an indicator of the internal cohesion of the material, which is appropriate to evaluate the mechanical performance of impregnation treatments and their penetration inside the substrate. The plain concrete (OC) showed an average penetration force of 37.9 ± 8.4 N, whereas the concrete specimens with hydrophobic admixtures, SP-B y HE-B had higher values (49.8 N ± 9.9 N and 48.0 N ± 7.1 N respectively) through the whole sample length (Fig. 5b), indicating that their addition did not promote segregation nor phase separation processes. This increase in SP-B despite its remarkably higher porosity indicates that the formation of hydration products by reaction of the sol with the cementing phases can effectively improve the cohesion of the material and compensate for the porosity-induced performance loss. For the HE-B concrete, the increase may be related with the more compact structure of the cement matrix observed by SEM (Fig. 2). It is worth mentioning that the observed trend is the opposite to the ul-trasound pulse measurements, which are more affected by the porosity and presence of voids than the drilling resistance.

The application of the hydrophobic sol as a surface treatment (SP–S) promoted an even higher increase in the drilling resistance (58 ± 9.0 N), though it was limited to the first 3.5 mm from the surface, where the product was able to penetrate in a sufficient proportion. The higher increase compared to the addition in bulk (SP–B) can be attributed to the chemical interaction of the reaction products (SiO2 and C–S–H gel, as seen by FTIR) with the concrete phases, combined with the formation of a more compact structure inside the material (as seen by SEM and MIP). The AQ-S concrete, on the other hand, presented virtually the same resistance as the untreated material. Although nano-silica dispersions have been used as consolidants on cementitious materials due to their ability to undergo pozzolanic reactions [42], the product forms a hydrophobic surface coating with low penetration.

The use of concrete on certain applications, such as floors and claddings, requires a sufficient impact resistance to prevent acci-dental damages during its service life. Fig. 5c shows the impact resistance of the concrete specimens, expressed as the energy required to fracture the material (normalized by sample thickness). The results showed no significant effect of the admixtures in bulk (SP–B and HE-B) over the impact resistance, which is in line with the opposed contributions of their higher porosity/presence of voids (which decreases impact resistance) and the higher cohesion of the components observed by the drilling resistance. Predictably, the nanoparticle-based hydrophobic coating (AQ-S) did not modify this parameter either. On the other hand, the application of the hy-drophobic sol as a surface treatment (SP–S) slightly increased the impact resistance of the concrete, which may result from the

Table 5 Water vapor permeability of the concrete specimens and %variation respect to the reference (OC) concrete.

Type of concrete Diffusivity coefficient ((10− 6 m2s− 1) % variation

OC − 1.20E-06 – SP-S − 8.66E-07 (− )28 SP-B − 1.29E-06 (+)7 AQ-S − 1.09E-06 (− )9 HE-B − 1.46E-06 (+)22

J. Gonzalez-Coneo et al.

Journal of Building Engineering 46 (2022) 103729

13

combination of a porosity decrease and the higher cohesion of the material, in line with the higher performance observed for the drilling resistance.

3.4. Durability evaluation

3.4.1. Rain simulation test Rain is one of the most common natural phenomena which promotes water ingress in the concrete, accelerating its decay and, due

to the impact of the water drops, it can erode the surface and affect the hydrophobic treatments if their adherence is low enough. Fig. 6a and b shows the evolution of the static contact angle and hysteresis of the hydrophobic concrete surfaces through the simulated rain test. In line with the measurements after the capillary absorption test (Table 3), static contact angle did not vary significantly on the SP-B surface, confirming the chemical stability of the hydrophobic component. The hysteresis, however increased above 10◦, which may indicate modifications of the surface roughness. In a similar note, the contact angles of HE-B were barely affected by the rain test. The surface treatments (SP–S and AQ-S) retained their hydrophobic properties with minor fluctuations on their contact angle and hysteresis. As seen in a previous study [43] by SEM, the simulated rain is able to cause minor physical damages to the surface and detach part of the nanoparticles of the AQ-S coating, but it is not aggressive enough to cause the coating failure.

3.4.2. Abrasion test Mechanical wearing of the surfaces, either by their regular use in the case of concrete floors or pavements or, to a lesser extent,

erosion by physical agents (e.g. wind/sand) gradually acts on the building elements during their service life. These processes can modify the topography, hydrophobic performance and, in a worst-case scenario, completely remove a hydrophobic coating. In order to study the mechanical stability of the hydrophobic surfaces, the materials were subjected to an abrasion test and their contact angles were monitored (Fig. 6c and d) along with their topography study by SEM (Fig. 2).

In all cases, there was a decrease of the static contact angle and an increase of the hysteresis, although the magnitude depended on the treatment. By comparing the hydrophobic sol as a surface treatment (SP–S) and in bulk (SP–B), it was observed that the latter presented higher contact angles and similar hysteresis after the abrasion, though both surfaces retained their hydrophobic character (SCA >90◦). As observed in the drilling resistance test, the hydrophobic sol penetrates up to 4–5 mm in SP-S, so the hydrophobic character (SCA >90◦) should not be compromised under the test conditions. The SEM micrographs, however, showed that the SP-S surface presented some cracking and a lower roughness, which explains the lower contact angles according to the Wenzel wetting model. In the case of SP-B, the surface still presented a high roughness, although its more irregular structure may have caused the transition from a Cassie-Baxter to a Wenzel wetting regime, thus explaining its higher increase of the hysteresis.

Fig. 6. Evolution of the Static contact angle and contact angle hysteresis of the concrete surfaces through the durability tests: (a) rain simulation, (b) abrasion test.

J. Gonzalez-Coneo et al.

Journal of Building Engineering 46 (2022) 103729

14

On the other hand, the SEM images evidenced a partial removal of the nanoparticles coating form the AQ-S surface, which resulted in a more pronounced decrease of its static contact angle. Previous studies showed that hydrophobic SiO2 nanoparticles weakly attach to the substrate unless they are incorporated in a matrix [43], although the coating may maintain its hydrophobic character due to the (limited) particles that penetrate in the pores near the surface. The HE-B concrete was the most affected by the abrasion test, as evidenced by the final SCA values below 90◦ (i.e. the surface becomes hydrophilic). Considering that the topography was not severely affected by the abrasion in this concrete, this limited stability of the hydrophobic surface could be attributed to the weak attachment of the calcium stearate to the cementitious matrix (see cross-section micrographs in Fig. 2), which would facilitate its removal by the mechanical action.

4. Conclusions

Hydrophobic properties were obtained in non-structural concrete cured at ambient conditions by using a sol containing PDMS and TEOS oligomers in two different application forms: (1) as a hydrophobic impregnation surface treatment (SP–S), (2) as an admixture added to the fresh paste (SP–B). Both strategies showed a better hydrophobic performance and durability in the rain and abrasion tests in comparison with concrete containing calcium stearate as an admixture or treated with a hydrophobic coating based on hydrophobic nano-silica.

Addition of the sol as an admixture promotes a significant increase of the concrete porosity and pore size along with an increased vapor permeability. However, these structural changes do not compromise the hydrophobic performance, with a reduction of capillary water absorption of 77% and static contact angles >120◦ on the surface, due to the low surface energy of PDMS. The increased porosity did not negatively affect the impact resistance of the concrete nor the material cohesion, as indicated by the drilling resistance measurements and the higher performance after the abrasion test. Furthermore, the reaction of the sol with the cement matrix components seems to promote the formation of C–S–H like phases. Aesthetics of the concrete are not altered by the addition of the admixture, as evidenced by the imperceptible color a change (ΔE* < 3).

Application of the sol as a surface treatment produces similar results regarding the hydrophobic performance (capillary absorption and contact angles), although the angles are slightly lower due to a reduced roughness and minor performance losses occurred after the abrasion test due to a loss of roughness. In contrast with the application as admixture, surface treatment leads to a porosity decrease and a subsequent ~28% vapor permeability reduction due to the formation of SiO2 and C–S–H inside the pore network. The treatment leads to a higher cohesion (drilling resistance) through the first 4 mm and promotes an increase of the impact resistance. Color change after the treatment was slightly higher but still within the acceptable range for restrictive applications (ΔE* < 5).

In light of the results, oligomeric alkoxysilane/alkyl siloxane-based sols can be used as admixtures to attain hydrophobic properties on non-structural concrete, with similar performance compared to their usual application as surface treatment and without compromising the material cohesion or impact resistance. Along with the homogeneous distribution of the hydrophobic component through the bulk material, this application form could potentially increase the service life of the treated concrete elements.

Author statement

Jorge Gonzalez-Coneo– Writing - Original Draft, Investigation, Formal analysis, Conceptualization. Rafael Zarzuela- Writing - Original Draft, Formal analysis, Visualization. Farid Elhaddad Investigation, Conceptualization, Luis M. Carrascosa Investigation, Conceptualization. M.L. Almoraima Gil- Writing - Review & Editing, Visualization. Maria J. Mosquera - Writing - Review & Editing, Supervision, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This project has received funding from the European Union’s Horizon 2020 - Research and Innovation Framework Programme under grant agreement No 760858; This work has been financed by the Spanish State Research Agency R&D program 2020 (Project reference: PID2020-115843RB-I00); This work has been co-financed by the European Union under the 2014-2020 ERDF Operational Programme and by the Department of Economic Transformation, Industry, Knowledge, and Universities of the Regional Government of Andalusia (Project reference: FEDER- UCA18-106613).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jobe.2021.103729.

J. Gonzalez-Coneo et al.

Journal of Building Engineering 46 (2022) 103729

15

References

[1] S.H. Kosmatka, B. Kerkhoff, W.C. Panarese, N.F. MacLeod, R.J. McGrath, Design and Control of Concrete Mixtures, fourteenth ed., Portland Cement Association, Chicago, 2002.

[2] Z. Bonic, G.T. Curcc, M. Trivunic, N. Davidovic, N. Vatin, Some methods of protection of concrete and reinforcment of reinforced-concrete foundations exposed to environmental impacts, Procedia Eng 117 (2015) 424–435, https://doi.org/10.1016/j.proeng.2015.08.189.

[3] I. Papayianni, G. Tsohos, N. Oikonomou, P. Mavria, Influence of superplasticizer type and mix design parameters on the performance of them in concrete mixtures, Cement Concr. Compos. (2005), https://doi.org/10.1016/j.cemconcomp.2004.02.010.

[4] M. Medeiros, P. Helene, Efficacy of surface hydrophobic agents in reducing water and chloride ion penetration in concrete, Mater. Struct. 41 (2007) 59–71, https://doi.org/10.1617/s11527-006-9218-5.

[5] E. Borrelli, in: C. McDowall, C. Rockwell (Eds.), ARC Laboratory Handbook: Conservation of Architectural Heritage, Historic Structrues and Materials, ICCROM, Rome, 1999, p. 23.

[6] A.A. Hilal, Microstructure of Concrete, in: H.B.-O. Salih Yilmaz (Ed.), High Perform. Concr. Technol. Appl., IntechOpen, Albania, 2016, p. 22, https://doi.org/ 10.5772/61562.

[7] X. Pan, Z. Shi, C. Shi, T.-C. Ling, N. Li, A review on concrete surface treatment Part I: types and mechanisms, Construct. Build. Mater. 132 (2017) 578–590, https://doi.org/10.1016/J.CONBUILDMAT.2016.12.025.

[8] E.W. Washburn, The dynamics of capillary flow, Phys. Rev. 17 (1921) 273–283, https://doi.org/10.1103/PhysRev.17.273. [9] R.N. Wenzel, Resistance of solid surfaces to wetting by water, Ind. Eng. Chem. 28 (1936) 988–994, https://doi.org/10.1021/ie50320a024.

[10] A.B.D. Cassie, S. Baxter, Wettability of porous surfaces, Trans. Faraday Soc. 40 (1944) 546–551, https://doi.org/10.1039/tf9444000546. [11] Z. Yoshimitsu, A. Nakajima, T. Watanabe, K. Hashimoto, Effects of surface structure on the hydrophobicity and sliding behavior of water droplets, Langmuir 18

(2002) 5818–5822, https://doi.org/10.1021/la020088p. [12] S. Muzenski, I. Flores-Vivian, K. Sobolev, Durability of superhydrophobic engineered cementitious composites, Construct. Build. Mater. 81 (2015) 291–297,

https://doi.org/10.1016/j.conbuildmat.2015.02.014. [13] V. Spaeth, J.P. Lecomte, Hydration Process and Microstructure Development of Integral Water Repellent Cement Based Materials, in: Hydrophobe 5th Int. Conf.

Water Repel. Treat. Build. Mater., Aedificatio Publishers, Freiburg, 2008, pp. 245–254. [14] S.W. Muzenski, I. Flores-Vivian, K. Sobolev, The development of hydrophobic and superhydrophobic cementitious composites, Proc. 4th Int. Conf. Durab. Concr.

Struct. ICDCS 1 (2014) (2014) 1–6. [15] F. Tittarelli, G. Moriconi, Comparison between surface and bulk hydrophobic treatment against corrosion of galvanized reinforcing steel in concrete, Cement

Concr. Res. 41 (2011) 609–614, https://doi.org/10.1016/j.cemconres.2011.03.011. [16] L. Ferrara, A. Pattarini, Siloxanes in Concrete: from Manual Application of Water-Proofing Treatments to Mix-Design Addition for Concrete Hydrophobicity, Key

Eng. Mater., 2016, https://doi.org/10.4028/www.scientific.net/KEM.711.263. [17] H. Feng, H.T.N. Le, S. Wang, M.H. Zhang, Effects of silanes and silane derivatives on cement hydration and mechanical properties of mortars, Construct. Build.

Mater. 129 (2016) 48–60, https://doi.org/10.1016/j.conbuildmat.2016.11.004. [18] I. Garcia-Lodeiro, P.M. Carmona-Quiroga, R. Zarzuela, M.J. Mosquera, M.T. Blanco-Varela, Chemistry of the interaction between an alkoxysilane-based

impregnation treatment and cementitious phases, Cement Concr. Res. 142 (2021), 106351, https://doi.org/10.1016/j.cemconres.2020.106351. [19] F. Sandrolini, E. Franzoni, B. Pigino, Ethyl silicate for surface treatment of concrete - Part I: pozzolanic effect of ethyl silicate, Cement Concr. Compos. 34 (2012)

306–312, https://doi.org/10.1016/j.cemconcomp.2011.12.003. [20] A. Moropoulou, A. Cakmak, K.C. Labropoulos, R. Van Grieken, K. Torfs, Accelerated microstructural evolution of a calcium-silicate-hydrate (C-S-H) phase in

pozzolanic pastes using fine siliceous sources: comparison with historic pozzolanic mortars, Cement Concr. Res. 34 (2004) 1–6, https://doi.org/10.1016/S0008- 8846(03)00187-X.

[21] R. Zarzuela, M. Luna, L.M. Carrascosa, M.P. Yeste, I. Garcia-Lodeiro, M.T. Blanco-Varela, M.A. Cauqui, J.M. Rodríguez-Izquierdo, M.J. Mosquera, Producing C-S- H gel by reaction between silica oligomers and portlandite: a promising approach to repair cementitious materials, Cement Concr. Res. 130 (2020), 106008, https://doi.org/10.1016/j.cemconres.2020.106008.

[22] A.M. Barberena-Fernandez, P.M. Carmona-Quiroga, M.T. Blanco-Varela, Interaction of TEOS with cementitious materials: chemical and physical effects, Cement Concr. Compos. 55 (2015) 145–152, https://doi.org/10.1016/j.cemconcomp.2014.09.010.

[23] J.F. Illescas, M.J. Mosquera, Producing surfactant-synthesized nanomaterials in situ on a building substrate, without volatile organic compounds, ACS Appl. Mater. Interfaces 4 (2012) 4259–4269, https://doi.org/10.1021/am300964q.

[24] A.C.M. Kuo, Poly ( Dimethylsiloxane ), second ed., Oxford University Press, Oxford, 1999. [25] F. Wang, S. Lei, J. Ou, W. Li, Effect of PDMS on the waterproofing performance and corrosion resistance of cement mortar, Appl. Surf. Sci. 507 (2020) 1–10,

https://doi.org/10.1016/j.apsusc.2019.145016. [26] L.A.M. Carrascosa, D.S.D.S. Facio, M.J.M.J. Mosquera, Producing superhydrophobic roof tiles, Nanotechnology 27 (13pp) (2016), 095604, https://doi.org/

10.1088/0957-4484/27/9/095604. [27] A. Maryoto, B.S. Gan, N.I.S. Hermanto, R. Setijadi, Effect of calcium stearate in the mechanical and physical properties of concrete with pcc and fly ash as

binders, Materials 13 (2020) 1–16, https://doi.org/10.3390/ma13061394. [28] J.D. Rodrigues, A. Grossi, Indicators and ratings for the compatibility assessment of conservation actions, J. Cult. Herit. 8 (2007) 32–43, https://doi.org/

10.1016/j.culher.2006.04.007. [29] M.J. Mosquera, D. Benıtez, S.H. Perry, Pore structure in mortars applied on restoration, Cement Concr. Res. 32 (2002) 1883–1888, https://doi.org/10.1016/

S0008-8846(02)00887-6. [30] S.D. Wang, Y.S. Jiang, The durability of superhydrophobic films, Appl. Surf. Sci. 357 (2015) 1647–1657, https://doi.org/10.1016/j.apsusc.2015.10.005. [31] B. Chen, H. Shao, B. Li, Z. Li, Influence of silane on hydration characteristics and mechanical properties of cement paste, Cement Concr. Compos. 113 (2020),

https://doi.org/10.1016/j.cemconcomp.2020.103743. [32] I.G. Richardson, The nature of CSH in hardened cements, Cement Concr. Res. 39 (1999) 1131–1147, https://doi.org/10.1016/S0008-8846(99)00168-4. [33] M. Nemati Chari, R. Naseroleslami, M. Shekarchi, The impact of calcium stearate on characteristics of concrete, Asian J. Civ. Eng. 20 (2019) 1007–1020,

https://doi.org/10.1007/s42107-019-00161-x. [34] D.S. Facio, M. Luna, M.J. Mosquera, Facile preparation of mesoporous silica monoliths by an inverse micelle mechanism, Microporous Mesoporous Mater. 247

(2017) 166–176, https://doi.org/10.1016/j.micromeso.2017.03.041. [35] J. Perko, R. Zarzuela, I. Garcia-Lodeiro, M.T. Blanco-Varela, M.J. Mosquera, T. Seemann, L. Yu, The importance of physical parameters for the penetration depth

of impregnation products into cementitious materials: modelling and experimental study, Construct. Build. Mater. 257 (2020), 119595, https://doi.org/ 10.1016/j.conbuildmat.2020.119595.

[36] L.A.M.M. Carrascosa, R. Zarzuela, N. Badreldin, M.J. Mosquera, A simple, long-lasting treatment for concrete by combining hydrophobic performance with a photoinduced superhydrophilic surface for easy removal of oil pollutants, ACS Appl. Mater. Interfaces 12 (2020) 19974–19987, https://doi.org/10.1021/ acsami.0c03576.

[37] M. Nosonovsky, B. Bhushan, Superhydrophobic surfaces and emerging applications: non-adhesion, energy, green engineering, Curr. Opin. Colloid Interface Sci. 14 (2009) 270–280, https://doi.org/10.1016/j.cocis.2009.05.004.

[38] T. Young, An Essay on Cohesion of Fluids, Royal Society, London, 1805. [39] H.W. Song, S.J. Kwon, Permeability characteristics of carbonated concrete considering capillary pore structure, Cement Concr. Res. 37 (2007) 909–915, https://

doi.org/10.1016/j.cemconres.2007.03.011. [40] M. Safiuddin, Concrete damage in field conditions and protective sealer and coating systems, Coatings 90 (2017) 2–22, https://doi.org/10.3390/

coatings7070090.

J. Gonzalez-Coneo et al.

Journal of Building Engineering 46 (2022) 103729

16

[41] Portland Cement Association, Types and Causes of Concrete Deterioration, Skokie, 2002. [42] X. Pan, Z. Shi, C. Shi, T.C. Ling, N. Li, A review on surface treatment for concrete – Part 2: Performance, Construct. Build. Mater. 133 (2017) 81–90, https://doi.

org/10.1016/j.conbuildmat.2016.11.128. [43] L.A.M. Carrascosa, R. Zarzuela, N. Badreldin, M.J. Mosquera, A simple, long-lasting treatment for concrete by combining hydrophobic performance with a

photoinduced superhydrophilic surface for easy removal of oil pollutants, ACS Appl. Mater. Interfaces 12 (2020) 19974–19987, https://doi.org/10.1021/ acsami.0c03576.

J. Gonzalez-Coneo et al.