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EFFECTS OF TYPE OF ACTIVATOR ON FIBER-MATRIX INTERFACE PROPERTIES
AND TENSILE BEHAVIOR OF STRAIN-HARDENING GEOPOLYMER COMPOSITES
Conference Paper · October 2018
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Behzad Nematollahi
Swinburne University of Technology
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EFFECTS OF TYPE OF ACTIVATOR ON FIBER-MATRIX INTERFACE PROPERTIES AND TENSILE BEHAVIOR OF STRAIN-HARDENING GEOPOLYMER COMPOSITES
Behzad Nematollahi1 and Jay Sanjayan2
1ARC DECRA Fellow, Center for Sustainable Infrastructure, Swinburne University of Technology 2Professor and Director, Center for Sustainable Infrastructure, Swinburne University of Technology
Abstract:
This study reports the quantitative influences of type of activator on the microscale fiber-matrix interface
properties, and their consequent effects on the tensile performance of a fiber-reinforced strain-hardening
geopolymer composite (SHGC). Single-fiber pullout tests were conducted to measure the fiber-matrix
interface properties of two fly ash-based SHGCs made by using one sodium (Na) and one potassium (K)
based activators. Effects of the experimentally measured interface properties on the crack bridging (σ-δ)
relation of the SHGCs were investigated using a micromechanics-based model to explain the
experimentally observed macroscopic tensile ductility of the SHGCs. The results indicated that the type
of activator had significant effects on the fiber-matrix interface properties. The chemical bond strength
of polyvinyl alcohol (PVA) fiber with the Na-based SHGC matrix was 83% lower than that of the K-
based SHGC matrix. In contrast, the frictional bond strength and slip hardening coefficient of the PVA
fiber with the Na-based SHGC matrix were about 2 times and more than 18 times, respectively higher
than those of the K-based SHGC matrix. The ultimate tensile strength and tensile strain capacity of the
Na-based SHGC were 139% and 50%, respectively higher than those of the K-based SHGC. The superior
tensile performance of the Na-based SHGC is attributed to its beneficial fiber-matrix interface properties,
and thereby its higher pseudo strain-hardening (PSH) performance indices, as compared to the K-based
SHGC.
Keywords: strain-hardening geopolymer composite (SHGC); type of activator, fiber-matrix
interface; tensile performance; PSH performance indices.
i. Introduction
Strain-hardening cementitious composite (SHCC) is a micromechanics-based design fiber-reinforced
brittle matrix composite which exhibits pseudo strain-hardening (PSH) behavior with extreme tensile
ductility in the range of 3-5% at a moderate fiber content of 2% or less by volume (Li, Wang & Wu
2001). Material sustainability, however, has not often been a concern in development of SHCCs. In
comparison to conventional concrete, large amount of cement is usually used in this composite resulting
in high material cost, autogenous shrinkage and heat of hydration (Wang & Li 2007). In addition, the
associated increase in the CO2 emissions and embodied energy arising from the production of ordinary
Portland cement (OPC) can compromise sustainability credentials of SHCCs. A sustainable approach to
significantly enhance the material sustainability performance of the composite is to incorporate a
geopolymer binder as ‘complete’ replacement of OPC in SHCC composition.
Geopolymer is an emerging OPC-less binder purported to provide an environmentally friendly
alternative to OPC. Previous studies reported that at least 80% less CO2 is emitted and about 60% less
energy is needed for manufacture of fly ash-based geopolymer in comparison to production of OPC (Li
& al. 2004; Duxson & al. 2007). Geopolymers can be manufactured by alkaline activation of fly ash
and/or slag, being industrial by-products coal power stations and iron manufacture, respectively which
contain high volumes of silica and alumina (Nematollahi, Sanjayan & Shaikh 2015a).
The researchers at Swinburne University of Technology, Australia have recently conducted a series
of studies aimed on multiscale development and investigation of properties of strain-hardening
geopolymer composite (SHGC). SHGC is a sustainable strain-hardening composite where OPC is
completely replaced by a geopolymer binder. The quantitative influences of the geopolymer matrix-
related parameters such as type of alkaline activator, water to geopolymer solids ratio (W/GP solids),
sand size and sand content on the matrix and composite properties of fly ash-based SHGCs have been
previously investigated by the authors of this paper. The fly ash-based SHGC developed by the authors
exhibited comparable properties to typical SHCCs (Nematollahi, Sanjayan & Shaikh 2014a&b;
Nematollahi, Sanjayan & Shaikh 2015b&c&d; Nematollahi, Sanjayan & Shaikh 2016). At the same time,
it had 52% less carbon emissions and 17% less energy consumption as compared to SHCC mix 45 (M45)
(Nematollahi & al. 2017a).
However, the fiber-matrix interface properties, and their consequent effects on crack-bridging and
tensile performance of the developed fly ash-based SHGC have not yet been investigated. Understanding
the microscale fiber-matrix interaction properties and mechanisms is of primary importance in design of
SHGCs. Therefore, this study as a follow up investigation aims to provide an in-depth understanding of
the quantitative influences of type of activator on the microscale fiber-matrix interface properties, and
their consequent effects on tensile performance of the developed fly ash-based SHGC. This
understanding presents the rational basis for design of such OPC-less strain-hardening composites.
ii. Experimental Procedures
a. Materials and mix proportions.
The low calcium fly ash was provided by Gladstone power station in Queensland, Australia. Two
different sodium-based (Na-based) and potassium-based (K-based) activator combinations were used in
this study. The Na-based activator combination was made of 8.0 M sodium hydroxide (NaOH) solution
and D Grade sodium silicate (Na2SiO3) solution (29.4 wt.% SiO2 and 14.7 wt.% Na2O) with Na2SiO3 to
NaOH mass ratio of 2.5. The K-based activator combination was made of 8.0 M potassium hydroxide
(KOH) solution and KASIL 2236 Grade potassium silicate (K2SiO3) solution (24.8 wt.% SiO2 and 11.2
wt.% K2O) with K2SiO3 to KOH mass ratio of 2.5. The PVA fiber was supplied by Kuraray Co. Ltd.,
Japan with a surface oil coating of 1.2% by weight. The PVA fiber has a density of 1.3 g/cm3, length of
8 mm, diameter of 39 μm, elongation of 6% and elastic modulus of 41 GPa. The nominal tensile strength
of the PVA fiber is 1600 MPa. As shown in Table 1, the SHGC-Na and SHGC-K mixtures compare the
effect of type of activator on the basis of the same compressive strength of the SHGC matrix.
Table 1: Mix proportions of fly ash-based SHGCs.
Mix ID Fly ash (kg/m3)
Activator (kg/m3)
PVA fiber (kg/m3)
Target compressive strength of SHGC matrix (MPa)
SHGC-Na 1240.0 558.0 26 32
SHGC-K 1335.6 467.4 26 32
b. Mixing, curing and testing of specimens
To make the mixtures, alkaline solution was added to fly ash in a Hobart mixer and thoroughly mixed
for about 4 minutes. The PVA fibers were gradually added to ensure uniform fiber dispersion. Heat curing
(60°C for 24 hours) was adopted in this study. The procedure for the heat curing can be found in
Nematollahi, Sanjayan & Shaikh (2015b).
For each mixture, a minimum of three 50 mm cube matrix specimens and three 50 mm cube composite
specimens for compression tests, three 100×200 mm cylindrical matrix specimens for matrix elastic
modulus tests, four matrix prisms (without addition of the fibers) with the dimensions of 60 mm×60
mm×280 mm for matrix fracture toughness tests, and three rectangular coupon specimens with the
dimensions of 400 mm×75 mm×10 mm for uniaxial tension tests were prepared. All specimens were
tested 3 days after casting, as the strength of fly ash-based geopolymer does not change considerably
after the completion of the heat curing period and three days compressive strength of heat cured fly ash-
based geopolymer is comparable to a typical OPC strength development after 28-days (Nematollahi &
Sanjayan 2014).
The compressive strength of matrix and composite was measured in accordance with ASTM C109
(2007). The elastic modulus of the SHGC matrix (Em) was measured in accordance with AS1012.17
(1997). The matrix fracture toughness (Km) was measured in accordance with effective crack model
developed by Karihaloo & Nallathambi (1990) using three-point bending test on single edge notched
matrix prisms. The procedure of the matrix fracture toughness test can be found in Nematollahi, Sanjayan
& Shaikh (2015b). Tensile performance was evaluated using uniaxial tension test on rectangular coupon
specimens under displacement control at the rate of 0.25 mm/min over a gauge length of about 200 mm.
Details of the uniaxial tension test are given in Nematollahi, Sanjayan & Shaikh (2015b).
Single-fiber pullout tests were conducted to measure the fiber-matrix interface properties, including
chemical bond strength (Gd), frictional bond strength (τ0), and slip-hardening coefficient (β). For each
mixture, at least four single-fiber pullout specimens were tested under displacement control at the rate
of 0.03 mm/min. Details of the single-fiber pullout test are given in Nematollahi & al. (2017b&c).
iii. Results and Discussions
a. Compressive strength
The average compressive strength of each mixture is given in Table 2. While the compressive strength
of the SHGC matrix in SHGC-Na and SHGC-K was comparable, the compressive strength of SHGC-
Na was 41% higher than that of SHGC-K. This could be attributed to the effect of type of activator on
the fiber-matrix interface properties.
Table 2: Compressive strength results.
Mix ID Compressive strength; (MPa)
Matrix Composite
SHGC-Na 31.6±1.5 52.6±1.6
SHGC-K 32.3±1.4 37.3±1.3
b. Matrix fracture properties
The fracture properties of SHGC matrices (without addition of the fibers) are summarized in Table 3.
The elastic modulus of SHGC-Na and SHGC-K matrices was identical, which corresponds to their
comparable compressive strength as reported in Table 2. However, the Km and thereby the 𝐽𝑡𝑖𝑝 of SHGC-
Na matrix were 14% and 28%, respectively higher than those of the SHGC-K matrix. The different Km
of SHGC-Na and SHGC-K matrices could be attributed to their different geopolymer microstructures,
owing to their different type of activator (Fernández-Jiménez & Palomo 2005). According to Fernández-
Jiménez & Palomo (2005), in fly ash-based geopolymer the type of activator has a significant effect on
the microstructure of the aluminosilicate gel. It is thereby concluded that the K-based activator increases
the brittleness of the fly ash-based SHGC matrix.
Table 3: Matrix fracture properties.
Mix ID
Matrix elastic modulus, Em;
(GPa)
Matrix fracture toughness, Km;
(MPa.m1/2)
Crack tip toughness, Jtip;
(J/m2)
SHGC-Na matrix 7.5 0.269 9.6
SHGC-K matrix 7.5 0.237 7.5
c. Fiber-matrix interface properties
The fiber-matrix interface properties of each mix are summarized in Table 4. The Scanning Electron
Microscope (SEM) images of the typical pulled-out fiber of each mix are shown in Figs. 1 and 2. As can
be seen in Table 4, the type of activator had significant effects on the interface properties. The chemical
bond strength of the oil-coated fiber in SHGC-Na was 83% lower than that of SHGC-K. On the other
hand, the frictional bond and slip hardening coefficient of the fiber in SHGC-Na were about 2 times and
more than 18 times, respectively higher than those of the fiber in SHGC-K.
Table 4: Fiber-matrix interface properties using oil-coated PVA fibers.
Mix ID Chemical bond
strength, Gd; (J/m2) Frictional bond
strength, τ0; (MPa) Slip hardening coefficient, β
SHGC-Na 0.59±0.67 1.73±0.51 0.463±0.200
SHGC-K 3.41±1.71 0.87±0.44 0.025±0.015
Fig. 1: SEM images of SHGC-Na using oil-coated PVA fiber.
Fig. 2: SEM images of SHGC-K using oil-coated PVA fiber.
It can be said that the K-based activator significantly enhances the chemical bond between the PVA
fiber and the SHGC matrix. As can be seen in Figs. 1 and 2, surface morphology of the pulled-out fibers
in SHGC-Na and SHGC-K is different. In SHGC-Na (Fig. 1), the pulled-out fiber tip is covered with
matrix debris and its diameter is relatively un-changed as compared to the original fiber diameter (40μm).
On the other hand, in SHGC-K (Fig. 2), the pulled-out fiber tip was sharpened and the fiber surface was
severely scratched. This explains the significantly higher chemical bond strength of the oil-coated fiber
in SHGC-K than SHGC-Na. This may be attributed to greater attack of the K-based activator to the oil-
coating agent on the surface of the PVA fiber. It has been reported that KOH solution can penetrate in oil
molecules faster than NaOH solution due its higher alkalinity and solubility in water (Diphare &
Muzenda 2013). As a result, it is believed that KOH breaks the oil coating agent on the fiber surface
quicker (Diphare & Muzenda 2013), which causes the increase of chemical bond. Without the protection
from the surface oil coating, oil-coated PVA fiber in the K-based SHGC matrix may also be weakened
due to strong alkalinity of the matrix, and delamination damage of the fiber during debonding may occur,
which results in the decrease of interface frictional bond, as shown in Table 4. As can be seen in Fig. 2,
the delamination surface of the oil-coated fiber in SHGC-K is relatively smooth as compared to the
surface of the oil-coated fiber in SHGC-Na (Fig. 1). This also explains the low interface frictional bond
of the oil-coated fiber in SHGC-K than SHGC-Na.
d. Uniaxial tensile performance
The uniaxial tensile stress–strain curves of each mixture are presented in Fig. 3. As can be seen, both fly
ash-based SHGCs, irrespective of the type of activator demonstrated PSH behavior. Table 5 presents the
uniaxial tension test results of each mixture. The first-crack strength of each mixture is estimated from
the tensile stress-strain curves following the method proposed by Kanda & Li (2006).
Fig. 3: Tensile stress-strain responses of fly ash-based SHGCs.
Table 5: Uniaxial tension test results.
Mix ID First-crack strength, σfc;
(MPa) Ultimate tensile strength,
σcu ; (MPa) Tensile strain
capacity, εcu; (%)
SHGC-Na 2.8 ± 0.40 4.3 ± 0.45 3.0 ± 0.19
SHGC-K 1.4 ± 0.062 1.8 ± 0.21 2.0 ± 0.26
As can be seen in Table 5, the type of activator had a significant effect on the uniaxial tensile behavior
of fly ash-based SHGC. The first-crack strength, ultimate tensile strength and tensile strain capacity of
SHGC-K were 52%, 58% and 33%, respectively lower than those of SHGC-Na. The lower first-crack
strength of SHGC-K corresponds to the lower fracture toughness of the K-based SHGC matrix, as
reported in Table 3 (Li, Wang & Wu 2001). The lower ultimate tensile strength of SHGC-K is due to the
significantly lower τ0 and β of the PVA fiber with the K-based SHGC matrix, as reported in Table 4,
which led to its inferior fiber-bridging strength (Nematollahi, Sanjayan & Shaikh 2015b; Nematollahi,
Sanjayan & Shaikh 2016).
The underlying reasons for the lower tensile strain capacity of SHGC-K could be elucidated based on
the PSH strength and PSH energy performance indices proposed by Kanda & Li (2006). The two PSH
performance indices should be determined from the crack bridging relation of the composite. In this
study, the micromechanics-based model developed by Yang & al. (2008) was used to compute the fiber-
bridging constitutive law σ(δ) of fly ash-based SHGCs. The applicability of this micromechanics-based
model for evaluating the tensile performance of fly ash-based SHGCs was verified in a recent study by
the authors of this paper (Nematollahi & al. 2017d). The resulting PSH performance indices are plotted
in Fig. 4. As can be seen, in all fly ash-based SHGCs both PSH performance indices are greater than one.
Thereby, it can be concluded that the necessary strength-based and energy-based conditions of steady-
state flat crack propagation which result in sequential multiple cracking of geopolymer matrix are
satisfied. Therefore, both fly ash-based SHGCs investigated in this study exhibited PSH behavior. In
addition, as can be seen in Fig. 4, the PSH strength and energy indices of SHGC-K were considerably
lower than those of the Na-based SHGC. As a result, it is not surprising that SHGC-K with lower PSH
performance indices exhibited inferior tensile ductility than that of SHGC-Na.
Fig. 4: PSH performance indices of fly ash-based SHGCs.
Fig. 5 presents the clear pattern of multiple cracking in each mixture. The multiple cracking pattern
was obtained by marking all visible cracks on an unloaded specimen after the uniaxial tension test using
a permanent marker. The crack spacing was determined based on the number of visible cracks counted
manually from an unloaded specimen, and the gauge length. As can be seen in Fig. 5, enormous number
of cracks with almost equal crack spacing ranging from 2.5 mm to 3.5 mm was observed in the SHGC-
Na specimen. The saturated multiple cracking behavior observed in the SHGC-Na specimen corresponds
to its superior tensile strain capacity, as reported in Table 5. In contrast, the lowest number of cracks
with un-even spacing ranging from 4-11 mm was observed in the K-based SHGC specimen. The
unsaturated multiple cracking behavior observed in the K-based SHGC specimen corresponds to its
inferior tensile strain capacity, as reported in Table 5.
Fig. 5: Typical multiple cracking pattern of fly ash-based SHGCs.
iv. Conclusions
The quantitative influences of type of activator on the microscale fiber-matrix interface properties, and
their consequent effects on the tensile performance of a fly ash-based SHGC were investigated in this
study. The following conclusions are drawn:
1) The fiber surface oil-coating does not provide much benefit when used with the K-based activator
compared to the Na-based activator. The chemical bond of the oil-coated fiber in the K-based SHGC was
considerably higher than that of the Na-based SHGC. It can be said that the K-based activator
significantly enhances the chemical bond between the PVA fiber and the geopolymer matrix, which may
result in delamination damage of the fiber during the debonding stage. On the other hand, the frictional
bond and slip hardening coefficient of the oil-coated fiber in the K-based SHGC were considerably lower
than those of the Na-based SHGC, due to the relatively smooth delamination surface of the fiber.
2) Uniaxial tension test results indicated that the Na-based SHGC exhibited superior tensile
performance to the K-based SHGC, in spite of the higher fracture toughness of the Na-based SHGC
matrix. This is due to the significantly higher complementary energy of the Na-based SHGC, resulted
from its beneficial fiber-matrix interface properties.
3) The experimentally observed macroscopic composite tensile ductility corresponded well with the
two PSH performance indices of the composite calculated based on the computed crack bridging relation
of the fly ash-based SHGCs. It was found that the two PSH performance indices of the Na-based SHGC
were considerably higher than those of the K-based SHGC, thus providing a rational basis for the
observed superior tensile performance of the Na-based SHGC.
Acknowledgment The authors gratefully acknowledge Assistant Professor En-Hua Yang and Dr. Jishen Qiu from Nanyang
Technological University, Singapore for conducting the single-fiber pullout tests and micromechanics
modelling of the developed SHGC.
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