Environmental Engineering and Management Journal November 2013, Vol.12, No. S11, Supplement, 31-34

http://omicron.ch.tuiasi.ro/EEMJ/

“Gheorghe Asachi” Technical University of Iasi, Romania

NEW COMPOSITE PANELS WITH HEMP HURDS

FOR SUSTAINABLE BUILDINGS

Extended abstract

Stefania Manzi1, Enrico Sassoni1,2, Antonio Motori1,2, Matteo Montecchi3, Max Canti3

11Department of Civil, Chemical, Environmental and Materials Engineering (DICAM),

University of Bologna, via Terracini 28, 40131 Bologna, Italy 2Centro Interdipartimentale di Ricerca Industriale (CIRI) Edilizia e Costruzioni

University of Bologna, via Lazzaretto 15/5, 40131 Bologna, Italy 3CMF TECHNOLOGY S.P.A. Via Bottegone 73, 41026 Pavullo nel Frignano (Modena), Italy

Background

The issue of developing new sustainable building materials, having a reduced impact on the environment, has become a very important task, as the building sector is responsible for huge natural resources consumption and waste production. For reducing the use of not renewable natural resources, several strategies have been investigated through the years, such as the use of recycled aggregates from construction and demolition waste to replace natural aggregates for structural concrete (Manzi et al., 2013) or the use of natural fibers from vegetable renewable resources to replace inorganic mineral fibers for thermal insulating composites (Korjenic et al., 2011; Motori et al., 2013).

In particular, the use of natural fibers (such as hemp, jute, flax, sisal, etc.) for producing thermal insulating composites offers several advantages, as natural fibers generally exhibit good thermal and acoustic insulating properties (thanks to their cellular structure), low density, good specific tensile properties, and, from the environmental point of view, they are characterized by renewability, recyclability and high carbon storage potential (Ku et al., 2011; Zach et al., 2013; Zampori et al., 2013).

The above mentioned characteristics, as well as the rapid growing and the high dry biomass production (Motori et al., 2013), have made hemp one of the most widely studied natural fibers for sustainable building materials. Indeed, hemp-based panels for building thermal insulation, produced by bonding hemp fibers and hurds with polyester, Portland cement or slaked lime, have been investigated (Balčiūnas et al., 2013; Zampori et al., 2013). So-obtained panels exhibited promising thermal-mechanical properties (Balčiūnas et al., 2013; Franzoni, 2011). Moreover, a reduced environmental impact of hemp-based panels, compared to traditional rock/glass wool panels, was assessed by Life Cycle Assessment (Zampori et al., 2013). Objectives

In addition to the use of natural fibers and particles from plants, in place of inorganic mineral fibers, a further step towards obtaining composite materials with enhanced sustainability is using also binders with reduced environmental impact as composite matrix (La Mantia and Morreale, 2011). In the case of hemp-based composites for the building industry, hemp hurds are generally bonded with Portland cement, slaked lime or organic polymers (Balčiūnas et al., 2013; Zampori et al., 2013). In terms of environmental sustainability, these binders exhibit some limitations, as Portland cement and slaked lime require high firing temperature and petroleum-based polymers are produced from not renewable resources. To overcome such limitations, the use of alternative binders, such as starch, has been investigated (Balčiūnas et al., 2013).

In this study, the preliminary results of the physical-mechanical characterization of new hemp-based composites, obtained by using alternative binders with much reduced environmental impact, are reported. Four types of panels were produced for different applications: two panel types (P-1 and P-2), having relatively low density, were

Author to whom all correspondence should be addressed: e-mail: enrico.sassoni2@unibo.it

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investigated as possible composites for building thermal insulation; for the other two types (P-3 and P-4), having relatively high density, the possible use in the building/furniture industry, as substitutes for high density formaldehyde-bonded wood particles boards, was investigated. In the latter case, in addition to the environmental benefit, a further benefit in terms of impact on human health is expected, as the new composites are free from toxic organic binders, such as formaldehyde, classified as "carcinogenic to humans", which can be emitted into the indoor environment by formaldehyde-containing building materials (Gunshera et al., 2013). Materials

The panels were produced as described in the following and summarized in Table 1: panels P-1 were obtained by bonding coarse hemp hurds (about 3-6 mm in diameter and 10-30 mm in

length) with an inorganic binder, consisting of a mineral oxide which reacts when a proper aqueous salt solution is added; a binder/hemp hurds ratio of 1:1 wt./wt. was used; the composites were produced by pressing the fresh mixture at room temperature for 20 minutes with a compression pressure of 5 MPa; the final panels had a nominal thickness t = 50 mm; panels P-2 were produced by bonding coarse hemp hurds (about 3-6 mm in diameter and 10-30 mm in

length) with a new hybrid organic-inorganic binder, obtained by partly replacing the mineral oxide used for panels P-1 with a reactive organic fraction, coming from renewable vegetable resources (the new binder is currently under patent - Italian patent request MO2011A000244, filed on 26/09/2011); a binder/hemp hurds ratio of 1:1 wt./wt. was used; the composites were produced by pressing the fresh mixture at 80 °C for 3 minutes with a compression ratio of 1:3.3, after preliminary pre-heating at 80 °C in a micro-wave unit for 3 minutes; the final panels had a nominal thickness t = 50 mm; panels P-3 were obtained by bonding thin hemp hurds (about 3-4 mm in diameter and 10 mm in length)

with an inorganic binder, consisting of a reactive hydraulic compound; a binder/hemp hurds ratio of 1:6 wt./wt. was used; the composites were produced by pressing the fresh mixture at room temperature for 20 minutes with a pressure of 5 MPa; the final panels had a nominal thickness t = 50 mm; panels P-4 were produced by bonding thin hemp hurds (about 1-2 mm in diameter and 2-5 mm in length)

with the same hybrid organic-inorganic binder as P-4; a binder/hemp hurds ratio of 1:1.5 wt./wt. was used; the composites were produced by pressing the fresh mixture at 80 °C for 3 minutes with a compression ratio of 1:4, after preliminary pre-heating at 80 °C in a micro-wave unit for 3 minutes; the final panels had a nominal thickness t = 10 mm.

Methods

The panels were subjected to the following characterization: bulk density (ρ) [measured on the same panels used for mechanical tests]; water absorption (WA), measured according to EN 13755 Standard on 5 × 5 × t cm3 samples; thermal conductivity (λ), measured according to EN 12667 Standard on 50 × 50 × t cm3 samples; λ was not

determined for P-3 and P-4 panels, as their λ is expected to be relatively high because of their high density; compressive strength (σc) corresponding to a strain ε = 10%, measured according to EN 826 Standard on 5 ×

5 × t cm3 samples; σc was not determined for P-4 panels because, considering their thickness (t = 10 mm), the friction between the loading plates and the panel would make the test results scarcely reliable; flexural strength (σf), according to EN 12089 Standard on 26 × 12 × t cm3 samples; microstructural features, by means of a stereo-optical microscope (SOM, Olympus SZX10). Five measurements were performed for each physical and mechanical test.

Results and discussion

As reported in Table 1, P-1 and P-2 panels, designed as possible composites for building thermal insulation, have a bulk density of 620 and 640 kg/m3, respectively. As can be observed in Fig. 1, they are characterized by a highly porous microstructure, which is responsible for a high water absorption (Table 1).

As a consequence of the low bulk density, as well as the cellular structure of hemp hurds, panels P-1 and P-2 exhibit relatively low values of thermal conductivity: λ = 0.087 W/mK for P-1 and λ = 0.138 W/mK. Such values are in good agreement with thermal conductivity values reported in the literature for composite materials from natural resources with a similar bulk density (e.g. wood wool boards and wood chip boards) (Domìnguez-Muñoz et al., 2010).

Even if thermal insulation provided by P-1 and P-2 results lower than that obtained by using traditional materials (e.g., glass and rock wool, expanded polystyrene, polyurethane foam) with lower density, P-1 and P-2 are characterized by higher mechanical properties. As shown in Fig. 2, compressive strength reached 1.9 MPa and 3.0 MPa for P-1 and P-2 panels, respectively. As a comparison, for a hemp hurds-Portland cement-based composite with a comparable bulk density (ρ = 519 kg/m3) a compressive strength of 1.5 MPa was reported (Balčiūnas et al., 2013).

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As for flexural strength, P-1 and P-2 reached 3.5 MPa and 6.8 MPa, respectively, also in this case exhibiting very promising mechanical properties.

Table 1. Mix design and physical properties of the panels

Unit P-1 P-2 P-3 P-4 Hemp hurds - coarse coarse thin thin Binder/hemp hurds ratio wt./wt. 1:1 1:1 1:6 1:1.5 Bulk density (ρ) kg/m3 620 640 1260 1280 Water absorption (WA) wt.% 69 54 39 13

If a comparison among properties of P-1 and P-2 is made, it can be observed that the partial substitution of the

mineral oxide, used for bonding P-1, with the reactive vegetable fraction, used for bonding P-2, led not to a decrease in mechanical properties, but, on the contrary, an improvement was achieved (Fig. 2). In terms of environmental impact, the mineral oxide used for P-1 can be considered as an improvement with respect to Portland cement or slaked lime, as lower firing temperatures are needed for its production. Moreover, the reactive vegetable fraction used for P-2 represents a further significant improvement in terms of environmental sustainability, as this binder is obtained from renewable agricultural resources.

Fig. 1. Stereo-optical microscopy images of the panels (magnification 16x): (a) P-1, (b) P-2, (c) P-3 and (d) P-4

Fig. 2. Compressive and flexural strength of the panels

Also in the case of P-3 and P-4 panels, designed as possible substitutes for high density formaldehyde- bonded wood particle boards, promising results were obtained. The P-3 and P-4 panels exhibited a bulk density of 1260 kg/m3 and 1280 kg/m3, respectively, and a corresponding water absorption of 39 wt.% and 13 wt.% (Table 1). The densification in panel microstructure, with respect to P-1 and P-2, can be observed in Fig. 1, where a lower open porosity is present for panels P-3 and P-4. Correspondingly to the high density, very good mechanical properties were obtained, particularly in the case of P-4. The compressive strength of P-3 reached 5.5 MPa, a value sensibly higher than that of P-1 and P-2 panels (in the case of P-4, compressive strength was not measured, due to the low panel thickness). As for flexural strength, it reached 2.1 MPa for P-3 and 17.5 MPa for P-4 (Fig. 2). In the case of P-3, flexural strength results lower than that of panels P-1 and P-2, presumably because the coarse hemp hurds (used

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for P-1 and P-2) performed a more effective bridging action than thin hurds used for P-3, thus allowing a more effective hindering of crack propagation in the region subjected to tensile strength. Panel P-4 exhibited a definitely high flexural strength, which can be ascribed to the good adhesion between the binder and the hemp hurds, as noticed by SEM observation (not reported here).

Concluding remarks

The studied panels, obtained by bonding hemp hurds with different binders, characterized by a reduced

environmental impact, exhibited very promising properties, both in the case of the low density panels, intended as composites for building thermal insulation, and high density panels, designed as possible substitutes for formaldehyde-bonded wood particle panels.

Further tests are in progress to evaluate additional physical-mechanical properties (such as tensile strength and resistance to withdrawal of screws), as well as reaction to fire and physical-mechanical durability.

Keywords: building materials, green materials, hemp, sustainability, thermal insulation

Acknowledgements The financial support of Emilia-Romagna Region (POR-FESR funds) is gratefully acknowledged.

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