Synthesis and Characterisation of Extruded Alkali Activated Earth-Based Composites for Sustainable Building Construction

Abstract

Alkali activation is a rapidly developing area in the global materials research and development community. It is based on alkali aluminosilicate chemistry with primary focus on the activation of solid aluminosilicate materials under alkaline conditions to produce three-dimensional network of inorganic polymer binders. However, little research has been conducted to harness this technology in the development of earth-based composites. This work provides insights on the application of alkali activation in the synthesis of in situ binders in earth-based composites using uncalcined clays and low molarity alkaline solutions for the development of low-impact extruded building materials. To evaluate the effect of these alkaline soil matrices on the reinforcing effects of natural fibres, two ligno-cellulosic fibres were selected (sisal and eucalyptus pulp) as well as a synthetic fibre (polypropylene) as reinforcements. The compositional dependence on extrudability of the fresh pastes was studied by characterizing bulk and interfacial rheology behavior induced by fibres using the Benbow-Bridgwater model to evaluate the feasibility of extrusion moulding as a processing method. Physico-mechanical evaluation as well as microstructural analysis was conducted to evaluate reinforcing efficiency of fibres. The underlying strengthening and toughening mechanisms associated with the fibres were explored using a combination of in situ/ex situ observations of crack propagation and micro- mechanical models. The results show that with low molarity alkali activator solutions, partial dissolution of aluminosilicates in the soil result in the formation of amorphous sodium aluminosilicate gels which provide satisfactory binding within the soil matrix. Variation of synthesis conditions shows that initial curing temperatures plays a critical role in the synthesis of these binders. Mechanical and physical properties of plain earth matrices incorporating these binders were comparable and in some cases higher than other stabilized earth building materials. Interactions between the alkaline matrix and ligno-cellulosic fibres resulted in improved flexural strengths as well as reduced water absorption capacities of the composites. On the other hand, weak interactions between polypropylene fibre and the alkaline matrix resulted in marginal increase in flexural strengths but transformed brittle earth matrices into deflection hardening composites. Whilst, strengthening mechanism of composites reinforced with ligno-cellulosic fibres was via elastic shear stress transfer at the fibre-matrix interface, polypropylene fibres strengthened the matrix via frictional slip after debonding at the fibre-matrix interface. Predominant toughening mechanisms for all composites was via crack bridging as prediction of resistance curve behavior of composites was comparable to measured resistance curve in short scale and long scale bridge regimes. The implications of these results are discussed as potential building materials to meet increasing housing demands in line with targets set by the sustainable development goals (SDGs).