Recycling of wheat straw aggregates of end-of-life vegetal concrete: Experimental investigation to develop a new building insulation material

The recycling of end-of-life bio-composite materials contributes to reducing the environmental and economic costs of the building sector. The current research is about studying the feasibility of reusing wheat straw aggregates from crushed vegetal blocks after deconstruction, manufactured three years ago. These recycled materials are composed of wheat straw aggregates and interparticles of old lime. These aggregates are also partially covered by a gangue of old lime (called "surrounded lime). The objective is to investigate the possibility of incorporating recycled wheat straw a second time in a new biocomposite formulation with recycled wheat straw aggregates (RWSA). For the first time, RWSA is physically characterized (grain size distribution, densities, water absorption) to establish a comparison with the natural wheat straw. A second time, two formulation groups are defined and performed by varying the water/binder ratio (W/B = 1.1; 0.8) and considering the presence of old-lime inter-particles (equal to 55% per unit of particle mass). For each group, the quantity of RWSA in the formulations increases in mass proportion (0, 30, 50, 70, 100%). The physical bulk density, thermal (conductivity), and mechanical (compression strength) properties of the new recycled concrete are identified, with the valuation of the influence of the curing time (28 and 90 days) and formulation parameters on these properties. A third group with different straw/binder ratios (S/B ratios) is considered with formulations where the aggregates were entirely recycled without considering the presence of the old-lime inter-particles. The influences of curing time, W/B ratio, and different percentages of RWSA on the thermal and mechanical properties were highlighted. Through microstructural analysis, it can be observed that increasing the proportion of RWSA in biocomposites increases the carbonation rate and hence the strength of such compounds. The carbonation rate increased from 42.58% in biocomposites containing 100% normal straw to 81.8% in those containing 100% recycled straw due to the presence of old-lime inter-particles. This confirms the findings in terms of the increase in thermal conductivity and mechanical strengths with the substitution rate of RWSA.