Influence of biochar and recycled gypsum on the strength and microstructure of conventional and sustainable cementitious composites

The need for sustainable waste management practices to mitigate climate change emphasized discovering alternatives for environmentally unfriendly materials with high carbon footprints such as ordinary Portland cement (OPC). This study examined the effect of biochar and its presoaking on the compressive strength, porosity, and microstructure of sustainable mortars containing fly ash, recycled gypsum drywall, and OPC by manufacturing and testing 270 cubic specimens, 30 mix designs, at 7, 28, and 90 days. The main roles of biochar in the matrix were to provide water and an additional surface for internal hydration, boost nucleation, and increase interlock in the cement paste. The water retention potential of biochar particles, however, could contribute to an artificial increase in the early-age compressive strength of cementitious composites (CCs) containing biochar. This strength was lessened over time as the retained water was utilized by cementitious reactions. The low strength of the biochar particles and its effect on the porosity of CCs were the primary sources of strength reduction of specimens containing biochar. The influence of biochar presoaking relied on the initial reactivity of binders. Lower initial reactivity lessened the initial water demand, increasing free water in the mixture, retained by non-soaked biochar. The highly reactive binders, however, needed extra water, which was supplied by presoaked biochar. The data also indicated the good compatibility and bond between biochar particles and the cement paste. The effect of adding biochar on the properties of CCs strongly depended on the particle size distributions of the composite constituents and biochar particles.

Automated summary

This study examined the effects of biochar and its presoaking on the properties of sustainable mortars. Biochar provides water and enhances hydration in cement matrices, affecting the strength and porosity of the materials. The water retention potential of biochar has a positive impact on early-age compressive strength but weakens over time. The low strength of biochar and its effect on porosity are the main factors reducing the strength of the materials. The influence of presoaking depends on the initial reactivity of binders.