Effects of biochar on hydration, strength degradation, and alkali-silica reaction in sustainable waste glass sand-based mortars

The biochar in concrete structures has attracted a lot of attention, offering new biomass recycling strategies while improving mechanical and durability properties of the biochar-cement composites. This study investigated the alkali-silica reaction (ASR) in waste glass sand-based mortars with three biochar, including corn cob biochar (CCB), waste wood biochar (WWB), and rice husk biochar (RHB), using the accelerated mortar bar test (AMBT) up to 96 days. Due to the fine biochar size (<100 μm, D90 = 34.67 μm), samples with 5 wt% CCB exhibited the lowest mortar bar expansion and mass gain up to 96-day of exposure in 1M NaOH bath at 80 °C, while experiencing the lowest strength loss. X-ray diffraction patterns indicated increased intensity of ASR gels, including tobermorite-type C-S-H and alkali-silicate-hydrates (ASH) in all groups. Thermogravimetric analysis (TG) results revealed that CCB5 had the lowest mass loss of ASR gels after 96 days in the 1M NaOH bath. Biochar degradation due to ASR was observed using Backscattered Electron images. Finally, it was recommended that up to 10 wt% fine-size corn cob biochar (<100 μm, D90 = 34.67 μm) could be conservatively used to partially replace cement content for sustainable concrete design.