Carbon Sequestration\nof Silica-Rich Biochar in Cement\nAccompanied by the Pozzolanic Effect

Biochar is a promising carbon-negative supplementary\ncementitious\nmaterial (SCM). However, traditional biochar lacks active ingredients,\nand its porous structure severely weakens the mechanical properties\nof ordinary Portland cement (OPC), limiting its substitutability in\ncement. To meet the dual needs of decarbonization and strength in\ncement, this paper explored the potential of two types of silica-rich\nbiochar, rice husk biochar rich in amorphous silica and bamboo biochar\nrich in crystalline silica, as carbon-negative SCMs. Here, the hydration,\nmicrostructure, and mechanical properties of rice husk biochar and\nbamboo biochar blended cements were investigated. Similar to bamboo\nbiochar, rice husk biochar reduces flowability and drying shrinkage\ndue to its porous structure. However, rice husk biochar contains a\nlarge amount of amorphous silica, resulting in the promotion of hydration\nof clinker and a significant pozzolanic effect, which shortens the\nsetting time of cement, improves the weak structure and bonding interface\nof biochar, and further increases the hydration degree and polymerization\ndegree of calcium silicate hydrate. Compared with bamboo biochar,\nrice husk biochar blended cement exhibited significantly higher compressive\nstrengths at the same substitutions and ages. A 5% rice husk biochar\nblended cement at 28 days exhibited 12.9% and 29.8% higher compressive\nstrength than OPC and 5% bamboo biochar blended cement, respectively.\nThe compressive strength of 10% rice husk biochar blended cement was\nonly 4% lower than that of OPC at 28 days. From a life cycle assessment,\n10% rice husk biochar blended cement has the lowest unified carbon\nemissions (per MPa), 22% and 28% lower than that of 10% bamboo biochar\nblended cement and OPC, respectively, showing great potential for\nlower unified carbon emission cementitious materials.