Carbon Sequestration of Silica-Rich Biochar in Cement Accompanied by the Pozzolanic Effect

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