Promoting internal curing in concrete by replacing sand with sustainable biochar

Using biochar as a replacement for sand in concrete provides a sustainable solution to the ecological degradation and environmental pollution. This study investigates the use of biochar for internal curing when substituting sand in concrete. Various properties, including slump, mechanical strength, and hydration products, were assessed. The results show that prewetted biochar effectively preserves the slump of fresh concrete. The prewetted biochar provides internal curing by extending the hydration process, resulting in a 56 % increase in the duration between the first and third hydration peaks at the 100 % replacement level. This extended hydration process reduced the amount of unhydrated cement clinkers and enhanced the development of Calcium Silicate Hydrate (C-S-H) and CaCO₃, indicating a more complete degree of hydration of cement. Furthermore, the addition of biochar effectively reduces the total shrinkage of concrete by 65 % at a 100 vol% replacement level at 28 days and guarantees later strength development of concrete. • Biochar replacing sand only causes 16 % slump reduction at 100 % replacement level. • Biochar replacing sand ensured internal curing with more completed hydration degree. • The internal curing optimized the interfacial transition zone (ITZ). • The internal curing restrained the shrinkage and improved late-age mechanical strength of concrete.