Effects of controlled and ambient curing conditions on the mechanical and microstructural performance of RHA-based greencrete blocks

This study systematically investigated the engineering and microstructural performance of soil, rice husk ash (RHA), and ordinary Portland cement (OPC) composite Greencrete blocks. Sixteen different mix compositions were prepared and cured under ambient conditions at 30 °C without humidity to represent the natural environment, and under controlled curing at 24 °C with regulated humidity to represent proper hydration to evaluate the influence of curing conditions on the mechanical and physical properties. Moreover, the effects of RHA and cement ratio on bulk density, water absorption, and compressive strength were determined. The optimum mixture containing 5% RHA and 12% cement exhibited the highest compressive strength, reaching 3.67 MPa at 30 °C and 2.83 MPa at 24 °C. This matrix showed a water absorption of nearly 32% and a bulk density of approximately 1301 kg/m 3 , indicating improved durability and performance of the Greencrete blocks. SEM and EDS analyses revealed the formation of a calcium silicate hydrate (C–S–H) gel, resulting from the pozzolanic reaction between RHA-derived silica and the cementitious calcium phase, leading to matrix densification and increased bond strength. Overall, the soil, RHA, and cement composite represents a non-load-bearing, low-carbon, climate-resilient, and sustainable masonry unit that serves as an effective alternative to burnt bricks and promotes the development of carbon–neutral building technologies.