Thermo-responsive engineering of Greencrete blocks: a foundational study toward waste-based next-generation concrete

The relentless surge in the global population and urbanization is intensifying demands on the construction sector, underscoring the urgent need for sustainable masonry solutions with reduced environmental footprints. This study systematically investigated the structural, physical, and microstructural performance of soil, rice husk ash (RHA), and ordinary Portland cement (OPC) composite Greencrete blocks, focusing on two different curing conditions: 24°C with humidity and 30°C without moisture. Sixteen compositional matrices were prepared, focusing on low- and medium-RHA systems (0%, 5%, 10%, and 15% RHA; 0%, 8%, 10%, and 12% OPC), and subjected to qualitative evaluations—namely, bulk density, water absorption, compressive strength, and energy-dispersive X-ray spectroscopy (EDS) coupled with enhanced scanning electron microscopy (SEM).The results revealed that the water absorption of the 5RHA12C sample at 24°C was 30.65%, which increased to 32% at 30°C—indicating a more porous structure. Additionally, the density changed significantly, from 1301.50 kg/m³ at 24°C to 1301.99 kg/m³ at 30°C. This result proves that an appropriate ratio of rice husk ash to cement and slow curing improves the durability and performance of the resulting Greencrete blocks. However, the same sample exhibited the highest compressive strength of 2.83 MPa at 24°C and 3.39 MPa at 30°C, which is in direct contrast to the other results. SEM‒EDS analysis revealed that RHA and OPC formed a dense and homogeneous calcium silicate hydrate (C–S–H) gel matrix under different temperature and humidity conditions, resulting in increased compressive strength and durability.Overall, the study presents strong empirical support for the use of agro-industrial waste, such as RHA, in the production of low-carbon, climate-resilient masonry units. The results align with the Sustainable Development Goals (SDGs 11 and 12) and lay the foundation for next-generation concrete research, with an emphasis on long-term performance and environmental assessment under field conditions.