Effect of locally available rice husk ash on compressive strength of low-grade concrete

Agricultural waste management presents both environmental challenges and opportunities for sustainable construction material development in rice-producing regions. This research investigates the influence of locally sourced rice husk ash on the compressive strength characteristics of low-grade concrete intended for non-structural applications in rural construction projects across southeastern Brazil. Rice husk ash was prepared through controlled incineration of agricultural residues collected from processing facilities in the Ribeirão Preto region, followed by mechanical grinding to achieve particle sizes below 45 micrometers suitable for pozzolanic applications. Concrete specimens were prepared with cement replacement levels of 0%, 5%, 10%, 15%, and 20% by weight, maintaining constant water-to-binder ratios of 0.50 throughout all mix designs. Compressive strength testing followed Brazilian standard NBR 5739 procedures at curing ages of 7, 14, 28, 56, and 90 days using 150 millimeter cubic specimens. The experimental program was conducted at the Construction Materials Laboratory of São Paulo Institute of Engineering between March and December 2023. Results demonstrated that 10% rice husk ash replacement achieved optimal performance, yielding 28-day compressive strength of 34.5 MPa compared to 31.8 MPa for control specimens without supplementary cite materials. This 8.5% strength enhancement continued through extended curing, with 90-day values reaching 42.8 MPa for the 10% replacement mix versus 37.4 MPa for control concrete. Statistical analysis confirmed significant differences between replacement levels at all testing ages, with the pozzolanic reaction contributing increasingly to strength development beyond 28 days of curing. The findings establish rice husk ash as a viable supplementary cementitious material for low-grade concrete applications in Brazilian agricultural regions, offering simultaneous benefits of agricultural waste valorization, cement consumption reduction, and improved concrete performance. Economic analysis indicated potential material cost savings of 12-18% when transportation distances remain below 50 kilometers from ash production facilities to construction sites.