Evaluation of Strength and Durability Properties of Cementitious Composites with Rice Stubble Biochar as Partial Binder Replacement

Abstract

This study examines the impact of varying proportions of biochar produced from the pyrolysis of rice stubble waste on the strength and durability performance of cementitious composites. Ordinary Portland cement was partially replaced with finely ground biochar at replacement levels of 0%, 2.5%, 5%, 7.5%, and 10%. The resulting concrete mixes were prepared, cast, and cured under controlled conditions. The primary objective was to determine the optimal biochar dosage and evaluate the influence of biochar incorporation on fresh, mechanical, and durability characteristics of concrete through slump test, rebound hammer test, ultrasonic pulse velocity test, compressive strength test, splitting tensile strength test, flexural strength test, water absorption test, and rapid chloride permeability test. It was observed that increasing biochar concentration led to a progressive reduction in slump, indicating stiffer mixes due to the high porosity and water absorption capacity of the biochar particles. Compressive strength testing revealed that incorporating biochar enhanced the compressive strength, with a 7.5% replacement dosage emerging as the optimal dosage. However, higher dosages still yielded improved strengths relative to the control mix. In contrast, splitting tensile and flexural strengths decreased with increasing biochar content, attributed to the internal porosity introduced within the concrete matrix. Rebound hammer results exhibited agreement with the compressive strength trends, while ultrasonic pulse velocity outcomes similarly confirmed that M-3 (7.5% biochar) exhibited the highest pulse velocity, corresponding to its superior compressive strength. Rapid chloride permeability results further validated the enhanced performance of M-3, which demonstrated the lowest charge passed, indicating reduced chloride ion penetration. Conversely, water absorption showed an increasing trend with biochar content, with M-4 presenting the highest absorption value. Overall, it can be concluded that M-3, containing 7.5% biochar as partial cement replacement, represents the optimum mix composition, while higher dosages still provide improvements over the control mix in several aspects. This study highlights that incorporating biochar can promote matrix densification due to its fine particle size and filler effect; however, it may simultaneously increase overall porosity when introduced beyond the optimum dosage threshold.