Application of rice husk biochar and thermally treated low silica rice husk ash to improve physical properties of cement mortar

Rice husk ash, an industrial by-product from boilers for energy generation and furnace for parboiling rice, has been explored as admixture to replace part of cement in structural mortar. However, due to uncontrolled burning process, the produced rice husk ash (iRHA) has low amorphous silica and contain unburnt husk particles, which affects durability of mortar containing iRHA. This study investigates thermal treatment of iRHA (TRHA) to produce ash with improved physical and chemical properties, which can then be used to reduce cement content in mortar by 20% (by weight). Furthermore, combination of rice husk biochar (RHB) and iRHA, where RHB is used to replace 10% and 40% by weight of iRHA, is used to improve mechanical and durability properties of iRHA-RHB mortar. The performance was compared with control (without RHA) and mortar containing RHA produced under controlled laboratory condition (LabRHA). Experimental results showed that addition of TRHA increased the strength of mortar by 20% and 34% at early stage (after 7 days) and matured age (after 120 days) compared to mortar with iRHA respectively. Although strength development was similar to control, TRHA-mortar showed lower autogenous shrinkage and water permeability, indicative of its improved durability as building material. Addition of RHB, as 40% replacement of iRHA, offers improvement in long-term (120-day) compressive strength and water tightness by 17% and 23% respectively compared to iRHA; it also has 12% more water-tightness than the control. Strength afforded by the carbon content of RHB particles and its reservoir effect eliminated autogenous shrinkage over the 6-week monitoring period. Among the tested admixtures, LabRHA offered maximum improvement in water-tightness, leading to 23% lower capillary absorption compared to control. Findings from this study suggest that thermal treatment and application of RHB can valorise industrial grade RHA as admixture in cementitious building materials, which will also reduce the need for landfilling and sequester carbon in the built environment.