Evaluation of pozzolanic activity in rice husk biochar and ash activated by ball milling for enhanced cement-based composites

Abstract Rice husk, an abundant agricultural waste rich in silica (SiO 2 ), is a promising material for use in cement-based composites. When thermally treated, rice husk is converted into rice husk ash (RHA) and rice husk biochar (RHB), both containing silica in different forms (amorphous and crystalline) and ratios that may affect pozzolanic reactivity. This study evaluated the pozzolanic activity of rice husk biochar (RHB) produced at 450°C and 650°C, and rice husk ash (RHA) as well as the effect of ball milling activation, for use in cement-based composites. The X-ray diffraction (XRD) analysis of the samples showed that RHA had a high degree of crystallinity, indicated by the sharp peak at 2θ ≈ 22° representing crystalline silica. This suggested that RHA, although rich in SiO 2 (79.67%), may have limited pozzolanic reactivity due to its crystalline structure. In contrast, both RHB 450°C and RHB 650°C were predominantly amorphous, with broad, low-intensity humps around 2θ ≈ 22°. The amorphous nature of these structures, along with the SiO 2 content in RHB at 450°C (33.50%) and 650°C (41.30%), suggested a higher potential for pozzolanic reactivity compared to crystalline silica, confirming the presence of silica in a more reactive form conducive to pozzolanic activity. Furthermore, the ball mill activation could enhance the active surface area of RHA and RHB, resulting in enhancement of the pozzolanic reactivity. Such results could improve the hydration reaction by forming additional calcium silicate hydrate (C-S-H), a crucial component for early strength development where it could effectively fill voids within the cement matrix. This contributes to increased compressive strength, reduced porosity, and enhanced durability of the composite. This research provides insights into the use and preparation of RHB in low-carbon concrete composites, offering an innovative approach to enhancing material properties and reducing cement demand in construction.