Synergistic effect of binary metakaolin–sugarcane bagasse ash blends as supplementary cementitious materials

Supplementary cementitious materials (SCMs) are a key strategy to reduce CO 2 emissions in cement production. However, the limited availability of traditional SCMs highlights the need to identify alternative sources. Sugarcane bagasse ash (SA), a by-product of biomass incineration, and metakaolin (MK) have been individually investigated as SCMs, but their combined performance as binary blends has not been fully addressed. This study evaluates the pozzolanic behavior of raw SA and MK as binary mixtures using tests with hydrated lime and Portland cement, in both pastes and mortars. A total of 21 compositions were prepared with SA content ranging from 0% to 100%, combined with MK in 5% increments for lime-based tests and 10% increments for cement-based mixes, replacing the binder content. X-ray Diffraction (XRD) results revealed increased consumption of calcium hydroxide with higher MK content and lower SA proportions, indicating that the pozzolanic reaction is primarily governed by MK. This trend was also observed in the compressive strength tests: in lime-based blends, up to 55% SA and 45% MK contents met the criteria for pozzolanic materials, while in cement-based mixes, up to 60% SA and 40% MK maintained pozzolanic behavior. Electrical conductivity measurements further confirmed that higher MK contents enhanced reactivity. In contrast, SA exhibited a predominantly physical behavior, acting mainly as a filler with limited chemical contribution. Although higher SA levels decreased CO 2 emissions, the 100% SA mix showed limited compressive strength, restricting its practical application. Conversely, binary blends provided more balanced performance, benefitting from synergistic interactions between constituents. These findings demonstrate that combining a low-reactivity material, such as SA, with a highly reactive SCM, like MK, can yield effective pozzolanic blends, offering potential for sustainable binder systems. • Binary blends of SA–MK improved pozzolanic reactivity compared to raw SA. • MK content governed reactivity, increasing CH consumption. • Mixtures met pozzolanic criteria up to 55% SA (lime) and 60% SA (SAI). • Electrical conductivity confirmed higher reactivity with increasing MK. • SA–MK systems reduced CO₂ emissions while maintaining adequate strength.