Effect of activator-to-precursor ratio on the mechanical and durability performance of rice husk ash-based alkali-activated concrete composites using recycled aggregates

The usage of Portland cement-based natural aggregate concrete leads to significant environmental consequences, such as the exhaustion of natural resources and CO₂ emissions associated with the production of Portland cement. The present study investigates the essential role of the activator-to-precursor ratio in the design of structural-grade alkali-activated recycled aggregate concretes incorporated with rice husk ash as one of the precursors alongside ground granulated blast furnace slag as the main precursor, to attain the required strength and durability. These concrete mixes with activator-to-precursor ratios ranging from 0.3 to 0.8 were developed, and their performance was evaluated over time to assess the influence of the activator-to-precursor ratio. The findings from this study demonstrate that by limiting the activator-to-precursor ratio to 0.5 or below, higher compressive strengths in the range of 65–70 MPa may be attained, while maintaining drying shrinkage strains of the order 500 to 700 microstrains, within the allowable limits. Also, by restricting the activator-to-precursor ratio to 0.5 or below, these concretes exhibited a volume of permeable voids below 13 % and minimal weight loss ranging from 1.12 % to 2.5 % under acidic exposure. Additionally, other durability parameters, such as water penetration depth and sorptivity, ranged from 6 to 19 mm and 1.59–2.36 mm, respectively, and remained within acceptable limits, irrespective of the activator-to-precursor ratio. However, a consistent improvement in performance was observed in mixes with lower activator-to-precursor ratios. The superior performance of mixes with lower activator-to-precursor ratios can be attributed to their denser matrix and higher Ca/Si and Al/Si ratios, as evident in SEM images and EDS analysis. The contribution of these concretes towards the CO₂ emissions is roughly three times lower than that of Portland cement-based concretes.