Abstract
Alkali-activated slag (AAS) systems offer a sustainable alternative to Portland cement but suffer from rapid setting and high drying shrinkage, limiting their widespread use.
Although rice husk ash (RHA) has been explored as a supplementary cementitious material, the effects of its fineness on reaction mechanisms and gel development remain underexplored. This study introduces a novel strategy for tailoring AAS performance by concurrently adjusting the Si/Ca and Si/Al molar ratios through the incorporation of ultrafine RHA (URHA) and controlled alkali activation. Unlike previous studies focusing solely on mechanical strength or shrinkage reduction, this work simultaneously optimizes both by engineering gel composition and pore structure. A comprehensive factorial design was implemented to investigate the effects of URHA content (0–30wt.%), fineness (9,000 vs. 24,000m²/kg), and alkali dosage (4–6wt.% Na₂O). The results revealed that URHA significantly increased the Si/Ca and Si/Al ratios, facilitating the formation of highly crosslinked, Si-rich calcium (alumino)silicate hydrate (C–(A)–S–H) and sodium aluminosilicate hydrate (N–A–S–H) gels. The optimized mixture (20wt.% URHA, 6wt.% Na₂O) achieved a 28-day compressive strength of 90.3MPa (19.4% improvement over the slag-only system) and reduced drying shrinkage by 45.0%, addressing both strength and dimensional stability concerns. Advanced characterization (XRD, FTIR, TG-DTG, SEM-EDS) confirmed that systematic control of Si/Ca and Si/Al ratios not only promotes amorphous phase development and pore refinement but also facilitates internal curing via gradual moisture release from URHA’s porous matrix. This work provides a novel composition-guided design framework for alkali-activated systems by using Si/Ca and Si/Al as key tunable parameters to optimize both mechanical performance and durability, while valorizing biomass-derived ultrafine additives in a circular approach.
•Grinding and alkali activation of RHA synergistically boost its reactivity.•Slurry with added RHA extended the slag AAM’s initial setting time by up to 64%.•RHA admixed at 20wt.% improved rheology and flow behavior. of slag-based AAM.•RHA admixed at 20wt.% significantly reduced drying shrinkage of slag-based AAM.•High Si/Ca (1.8) and Si/Al (3.1) molar ratios led to high strength (90.3MPa).