The influence of relative humidity on drying shrinkage mechanisms in hybrid alkaline systems with bottom ash and rice husk ash

• Dry curing (<30% RH) increased shrinkage up to 7 times in HAC • Wet curing reduced shrinkage and refined pores below 5 nm. • 60 % RH increased mesopore volume. • Shrinkage correlated with cumulative desorption pore volume. This study investigated the influence of relative humidity (RH = 30 %, 60 %, and 90 %) on the drying shrinkage behaviour of hybrid alkaline cement (HAC) systems, composed of 30 % Portland cement and 70 % bottom ash (BA) and/or rice husk ash (RHA). At RH < 30 %, HAC mortars exhibited shrinkage strains up to 7 times higher than Portland cement, whereas curing at RH > 90 % reduced shrinkage by nearly 15-fold compared to dry conditions. The 70RHA mixture demonstrated the highest shrinkage under low RH, associated with its refined micropore structure and elevated internal surface area. Nitrogen adsorption-desorption analysis revealed that low RH hindered pore refinement, while wet curing promoted gel densification, increasing the fraction of pores below 5 nm and improving compressive strength. At intermediate RH, mesopore volume increased after 28 days, consistent with microstructural changes associated with moisture exchange processes. This work provides a combined analysis of humidity-dependent shrinkage mechanisms and pore structure evolution in hybrid alkaline systems incorporating industrial and agricultural residues. The main contribution lies in identifying two distinct shrinkage regimes governed by pore-scale mechanisms under different RH conditions, offering practical guidance for curing optimisation and dimensional stability control in sustainable cementitious materials.