Influence of mortar composition and pore structure on the volume, kinetics and location of DEF and induced expansion

This study analyzes the interaction between delayed ettringite formation (DEF), expansion, and pore size distribution in mortars with varying sand-to-binder (s/b) ratios using an accelerated DEF setup. Measurements of length change, ettringite content, and pore structure show that DEF behavior depends strongly on s/b ratio and microstructural characteristics. Ettringite preferentially precipitates within larger pores (250–1000 nm) in sand-rich mortars, while in paste and low-sand mortars precipitation also extends into smaller pores (70–250 nm). Increasing sand content reduces interparticle spacing, promoting ettringite precipitation. A clear correlation is established between expansion and effective ettringite volume: paste reaches the 0.1% expansion threshold at ~2% ettringite, whereas mortars require ~3% due to greater pore volume available to accommodate precipitation. An increase in sub-70 nm pores suggests chemical–mechanical processes such as C–S–H decalcification, crystallization pressure, and water redistribution. Overall, DEF-induced expansion is governed by both ettringite volume and its preferential precipitation sites within the specific pore size domains, highlighting the critical role of pore structure in governing DEF-related deterioration.