Microstructural verification of delayed ettringite formation (DEF) mitigation in GGBFS-blended high-early-strength cement mortars

Delayed ettringite formation (DEF) poses a serious durability risk in precast concrete elements, particularly when exposed to high-temperature curing and cyclic environmental conditions. Although DEF mechanisms have been reported, the combined influence of steam curing and cyclic exposure, as well as the mitigating role of ground granulated blast-furnace slag (GGBFS), remains insufficiently clarified. This study evaluated Portland cement mortars incorporating 0, 20, and 40 % GGBFS subjected to steam curing at up to 90 °C followed by cyclic dry–wet exposure. Mortars without GGBFS exhibited visible surface cracking and up to 0.16 % expansion after 196 days, whereas GGBFS mixtures remained crack-free with expansions below 0.025 %. Microstructural analyses revealed that DEF suppression was associated with reductions in the SO3/Al2O3 molar ratio (1.41→0.88), Na2Oeq (1.32→1.04), pozzolanic consumption of Ca(OH)2, and microstructural densification. In addition, the S40–90C mixture retained over 98 % of its 28-day strength at 196 days, in contrast to the ∼8 % strength loss observed in the control. These findings clarify the mechanisms by which GGBFS mitigates DEF and provide practical guidance, suggesting that incorporating ≥ 20 % GGBFS is an effective strategy for durable precast high-early-strength mortars.