Performance deterioration of SAP-modified concrete under combined hot-dry curing and sulphate attack

In saline soil regions, high-temperature and low-humidity (hot-dry) summers accelerate concrete moisture loss, hinder cement hydration, and induce shrinkage cracking, thereby increasing permeability and reducing sulphate resistance. This study investigated superabsorbent polymer (SAP)-modified concrete under combined hot-dry curing and sulphate attack. Based on environmental surveys, a representative hot-dry curing regime was designed. Mass loss and compressive strength were measured, and four SAP particle sizes and three dosage levels were evaluated over 150 sulphate dry-wet cycles. Mercury intrusion porosimetry and scanning electron microscopy were used to reveal the mechanisms. A grey GM(1,1)-Markov model was established to predict service life. The results indicate that after 150 cycles, combined exposure increased deterioration in compressive strength by 24.3% relative to sulphate attack alone. Compared with benchmark concrete, the optimal SAP (100–120 mesh, 0.17%) reduced compressive-strength deterioration by 62.4%. Microstructural analysis showed that SAP promoted cement hydration, densified the microstructure under hot-dry curing, and mitigated later-stage deterioration of pore structure and the interfacial transition zone. Model predictions indicated that combined hot-dry curing and sulphate attack accelerated concrete failure, whereas SAP extended service life by 1.2 times. These findings clarify SAP effects under combined severe conditions and support durability prediction.