Degradation of Shotcrete Materials Subjected to Sulfate and Chloride Attack in Varying Exposure Regimes

Durability of in situ shotcrete under external sulfate attack was investigated, taking into consideration the addition of mineral admixtures, along with the presence of chloride ions. Three water‐to‐binder ratios (w/b), i.e., 0.35, 0.45, and 0.55, and two types of supplementary cementitious materials (SCMs), namely, fly ash (FA) and silica fume (SF), were considered in the current study. Two different laboratorial immersion regimes (continuously full immersion and partial immersion with cycling temperature and relative humidity) were carried out to induce chemical/physical sulfate attack. Results show that loss of strength was the typical feature of chemical sulfate attack on shotcrete, while surface spalling dominated in deterioration caused by physical sulfate attack. The presence of chloride ions can globally mitigate these deteriorations. Meanwhile, the lower w/b ratio proved to be efficient in increasing the resistance to both sulfate attacks. Adding fly ash (FA) in shotcrete mixtures enhanced the long‐term performance but invited massive white efflorescence on surface layer under partial‐immersion exposure condition. Silica fume (SF) admixture can compensate the undesired reduction of early‐age strength caused by FA addition, but make these specimens more susceptible to sulfate attack. Mercury intrusion porosimetry (MIP) analysis, scanning electron microscopy (SEM), and X‐ray diffraction (XRD) tests reveal that these consequences were strongly related to the refinement of microstructure resulted from pozzolanic reactions and hydration kinetics.