Evaluation of the efflorescence resistance of calcium sulfoaluminate cement mortar: from indoor accelerated testing to outdoor exposure

Efflorescence is an inevitable phenomenon in all cementitious materials, particularly for Portland cement (PC) - based materials. Considering the relations between ion availability and crystallization, the factors influencing efflorescence involve the compositions of binders, pore structure of the hardened matrix, and so on. This study investigated the efflorescence of calcium sulfoaluminate cement (SAC) mortar at 5–40 °C from perspectives of carbonation potential and pore structure, while PC mortar was chosen as a reference. The microstructure of the efflorescence substances, such as phase assemblage and distribution, was systematically unraveled using optical microscope, in-situ Raman spectroscopy, SEM and EDS at multi scale-level. Water absorption test and XCT were employed for the pore structure characterization. Additionally, an outdoor exposure was carried out to compare with the results from the indoor accelerated experiments. Results reveal that a lower temperature has a limited effect on the efflorescence potential. The dominant efflorescence ingredient for SAC and PC mortars cured at 5–40 °C is CaCO3, with varied polymorphs. However, both primary and secondary efflorescence of SAC mortar is much slighter than that of PC mortar. The volume of fine pores (less than 0.1 mm3) in SAC mortar is higher than that of PC, accompanied by higher capillary absorption coefficient and water absorption rate. However, the low carbonation potential of ettringite tends to be more effective in reducing efflorescence risk for SAC mortar in both lab tests and outdoor exposure. The understanding of the relationship between efflorescence and carbonation provides important insights into the inhibition of efflorescence in cementitious materials