Abstract The overall aim of this paper was to establish sulfate resistance of cement mortars and pastes incorporating limestone exposed to severe sulfate attack at 20 and 4°C. Specimens with 0, 10, 20, and 30 % replacement levels of cement by limestone were continually exposed to a sodium sulfate solution with 33,800 ppm of SO42– concentration for 44 weeks. Sulfate exposure tests included compressive and flexural strengths, expansion, and mass loss measurements of mortar specimens. Additionally, in order to identify products formed by sulfate attack, microstructural observations such as X-ray diffraction (XRD) and scanning electron microscopy (SEM) were performed on the paste samples. Experimental results indicated that the mortar specimens incorporating limestone were much more susceptible to sodium sulfate attack compared to those without limestone, especially when exposed to a 4°C sulfate solution. It was found that the main deterioration product of the damaged samples with higher limestone contents was identified as thaumasite, not ettringite or gypsum, by means of XRD and SEM analysis. Furthermore, data on mechanical characteristics confirmed that the sulfate deterioration of mortar specimens was greatly dependent on the replacement levels of limestone.