The effect of calcined clays of the swelling type, on Portland limestone cement at 5oC is studied. Local clays were thermally activated by burning one hour at 900 °C and ground to a Blaine of 2842 (cm2/g). The pozzolanic reactivity of the activated clays was defined chemically and according to ASTM C311/C311M-13. Portland limestone cement was prepared by replacing 30% CEM I 42.5 N by limestone powder. 10 and 20% of limestone were further replaced by the calcined clays. Parallel series of mortars specimens pre-cured for 28-day long as well as for a short period of one day, were immersed in water and sulfate solutions at 5oC for time up to 90 days. Compressive strength and length change were measured for the samples. The results show that replacement of limestone with calcined clays improves the compressive strength and the expansion of Portland limestone cement mortars in water and sulfate solutions. Ettringite was detected in the expanded mortars and no thaumasite formed. The results are interpreted in terms of the mechanism of thaumasite formation.