Effects of carbonation and ageing of slag concrete on salt freeze-thaw scaling, porosity, and portlandite consumption

This study investigated the combined effects of ground granulated blast furnace slag (GGBFS) content (0 – 46%) and carbonation conditions on salt freeze-thaw scaling resistance of concrete. Standard 7 days curing resulted in minimal scaling (< 1.0 kg/m 2 ), while accelerated carbonation for similar duration amplified scaling values to 1 kg/m 2 , 3.3 kg/m 2 , and 4.5 kg/m 2 in 0%, 15%, and 46% GGBFS concretes, respectively. Extended carbonation (30 days) further increased scaling to 1.2, 5.7, and 7.4 kg/m 2 for these mixtures, with 70% of total scaling occurring within the first 14 freeze-thaw cycles. The strong correlation (R 2 = 0.873) between carbonation depth and scaling depth confirmed that approximately 50% of the carbonated layer was removed during the slab test. Specimens with 0% and 46% GGBFS showed 56% and 8% reduction in large capillary pores at paste level (2 µm/voxel), respectively. While concrete specimens with 46% GGBFS exhibited 22% increase in macropores (≥15 µm), indicating carbonation-induced microcracking. Despite these porosity changes, capillary sorptivity remained unchanged across all concretes, while total water absorption decreased between 12 – 16%, suggesting that carbonation fills pore bodies without blocking connecting throats. Cement paste with 46% GGBFS contained 47% less portlandite than paste with 0% GGBFS. Thermogravimetric analysis showed the formation of calcium carbonate (27% in paste with 46% GGBFS after 30 days carbonation) greater than the amount attributable to portlandite alone, confirming decomposition of calcium silicate hydrate. This decalcification combined with carbonation induced microcracking and deterioration of air-void system, might weakened the binding matrix and explained the severe salt freeze-thaw scaling in carbonated slag concretes.