Developing new sulfate-resistant cements: a BaCO3 approach

Abstract

OPC mortar and concretes can be attacked by sulphate solutions giving rise to different decaying processes, related with the formation of ettringite, gypsum, thaumasite etc. At present deterioration due to ettringite formation is avoided by limiting aluminate phase in clinker Portland, however it does not prevent other forms of sulphate attack. Barium carbonate has been used in several fields to eliminate sulphate ion from solutions due to it reacts with them and produces a very insoluble salt, BaSO4. There are in the literature studies demonstrating that ettringite as well as gypsum decompose in the presence of barium carbonate. Studies on the hydration rate of synthetic C3A in the presence of varying percentages of gypsum, BaCO3 or gypsum+BaCO3 revealed that BaCO3 neither regulated the speedy reaction of C3A with water nor reacted with the aluminate. Blends of gypsum plus BaCO3 proved able to regulate C3A hydration. The objective of this paper was to know the behavior of mortars elaborated with optimized mixes of clinker-gypsum-BaCO3 in 4.4 wt% Na2SO4 solution. For this four cements were elaborated by mix of: a) clinker M with 5% of gypsum; b) Clinker M with 3% of gypsum and 15% of BaCO3; c) clinker V and 7 wt% of gypsum; d) clinker V plus 3wt% of gypsum and 15% of BaCO3. Studies on durability were performed on mortar prisms of 10x10x60 mm size, with cement/sand ratio = 1/3 and water/cement ratio = 0.5. Cement Mortar prisms remained submerged under water and under sodium sulfate solution for one year. Samples were extracted from solution after 1,3,5,7 and 12 months and then mechanically tested. XRD and SEM/EDX were performed on the said samples. The compressive but mainly flexural strengths of mortars a), decreased from 5 months in contact with sulphate solution while in the mortars b) strengths remain unaltered after 12 months. Mechanical strengths of mortars c), decreased from 3 months in contact with sulphate solution and were completely destroyed after 12 months of exposition. BaCO3 fails as protector from sulphate attack in d) mortars which were cracked after one year of sulphate exposition. Correlations between mineralogical composition and microstructural changes in mortars along time of test were established.