Some Aspects of Chloride Corrosion of Cement Paste and Mortar

The durability of different kinds of cement i.e. PC 45 and PC 55, slag cement (SC) and alkali activated slag (AAS) has been investigated. The mortar were cured in a strong chloride solution and the expansion and strength were measured. Additionally, the frost resistance was tested according to ASTM Standard. The obtained results showed that PC was rather quickly destroyed in the chloride solution and the reason was the formation of expansive basic calcium or magnesium chlorides. The crystallisation of secondary ettringite seems to have a rather minor influence. Independently of weak durability of PC the concrete of these cements had a good frost resistance according to ASTM test. Slag cement with 50% of slag addition had a better durability and a longer resistance time in these corrosive environments. The AAS mortar showed the best durability, which had a very good strength after 11 years of immersion in strong chloride solution. The main property of the paste, determining the corrosion rate, is porosity, mainly the content of capillary pores. In case of AAS the unhydrated slag grains play also a very positive role. It is of great importance because the first phenomenon of action of the strong chloride solution was the paste shrinkage due to Osmotic pressure which caused microcracks. The microcracks in the paste were filled by the products of the delayed slag hydration.