Sulphate attack Portland cements

Destructive expansion from reaction with sulphates can occur not only if the latter are present in excessive proportion in the cement, but also from attack on concrete by sulphate solutions. The reaction involves the Al,0,-containing phases in the hardened cement, and in sulphate-resisting Portland cements, its effects are reduced by decreasing the proportion of the aluminate phase, sometimes to zero. This is achieved by decreasing the ratio of AljOj to Fe203 in the raw materials. In the USA, sulphate-resisting Portland cements are called Type V cements. 

The role of the ferrite phase, generally identified as brownmillerite, should be mentioned too. In the case of sulphate attack this phase can be the source of almninate ions [237] moreover the ferrite ions can form the analogue of ettringite or to substitute the aluminate ions in all calcium aluminate phases [222]. The latter case is undoubtedly the most common one in the Portland cement paste. However, the reaction of sulphate ions with ferrites is slower. There is a view that the F/Al ratio in the hydrated phases is lower than in brownmillerite hence, some amount of iron(in) hydroxide is always present [222] (see also Sect. 4.1.1.). This hydroxide occurs in the gel-like form and therefore the diffusion of ions through the gel layer is slowed down. Therefore, the corrosion process is hindered. The other phases containing the Fe ions can be produced too, it is discussed in Chap. 3. 

Sersale, R. et al. (1997) Sulphate attack of carbonated and uncarbonated Portland and blended cement mortars, in Proceedings 10th ICCC, Goteborg, paper 4iv017. 

Mortars and concretes made with high alumina cement are resistant against sulphate attack and also are better at resisting CO2 from ordinary drinking or mineral water. In comparison, the resistance against alkalis is lower than that of Portland cements. 

The sensitivity of concrete structures to sulphate attack is strongly related to the exposure conditions. Structures in an environment of high sulphate content in the air or in w ater, for example sewage tunnels, are particularly vulnerable. After sulphate ions penetrate the pore system of cement paste, complex reactions start with C3 A leading principally to two kinds of processes gypsum corrosion and sulphoaluminate corrosion (Mindess etal. 2003). The products of sulphate reactions with cement expand and can cause cracking and destruction. The permeability of the material s structure and the quality of cement decide upon the rate of these processes. Special Portland cements as well as high alumina cements may be used for elements exposed to sulphates (cf. Section 4.1.1). 

---