Sulphate attack calcium aluminate cements

The sulphate attack has been known from a long time, and already in 1858 Vicat [247] studied the chemical causes of hydraulie eompounds corrosion in sea water [247]. Bied [248] invented the teehnology and developed the production of calcium aluminate cement, as a remedy for rapid destmetion of concrete in France, caused by the sulphate ground water attaek, from the dissolution of gypsum and anhydrite. 

As it can be concluded from the information relating to the chemical mechanism of cement paste corrosion under sea water, its resistance depends primarily on the Ca(OH)2 and calcium aluminates content it means that the resistance increases with the C-S-H phase content in the paste. Therefore cements poor in C3A, with low C3S/C2S ratio, that is those with high sihca ratio and low lime saturation factor show the best resistance. Cements with high slag and pozzolana content are extremely resistant, particularly when they are produced from clinker poor in C3A. Very good results were assured by the three component cements, with 30% pozzolana addition, beside of slag [98]. Calcium aluminate cement reveals, as in the case of sulphate attack, the highest corrosion resistance in marine environment (see Chap. 9). 

Calcium aluminate cements are extremely highly resistant to various aggressive media. They were invented as a result of concrete searching, resistant to sulphate solutions attack. Besides the sulphates, they are resistant also in the acidic waters enviromnent (see Chap. 6) as well as in sea water. This was proved by examination of calcium aluminate cement concretes after many years of exploitation in different conditions. For example Lea [5] reports that the concrete samples, moreover those with anhydrite aggregate, stored many years in gypsum water, do not show any symptoms of destructiom The other concrete samples were successfiilly stored for 20 years in Medicine Lake in Dakota, where the concentration of sulphates was... 

The calcium aluminate cement (CAC) was developed as a solution to the sulphates attack in OPC, and was patented in France in 1908. The CACs are cements consisting predominantly of hydraulic calcium aluminates mainly monocalcium aluminate, CA, but also contains minor amounts of Ci24y, C 2 and C AF. 

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. 

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