Calcium sulphate cements

L. V. Palij, V. E. Akhrimenko, A. K. Kuksov, V. M. Medentsev, and V. I. Panov. Plugging solution for oil and gas wells—contains Portland cement, fly ash, sodium or calcium sulphate and additionally hydrosil, to increase adhesion of cement rock to casing string. Patent SU 1802087-A, 1993. 

Portland cement clinkers contain small amounts of alkalis and sulphates derived from the raw materials and fuel. Both alkalis and SO3 can be present in the major clinker phases, but tend to combine preferentially with each other to form alkali or potassium calcium sulphates, and it is necessary to consider these components together. In addition, silicate and aluminate phases containing sulphate can form either as intermediates or in undesirable deposits in eement making, and a calcium aluminate sulphate is a major constituent of some expansive and other speeial cements. 

Unless otherwise stated, this chapter relates to ordinary Portland cements hydrated in pastes at 15-25°C and w/c ratios of 0.45-0.65. XRD powder studies on such pastes have been reported by many investigators (e.g. C38,M67). The rates of disappearance of the phases present in the unreacted cement are considered more fully in Section 7.2.1. Gypsum and other calcium sulphate phases are no longer detectable after, at most, 24 h, and tbe clinker phases are consumed at differing rates, alite and aluminate phase reacting more quickly than belite and ferrite. The ratio of belite to alite thus increases steadily, and after about 90 days at most, little or no alite or aluminate phase is normally detectable. 

Concrete made with calcium aluminate cement at a properly low w/c ratio is highly resistant to sulphate solutions, sea water or dilute acid solutions with pH > 4, including natural waters in which CO2 is the only significant solute. Resistance may extend to pH 3 if the salt formed is of sufficiently low solubility. Midgley (M96) showed that, for fully converted material exposed to a sulphate ground water for 18 years, penetration with formation of a substituted ettringite was limited to a depth of 5 mm. These properties are consistent with Lea s (L6) view that the resistance is due to the formation of a protective coating of alumina gel, coupled with the absence of CH. No fundamental studies, e.g. on microstructural effects, appear to have been reported. 

Type S cements are Portland cements high in C3A and with suitable contents of calcium sulphate they have found little favour as they are too diHicLilt to control. The A1,0, has also been supplied in forms other than those mentioned above. Impure alunite [KAl3(S04),(0H)(,]. which occurs as a natural rock, has been used, either after calcination (V6) or uncalcined (W30). In the latter case it was mixed with Portland cement clinker, anhydrite and pfa or slag and was found to dissolve relatively slowly, thus suitably delaying the expansive reaction. 

Eogenetic magnesite cement in sandstones is relatively rare because its formation requires pore waters to be enriched in Mg " " and depleted in Ca " ", S04 and Cl". These conditions may occur in arid climates in which marine pore waters evaporate and become successively saturated with respect to calcium carbonates, calcium sulphates and halite, such as in sabkha settings (Kinsman, 1969 Morad et al., 1995). Continental brines enriched in Mg + are also suitable for the formation of eogenetic magnesite due to the low sulphate and chloride ion concentrations. Most recent magnesite cements form in the fine-grained sediments of alkaline/saline lakes (Last, 1992 Warren, 1990) and, less commonly, in freshwater lacustrine sediments (Zachmann, 1989). 

In coohng water the content of calcium, sulphates and hydrogen carbonates is hmited. Sulphates should be avoided because of the problems with concrete structures. Calcium sulphate can react with a component of cement — calcium aluminate — and form with it etteringite (CagAl2[(0H)4(S04)]3.2H20) whose crystals are more voluminous than those of the original aluminate. Changes in the volume can cause destruction of concrete. 

Lieber [85] studied the effect of PbO and ZnO on cement hydration. PbO affects only the C3S hydration rate there is no CafOH) for many hours or even days, depending on its addition. However, after the delay on the beginning, a quite intensive reaction is then continued. The reaction of C3A with gypsum is retarded but after 24 h there is no soluble calcium sulphate. On the other side, PbO does not affect the... 

This classification has a long tradition and reflects rather the problems to be resolved by the specialists in the field of cement chemistry, with aim to improve the durability of concrete in the more fiequently occurring aggressive enviromnents. The sulphate corrosion is here a typical, common example, which led to the invention of calcium aluminate cement by Bied (ciment fondu). The deterioration of concrete by de-icers, used in millions tons (for example in the USA in winter 1966/1967 6.3 milliont [62]), became a serious problem. The cost of bridges repairs in USA in 1975 was 200 million [63]. 

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... 

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