Sulphate calcium aluminate cements

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. 

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... 

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. 

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. 

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. 

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. 

Hardened cement paste mainly consists of a porous, semi-colloid phase of calcium sihcate hydrates physically incorporating crystalline calcium aluminate sulphate hydrates, the so-called cement gel. The cement gel has a very large active internal surface area that can adsorb water molecules it has been found through measurements that the specific surface S of hardened cement paste is of the magnitude of 200000 m /kg of the material. The specific surface is a measure of the internal, free surface in m per kg of the material. 

Cements are commonly made by heating a mixture of limestone and clay to about 1700 C. The product is ground with gypsum. Chemically cements consist of a mixture of calcium silicates and aluminates with some sulphate present. World production 1976 730 megatonnes. 

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. 

Many salts precipitate additional phases when added to CjS or cement pastes. If the hydroxide of an added cation is less soluble than CH, either it or a basic or complex salt is precipitated. Examples of such precipitates are Mg(OH)2 or AFm phases (K57), Zn(OH)2 or CaZn2(0H) -2H20 (A27) and basic lead nitrate or sulphate (T54). Precipitation will leave in solution all or part of the added anion and an equivalent amount of Ca. The calcium salt of an added anion, if of sufficiently low solubility, is similarly precipitated examples are provided by SO , COj", PO4", F, silicate, aluminate and borate. In this case, the added cation remains in solution with an equivalent amount of OH... 

Gypsum is important as it reacts with aluminate to give etringite - calcium aluminium sulphate hydrate, Ca6Al2S30ig 32H2O - on the surface of the aluminate grains. This slows the reaction of aluminate with water and allows the wet cement paste to be worked for longer. 

The alkali content in clinker is usually on the level of 1 % Na O with dominating soluble alkali sulphates. In the solution rich in CH, they increase the solubility of gypsum, while the solubility of calcium hydroxide decreases. In this condition the SO4 ions can be bonded with C-S-H. That is why the gypsum addition should be higher. When the specific surface area of cement is high and the reaction of tricalcium aluminate is therefore accelerated, the gypsiun addition should be higher too. 

The most common type of cement used by concrete manufacturers is Portland cement, which is prepared by igniting a mixture of raw materials mainly composed of calcium carbonate or aluminium silicates. Portland cement can be defined as hydraulic cement produced by pulverizing clinker consisting essentially of hydraulic calcium silicates, usually containing one or more of the forms of calcium sulphate as an interground addition . The phase compositions in Portland cement are denoted as tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A), and tetracalcium aluminoferrite (C4AF). 

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