Calcium aluminate, expansive and other cements

Calcium Aluminate. Expansive and Other Cements 323 10.1.5 Thermodynamic calculations... 

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. 

Pastes inpregnated with PMMA or sulphur are still sufficiently permeable to water that expansion occurs on long exposure (F46). In polymer-impregnated (S108) and MDF (R64) cement pastes, there is evidence of interaction between Ca ions and carboxylate and possibly other groups of the polymer. In MDF pastes made with calcium aluminate cement, the polymer (PVA) was found to inhibit the normal hydration reactions of the cement, but to react with Ca and AH to give an ionically cross-linked polymer and calcium acetate. TEM showed the material to be essentially a dispersion of grains of clinker or hydration products in a continuous polymer matrix. 

Ettringite cements contain an aluminate donor such as monocalcium almninate, tricalcium aluminate, tetracalcium trialmninate sulfate, or tetracalcium aluminate ferrite, together with calcium sulfate dihydrate and in some instances also calcium hydroxide. In the hydration of such mixes ettringite is formed as the main or sole reaction product. If allowed to react umestricted, the hydrating paste exhibits a significant expansion however, hardened pastes with strengths comparable to those of other cements may be produced if the hydration is allowed to take place under mechanically restricted conditions. This may occur in completely closed steel molds, by which measure undesired expansion of the paste m be effectively prevented (Odler and Yan, 1994). 

In the most widely used expansive cements the cementitious component consists of ordinary Portland cement, but other binders such as calcium aluminate cement may also be employed. Most commonly, the expansive stresses in the course of hardening are created by the formation of ettringite (C3A.3CS.32H). This phase is formed in the reaction of a constitnent containing alutninnm oxide with calcinm snlfate, calcinm oxide or hydroxide and water. Less common than expansive cements based on ettringite... 

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