Mechanism of expansion in Type K cements

In a Type K cement, the formation of ettringite from C 4A,S represented by the equation 

Any attempt to understand the expansive process must thus consider the manner in which the deposition of the products affects the microstructure. The production of ettringite can cause expansion, but does not necessarily do so in a supersulphated cement (Section 9.2.9), much ettringite is formed, but there is no marked expansion. Comparable amounts of ettringite are reported to be formed when ordinary or sulphate-resisting Portland cements 

Cohen (C55) reviewed theories of the expansion mechanism. Probably a majority of workers (e.g. Refs I9,I10,O16,O24,BI33) have attributed expansion to forces exerted by the growth of the ettringite crystals. Of other theories, the most significant is that proposed by Mehta (MlOO). who attributed it to imbibition of water by the gelatinous layer of colloidal ettringite. On this hypothesis, expansion does not occur with the larger crystals formed at low CaO concentrations because these do not form 

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