Cement                 Principal                                                            Ettringite Forming

Type                     Constituents                                                             Aluminates

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                            Portland Cement

K                         Calcium Sulfate                                                             C4AS

                            High C4AS Cementitious Material

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                            Portland Cement

M                         Calcium Sulfate                                                     CA and C12A7

                            Calcium Aluminate Cement

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S                         Portland Cement High in C3A                                         C3A

                            Calcium Sulfate

The three types of expansive cements used in shrinkage-compensating concrete are listed above. Each differs from the others in its aluminate compound from which the expansive ettringite is formed. The aluminate compounds are added before or after the portland cement clinker is made, hence 90% (approximately) of the constituents in each are identical to regular portland cement. For this reason, expansive cements have almost identical properties, except for higher percentages of sulfates and aluminates, as those of portland cement.

The proper type and amount of aluminate must be proportioned into portland cement to determine the desired rate and amount of ettringite formed at early ages and thus the total expansion that is equal to the predicted shrinkage. The key is to have the correct material proportions of Al2O3 and CaO*SO3 available in the concrete mixure for the formation of ettringite at the appropriate time. The Al2O3 is the limiting reagent needed to produce ettringite, and the SO3 provides a sulfate-rich environment to stabilize the ettringite crystals.

Timing becomes a factor, because ettringite begins to form almost immediately after water is added to the cement. In order for expansive cement to work, a large percentage of the ettringite must form after some degree of strength has been achieved. Otherwise, the expansion will produce a non-recoverable deformation in the concrete, because the concrete is still to plastic in nature. Over-mixing will increase the chances of this to occure.