Phase development, heat of hydration and expansion in a blended cement with b-hemihydrate and anhydrite as calcium sulfate

Abstract: Expansive cements are utilised in special applications to improve properties such as cracking caused by drying shrinkage. Several types of expansive cements exist and the main hydration product responsible for the expansion is ettringite, [Ca₃Al(OH)₆]₂(SO₄)₃·26H ₂O. The importance of the cement phase containing alumina is well known, in this study the source of sulfate is investigated. Pastes containing 50 wt% calcium aluminate cement, 25 wt% Portland cement and 25 wt% calcium sulfate mixed with water at a water to solid weight ratio of unity have been investigated. The calcium sulfate was b-hemihydrate, anhydrite or mixes of the two. The phase development was studied by in-situ synchrotron X-ray diffraction, the heat of hydration by isothermal calorimetry and the dilatation was followed by a unidimensional dilatometer. The microstructure of hydrated pastes was investigated by scanning electron microscopy. The source of calcium sulfate was shown to be important to the kinetics of ettringite formation and the expansive properties of ettringite. In the paste with b-hemihydrate, ettringite forms continuously over a time period of approximately three hours parallel to the formation and re-dissolution of gypsum. The last 50% of the ettringite formation is followed by an expansion caused by interacting spheres of ettringite crystals growing radially on the surfaces of the cement grains. When anhydrite is utilised, the ettringite formation is slower and not accompanied by expansion. The kinetics of ettringite formation and the expansive properties can be controlled by blending the two calcium sulfates. When 75 and 50 wt% of the calcium sulfate was b-hemihydrate, the kinetics of ettringite formation can be described as a linear addition of the kinetics in the pure pastes. In the remaining mixed paste, containing 25 wt% b-hemihydrate, the phase development differed from the other two mixes and sulfate-AFm was detected. Addition of lithium carbonate accelerates the dissolution of anhydrite and appears to alter the hydration reactions at high additions. Tartaric acid retards all stages of hydration in all pastes. The utilisation of dynamic corrections in isothermal calorimetry has been investigated. A correction with two time constants has been found successful in order to study the very early reactions in cement hydration.