Expansive Strontium Aluminate Cements

Abstract

Cement hydration is a complex set of exothermic chemical reactions. Each chemical compound may react with water in different ways, each reaction has its own volume stoichiometry, and the reaction (or hydration) products may undergo the transition to different forms. There are several mechanisms by which a cement paste changes its volume (shrinks or swells) during the hydration process under sealed conditions, or without the moisture transfer from paste to the environment. The major mechanisms of total shrinkage include [12,278,673-676]: • Hydration shrinkage; • Self-desiccation shrinkage; • Thermal contraction or expansion; • Crystallization swelling; • Shrinkage during the phase transition or expansion. Autogenous shrinkage is a phenomenon in which cementitious materials shrink at constant temperature without any changes in weight [677]. It is well-known that the volume changes in a cement paste or concrete mentioned above may occur simultaneously. It is the overall shrinkage behaviour that controls the concrete performance [278]. Furthermore, the influence of this phenomenon on total shrinkage depends on the size of specimen [414]. Most chemical shrinkage measurements reported are often performed at short duration and suggested water to cement ratio of 0.40 as required by ASTM C 1608 [678]. Autogenous shrinkage in concrete with low water-cement ratio is the major factor that generates larger shrinkage and early cracking [673,677,679]. A small increase in the chemical shrinkage at "infinite time" was found with increasing water-cement ratio. The influence of temperature was found to be twofold: Increasing temperature caused increasing rate of the development of chemical shrinkage and the decrease of the chemical shrinkage at "infinite time" [680]. The mineral admixtures in concrete affect the concrete shrinkage in different ways. Fly ash and limestone powder can reduce the autogenous shrinkage while silica fume may lead to increased shrinkage [679,681].