The reaction of slag in cement, theory and computer modelling

For a better understanding of the performance of slag in concrete, evaluating the feasibility of using one certain type of slag and possible improvement of its use in practice, fundamental knowledge about its reaction and interaction with other constituents is important. While the researches on hydrating Portland cement paste is quite abundant, researches on theory of slag cement reaction are rather scarce. At least three difficulties impede the advancement: (a) The lack of knowledge about the chemistry of slag reaction; (b) The complexity involved with respect to the interaction between the two constituents in slag cement (slag and Portland cement); (c) The reactivity of slag in cement. Efforts on clarifying these three factors will be proven valuable when evaluating the reactivity of a slag, predicting the microstructure development and investigating the durability aspects of the concrete made with slag. Microstructural modelling of cement hydration is expected to provide a reliable representation of the real hydration process. It can on the one hand deepen the understanding of the material, and on the other hand extrapolate properties outside the available data. The ultimate goal of microstructural modelling is to predict the performance of cement-based materials throughout its service life. Benefiting from the rapid development in computer science, and the fundamental knowledge about the chemistry of cement hydration, several computer models have been developed in various groups to simulate the microstructure development of hydrating cement paste. Most of the models available now are focused on the Portland cement hydration, while very few models are able to deal with slag cement hydration, yet. In this· study, theoretical models available for the reaction of both pure slag (alkali-activated) and slag-blended cement are reviewed, They were developed by using stochiometric computations. For the slag-blended cement, models accounting for the interaction etween the slag and clinker reactions were proposed, involving the CH (notation in cement chemistry is used, e.g. C = CaO, S = Si02, A = Al203, H = H20 etc.) consumption and the composition equilibrium of the C-S-H. These theoretical models are further incorporated into a 3-D computer model, CEMHYD3D, originally developed for simulating the Portland cement hydration, with special consideration to the reactivity of slag in cement.