Here - Institute for Building Materials - ETH Zürich

Dissolution kinetics and rate-control of alite and portland cement clinkerBisschop, J., and Kurlov, A.Institute for Building Materials, ETH-Zürich,Schafmattstr. 6, 8093 Zürich, SwitzerlandHydration of portland cement is a complex chemical process involvingdissolution and precipitation reactions. An understanding of what the ratecontrollingreactions in cement hydration are, will benefit cement hydrationmodeling and ultimately the development of improved concretes. In thiscontribution we address the topic of the type of dissolution rate control for alite(C3S), i.e., by aqueous diffusion or by surface reaction rate. This will partlydetermine if C3S dissolution has a potential to act as a rate-limiting step duringportland cement hydration.A new flow-through dissolution method has been developed for the purpose ofmeasuring dissolution rates of single- or multiphase cements. Dissolution rateswere measured from SEM-BSE images of sample cross-sections which has anumber of advantages over existing dissolution measurement techniques. Thecapability of the method is demonstrated by experiments on flat-groundembedded fragments of raw portland cement clinker and synthesized alite pelletsin deionised water. Dissolution rates increased with water flow velocity (0, 0.45,4.5, 16.4 m/s) and this shows that the alite/C3S dissolution rate was transport(diffusion) controlled under the studied conditions, at least at lower flow rates.This result is supported by preliminary predictions of a diffusion boundary layermodel for the laminar flow regime in our experiments.Widespread precipitation of hydrates occurred during dissolution of C3S inclinker. In the synthesized alite fibrous/sheet-like precipitates on the dissolutionfront were sometimes observed as a result of water penetrating the material alongcrack or grain boundary surfaces. The amount of precipitates generally decreasedwith increasing flow rate, but no evidence was found that the precipitationaffected the initial dissolution rate. Hydrate precipitation is an expectedphenomenon during diffusion-controlled dissolution. After one hour ofdissolution, the dissolution rate of alite became larger than the one of C3S inclinker. This showed that dissolution of the latter was no longer limited byaqueous diffusion, but became limited by transport through a layer of precipitatesand non-dissolved clinker components.48