Effect of retarders on the early hydration of calcium-sulpho-aluminate (CSA) type cements

The retardation of calcium-sulpho-aluminate (CSA) type cements by three common retarders—sodium gluconate, sodium–potassium tartrate and borax is investigated at a 2% addition rate. Each of the retarders has a different effect on the early-age hydrate assemblage, as shown by quantitative X-ray diffraction, thermogravimetric analysis, scanning electron microscopy and pore solution analysis. Regardless of the retarder used, the hydration starts with mainly the dissolution of ready soluble (calcium) alkali sulphates and ye'elimite, followed by the formation of ettringite and aluminium hydroxide, whereby the specific clinker mineral dissolution as well as ettringite-forming reaction is delayed in different ways depending on the retarder used. The results showed that the retardation is mainly caused by preventing hydrates formation in the case of tartrate and gluconate. Contrary, borax retards the hydration by preventing to a certain extent the dissolution of ye'elimite and lowers the initial pH. Furthermore, the morphology of ettringite is altered depending on the type of the retarder used.