Composite cement based on portland cement clinker, limestone and calcined clay

Composite cements consisting of CEM I Portland cement and 30 % replacement by mixtures of calcined clays and limestone have been studied. The calcined clays were prepared from kaolinite and smectitic clay by heating to the minimum temperature needed for complete dehydroxylation. The limestone was upper Maarstricht chalk from Northern Denmark ground to approx. 12000 cm2/g. Replacement of the CEM I Portland cement with 30 % limestone resulted in a relative 28 day standard mortar strength of 78 % compared to the CEM I. Replacement with 30 % calcined clay resulted in relative 28 day mortar strengths of between 60 and 84 % depending on the reactivity of the calcined clay. Mixtures of the same clays and limestone at the same replacement level invariably resulted in higher strengths than would be predicted by the results of the binary systems with relative 28 day strengths as high as 109 % of the CEM I strength, although a linear effect was observed when the limestone was mixed with an inert filler (aplite) rather than (aluminosilicate) calcined clays. Mineralogical analyses (NMR and XRD) performed on paste specimens showed a strong correlation between the amount of monocarboaluminate hydrate formed and the high strengths described above. This is in good agreement with thermodynamic calculations which predict higher monocarboaluminate contents in the composite systems, where more limestone is consumed in the presence of higher amounts of available alumina from the calcined clays.

Author: M. Steenberg, D. Herfort, S.L. Poulsen, J. Skibsted, J.S. Damtoft

Section: Science and production

Keywords: composite cement, calcined clay, limestone, CO2 reduction