Contents lists available at ScienceDirect Cement and Concrete Research journal homepage: www.elsevier.com/locate/cemconres Influence of calcium and magnesium carbonates on hydration kinetics, hydrate assemblage and microstructural development of metakaolin containing composite cements Maciej Zajac a, ⁎ , Pawel Durdzinski b , Christopher Stabler a , Jan Skocek a , Dominik Nied a , Mohsen Ben Haha a a HeidelbergCement Technology Center GmbH, Rohrbacher Str. 95, 69181 Leimen, Germany b École Polytechnique Fédérale de Lausanne (EPFL), Laboratoire des Matériaux de Construction, Station 12, CH-1015 Lausanne, Switzerland ARTICLE INFO Keywords: Composite cement (D) Mechanical properties (C) Microstructure (B) ABSTRACT The hydration of metakaolin composite cements containing quartz, natural limestone and dolomite rock is studied using a multi-method approach and modelling. The study demonstrates that the calcite present in limestone and dolomite rock is very reactive. Contrary, dolomite does not dissolve. Additionally to the previously reported stabilization of ettringite, the reaction of calcite introduces several changes to the mechanism of metakaolin pozzolanic reaction. Namely, the silicate and aluminate distribution among the hydrates is different. In carbonate containing samples, experimental results supported by the thermodynamic modelling suggest that the silicate mainly precipitates as low Ca/Si C-S-H and additionally the ettringite content is higher. In the case of quartz analogue, the silicates precipitate as strätlingite and C-S-H of higher Ca/Si. These changes cause lower porosity as observed by SEM-BSE and higher strength. High metakaolin reactivity results in a very dense matrix that in turn enables co-existence of phases that are thermodynamically non-compatible. 1. Introduction Current cements incorporate significant amounts of supplementary cementitious materials (SCMs) as a lever to reduce the environmental footprint associated with the cement production [1][2]. Supplementary cementitious materials contribute to the mechanical properties of hardened concrete through their hydraulic or pozzolanic properties. Hydration of the cement clinker and the pozzolanic reaction of the SCMs occur simultaneously and may influence the reactivity of each other [3]. The presence of a pozzolan increases the early reactivity of Portland clinker due to the so-called filler effect [4][5]. Typical pozzolans react slowly achieving substantial reaction degrees only after several weeks [6]. The exception is metakaolin that shows a rapid re- action [7]. The C–S–H with a reduced Ca/Si and increased Al/Si ratio is formed as a result of the pozzolanic reaction [6]. In parallel, additional AFm phases precipitate when reactive alumina is also provided by the SCMs [6]. The presence of reactive pozzolans may limit the late hy- dration rate of the Portland clinker, i.e. after 28 days [8]. The phase assemblage and microstructure of composite cements can be further changed by the presence of limestone. In Portland cements, the limestone addition stabilizes monocarbonate in favor of monosulfate hindering the decomposition of ettringite [9]. This effect is even more pronounced in the case of composite cements containing high alumina pozzolans like fly ash or calcined clay [6][10][11]. This study reports on the hydration of composite cements con- taining metakaolin blended with two carbonate rocks (limestone and dolomite) and an inert quartz powder. Dolomite was used since it is frequently associated with calcite in carbonate rocks [12] and is im- portant for industrial applications. In order to reproduce the real in- dustrial conditions, a natural dolomite rock containing also some calcite was used. Additionally, the dolomite mineral is expected to react slower than limestone [13][14][15]. As an additional reference, the plain Portland cement was investigated. Multi-technique approach was used to characterize the mortars and pastes. Mechanical testing of the compressive strength was done on mortar cubes. The hydration of paste samples was investigated by means of thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX) and mercury intrusion porosimetry (MIP). In order to enrich the discussion, thermodynamic modelling was carried out. https://doi.org/10.1016/j.cemconres.2018.01.008 Received 26 April 2017; Received in revised form 5 January 2018; Accepted 10 January 2018 ⁎ Corresponding author. E-mail address: maciej.zajac@htc-gmbh.com (M. Zajac). Cement and Concrete Research 106 (2018) 91–102 0008-8846/ © 2018 Elsevier Ltd. All rights reserved. T