Author's personal copy Rheological and hydration characterization of calcium sulfoaluminate cement pastes Marta García-Maté a , Isabel Santacruz a , Ángeles G. De la Torre a , Laura León-Reina b , Miguel A.G. Aranda a,⇑ a Departamento de Química Inorgánica, Cristalografía y Mineralogía, Universidad de Málaga, 29071 Málaga, Spain b Servicios Centrales de Investigación, Universidad de Málaga, 29071 Málaga, Spain article info Article history: Received 20 October 2011 Received in revised form 13 January 2012 Accepted 18 January 2012 Available online 1 February 2012 Keywords: Rheology Hydration X-ray diffraction Compressive strength Cement paste Sulfoaluminate abstract Calcium sulfoaluminate (CSA) cements are currently receiving a lot of attention because their manufac- ture produces less CO 2 than ordinary Portland cement (OPC). However, it is essential to understand all parameters which may affect the hydration processes. This work deals with the study of the effect of sev- eral parameters, such as superplasticizer (SP), gypsum contents (10, 20 and 30 wt.%) and w/c ratio (0.4 and 0.5), on the properties of CSA pastes during early hydration. This characterization has been per- formed through rheological studies, Rietveld quantitative phase analysis of measured X-ray diffraction patterns, thermal analysis and mercury porosimetry for pastes, and by compressive strength measure- ments for mortars. The effect of the used SP on the rheological properties has been established. Its addi- tion makes little difference to the amount of ettringite formed but strongly decreases the large pore fraction in the pastes. Furthermore, the SP role on compressive strength is variable, as it increases the val- ues for mortars containing 30 wt.% gypsum but decreases the strengths for mortars containing 10 wt.% gypsum. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Calcium sulfoaluminate (CSA) cements are receiving increasing attention nowadays since their manufacture produces less CO 2 than ordinary Portland cement (OPC) [1–4]. These binders may have quite variable compositions, but all of them contain Ye’eli- mite phase, also called Klein’s salt or tetracalcium trialuminate sul- fate (C 4 A 3 S ) [5]. Cement nomenclature will be used hereafter C = CaO, S = SiO 2 , A = Al 2 O 3 , F = Fe 2 O 3 ,S = SO 3 , M = MgO, T = TiO 2 and H = H 2 O. The term CSA cements is usually reserved for those clinkers containing more than 50 wt.% of Klein’s salt and they may also have minor amount of phases such as C 2 S, CA, C 4 AF, CS, CS H 2 , and so on [6]. Cements with large amounts of Ye’elimite may have special applications such as high strength developments at early-ages [7,8] and radioactive element encapsulation in a high-density cement paste [9,10]. On the other hand, a new related type of cement is emerging, aimed to replace OPC in the long term by combining approximately 20–30 wt.% of Ye’elimite (for early- age strength development) and 50–60 wt.% of activated belite (for medium-age strength development) [11–13]. This type of ce- ment, initially developed in China but without the key activation of belite, is known as sulfobelite and also as belite-calcium sulfoa- luminate cement to stress the importance of belite in its composi- tion and performance. CSA cements are prepared by mixing the clinker with different amounts of a calcium sulfate set regulator such as gypsum, bassa- nite or natural anhydrite, or mixtures of them. Their main proper- ties are high early strengths, short setting times, impermeability, sulfate and chloride corrosion resistance and low alkalinity [5]. The early hydration of the CSA cements is mainly governed by the amount and reactivity of the added calcium sulfate [8,14– 16]; it gives as main crystalline phases ettringite and monosulfate [17] and most of the hydration heat is released during the first 12– 24 h of hydration [18]. The w/c ratio needed for full hydration is higher than that for an OPC. For instance, pure Ye’elimite reacting with the stoichiometric amount of anhydrite to yield ettringite re- quires a 0.78 w/c ratio [17,19] which may yield pastes with larger pore diameters than OPC pastes. Moreover, both high and low w/c ratios may involve severe expansion [20] with high w/c ratios also resulting in final strength development problems [21]; however, lower particle size [20] or the use of additives may eliminate some of these undesirable properties [22]. The dispersion of agglomerated cement particles is a key point to improve the workability of concrete to obtain more homoge- neous mixtures and to reduce the amount of mixing water [23,24]. This is the role of superplasticizers and their use has be- come a common practice in OPC. It has been demonstrated that a lower water demand, within certain limits, is related with im- proved mechanical properties [25,26]. This is because the adsorp- tion of the superplasticizers onto the surface of cement particles causes the repulsion of the particles (electrostatic or electrosteric). This repulsion depends [23] on the type and amount of 0958-9465/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconcomp.2012.01.008 ⇑ Corresponding author. Tel.: +34 952131874; fax: +34 952131870. E-mail address: g_aranda@uma.es (M.A.G. Aranda). Cement & Concrete Composites 34 (2012) 684–691 Contents lists available at SciVerse ScienceDirect Cement & Concrete Composites journal homepage: www.elsevier.com/locate/cemconcomp