Author's personal copy Hydration studies of calcium sulfoaluminate cements blended with fly ash M. García-Maté a , A.G. De la Torre a , L. León-Reina b , M.A.G. Aranda a,c , I. Santacruz a, ⁎ a Departamento de Química Inorgánica, Cristalografía y Mineralogía, Universidad de Málaga, 29071 Málaga, Spain b Servicios Centrales de Apoyo a la Investigación, Universidad de Málaga, 29071 Málaga, Spain c CELLS-Alba synchrotron, Carretera BP 1413, Km. 3.3, E-08290 Cerdanyola, Barcelona, Spain abstract article info Article history: Received 11 February 2013 Accepted 4 July 2013 Keywords: X-Ray diffraction analysis Hydration (A) Compressive strength (C) Sulfoaluminate (D) Blended cement (D) The main objective of this work is to study the hydration and properties of calcium sulfoaluminate cement pastes blended with fly ash (FA) and the corresponding mortars at different hydration ages. Laboratory X-ray powder diffraction, rheological studies, thermal analysis, porosimetry and compressive strength measurements were performed. The analysis of the diffraction data by Rietveld method allowed quantifying crystalline phases and overall amorphous contents. The studied parameters were: i) FA content, 0, 15 and 30 wt.%; and ii) water addi- tion, water-to-CSA mass ratio (w/CSA = 0.50 and 0.65), and water-to-binder mass ratio (w/b = 0.50). Finally, compressive strengths after 6 months of 0 and 15 wt.% FA [w/CSA = 0.50] mortars were similar: 73 ± 2 and 72 ± 3 MPa, respectively. This is justified by the filler effect of the FA as no strong evidences of reactivity of FA with CSA were observed. These results support the partial substitution of CSA cements with FA with the economic and environmental benefits. © 2013 Elsevier Ltd. All rights reserved. 1. Introduction Calcium sulfoaluminate (CSA) cements are receiving increasing at- tention since their manufacture produces much less CO 2 than ordinary Portland cement (OPC) [1–5]. In addition, they show interesting proper- ties such as high early-age strengths [6–8], short setting times, imperme- ability [9,10], sulfate and chloride corrosion resistance and low alkalinity. The main uses of these CSA cements, or blended with Portland cements, are for quick repairs and pre-cast products or floor concrete applications [11]. All these properties are related to the resulting hydration phases at the first hours of hydration. Early hydration of CSA cements [12,13] gives ettringite also named as AFt (C 6 AS 3 H 32 ), and monosulfate also known as AFm (C 4 ASH 12 ) [14], as main crystalline phases, depending mainly upon the sulfate availability. CSA binders may show variable compositions, but all of them contain ye'elimite, also called Klein's salt or tetracalcium trialuminate sulfate (C 4 A 3 S) [1,15]. Ye'elimite synthesis produces only a third part of the CO 2 released by the production of alite content in OPCs [16]. Moreover, its clinkering temperature (~1250 °C) is lower than that for OPC clinkers, consequently there are environmental bene- fits due to fuel savings. Ye'elimite-containing clinkers are easier to be ground than OPC clinkers, and waste materials or industrial by- products can be used in the kiln feed as raw material. Another environmental strategy for reducing the negative impact of the cement industry is the partial substitution of cement by by-products materials (such as fly ash or slag) also known as supplementary cementitious materials (SCMs) [17–19]. The environmental benefits of the use of waste materials are two folds, lower CO 2 emissions because of the clinker reduction, and the valorization of a “useless” product. Furthermore, the addition of SCMs to cement may give the possibility of the modification of their properties [20] and mortars/concretes prepared from blends, such as decreased hydration heat, improved workability of fresh mortar/concrete mix, improved chemical resis- tance, ultimate strength increase, and improvement of other engineer- ing properties of concrete [21]. However, the extent of replacement is limited by the following problems: reduced early strength, limited amounts of reactive SCMs and limited replacement by non-reactive SCMs without compromising final strength. In any case, it is essential to control/understand the hydration process [14,22,23] without or with SCMs. The effect of the addition of by-products to OPC pastes has already been well described in literature [24,25]. However, for CSA cements, it is needed to understand all parameters involved in the CSA-SCMs hydration, and the key roles of water and SCMs contents. The w/c ratio required for full CSA hydration is higher than that for an OPC [26]. The specific water demand for complete hydration will de- pend on CSA phase assemblage (mainly ye'elimite and belite contents). For instance, pure ye'elimite reacting with the stoichiometric amount of anhydrite to yield ettringite requires a 0.78 w/c mass ratio [14,27]. This means that a CSA with ~ 50 wt.% of ye'elimite would need a w/CSA ratio of 0.5, which may yield pastes with larger pore diameters than OPC pastes. Moreover, high w/c ratios may involve severe expansion [28] also resulting in poor final strengths; however, lower initial particle sizes or the use of additives may reduce some of these undesirable properties [26,29,30]. Cement and Concrete Research 54 (2013) 12–20 ⁎ Corresponding author. Tel.: +34 952131992; fax: +34 952132000. E-mail address: isantacruz@uma.es (I. Santacruz). 0008-8846/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cemconres.2013.07.010 Contents lists available at ScienceDirect Cement and Concrete Research journal homepage: http://ees.elsevier.com/CEMCON/default.asp