Contents lists available at ScienceDirect Journal of Building Engineering journal homepage: www.elsevier.com/locate/jobe Phase assemblage in ettringite-forming cement pastes: A X-ray diffraction and thermal analysis characterization Elsa Qoku ⁎ , Thomas A. Bier, Torsten Westphal Institut für Keramik, Glas, und Baustofftechnik, TU Bergakademie Freiberg, Germany ARTICLE INFO Keywords: Cement pastes Hydration Phase assemblage X-ray amorphous hydrates ABSTRACT The study attempts to describe the evolution of the solid phase composition with ongoing hydration in three different calcium aluminate rich cement paste mixtures by means of XRD and thermal analysis. The phase assemblage was followed quantitatively at discrete ages of 1, 7, 28, 56 and 90 days. Ettringite was the main crystalline hydration product. Quantification of amorphous fractions using the external standard method was performed and relatively high amounts of amorphous fractions were reported in all the cases. Thermal analysis revealed that the X-ray amorphous hydrate fraction was mainly composed of monosulphate, AH 3 and C-S-H. The presence of strätlingite, was not clearly manifested in any of the DTG curves. A mass balance calculation based on stoichiometric reactions was performed in order to estimate the amounts of monosulphate, AH 3, and C-S-H. The quantities of the amorphous portions obtained from QXRD were observed to be higher as those estimated from mass balance calculations. Additional calculations from oxide balance suggested that besides AH 3 monosulphate and C-S-H, an X-ray amorphous AFm or/and C-S-H type like phase might form in the early age of hydration. During Rietveld refinement, the impact of the number of Chebyshev background polynomials in the determination of amorphous content was investigated. 1. Introduction 1.1. Background Fast setting binders are often composed of three mineral compo- nents which are Portland cement (PC), calcium aluminate cement (CAC) and calcium sulphate (CS ̄ Hx). In PC/CAC/CS ̄ Hx ternary systems, two mixes can be distinguished, a CAC richer mix and a PC richer mix. As reported by several researchers [1–3] these compositions are mainly used as technical mortars for concrete fast repair and protection, or flooring installation with selfleveling compounds. Ettringite is one of the main phases formed in such systems. Although not really proven, several studies suggest that fast setting is related with ettringite formation [4–7]. The formation of ettringite occurs through solution [8]. Besides ettringite, other hydrates such as, C-S-H, AH 3 and AFm constituents are formed (cement notation will be used throughout the text with A: Al 2 O 3 , C: CaO, F: Fe 2 O 3 , H: H 2 O, M: MgO, S: SiO 2 ,S ̄ : SO 3 , T: TiO 2 ). The hydration steps of ternary systems composed of OPC, CAC and CS ̄ are complex. Some of the main reactions occurring during the hydration process are described below. When a ternary binder system (PC / CAC /CS ̄ Hx) encounters water, the phases dissolve and the following reactions take place: CA CSH xH A CSH AH 3 +3 + (38 − 3 ) →C 3 +2 x 3 32 3 (1) CA CSH xH A CSH +3 + (32 − 3 ) →C 3 X 3 3 32 (2) CA C CSH xH A CSH +2 +3 + (32 − 3 ) →C 3 X 3 32 (3) CS + 5.3H → C SH + 1.3CH 3 1.7 4 (4) where x=0 for anhydrite, x=0.5 for hemihydrate and x=2 for gypsum. When the calcium sulphate is depleted, ettringite reacts with remaining anhydrous CA to form calcium monosulphate (AFm phase). A CSH CA H ACS H AH C 3 +6 + 16 → 3C +4 3 32 3 12 3 (5) Other coupled reactions between hydrates and anhydrous phases coming from the different cements, are also relevant and most of them can be found in the literature [6]. The complexities of the mechanisms involved in these systems have been reported by various studies [9–11]. 1.2. Scope of the present study Despite the available data on the hydration mechanisms of ternary http://dx.doi.org/10.1016/j.jobe.2017.05.005 Received 13 January 2017; Received in revised form 5 May 2017; Accepted 5 May 2017 ⁎ Corresponding author. E-mail address: elsa.qoku@ikgb.tu-freiberg.de (E. Qoku). Journal of Building Engineering 12 (2017) 37–50 Available online 08 May 2017 2352-7102/ © 2017 Elsevier Ltd. All rights reserved. MARK