CEMENTand CONCRETE RESEARCH Pergamon Cement and Concrete Research 32 (2002) 1169-1175 The influence of the alite polymorphism on the strength of the Portland cement T. Staněk3, *, P. Sulovskýb •f 3Research Institute of Building Materials, Hněvkovského 65, 617 00 Brno, Czech Republic h'Faculty of Natural Sciences of the Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic Received 3 January 2001; received in revised form 5 February 2002 Abstract The influence of the alite polymorphism on the strength of cement was monitored in a set of laboratory-prepared clinkers with equal quantitative phase composition and different ratio of modifications. The alite polymorphism in clinkers was influenced by the change of the MgO and S03 side oxides in clinker, raw meal reactivity change, raw meal preheating, burning temperature or by the adding of crystallisation nuclei. The differences in the hydraulic properties of the M] and M3 modifications were determined. In the case of all the hydration periods monitored, the strength of cements with the M| modification was 10% higher than the strength of cements with the M3 modification. © 2002 Elsevier Science Ltd. All rights reserved. Keywords: Crystal structure; Compressive strength; Clinker; Cement paste 1. Introduction ically trigonal pseudomorphs in clinker. The most common modifications are IV^ and M3, and T2 occurs only rarely [3]. The ratio of the nucleation processes and alite crystallisa tion, and the contents of minor oxides in the melt are the decisive stabilisation factors. High intensity of nucleation and slow, stable crystal growth together with increased MgO stabilise M3. On the contrary, high growth rate and non stable crystal growth at low nucleation and increased S03 lead to Mi stabilisation [4-6]. At high melt oversaturation, the slow steady crystallisation during an intense nucleation results in the growth of smaller idiomorphic crystals of M3 with a low number of inclusions. In the case of a low oversaturation of the melt, crystal growth predominates over nucleation; low number of nuclei leads to fast growth of large irregularly shaped M, crystals with abundant inclu sions of belite and interstitial matter. These genetic markers help to identify alite microscop ically in clinker. The distinguishing features are different birefringence and type of twinning. The M3 form has two times higher birefringence than Mn T2 is characterised by the lowest birefringence and inverse polysynthetic twinning. The polymorphs can also be identified by X-ray diffraction [7,8], Alite modifications can also be reliably identified by high-temperature microphotometry, giving the variation of birefringence as a function of temperature [2,9,10]. Alite is the principal hydraulic phase of clinker, having the decisive impact on the strength of Portland cement. Alite is a solid solution of Ca3Si05 with minor oxides. Ca3Si05 is characterised by an extensive polymorph ism. Seven structural modifications have been identified [1,2] with enantiomorphic transformations—three triclinic (T), three monoclinic (M) and one trigonal—the trans formation temperatures of which (°C) can be observed in the following scheme: 620 920 980 990 1060 1070 Ti <-----'T2 <-^T3 <^+M, —>M2 M3 <—4 R. n In clinker, alite always crystallises from the melt in a trigonal form. The stabilisation factors decide during the cooling process which modification stable at ambient tem perature is formed. For this reason, alite forms morpholog- * Corresponding author. Tel.: +42-5-43-52-93-48; fax: +42-5-43-27- 60-29. E-mail address-, t.stanek@email.cz (T. Staněk). 0008-8846/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII: S0008-8846(02)00756-l