Assessment of mechanical, thermal properties and crystal shapes of monoclinic tricalcium silicate from atomistic simulations

Abstract

The two most common polymorphs in industrial alite, M1 and M3, were characterized at the molecular scale. Different methods were employed and discussed to assess mechanical properties and specific heat of both polymorphs. The calculated homogenized elastic moduli and specific heat were found in good agreement with experimental measurements. A comparative analysis of spacial Youngs modulus reveal isotropic and anisotropic spacial distribution for M₁ and M₃ respectively. A more isotropic compressive strength is also reported for M₁ when compared to M₃ polymorph. Cleavage energies computation allowed to proposed equilibrium shapes for both polymorph, with significant differences. While the lowest cleavage energies were found along (100) and (001) for both polymorphs, the constructed M1 crystal possesses 3 independent facets, against seven for the M3 polymorph.

Introduction

The research in cementitious materials is experiencing new challenges mainly due to the need to preserve the environment and save energy. To reduce CO₂emissions and energy cost of production, alternative binders are under study [1]. However, ordinary Portland cement (OPC) should continue to be employed for a long time and understanding of its principal constituents is primordial for its improvement. Another way to reduce the environmental impact of Portland cement is to enhance its reactivity which will involve less content of cement in concrete for the same strength. Since alite is the principal phase of Portland clinker that most contributes to strength development of Portland cement and particularly at early ages, a deep understanding of its properties is crucial to improve its quality and reactivity. Alite is a tricalcium silicate (C₃S) with minor oxides usually called impurities. It presents a large grade of polymorphism depending on different factors, among them: the nature and the amount of impurities, the temperature of preheating or burning [2]. Seven structures were reported in industrial alite: three triclinic (T₁, T₂and T₃), three monoclinic (M₁,M₂and M₃), and a rhombohedral form R. These polymorphs appear via successive and reversible phase transitions [3]:

$${\mathrm{T}}_1\stackrel{620°\mathrm{C}}{\leftrightarrow }{\mathrm{T}}_2\stackrel{920°\mathrm{C}}{\leftrightarrow }{\mathrm{T}}_3\stackrel{980°\mathrm{C}}{\leftrightarrow }{\mathrm{M}}_1\stackrel{990°\mathrm{C}}{\leftrightarrow }{\mathrm{M}}_2\stackrel{1060°\mathrm{C}}{\leftrightarrow }{\mathrm{M}}_3\stackrel{1070°\mathrm{C}}{\leftrightarrow }\mathrm{R}$$

It is well known that impurities in alite stabilize high temperature polymorphs at low temperatures [3,4]. The two main C₃S polymorphs present in industrial clinker are M₁ and M₃. According to Maki and Goto [5], MgO in clinker promotes the stable growth of alite in favor of the occurrence of M₃. In contrary, decreased MgO/SO₃ratio lead to M₁ stabilization [2,5]. It was also reported that preheating of the raw meal may result in the disappearance of the M₃ polymorph [2]. This modifications in the structure of C₃S can have a significant impact on strength as reported in few experimental results in the literature [2,6]. The transformation of M₃ to M₁ polymorph may result in a 10% increase in the compressive strength [2]. The origin of this observed variation in strength could be explained by a greater amount of non-bonding electrons in oxygens of M₁ C₃S, leading to a higher reactivity when compared to the M₃ polymorph. The impact of structure at the nanoscale on the properties like strength is a complex topic and investigation at the atomic level via molecular modelling and simulation should be helpful to improve our understanding of Portland cement.

Over recent years, the properties of cementitious materials were addressed using atomistic models, with particular attention on the main hydration product of OPC: calcium silicate hydrates (C-S-H) [7,8]. In comparison, only few studies at the atomic scale focused on OPC clinker phases [9]. Computation of thermal and mechanical properties at the molecular scale can provides important information on the behaviour of OPC clinker and hydrated product. From the computation of surface energies, crystal shapes can be theoretically constructed for different polymorphs and help to understand morphological changes [10]. The knowledge of preferential cleavage planes and crystal shapes of C₃S polymorphs is fundamental to understand their growth during the clinkering process [11]. It could also explain C₃S dissolution mechanisms [12] or cleavage modes during clinker grinding [13,14]. Determination of such properties by experimental methods are most of the time limited, especially in the case of surface energies [15]. In all cases, a proper synthesis procedure of pure C₃S is necessary, and the determination of the amount of each polymorph in a sample is neither trivial nor accurate [16].

In this work, the mechanical, thermal and surface properties of M₁ and M₃ C₃S (the main forms of alite encountered in industrial OPC [17]) were characterized by molecular dynamics (MD) simulations. The present article is divided into four sections: crystal structures and force fields, mechanical properties, thermal properties, and cleavage energies and equilibrium shapes. The last three sections include a description of the method employed and a presentation and discussion of results. To finish, a general conclusion resumes the different findings.