Effect of temperature and aluminium on calcium (alumino)silicate hydrate chemistry under equilibrium conditions

Abstract

There exists limited information regarding the effect of temperature on the structure and solubility of calcium aluminosilicate hydrate (C–A–S–H). Here, calcium (alumino)silicate hydrate (C–(A–)S–H) is synthesised at Ca/Si = 1, Al/Si ≤ 0.15 and equilibrated at 7–80 °C. These systems increase in phase-purity, long-range order, and degree of polymerisation of C–(A–)S–H chains at higher temperatures; the most highly polymerised, crystalline and cross-linked C–(A–)S–H product is formed at Al/Si = 0.1 and 80 °C. Solubility products for C–(A–)S–H were calculated via determination of the solid-phase compositions and measurements of the concentrations of dissolved species in contact with the solid products, and show that the solubilities of C–(A–)S–H change slightly, within the experimental uncertainty, as a function of Al/Si ratio and temperature between 7 °C and 80 °C. These results are important in the development of thermodynamic models for C–(A–)S–H to enable accurate thermodynamic modelling of cement-based materials.

Introduction

Temperatures experienced by cement and concrete based construction materials in service can vary greatly, due to heat evolution from cement hydration, variable ambient environmental conditions, steam curing, and other factors. The effects of temperature on hydrated blended and neat Portland cement (PC) material properties are important, and can include the following: increased reaction rate and density of calcium silicate hydrate (C–S–H)^a [1], coarsening of paste microstructures [2], and decreasing compressive strengths [3] with increasing temperature. Despite the wealth of engineering information available in this area, only a few studies are available in the literature regarding the equilibrium phase assemblages and aqueous chemistry of PC systems as a function of temperature [1], [4], [5]. However, a good understanding of the nature of C–S–H and other constituent phases in these systems at equilibrium [6], [7], [8], [9] has meant that hydrated neat PC materials can be accurately described by thermodynamic modelling at temperatures from 5 °C to above 80 °C [10]. Extending this analysis to the CaO–Al₂O₃–SiO₂–H₂O system represents a major step towards applying this technique to hydrated PC blends with high replacement levels of supplementary cementitious materials, which are not fully described by existing thermodynamic models [11]. This will enable a much deeper understanding of the chemistry and phase composition, and hence durability, of these materials in service.

The chemistry and structure of calcium (alumino)silicate hydrate (C–(A–)S–H) products at ambient conditions have been the subject of sustained research for more than half a century [12]. These products are structurally similar to the tobermorite group of minerals, which contain aluminosilicate chains in ‘dreierketten’-type arrangements that are flanked on either side by an ‘interlayer’ region and a calcium oxide sheet (Fig. 1) [13], [14]. Al substitutes into bridging sites with strong preference over paired sites in these chains [15], [16]. It has also been suggested that the aluminosilicate chains in C–(A–)S–H products can cross-link in low-Ca (Ca/Si < 1) cements [17] to form disordered analogues of ‘double chain’ calcium silicate minerals, e.g. 11 Å tobermorite [18] (Fig. 1A).

Studies analysing laboratory-synthesised C–(A–)S–H specimens have identified that phase-purity decreases as the Al/Si and Ca/(Al + Si) molar ratios of the solid phase increase, suggesting that a ‘soft’ upper bound on the Al content of C–(A–)S–H exists in the composition range relevant to cementitious materials of Al/Si ≈ 0.2 [19], [20], [21]. The secondary phases formed in these systems are typically AFm (Al₂O₃–Fe₂O₃-mono) type phases such as strätlingite (C₂ASH₈), katoite (C₃AH₆, which is the Si-free end member of the hydrogarnet series C₃AS_yH_6−2y, 0 ≤ y ≤ 3) and/or the ‘third aluminate hydrate’ (TAH) [19], [21].

Considering the aqueous phases in equilibrium with C–(A–)S–H at different temperatures, it has been observed that the dissolved concentrations of Ca and Si are inversely related [20], [21], similar to the solubility of these elements in C–S–H systems [22], [23]. The dissolved Al content is closely linked to the amount of Al incorporated into C–(A–)S–H, and the nature and quantity of secondary phases formed. However, more experimental work is needed to provide data covering the full range of compositions and temperatures relevant to modern cementitious materials. Therefore, this paper aims to clarify the effects of temperature and Al on the chemistry, structure and solubility of equilibrated C–(A–)S–H systems at 7 °C, 50 °C and 80 °C, which are not yet well-described in the literature, and also utilises a recently published data set collected at 20 °C [21] to complete the temperature series.