Effect of temperature and aluminium on calcium (alumino)silicate hydrate chemistry under equilibrium conditions
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 C3ASyH6−2y, 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.
Section snippets
Materials and methods
C–(A–)S–H samples were prepared by mixing Milli-Q water (Merck Millipore), SiO2 (Aerosil 200, Evonik), CaO (obtained by burning CaCO3 (Merck Millipore) at 1000 °C for 12 h) and CaO·Al2O3 at a water/solid ratio of 45 in an N2-filled glovebox to obtain bulk molar Al/Si ratios (Al/Si*) of 0 to 0.15, with all experiments conducted at a bulk Ca/Si ratio of 1. The CaO·Al2O3 (99.1 wt.% determined by X-ray diffraction (XRD) with Rietveld analysis) was made from CaCO3 and Al2O3 (Sigma Aldrich) by heating
X-ray powder diffraction with Rietveld analysis
The XRD results show that C–(A–)S–H phases are the dominant reaction products in each sample (Fig. 2). Katoite (C3AH6, PDF# 00-024-0217) and strätlingite (C2ASH8, PDF# 00-029-0285) are also observed in some systems. Siliceous hydrogarnet (C3ASyH6−2y, 0 < y ≤ 3) is not identified in any of the samples. Katoite and strätlingite are more commonly found as secondary products in the systems with higher bulk Al/Si ratios and lower equilibration temperatures: strätlingite and katoite are observed in every
Conclusions
This paper has analysed the structure and solubility of calcium (alumino)silicate hydrates, with and without the inclusion of Al, as a function of temperature. The long-range order and degree of polymerisation of the C–(A–)S–H products, and the type and quantity of secondary phases formed in the equilibrated CaO–Al2O3–SiO2–H2O systems studied here, were significantly influenced by the synthesis temperature. The supernatants in these systems were close to saturation with respect to strätlingite
Acknowledgements
The authors thank Salaheddine Alahrache and Daniel Rentsch for the assistance with NMR spectroscopy, Luigi Brunetti and Boris Ingold for the assistance in the laboratory and the Swiss National Science Foundation grant no. 130419 for the financial support of E. L'Hôpital.
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