Composition-solubility-structure relationships in calcium (alkali) aluminosilicate hydrate (C-(N,K-)A-S-H).

Rupert J Myers,Emilie L'Hôpital,John L Provis,Barbara Lothenbach

doi:10.1039/c5dt01124h

Abstract

The interplay between the solubility, structure and chemical composition of calcium (alkali) aluminosilicate hydrate (C-(N,K-)A-S-H) equilibrated at 50 °C is investigated in this paper. The tobermorite-like C-(N,K-)A-S-H products are more crystalline in the presence of alkalis, and generally have larger basal spacings at lower Ca/Si ratios. Both Na and K are incorporated into the interlayer space of the C-(N,K-)A-S-H phases, with more alkali uptake observed at higher alkali and lower Ca content. No relationship between Al and alkali uptake is identified at the Al concentrations investigated (Al/Si ≤ 0.1). More stable C-(N,K-)A-S-H is formed at higher alkali content, but this factor is only significant in some samples with Ca/Si ratios ≤1. Shorter chain lengths are formed at higher alkali and Ca content, and cross-linking between (alumino)silicate chains in the tobermorite-like structure is greatly promoted by increasing alkali and Al concentrations. The calculated solubility products do not depend greatly on the mean chain length in C-(N,K-)A-S-H at a constant Ca/(Al + Si) ratio, or the Al/Si ratio in C-(N,K-)A-S-H. These results are important for understanding the chemical stability of C-(N,K-)A-S-H, which is a key phase formed in the majority of cements and concretes used worldwide.

Highlights

The alkali concentration in cement-based materials varies greatly as a function of the cement formulation and type: Portland cement (PC) typically contains up to 1 wt% alkali oxide equivalent;[1] fly ash, a common supplementary cementitious material (SCM), generally contains >1 wt% alkali (Na + K) oxide equivalent;[2] and alkali-activated cementitious materials typically involve the use of highly concentrated Nabased solutions.[3]
C-(N,K-)A-S-H samples were synthesised at bulk molar Al/Si ratios (Al/Si*) of 0 to 0.1 and bulk molar Ca/Si ratios (Ca/Si*) of 0.6 to 1.6 using Milli-Q water (Merck Millipore) and 0 to 1 M solutions of NaOH and/or KOH (Merck Millipore), at a solution/solid ratio of 45 in a N2-filled glovebox by the method described in ref. 8 and 14
Reflections assigned to portlandite (Ca(OH)[2], Powder Diffraction File (PDF)# 00-044-1481) are only present in the data for the Al-free sample synthesised with 0.5 M NaOH/ 0.5 M KOH at a Ca/Si* ratio of 1.4, portlandite is identified in some other Al-free samples synthesised with alkali hydroxide solutions and Ca/Si* ratios ≥1.2

Summary

Introduction

The alkali concentration in cement-based materials varies greatly as a function of the cement formulation and type: Portland cement (PC) typically contains up to 1 wt% alkali (mainly K) oxide equivalent;[1] fly ash, a common supplementary cementitious material (SCM), generally contains >1 wt% alkali (Na + K) oxide equivalent;[2] and alkali-activated cementitious materials typically involve the use of highly concentrated Nabased solutions (up to or exceeding 5 M).[3]. The main reaction product in hydrated Portland cement (PC) materials is calcium (alkali) silicate hydrate (C-(N,K-)S-H‡) with a structure analogous to the natural mineral tobermorite, usually with Ca/(Al + Si) ≥ 1.5, and sometimes with minor Al substitution up to an Al/Si ratio not exceeding 0.1.12 This phase contains silicate (or aluminosilicate if Al is present) chains in dreierketten type arrangements, flanked on either side by a Ca–O sheet and an interlayer region (Fig. 1).[13] These aluminosilicate chains can cross-link to form Q3-type structures,[14] and if present, Al is bound in bridging sites with a strong preference over the paired sites.[15] Al is thought to be incorporated in C-(N,K-)A-S-H in five- and six-fold coordination, e.g. in the interlayer (Fig. 1), no consensus. A good understanding of C-(N,K-)S-H solubility currently exists up to bulk Na concentrations of 0.8 M NaOH,[8,9,21,22,23,24] which represents the pH range relevant to most cement-based materials ( pH ≤ 13.5), but fewer solubility data for this phase are available for K-containing materials in the corresponding composition range.[21,22,25,26] These data are essential in understanding the long-term stability of C-(N,K-)S-H and in the development of thermodynamic models for this phase, enabling simulation of the chemistry of cementbased materials in service.[27,28]

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Results