I.G. Richardson

The characterization of hardened alkali-activated blast-furnace slag pastes and the nature of the calcium silicate hydrate (C-S-H) phase

Abstract The C-S-H gels present in commercial blast-furnace slag and synthetic-slag glass pastes produced by hydrating with 5M KOH solution have been studied by a combination of transmission electron microscopy (TEM) and 29 Si and 27 Al nuclear magnetic resonance (NMR) spectroscopy. They are related by both composition and morphology to the C-S-H gels present in slag-OPC pastes but are more crystalline. The inner product C-S-H is intermixed on a fine scale with a Mg,Al-rich phase with a Mg/Al ratio of ≈2.5. The C-S-H in both inner and outer product contains substituted Al in tetrahedral co-ordination sites. The data are analysed in terms of a model for the structure of C-S-H gel.

The incorporation of minor and trace elements into calcium silicate hydrate (CSH) gel in hardened cement pastes

Abstract The General model for CSH gel described by Richardson and Groves (1) has been extended to incorporate elements other than Ca, Si, O and H which have been detected by X-ray microanalysis of gels in hardened Portland cement and blended cement pastes.

Models for the composition and structure of calcium silicate hydrate (CSH) gel in hardened tricalcium silicate pastes

Models for the structure of CSH gels occuring in hardened C3S cement pastes are considered and compared to some examples in which composition and silicate anion structure have been investigated experimentally.

In situ solid-state NMR studies of Ca3SiO5: hydration at room temperature and at elevated temperatures using 29Si enrichment

Abstract29Si isotopic enrichment was used for acquisition of multiple 29Si magic-angle spinning (MAS) and cross-polarization magic-angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectra, in situ in an NMR probe, from a single sample of hydrating Ca3SiO5 (C3S). Data with excellent signal-to-noise ratios were obtained at 20, 50 and 75 °C, with minimal use of spectrometer time, and without the need for the quenching of multiple samples. Spectral line widths and polymer-chain lengths derived from the spectra had no detectable differences from experiments in which the quenching was carried out with propan-2-ol. Furthermore, the effects of the MAS technique on the hydration reaction appeared to be minimal. At 20 °C, the bulk hydrate initially produced was dimeric; at later stages of the reaction, polymerization occurred. Arrhenius energies of 35 and 100 kJ mol−1, respectively, were calculated for these two reactions. The cross-polarization (CP) spectra acquired throughout the hydration showed that at 20 °C, 2% of the hydrated monomeric Qo(H)species persisted from after the induction period through to the late stages of the hydration reaction; this indicates that this species is unlikely to result from surface hydroxylation of C3S; an upfield shift of this species occurred with increasing hydration, indicating a possible change of environment for the silicate species. The amount of Qo(H)produced was found to increase at higher temperatures. Potential mechanisms for polymerization were assessed and a model in which dimeric-silicate units are linked together by insertion of monomers (dimer → pentamer → octomer) was found to give the best fit to the observed data; these results support a dreierketten model for the structure of the hydrate.

A study of the pozzolanic reaction by solid-state 29Si nuclear magnetic resonance using selective isotopic enrichment

The hydration of a mixture of tricalcium silicate and silica has been studied by 29Si solid-state nuclear magnetic resonance, using selective enrichment of the reactants with 29Si in order to follow and compare the behaviour of the silicon nuclei originating from either source. This approach shows for the first time that the silicon atoms from the two components are not equilibrated throughout the hydration products but are preferentially located in distinct species. In particular, from the distinctive spectra observed when the silica only is enriched, it is concluded that the part of the calcium silicate hydrate gel formed which incorporates silicon from this source has a longer chain length and a slightly better-ordered structure than the remainder. The spectra obtained with selective enrichment are interpreted in terms of a model based on a dreierkette chain structure for C-S-H.

Alkali Activation of Reactive Silicas in Cements: In Situ29Si MAS NMR Studies of the Kinetics of Silicate Polymerization

In the highly alkaline environment of the cement paste of a concrete, a source of silica can potentially react in two ways. In the pozzolanic reaction, it can combine with free lime to generate additional calcium silicate hydrate binding phase. Alternatively, reaction with alkali to form a gel can occur; this gel may swell and degrade the concrete. 29Si magic angle spinning (MAS) and cross-polarization (CP) MAS nuclear magnetic resonance (NMR) studies have been performed to determine the silicate connectivity in some model cement systems; 29Si enrichment was utilized to enable a series of spectra to be acquired in situ from a single sample.The hydrate from pozzolanic reaction of lime with silica was similar to the hydrate formed around silica in blended pozzolanic cements, with a relatively high crystallinity and long silicate chains. In the absence of lime, silica reacted with an alkaline solution to produce a gel having a high degree of cross linking, and a range of silicate mobilities. Tricalcium silicate hydration was found to be accelerated significantly by high levels of alkali (KOH) in solution; the hydrate formed had shorter silicate chains and was more crystalline than that produced by reaction in pure water. Hydration in alkali solution of a model blended cement, comprising a mixture of tricalcium silicate and silica, gave rise to two products, a long chain calcium silicate hydrate (C-S-H) and an alkali silicate of low rigidity. The alkali silicate phase gradually polymerized; at later ages it underwent a phase change, although no crystalline phase appeared to be formed. Silicate exchange took place between the C-S-H and the alkali silicate phase at a slow rate.

Parallel electron energy loss spectroscopy study of al-substituted calcium silicate hydrate (CSH) phases present in hardened cement pastes

Abstract The local environment of aluminium in Al-substituted calcium silicate hydrate phases in cement pastes has been determined by measurement and modelling Al K-ELNES. The results are in agreement with bulk 27Al and 29Si MAS NMR data and predictions derived from compositional trends.

Microcrystalline calcium hydroxide in pozzalanic cement pastes

Abstract A microcrystalline form of calcium hydroxide consisting of clusters of lamallae, ∼10nm in thickness, intimately mixed with CSH gel, has been observed in a lime-silica cement paste by transmission electron microscopy. The volumes of calcium hydroxide and CSH in the clusters are comparable. The implications of this are discussed.

29Si Enrichment and Selective Enrichment for Study of the Hydration of Model Cements and Blended Cements

29Si enrichment has been used to enable acquisition of multiple 29Si MAS and CP-MAS NMR spectra from samples of C3S hydrating in situ in a MAS NMR probe. Data with excellent signal-to noise ratios were obtained at 20, 50, and 75°C, with minimal use of spectrometer time, and without the need for quenching of multiple samples; the data were however consistent with those obtained conventionally. The bulk hydrate initially produced was dimeric; at later stages of reaction, polymerization occurred. Arrhenius energies of 35 and 100 kJmol−1 were calculated for formation of the two products. A model of hydration in which dimeric silicate units are linked together by monomers was found to be consistent with the observed results. The pozzolanic reaction with C3S and SiO2 has also been studied with enrichment of both starting materials — the hydrate produced is more ordered and of longer chain length than that produced by hydration of C3S alone. By enrichment of the SiO2 only it was possible selectively to observe hydrate which contains silicate species which originate from the SiO2; this material had a much higher degree of order, and a much longer chain length than the remainder of the hydrate. Thus isotopic enrichment and especially selective isotopic enrichment are valuable tools for the study of cementitious systems.

Determining the local coordination of aluminium in cement using electron energy loss near-edge structure

The local site symmetry of aluminium in Al-substituted calcium silicate hydrate phases, similar to those found in hardened Portland cement pastes, has been determined by experimental measurement and theoretical modelling of the electron energy loss near-edge structure associated with the aluminium K-edge measured using parallel electron energy loss spectroscopy in the transmission electron microscope. Changes in the local aluminium environment were observed between different regions of the microstructure which are explained in terms of aluminium substitution into both tetrahedral silicon and octahedral magnesium sites. This spatially-resolved information is in excellent agreement with27Al and29Si magic angle spinning nuclear magnetic resonance data on bulk samples as well as the predictions based upon compositional trends derived from the results of energy dispersive X-ray and electron energy loss elemental microanalysis. In a wider context we wish to stress the complementarity of both spatially-resolved and bulk spectroscopic techniques, which possess differing degrees of sensitivity, in the analysis of materials science samples.