G. W. Groves

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 Cue5f8Sue5f8H 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 Cue5f8Sue5f8H 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.

The structure of the calcium silicate hydrate phases present in hardened pastes of white Portland cement/blast-furnace slag blends

The C-S-H gels present in both water- and alkali-activated hardened pastes of white Portland cement/blast-furnace slag blends have been studied by solid-state 29Si magic angle spinning nuclear magnetic resonance (NMR) spectroscopy and analytical transmission electron microscopy (TEM). Structural data are obtained by NMR for the semi-crystalline C-S-H gels in the alkali-activated systems and extended to the nearly amorphous gels in the water-activated systems by peak broadening; unambiguous chemical analyses are determined in the TEM. The following conclusions apply to both the semi-crystalline and nearly amorphous C-S-H gels: (1) aluminium substitutes for silicon at tetrahedral sites; (2) aluminium only substitutes for silicon in the central tetrahedron of pentameric silicate chains; (3) the results strengthen confidence in dreierkette-based models for the structure of C-S-H. Compositional similarities suggest that these conclusions will be true for OPC/slag blends, and possibly also for OPC/pulverized fuel ash blends indicating that the same structural model applies to C-S-H gels in a wide range of hardened cement pastes.

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.

High strength cement pastes: Part 2 Reactions during setting

A study of the chemical reactions occurring in high strength cements is presented with particular reference to the cement-polymer interaction. The addition of small amounts (1.6%) of polyvinyl (alcohol/acetate) to the high alumina cement Secar 71 significantly retards the normal hydration reactions. At much larger doses (10% PVA) the crystalline hydrates are completely suppressed and the polymer reacts with the cement solution to form calcium acetate and a crosslinked polymeric product. In the case of high strength OPC/polyacrylamide pastes, the high pH of the cement solution converts the polymer to polyacrylic acid which reacts with the cations from the hydrated cement to form a crosslinked metal-polyacrylate. This latter system is analogous to reactions which occur in certain polyacrylic acid-based dental cements.

Microcrystalline calcium hydroxide in Portland cement pastes of low water/cement ratio

Abstract A microcrystalline form of calcium hydroxide has been observed in Portland cement pastes of low water/cement ratio by transmission electron microscopy of sections thinned from bulk, hardened pastes. The typical morphology is of clusters of microcrystals in the form of lamellae ∼10nm thick, parallel to the basal plane. The lamellae tend to be aligned although the perfection of the alignment is variable. The clusters are embedded in the hydrate gel.

Twinning in β-dicalcium silicate

Abstract βC 2 S was prepared by cooling α′C 2 S or αC 2 S. In both cases the βC 2 S contained many twin lamellae with a (100) twin plane. βC 2 S produced from a′C S contained lamellae on a single plane in each grain, but βC 2 S produced from αC 2 S contained smaller domains of twins. Dislocations were observed at a domain boundary. The observations are explained by the hypothesis that the twin lamellae form by lattice transformation shear strains.

Physical properties of high strength cement pastes

Abstract Recent developments in methods of processing ordinary Portland cement have shown that above average strengths (in flexure) are easily obtained without expending energy on high pressure compaction techniques. This paper reports strengths and other physical properties of both ordinary pastes and modified (macro defect free) pastes and attempts to compare and contrast the two types. The apparently greater notch sensitivity of the modified pastes is explained in terms of a reduced inherent flaw size. Optical microscopy shows that large pores are absent in the modified paste but transmission electron microscopy (TEM) shows that the fine scale microstructures of ordinary pastes and high strength pastes are very similar.

TEM Studies of Cement Hydration

The application of transmission electron microscopy to the study of the early stages of hydration C3S and the microstructure of very mature C3 S paste is reported. In the former case the effect of water-cement ratio appears to be very important. In the latter, a detailed evaluation of the microstructures of inner C-S-H, outer C-S-H and CH is possible and the relationship of these to models of cement hydration is discussed.