Adrian R. Brough

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.

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.

Effects of an early or a late heat treatment on the microstructure and composition of inner C-S-H products of Portland cement mortars

The influence of both early and late heat treatments on the microstructure and on the hydration products of Portland cement mortars has been investigated. The mortars were given either a 4-h or 28-day precure at 20 °C before heating at 90 °C for 12 h and were subsequently stored in distilled water at 20 °C. The microstructure, studied by backscattered electron (BSE) imaging, shows the formation of distinct rims of inner C-S-H with different grey levels during the different stages of the curing cycles. The grey levels and corresponding BSE coefficients of these different rims were determined by image analysis and their chemical compositions by EDS microanalysis. It was found that the compositions depend on the temperature and time at which the rims had developed. The lighter C-S-H formed at 90 °C was denser and contained much more sulfate than the darker C-S-H formed at 20 °C, especially when the heat cure took place at early ages. The sulfate incorporated within the lighter C-S-H was released gradually over time.

The C-S-H gel of Portland cement mortars: Part I. The interpretation of energy-dispersive X-ray microanalyses from scanning electron microscopy, with some observations on C-S-H, AFm and AFt phase compositions

Abstract Scanning electron microscopy (SEM) microanalyses of the calcium-silicate-hydrate (C-S-H) gel in Portland cement pastes rarely represent single phases. Essential experimental requirements are summarised and new procedures for interpreting the data are described. These include, notably, plots of Si/Ca against other atom ratios, 3D plots to allow three such ratios to be correlated and solution of linear simultaneous equations to test and quantify hypotheses regarding the phases contributing to individual microanalyses. Application of these methods to the C-S-H gel of a 1-day-old mortar identified a phase with Al/Ca=0.67 and S/Ca=0.33, which we consider to be a highly substituted ettringite of probable composition C 6 A 2 S 2 H 34 or {Ca 6 [Al(OH) 6 ] 2 ·24H 2 O}(SO 4 ) 2 [Al(OH) 4 ] 2 . If this is true for Portland cements in general, it might explain observed discrepancies between observed and calculated aluminate concentrations in the pore solution. The C-S-H gel of a similar mortar aged 600 days contained unsubstituted ettringite and an AFm phase with S/Ca=0.125.

Purification and properties of the Escherichia coli nucleoside transporter NupG, a paradigm for a major facilitator transporter sub-family

NupG from Escherichia coli is the archetype of a family of nucleoside transporters found in several eubacterial groups and has distant homologues in eukaryotes, including man. To facilitate investigation of its molecular mechanism, we developed methods for expressing an oligohistidine-tagged form of NupG both at high levels (>20% of the inner membrane protein) in E. coli and in Xenopus laevis oocytes. In E. coli recombinant NupG transported purine (adenosine) and pyrimidine (uridine) nucleosides with apparent Km values of ∼20–30 μM and transport was energized primarily by the membrane potential component of the proton motive force. Competition experiments in E. coli and measurements of uptake in oocytes confirmed that NupG was a broad-specificity transporter of purine and pyrimidine nucleosides. Importantly, using high-level expression in E. coli and magic-angle spinning cross-polarization solid-state nuclear magnetic resonance, we have for the first time been able directly to measure the binding of the permeant ([1′-13C]uridine) to the protein and to assess its relative mobility within the binding site, under non-energized conditions. Purification of over-expressed NupG to near homogeneity by metal chelate affinity chromatography, with retention of transport function in reconstitution assays, was also achieved. Fourier transform infrared and circular dichroism spectroscopy provided further evidence that the purified protein retained its 3D conformation and was predominantly α-helical in nature, consistent with a proposed structure containing 12 transmembrane helices. These findings open the way to elucidating the molecular mechanism of transport in this key family of membrane transporters.

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.

High temperature ceramics for use in membrane reactors: the development of microporosity during the pyrolysis of polycarbosilanes

The pyrolysis of polycarbosilane (PCS), a ceramic precursor polymer, at temperatures up to 700 °C under an inert atmosphere results in the development of amorphous microporous materials which have a number of potential applications, such as gas separation membranes. This paper investigates the development of microporosity during pyrolysis under nitrogen, at temperatures ranging from 300 to 700 °C, of both the cross-linked and non-cross-linked starting materials. The products are characterised by nitrogen adsorption, to determine surface areas and pore volumes, solid-state NMR, electron microscopy and FTIR, and their formation is studied using thermal analysis and evolved gas analysis with on-line mass spectrometry. The cross-linked and non-cross-linked PCSs have a maximum micropore volume of 0.2 cm3 g−1 at pyrolysis temperatures of between 550 and 600 °C. The microporosity is stable in air at room temperature, but is lost in oxidising atmospheres at elevated temperatures.

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.

On the para-selective chlorination of ortho-cresol

Abstract Merrifield-bound o-cresol undergoes electrophilic aromatic chlorination using SO2Cl2 leading to para/ortho ratios in excess of 50, the highest such ratio reported for this chlorinating agent. Model studies suggest that this significant para/ortho ratio results not just from steric effects but that considerable electronic influence can be detected.

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.