Donald E. Macphee

XRD, EDX and IR analysis of solid solutions between thaumasite and ettringite

Solid solutions between thaumasite and ettringite were prepared by methods analogous to those well established for the preparation of thaumasite and ettringite. The extent of immiscibility in this system is investigated by varying the Al:Si and SO42−:CO32− ratios in reactant mixtures. The solids produced were analysed by quantitative X-ray diffraction, with Rietveld refinement also providing accurate unit cell dimensions, energy-dispersive X-ray analysis and infrared spectroscopy. The compositional and unit cell variations in the solid solution are discussed. A wide variety of solid solution compositions were produced with both the thaumasite and ettringite structures, but all preparations were considerably diluted by secondary amorphous products.

Studies using 27Al MAS NMR of AFm and AFt phases and the formation of Friedel's salt

This paper describes the application of the magic angle spinning (MAS) NMR spectroscopy to study the chemical environment of 27Al-bearing phases in Portland cement-based concrete. A specific methodology is described that allows reliable spectra to be determined for combinations of different types of cements and fillers (in this case, Portland cement, fly ash, slag, silica fume, metakaolin and limestone filler). As well as the study of ‘molecular structure’ of cement matrix, the paper reviews the mechanism of Friedels salt formation in cement systems. Mechanisms based on ion exchange of chloride for hydroxide in hydroxy-AFm and on chloride absorption on formation are discussed. Finally, the nature of the chloride/hydrate binding phenomena are described to provide a reasonable robust and fundamental picture of the role different cements can play in the provision of overall concrete durability to chloride ingress from a chemical perspective.

Photocatalytic NOx abatement: why the selectivity matters

Titanium dioxide photocatalysis offers an excellent way to oxidise NOx to nitrate and thus reduce air pollution. However, unmodified titanium dioxide also releases a significant amount of the toxic intermediate nitrogen dioxide in the process, a problem that is rarely discussed in previous literature. Herein, we highlight this issue by presenting systematic data on the activity and selectivity of a number of commercial titania powders. The photocatalytic performance of a previously developed W/N-codoped titanium dioxide is also reported which, for the first time, offers a way to eliminate this problem as it exhibits an exceptionally high selectivity towards nitrate. The selectivity appears to be solely dependent on the tungsten content, a concentration of 4.8 at.% is sufficient to induce a very high selectivity. Furthermore, the high selectivity could also be replicated by a W/N-codoped sample derived from the industrial sulphate synthetic process. The increased selectivity comes at the expense of absolute activity, which is lower than in the reference titania samples. This raises the question of how to properly evaluate NOx abatement photocatalysts when there are two factors to consider, activity and selectivity. To resolve this, we propose to define a new figure of merit for the evaluation of NOx abatement photocatalysts by distilling total NOx removal and selectivity into one value, the DeNOx index. It is derived by assigning a toxicity value to both NO and NO2 and then expressing the change in total toxicity rather than the concentration change of the individual nitrogen oxides.

Polymerization effects in C-S-H: implications for Portland cement hydration

Synopsis 29Si NMR has been applied to the study of calcium silicate hydrate (C-S-H) gels. The degree of silicate polymerization in young gels is found to be sensitive to the curing humidity; higher degrees of polymerization are observed for gels cured in drier environments. Compositional effects on polymerization have been investigated and found to be insignificant at CaO/SiO2 ratios greater than about 1·2. At lower lime contents, the predominant silicate species are long chain polymers. The results of the studies on synthetic C-S-H are discussed in relation to the hydration of Ca3SiO5 and historical cement specimens.

Reactions between cement and As(III) oxide: The system CaOSiO2As2O3H2O at 25°C

Arsenic-bearing wastes are common by-products of mineral processing and smelting. As arsenic is well known to be highly toxic, it is important that these wastes should be treated in an ecologically acceptable manner. As(III) is the species of principal concern. One treatment system uses Portland cement as a containment and chemical immobilization matrix. A review of the literature revealed the solid phases of relevance, and solubility data were critically evaluated. Based upon these data, computer-based modeling was used to identify stable phase assemblages in the CaO-SiO{sub 2}-As{sub 2}O{sub 3}-H{sub 2}O system at 25 C and calculate equilibrium solubilities. An experimental program was undertaken to verify the modeling predictions within the CaO-As{sub 2}O{sub 3}-H{sub 2}O sub-system, and experimentally-obtained solubility data were, in turn, used by the modeling program. Solubility data thus derived were critically compared with those of the literature and although there are some differences between the data sets recast in the form of phase diagrams, the two are numerically in good agreement. The solubility of As is limited by reaction with calcium hydroxide and C-S-H gel to {approximately}0.0007 mmol kg{sup {minus}1}, or approx. 0.075 mgl{sup {minus}1} as arsenite (AsO{sub 2}{sup {minus}}), by formation of CaAsO{sub 2}OH.

Acid attack on pore-reduced cements

Because the durability of high-performance cements is as important as their strength, the performance of pore-reduced cement (PRC) in aggressive media such as sulfuric, hydrochloric and ethanoic acids, was studied and compared with that of ordinary Portland cement (OPC). The effects of exposure to these media on these cements were monitored by periodic visual inspection and sample weighing. Specific interactions with regard to interconnected porosity were addressed and the corrosion products characterized. PRC is less susceptible than OPC against hydrochloric and ethanoic acids. However, sulfuric acid damages PRC and OPC to almost the same extent. It is shown by electron microprobe analysis that the hydrochloric and ethanoic acids quickly penetrate the interior of normal cement pastes by acid leaching of the interconnected porosity. The reduced porosity of PRC reduces the susceptibility to attack by this mechanism. Sulfuric acid exposure causes extensive formation of gypsum in the cement surface regions, which results in mechanical stress and ultimately leads to spalling. Thus fresh surfaces are exposed regularly and therefore the relatively closed microstructure of PRC is no hindrance to this kind of attack.

Extent of immiscibility in the ettringite-thaumasite system

Abstract Solid solutions between thaumasite and the related phase ettringite were prepared and characterised by X-ray diffraction and infrared spectroscopy. A miscibility gap in the solid solution is identified and defined. Crystallographically, the miscibility gap is identified as a gap in the unit cell dimensions of the solid phases formed between c≅20.95 (10.475) and c≅21.3 A. A combination of quantitative X-ray powder diffraction and infrared spectroscopy enabled us to define the miscibility gap in terms of Al:Si ratio. Ettringite can accept the replacement of 1/2 its Al by Si, while thaumasite tolerates little or no Al in its structure.

Toughening cement-based materials through the control of interfacial bonding

Mortars are made from inherently brittle components: sand grains and hardened cement paste. Under normal circumstances, cracks will propagate rapidly through the cement matrix, bypassing the strong sand grains but fracturing some of the weakest. The approach of the work described in this paper was to modify the mortar in order to alter this process. These modifications produced tensile residual stresses between the matrix and the aggregate, which when released by an additional applied tensile stress produced microcracking, debonding of matrix from aggregate, a small expansion and increased toughness. This work demonstrates toughening in sand/Portland cement mortars modified with different expansive admixtures: sodium sulphate or dead-burnt lime. Additionally, mortars of sand/ASTM Type K cement were tested. In order to give additional insight into the toughening mechanism, spherical and angular aggregate have been used to ascertain the consequences of microcracking and aggregate-bridging. The role of aggregate-bridging, especially when related to fracture paths, is also discussed and suggests that the bond between the aggregate and the matrix has been found in some cases to control not only the crack path but consequently the apparent toughness.

Modelling approach to the prediction of Equilibrium Phase Distribution in Slag-Cement Blends and their Solubility Properties.

Blended cements containing mixtures of granulated blast-furnace slag (BFS) and Portland cement give low permeability matrices with initially favourable leach characteristics. Their retention arises from a combination of physical and chemical effects which include high pH and sorption. It is not practical though, in either laboratory or site-based experimentation, to determine changes in matrix chemistry over long timescales and the development of realistic models for long-term property predictions therefore becomes increasingly important. This paper pursues the development of such a model enabling changes in chemical and mineralogical balances during ageing to be predicted. Phase development is assessed in the system CaO-Al 2 O 3 -SiO 2 -MgO-H 2 O. In the relevant composition range, the phases occurring include crystalline hydrates: portlandite, (Ca(OH) 2 ); gehlenite hydrate, (2CaO.Al 2 O 3 .SiO 2 .8H 2 O); a hydrotalcite-structured phase (nominally 6MgO.Al 2 O 3 .(OH) x .yH 2 O). an AF m type phase, (nominally 4CaO.Al 2 O 3 SO 3 .12H 2 O); and a poorly crysiallised c~lcium silicate hydrogel, C-S-H. All five phases are observed to occur together in slag-cements which are still hydrating. Given, as input, the chemical analyses of both the slag and the cement, and the initial blending proportions, the model predicts the equilibrium distribution between the five components and additionally, the Ca/Si ratio of the C-S-H. The aqueous chemistry in the system is predicted from the calculated phase distribution and appropriate solubility products.

Remediation of a chlorinated aromatic hydrocarbon in water by photoelectrocatalysis

Photoelectrocatalysis driven by visible light offers a new and potentially powerful technology for the remediation of water contaminated by organo-xenobiotics. In this study, the performance of a visible light-driven photoelectrocatalytic (PEC) batch reactor, applying a tungsten trioxide (WO(3)) photoelectrode, to degrade the model pollutant 2,4-dichlorophenol (2,4-DCP) was monitored both by toxicological assessment (biosensing) and chemical analysis. The bacterial biosensor used to assess the presence of toxicity of the parent molecule and its breakdown products was a multicopy plasmid lux-marked E. coli HB101 pUCD607. The bacterial biosensor traced the removal of 2,4-DCP, and in some case, its toxicity response suggests the identification of transient toxic intermediates. The loss of the parent molecule, 2,4-DCP determined by HPLC, corresponded to the recorded photocurrents. Photoelectrocatalysis offers considerable potential for the remediation of chlorinated hydrocarbons, and that the biosensor based toxicity results identified likely compatibility of this technology with conventional, biological wastewater treatment.