C.J. Lynsdale

Strength, permeability, and carbonation of high-performance concrete

Abstract This investigation is aimed at developing high-performance concrete and form part of an investigation into the optimization of a blended cementitious system for the development of high-performance concrete. Binary and ternary blended cementitious systems based on ordinary Portland cement (OPC), pulverised fuel ash (PFA) and silica fume (SF) were investigated. PFA up to 40% was used, and to these blends, 0%, 5%, 10% and 15% SF were incorporated as partial cement replacements. Results of compressive strength, tensile strength, oxygen permeability and carbonation of concrete are reported. A water–binder (w/b) ratio of 0.27 was used for the main group of mixes and w/b ratios of 0.40 and 0.50 were used for some selected mixes. Based on the experimentally obtained results, prediction models were developed which enabled the establishment of isoresponse contours showing the interaction between the various parameters investigated. It was found that the incorporation of 8–12% SF as cement replacement yielded the optimum strength and permeability values.

Aggregate-cement paste interface: Part I. Influence of aggregate geochemistry

The influence of aggregate geochemical properties on the development of the microstructure and bond strength at the interfacial transition zone (ITZ) between aggregate and the hydrating cement paste was studied. The mineral phases and microstructure at the ITZs of three commonly used concrete aggregates (quartzite, basalt, and limestone) with contrasting chemical properties were analysed using scanning electron microscopy with an energy dispersive X-ray analyser and X-ray diffraction. It was observed that differences in microstructure and mineral phases exist at ITZ between the three different aggregates studied. The limestone in particular produced a porous ITZ as a result of chemical interaction.

Porosity and strength of PFA/SF/OPC ternary blended paste

Abstract The results presented in this paper form part of an investigation into the optimisation of a ternary blended cementitious system based on ordinary Portland cement (OPC)/pulverised fuel ash (PFA)/silica fume (SF) for the development of high-performance concrete. Cement pastes covering a wide range of PFA/SF blending proportions were investigated. Compressive strength and porosity at the ages of 7, 28, and 90 days for cement paste specimens containing 0%, 15%, 20%, 25%, 30%, 35%, 40% and 45% PFA along with 0%, 5%, 10% and 15% SF as partial cement replacement at a water–binder ratio of 0.30 were investigated. A statistical approach was used which permitted the calculation of the isoresponse curves for the parameters under study over the experimental domain and the optimisation of their effect.

Long term durability of Portland-limestone cement mortars exposed to magnesium sulfate attack

Mortar prisms made with Portland-limestone cement have been stored in air and in 1.8% magnesium sulfate solution at 5 °C and have been examined over a period of 5 years. This paper is primarily concerned with the results obtained at the end of this period. The limestone content in the samples varied from 0% to 35%, but the water to cement plus limestone powder ratio was kept constant. The status of the samples after storage for 5 years is reported based on visual examination and a thorough characterisation using X-ray diffraction, infra-red spectroscopy and scanning electron microscopy. The prisms stored in magnesium sulfate solution were all showing clear signs of deterioration, increasing in intensity with limestone content. The mortar prism with 5% limestone replacement was, however, seriously degraded in comparison with the ordinary Portland cement control prism, and it is shown that this was due to the thaumasite form of sulfate attack.

Aggregate-cement paste interface. II : Influence of aggregate physical properties

This paper describes part of a large project on aggregate-cement interactions and interface bonding mechanisms in concrete. This part of the study investigates the influence of aggregate physical properties on the nature of aggregate-cement paste interfacial bonding with the aim of establishing the bonding mechanisms as controlled by aggregate physical properties. A newly developed experimental technique to characterise quantitatively the aggregate surface texture using a surface profilometer is also presented. Three commonly used concrete aggregate rocks (basalt, limestone, and quartzite) were investigated. Significant differences in the measured bond strength and modes of failure of rock-hardened cement paste composite specimens under uniaxial tension were observed between the different rock types. Based on the results presented, it is apparent that for a given cement paste, the interfacial bond strength cannot be predicted from aggregate surface roughness alone.

Delayed ettringite formation in heat-cured Portland cement mortars

Abstract A Portland cement mortar was subjected to elevated temperature curing at 100°C for 12 h and then stored under water at room temperature. Expansions, attributable to delayed ettringite formation, were found to develop over a period of 1 year. Sulfate ions released to the pore fluid at elevated temperatures, and partly sorbed by C-S-H gel, evidently formed ettringite in the outer products and the paste–aggregate transition zones during subsequent water storage at room temperature. The results of X-ray microanalyses implied that a potential for ettringite band formation had been established in the mortar. Investigations of microstructural features by backscattered electron imaging indicated that the expansion was caused by generation and extension of these ettringite bands. No evidence in support of an alternative mechanism based on a homogeneous expansion of the cement paste could be found.

Engineering Properties of Alkali-Activated Natural Pozzolan Concrete

The development of alkali-activated binders with superior engineering properties and longer durability has emerged as an alternative to ordinary portland cement (OPC). It is possible to use alkali-activated natural pozzolans to prepare environmentally friendly geopolymer cement leading to the concept of sustainable development. This paper presents a summary of an experimental work that was conducted to determine mechanical strength, modulus of elasticity, ultrasonic pulse velocity, and shrinkage of different concrete mixtures prepared with alkali-activated Iranian natural pozzolans—namely Taftan andesite and Shahindej dacite, both with and without calcining. Test data were used for Taftan pozzolan to identify the effects of water-binder ratios (w/b) and curing conditions on the properties of the geopolymer concrete, whereas the influence of material composition was studied by activating Shahindej pozzolan both in the natural and calcined states. The results show that alkali-activated natural pozzolan (AANP) concretes develop moderate-to-high mechanical strength with a high modulus of elasticity and a shrinkage much lower than with OPC.

Aggregate-cement chemical interactions

This paper highlights the role played by aggregate with respect to the chemical interactions that take place within the interfacial transition zone between aggregates and cement paste matrix. Four commonly used aggregates with different chemical properties, basalt, limestone, silica sand, and quartzite, were investigated. It was observed that ions are both absorbed and released by the aggregates in all the aggregate-cement solution systems, with basalt being the most active in this respect. The findings reveal that aggregates are more chemically active than has been supposed and that a wide range of chemical interactions between the aggregates and cement should be anticipated.

Oxygen and Chloride Permeability of Alkali-Activated Natural Pozzolan Concrete

This paper describes how one of the most important factors in the use of portland cement concrete is its durability. Most of the situations where durability is lacking have been identified and strategies to manage durability have been implemented. Geopolymer concrete, made from an alkali-activated natural pozzolan (AANP), provides an important opportunity for the reduction of carbon dioxide (CO2) emissions associated with the manufacture of concrete. However, there is a limited history of durability studies. Until its different properties are well understood there is no desire to adopt this new technology of unknown provenance by the concrete industry. This paper presents an experimental study of oxygen and chloride permeability of AANP concrete prepared by activating Taftan andesite and Shahindej dacite (Iranian natural pozzolans), with and without calcining, and the correlations between these properties and compressive strength. The results presented in the paper show that compared to ordinary portland cement (OPC) concrete, AANP concrete has lower oxygen permeability at later ages; but it shows moderate to high chloride ion penetrability.

Chloride and Oxygen Permeability of Concrete Incorporating Fly Ash and Silica Fume in Ternary Systems

Results presented herein form part of an investigation into the optimization of ternary blend systems based on normal portland cement, fly ash, and silica fume, for the development of high performance concrete. Chloride and oxygen permeability values at ages of 7, 28, 90, and 180 days of concrete containing portland cement, fly ash, and silica fume are reported. Fly ash, up to 40%, and silica fume, up to 15%, were incorporated as partial cement replacements for the preparation of various combinations of ternary blended systems. A water-binder ratio of 0.27 was used for the main group of mixtures. Two other water-binder ratios, 0.40 and 0.50, were used with selected concrete mixtures to show the effect of this parameter. Based on experimentally obtained results, prediction models were developed enabling the establishment of isoresponse contours showing the interaction between the various parameters studied.