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Cement replacement materials are by-products used to produce high performance concrete. Published data on the effects of combinations of mineral admixtures in concrete on the microstructural and performance-related properties under different curing regimes are comparatively little. Further the correlation of strength of concrete to its permeability and pore structure is also not clear. The main objective of this research is to study the performance of various combinations of fly ash/silica fume and slag/silica fume concretes under three different curing regimes, viz. continuous moist curing, no moist curing after demolding and air drying after 7-days of initial moist curing. Six different concrete mixes were prepared with ordinary portland cement and a blend of portland cement and combinations of fly ash+silica fume and slag+silica fume The water-to-cementitious materials ratio of all the concrete mixtures was kept constant at 0.45. The properties investigated included workability of the fresh concrete, engineering properties such as cube and modified cube compressive strength, flexural strength, dynamic modulus of elasticity, pulse velocity, shrinkage and swelling, permeability and microstructural properties such as porosity and pore size distribution. The results show that prolonged dry curing results in lower strengths, higher porosity, coarser pore structure and more permeable concretes. It was found that the loss in early age compressive strength due to incorporation of fly ash or slag can be compensated for by the addition of small amounts of silica fume. The engineering and microstructural properties and permeability of concretes containing fly ash or slag appear to be more sensitive to poor curing than the control concrete, with the sensitivity increasing with increasing amounts of fly ash or slag in the mixtures. The incorporation of high volumes of slag in the concrete mixtures refined the pore structure and produced concretes with very low porosity and threshold diameters. The results emphasize that a minimum 7-day wet curing is needed for concrete with mineral admixtures to develop the full potential, and that continued exposure to a drying environment can have adverse effects on the long-term durability of inadequately cured slag or fly ash concretes. The results also confirm that compressive strength alone is not an adequate index to judge the performance of concrete, and the knowledge of the strength, pore structure and permeability are required for this purpose. Slag/silica fume concrete mixtures showed better performance than fly ash/silica fume concrete mixtures as regards the development of engineering and microstructural properties.

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Extensive literature reviews are presented in this thesis with respect to the hydration of slag, especially on the mechanism of hydration. The range of work in the thesis includes: 1) compressive strength testing on slag cement pastes and slag pastes with other alkaline activators; the effect of curing temperatures and slag compositions were specially discussed. 2) Analysis of pore fluid from slag/NaOH pastes. 3) Porosity and pore structure analysis of slag cement pastes as a function of age. 4) Phase development in hydrating slag or slag cement pastes. 5) Microstructural and microchemical development of slag hydration products. Electron microscopy has shown that several chemically and microstructurally distinct zones quickly develop in a hydrating slag cement paste. Initially, a dense layer of gel-like hydration product forms around slag grains. The microstructure and chemistry of the gel are not constant, but evolve with time. Microstructural evolution is manifested by the crystallization of the previously formed gel hydrate, with the formation of a hydrotalcite-like phase. This crystallization is accompanied by distinctive chemical evolution, in which Ca, Si and some A1 migrate into the outer matrix; however, Mg appears to be virtually immobile. The evolution results in the creation of pores in the in-situ slag hydration zones, and at the same time, the marked densification of the outer matrix. Mass balance calculations are used to support the microstructural observations and to generalize on them, so the extent of the densification potential can be assessed. The ability to calculate the potential for densification, at least in principle, is regarded as an important step forward in the design for durability. A theory, based on the microstructural observations, is proposed to account for the differences between the calculated and observed porosities.

The microstructure and performance of calcium sulfoaluminate (CSA) cements are described. CSA cements contain C₄A<sub>3S, 4CaO.3Al₂O₃.SO₃,and are interground with gypsum/anhydrite. They have been used structurally in China for more than 20 years and elsewhere as special cements in non-structural applications. Their long-term behaviour of depends on ettringite, a relatively fragile substance, and a novel matrix former compared with Portland cement. The mineralogy, evolution of hydrate and microstructure of CSA cement pastes and concretes were studied to relate hydration products and microstructure with performances. Samples taken from in-service structures in China, ranging from several to 25 years old, were investigated. Drying methods for cement pastes affect experimental results, especially for ettringite-based materials and are reported. Calorimetry, X-ray diffraction, microprobe and porosimetry are used to investigate early (<24 hours) and late hydration processes in different curing regimes. A "shrinking sphere" model is proposed to explain hydration processes. At early stages, mineralogy differs between inner and outer products: the former contain mainly AFm and C-S-H while the latter contains mainly ettringite and gibbsite. At 25°C, these differences persist but inner products densify the paste. At 55°C, and at 85°C, substantial siliceous hydrogarnet, forms after ~3 d. It is a major inner product. This results in a volume decrease and coarsening of pores. Long-term exposure to higher temperatures ~85°C is harmful. The impact of water:cement ratios on hydration mechanisms is reported. Depths of carbonation of CSA cement concrete are reported: results show similar resistance Portland cement concrete. Pipe immersed in seawater for 16 years shows that CSA cement has excellent resistance to seawater attack. The steel in the pipe is uncorroded even though chloride ions have penetrated the cover. Long-term exposure to high temperatures, 85°C, of CSA cement should be avoided but under normal in-service conditions its long-term behaviour is satisfactory.

Concrete has the largest production of all man-made materials. Compared with other construction materials, it possesses many advantages including low cost, general availability of raw materials, low energy requirement and utilization under different environmental conditions. Therefore, concrete will continue to be the dominant construction material in the foreseeable future. However, durability of concrete and reinfored concrete structures are still of worldwide concern, so producing a good quality concrete which impedes the ingress of harmful substances into it is of paramount importance. Cement replacement materials have been introduced into concrete mixtures for the purpose of improving the durability performance. Hence, the aim of the present investigation is to study the durability of concrete with and without cement replacement materials under various initial curing conditions. In this thesis various concrete mixes with and without cement replacement materials were considered. The cement replacement materials were, pulverised fuel ash, condensed silica fume, and ground granulated blast furnace slag. Superplasticiser was added to the majority of the mixes considered and air entraining agent to some of the mixes. Various curing regimes were employed which comprised hot dry curing to simulate concrete in the hot arid areas in the world and curing at normal temperature. Curing involved air curing, membrane curing and moist curing for fourteen days followed by air curing. A number of tests were conducted at either one particular age or at various ages. These included tests on porosity and pore structure of pastes obtained by mercury intrusion porosimetry technique, water absorption which covers the water absorption of concrete obtained by shallow immersion and the water absorbed by capillary action when the concrete surface is in contact with water, sulphate resistance of concrete which is performed by immersing the concrete specimens in sulphate solution, and monitoring the change in length at various periods of immersion, chloride penetration profiles of concrete at various ages of exposure. In addition to these tests on durability related properties, tests on compressive strength were also performed. Throughout the study a correlation between pore structure and durability related properties is investigated. A comprehensive compilation of chloride penetration data is made and an empirical expression is derived for the prediction of long term diffusion coefficients. At the end of the investigation, limitations of the present study, conclusions and suggestions for future research are made.

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