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.

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.

[pt] Os problemas à saúde acarretados pela exposição às fibras minerais do amianto (asbesto) têm motivado esforços para a substituição destas fibras nos diversos componentes que as empregam como matéria prima. Devido às propriedades físicas e mecânicas e estabilidade química do amianto, bem como sua afinidade natural com a matriz cimentícia, o cimento- amianto é um compósito com excepcionais características de resistência e durabilidade a um custo relativamente baixo. Tais características fazem da busca por um reforço alternativo ao asbesto um desafio, mobilizando indústria e pesquisadores desde a década de 70. Neste contexto, considerando sua disponibilidade e características mecânicas, as fibras celulósicas se mostram como alternativa viável, tendo sido empregadas industrialmente como reforço em fibrocimentos há mais de duas décadas. Entretanto, mesmo com a industrialização, alguns aspectos de seu comportamento, principalmente aqueles relacionados à durabilidade, são ainda foco de intensos esforços de pesquisa (no Brasil, o estudo do emprego de fibras celulósicas como alternativa ao amianto teve início em 79, com os trabalhos pioneiros realizados na PUC-Rio). Considerando os principais mecanismos causadores de degradação nos fibrocimentos, todos eles relacionados ao transporte de fluidos pela rede porosa do material, tem-se que o principal método empregado para melhoria nas características de durabilidade é a substituição parcial do cimento por aditivos com alto teor de sílica amorfa finamente moídos. As melhores características assim obtidas decorrem de modificações na estrutura da matriz e, principalmente, da interface. A casca de arroz, é um resíduo agrícola produzido em grande quantidade no Brasil. Quando não empregada como combustível no próprio eneficiamento do arroz ou em outras atividades rurais, a casca de arroz é disposta sem qualquer controle, apresentando-se assim como um problema ambiental. Entretanto, se queimada em condições controladas, a casca de arroz resulta em cinza, CCA, com alto teor de sílica (80- 90 por cento) altamente amorfa, apresentando boa reatividade com o cimento. Portanto, a CCA foi empregada neste trabalho como material de substituição parcial do cimento em compósitos reforçados por polpas de bambu, buscando com isso melhorar as características relacionadas à durabilidade destes fibrocimentos. Foi observado que o emprego de até 30 por cento de CCA com baixo teor de carbono como substituição parcial do cimento resultou em um substancial decréscimo na porosidade da matriz e interface do compósito. Por conseqüência, estes compósitos apresentaram permeabilidade significativamente inferior à daqueles produzidos sem CCA. Com o emprego de CCA com alto teor de carbono (simulando a cinza obtida de queima não controlada, como a realizada no beneficiamento do arroz) é possível obter resultados semelhantes, uma vez que o compósito seja submetido à cura acelerada em autoclave. Neste caso, devem ser empregadas taxas ainda maiores de substituição parcial do cimento por CCA, com os melhores resultados observados em compósitos cujas matrizes compunham-se por 50 por cento da CCA. Além destes aspectos intimamente ligados aos principais mecanismos de degradação dos compósitos, foi observado que a CCA também favorece a aderência interfacial nos compósitos, acarretando em maior resistência mecânica. / [en] Asbestos is regarded as a hazardous material since the 60 s, motivating the efforts for the replacement of these mineral fibres in the vast range of materials in which they are applied as a raw material. Asbestos-cement was the first building material produced in large scale applying natural fibres as reinforcement in cement-based materials. Due the physical and mechanical behaviour and chemical stability of asbestos fibres, as well as their natural affinity with the cementitious matrix, asbestos-cement presents remarkable strength and durability, associated to a relative low cost. Such characteristics make the search for a suitable replacement to asbestos in fibre-cements a challenge, mobilizing industry and researchers since the early 70 s. Considering their availability and mechanical strength, cellulose fibres have proven to be a viable alternative to asbestos, being employed by the industry as reinforcement in fibre-cements for more than two decades. However, in spite of their well established production and commercialization in many parts of the world, some aspects of the cellulose-cement composites behaviour still motivates research efforts, which are mainly focused on durability aspects. The main deterioration mechanisms acting in cellulose-cement composites are all related to fluid transport within the pore network of the composites and the most applied treatment method is the partial replacement of cement by finely ground admixtures with high active silica content. The improvements in the durability aspects of composites are achieved by modifying the characteristics of the matrix and, mainly, the interfacial region. Rice husk is an agricultural residue produced in large scale in Brazil. If not applied as fuel in the rice mills or in others rural activities, the rice husk is disposed without control, resulting in an ecological problem. However, the pyrolysis of rice husk yields ash with high silica content, (80-90 percent). When burned in a proper way, this silica remains amorphous, presenting high reactivity with cement. Due to these characteristics rice husk ash, RHA, is applied in this PUC-Rio - Certificação Digital No 9924941/CA research as the treatment method in cement composites reinforced by bamboo pulp. It was observed that blended cement with up to 30 percent RHA with low carbon content resulted in a significant decrease in the porosity of the matrix and interface of the composite. As a consequence, these blended-cement composites presented water permeability expressively lower than that of the composites produced without RHA. High carbon content RHA was also applied, simulating the use of ash obtained by a non-controlled burning process. Similar results as those observed in composites with low-carbon-content RHA were achieved, once accelerated autoclave curing was applied to the composites. In this case, for better composite properties, higher RHA content must be used, with the best results being observed in composites with 50 percent RHA. Also, besides these aspects closely related to the main deterioration mechanisms of the composites, it was observed that RHA enhances the fiber-matrix interaction in the interface, improving the mechanical behaviour of the composites.