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Utilization of ultrafine quarry by-products for the production of non-autoclaved cellular micro-concrete

Soultana Athanasia

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Summary

This study aims to the production of non-autoclaved cellular micro-concrete with the use of ultrafine quarry by-products (quarry dust). Aggregate quarrying operations produce a large amount of quarry dust, which is difficult to handle and poses several environmental and economic problems. Valorization of quarry dust in construction applications could lead to economic and environmental benefits for the quarrying industry.Over the past few years many research efforts regarding the utilization of quarry dust for the production of several types of building elements (mostly self-compacting concrete and cement-based building elements), have been conducted. On the other hand, research studies related to the use of quarry dust for the production of lightweight building elements such as cellular concrete, are very limited.The production of cellular micro-concrete using quarry dust to replace quartz sand, as well as fly ash for partial substitution of lime and cement, could reduce the environmental impact of raw materials extraction industry.This dissertation focuses on the production of cellular micro-concrete, using quarry dust for total replacement of quartz sand, fly ash for the highest possible lime and cement substitution, and aluminum powder as aerating agent, under conventional curing processes used for cement-based building elements.Regarding the mixture design methodology, due to the fine and similar grading of aggregates and binders used, specialized synthesis studies are required. The proposed mixture design methodology is based on Andreasen particle packing model, while Box-Behnken fractional factorial design of experiments, in combination with the response surface methodology are also used to determine optimal composition.Due to the assumptions and simplifications introduced in the Andreasen model (e.g., particle characteristics like shape are not taken into account), its estimations are considered approximate and therefore is used in the initial mixture design stage. Consecutively, based on the initial estimations, the levels of design parameters are then selected for the determination of the optimum composition based on factorial design of experiments and response surface methodology.Based on the binding material used, two mixture groups (OPFA and WCHL) are designed. OPFA group refers to the mixtures produced using common Portland cement CEM I 42.5N and calcareous fly ash, while WCHL to the mixtures produced using white Portland cement CEM I 52.5N and hydrated lime.Specimens’ preparation includes the mixing of raw materials and casting into molds, followed by conventional moist-curing instead of the energy intensive autoclaving curing process. The specimens are tested for their compressive and flexural strength, density and water absorption. Additionally, the thermal conductivity, capillary rise coefficient, modulus of elasticity, and linear shrinkage of the produced specimens, are determined.The microstructure of the produced cellular micro-concrete is also studied by means of optical and electron microscopy, mercury intrusion porosimetry, image analysis and X-ray microtomography.Moreover, elevated temperature and accelerated carbonation curing techniques are investigated, in order to improve the mechanical characteristics of cellular micro-concrete, such as early-age compressive strength and linear shrinkage.Results indicate that the density of both OPFA and WCHL hardened specimens varies from 680 to 1080 kg/m3 and is considerably lower than that of reference specimens (produced without Al powder). Density is considered to be the most characteristic physical property of all the produced specimens which largely determines all the measured properties.More specifically, a decrease in density of about 50%, causes a compressive strength and flexural strength decrease of 60 and 85 %, respectively. On the other hand, a 50% decrease in density provokes a water absorption increase of about 90%, whereas the decrease rate of thermal conductivity is similar to the one of density.Box-Behnken factorial design results show that the aerating agent has a significant effect on the density of the hardened specimens, since it affects the aeration degree of both OPFA and WCHL mixtures. The amount of binder used has a similar effect with aerating agent on response variables, whereas superplasticizer dosage and mixer speed (revolutions per minute) effect on response variables is significantly lower.Regarding the developed polynomial models, it seems that they can adequately describe the influence of the design factors to response variables and can be successfully used for the production of cellular micro-concrete of predetermined properties.Linear shrinkage of OPFA and WCHL specimens is high (1.1 and 1.7 mm/m, respectively) and reaches a stable value after 140 days of curing. OPFA and WCHL specimens meet the linear shrinkage requirements according to Eurocode 6 (0.5 mm/m) after 42 and 35 days of curing, respectively.Regarding microstructural observations, it seems that the median pore size of OPFA specimens is lower than the WCHL specimens, whereas pore size distribution of WCHL specimens appears to be more uniform than that of OPFA specimens. Moreover, pore shape of OPFA specimens appears to be elongated, as a result of the synthesis conditions and the mixing procedure used. Pore size distribution obtained through 3D microstructural analysis, is different to the one determined using 2D image analysis technique. This is attributed to the fact that the aeration pores are connected forming pore tubes/channels.Moreover, 3D analysis shows a significant difference between the size of the pores formed in the basis compared to the ones formed in the top of the specimens as a result of the hydrostatic pressure applied by the fresh mixture on the walls of macropores.Consequently, in order to better elucidate the microstructure of cellular micro-concrete three-dimensional pore system analysis is required.High-temperature curing results in a more than double increase of OPFA specimen’s early-age compressive strength, while WCHL specimen shows an early-age compressive strength increase of 60%. On the contrary, the final strength (28 days) of OPFA and WCHL specimens cured under high-temperature is 7 and 5 % lower (crossover effect) than the ones cured under conventional curing, respectively. Moreover, high-temperature curing decreases the linear shrinkage of OPFA and WCHL specimens by 34 and 37 %, respectively. The required high-temperature curing days of OPFA and WCHL specimens according to Eurocode 6 decrease from 42 and 35 to 25 and 27 days, respectively.Regarding accelerated carbonation, OPFA specimen shows a more than double early-age compressive strength increase, whereas its final strength increases by 30%. WCHL specimen shows an early-age compressive strength increase by 60% and a 28-days compressive strength increase by 26%.Accelerated curing significantly reduces the linear shrinkage of OPFA and WCHL specimens by 70 and 67 %, respectively. Consequently, OPFA and WCHL specimens meet the Eurocode 6 requirements at 7 and 12 days of curing, respectively.

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