In the concrete manufacturing industry, a large amount of waste is generated. Such waste can be utilised in the production of more sustainable products with a low carbon footprint. In this study, concrete sludge, a difficult-to-utilise waste that is obtained from residual concrete by washing a concrete truck, was investigated. During washing, aggregates from the concrete mixture are separated, and the remaining insoluble fine particles combine with water to form concrete sludge. Dried and wet concrete sludge were used in the tests. Samples with different compositions were produced with dried and wet concrete sludge, cement, superplasticiser, and tap water. Seven cement pastes with different compositions were made by partially replacing cement with dried concrete sludge (0%, 5%, 10%, 15%, 20%, 25%, and 30%). In compositions with wet concrete sludge, cement was replaced by the same amounts as in the case of dried concrete sludge. The slump, setting time, and their changes with different amounts of concrete sludge were determined for fresh cement pastes. It was found that with different forms of concrete sludge, the technological properties of the mixtures change, and the setting time decreases. The density and compressive and flexural strength results were confirmed by SEM and XRD tests. The research results show that dry concrete sludge causes the deterioration of the mechanical properties of cement stone, while wet concrete sludge improves the mechanical properties of cement stone. However, it was found that replacing 5% cement with dry concrete sludge does not significantly affect the properties of hardened cement stone. In mixes with wet concrete sludge, the recommended amount of replaced cement is 10%, because the technological properties of the mixture are strongly influenced by larger amounts.
The article analyses what effect crystallization additive usage has on concrete physical and mechanical properties and shrinkage to find most effective concrete additive. Investigations of concrete properties with various amounts of crystallization admixture were performed. 9 concrete mixtures with different amounts of crystallizing admixtures were formed and admixtures were added to the concrete mixes from 0.6% to 1.4% of the cement mass, in increments of 0.1%. During the research, the influence of the amount of crystallization admixture the density of hardened concrete, flexural strength, compressive strength, and shrinkage was determined. After evaluating the physical, mechanical properties and shrinkage of the concrete, it was determined that the optimal amount of crystallizing admixture in the concrete mixture is 1% of the cement mass. Thus, it can be argued that the use of a crystallizing admixture has a positive effect i.e., reduces the shrinkage of concrete, because of which cracks in concrete structures can be avoided.
Two different mix compositions were prepared to compare the effect of waste glass sludge, with and without a crystallising admixture, on the properties of fresh cement paste and hardened cement paste. In Composition 1 cement was replaced by different percentage of waste glass sludge: 0 %, 5 %, 10 %, 15 %, 20 %, 25 %, and 30 %. In Composition 2 the same cement content was used with the addition of the crystallising admixture added at 1 % by weight of the binder. The tests showed that the replacement of 20 % of cement with waste glass sludge improved the properties of hardened cement paste, but the properties of fresh cement paste deteriorated. The results of the tests with composition 2 containing the crystallising admixture showed that the crystallising admixture had a negligible effect on the properties of hardened stone, while the properties of fresh cement paste improved in comparison with the specimens without the crystallising admixture. The overall results showed that 20 % was the optimum amount of cement to be replaced with waste glass sludge and that crystallising admixture could be one of the solutions to improve the workability of the mixtures modified with waste glass sludge.
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