The Influence of Calcareous Fly Ash on the Effectiveness of Plasticizers and Superplasticizers

Due to the rational shaping of the environment and the management of environmental resources in accordance with the principle of sustainable development, calcareous fly ash (CFA)—high-calcium as a by-product of lignite combustion—is a valuable addition to concrete. This additive, however, due to its high-water demand lowers the workability of the concrete mix, which is a problem, especially in the first 90 min after mixing the components of the mix. In order to meet this challenge, plasticizers (P) and superplasticizers (SP) for concrete are used with various effects which are designed to reduce the yield value and plastic viscosity. To check the technical efficiency of admixtures P and SP with different chemical bases, the main objective of this research was to investigate the influence of raw and ground CFA on the rheological properties and other side effects of admixtures, such as the amount of air in the mixture and the amount of heat of hydration. The use of P, particularly SP, effectively improves the workability of the mortar containing CFA, especially ground CFA. With these admixtures, it is possible to obtain mortars containing ground CFA with similar rheological properties to mortars without its addition. To obtain a specific workability of mortar with CFA, it is usually necessary to introduce a higher dose of P or SP than used for mortars without CFA. The presence of raw CFA does not alter the effectiveness of P and strongly reduces the effectiveness of SP. The reduced effectiveness of SP manifests primarily as a high workability lost. The presence of ground CFA does not change the effectiveness of P (or is higher). The effectiveness of the superplasticizer SNF (with a chemical base of naphthalene sulfonate) and PE (with a chemical base of polycarboxylate ether) is slightly lower or does not change. The effectiveness of the superplasticizer SMF (with a chemical base of melamine sulfonates) is significantly lower. We found that the presence of ash affects the efficiency of P and SP, while processing via the grinding of ash makes the effect negligible. These results are novel in both their cognitive and practical aspects.