Self-compacting concrete for monolithic constructions with highly dispersed silica-based additives

Introduction. Self-compacting concrete offers broad potential in construction due to its operational reliability and durability. However, the high cost of self-compacting concrete and the technological complexity of its production require the development of new concrete mix designs and improved placement technologies. When selecting concrete mix designs for self-compacting concrete, chemical additives can be used to reduce cement consumption and improve the concrete's properties. Materials and methods. To ensure the rheological and technological properties of the concrete mix, fly ash, a dry polycarboxylate-based superplasticizer, and ultrafine additives were used. The ultrafine additives were produced by hydrolysis, using pure Portland cement as a precursor, with the concentration in the solution varying from 1 to 5%. The cement hydrolysis reaction results in the formation of a multicomponent sol containing silicic acid, aluminum hydroxide, iron hydroxide and calcium hydroxide. Results and discussion. Experiments were conducted to modify self-compacting concrete with an ultrafine additive obtained using sol-gel technology. An optimal composition of modified cement containing an ultrafine additive with an average particle size of up to 100–150 nm was developed. The use of the ultrafine additive accelerated the cement hardening kinetics and improved the physical and mechanical properties of cement stone by 1.4–1.8 times compared to cement without the additive due to water accumulation, an increase in the volume of cement gel, and a decrease in capillary porosity. Based on an assessment of the technological and rheological properties of concrete mixtures containing fly ash as a microfiller and a modifying additive, compositions corresponding to strength classes B40–B60 were established, containing 7.5–44% fly ash and an additive in an amount of 0.1% of the cement weight on a dry matter basis. It has been established that the introduction of ultrafine fly ash and fly ash into the concrete mix reduces segregation by 17–19%, increases viscosity by 13% to 20%, and reduces flowability only slightly by 5–10%. It has been established that self-compacting concrete with the combined use of fly ash with medium pozzolanic activity, a chemically active ultrafine additive, and a polycarboxylate-based superplasticizer is characterized by intensive strength gain within 1–3 days of curing and an increase in strength by 15–17% within the design curing period. Conclusion. As a result of the research, a low-temperature method for producing a modifying additive using sol-gel technology was developed. This method is simple to synthesize, does not require complex process equipment, and can be added along with mixing water and uniformly distributed throughout the concrete mix. Concrete mix formulations for self-compacting concrete of strength classes B40–B60 were developed using an ultrafine additive, which improves the process properties, quality indicators, and physical and mechanical properties of the concrete.