Influence of Processing and Mix Design Factors on the Water Demand and Strength of Concrete with Recycled Concrete Fines

The paper investigates the impact of processing and mix design factors that determine the effectiveness of adding crushed and sieved concrete rubble (hereafter – recycled concrete fines, RCF) into cement-based concrete. The set of factors includes the dosage and specific surface area of RCF, cement content, superplasticizer dosage, temperature of thermal treatment of RCF, and dosage of accelerating admixture. The compressive strength of the concrete from which the rubble was obtained was preliminarily established based on its correlation with mass loss during crushing. XRD, SEM, and EDS tests were used to determine the chemical and mineralogical composition of RCF and the morphology of the particles. There were specific surface areas of RCF, pozzolanic activity, and correlations with fineness and thermal treatment temperature. Using the experimental design, experiments were carried out by varying six factors: RCF specific surface area, RCF content, thermal treatment temperature of RCF, cement content, superplasticizer dosage, and hardening accelerator (Na₂SiF₆) content in concrete containing RCF. Statistical processing of the experimental data provided adequate polynomial regression models for the water demand of the fresh concrete and the compressive strength of hardened concrete at 7 and 28 days. Analysis of the models made it possible to quantitatively assess the influence of the studied factors on the output parameters and rank them by their degree of influence. The greatest increase in water demand was attributed to cement content, especially above 400 kg/m³, and to RCF content. It was established that the addition of a superplasticizer allows for compensating additional water demand and the reduction of compressive strength caused by increased RCF dosage. The influence of different RCF activation methods on compressive strength was ambiguous. Increasing the specific surface area up to a specific surface area of 250 m²/kg of RCF improved strength, but further grinding caused strength reduction due to increased water demand. The positive effect of the superplasticizer on RCF-modified concrete strength was enhanced by the introduction of a chemical activator (hardening accelerator) and thermal treatment of RCF. The obtained models of water demand and compressive strength of concrete with RCF can, under certain conditions, be applied for the optimisation of mix design. The paper proposes a method of mix design and provides an example of calculation.