INFLUENCE OF ACTIVATION METHODS OF THE FINE FRACTION OF RECYCLED CONCRETE WASTE USED AS A MINERAL ADDITIVE FOR CONCRETE

Abstract

This paper investigates the influence of mechanical, thermal, and chemical activation of the fine fraction of recycled concrete waste on its performance as a mineral additive for concrete. The fine recycled concrete powder (CF < 1 mm) was obtained from demolished structures, and its chemical and mineralogical composition was determined. The pozzolanic activity and pH of CF increased with fineness and thermal treatment temperature. A four-factor three-level experimental design (B⁴) was applied to evaluate the combined effects of specific surface area (130–370 m²/kg), thermal treatment temperature (0–600 °C), dosage of polyacrylate superplasticizer (0–1 % by cement mass), and sodium silicofluoride (0–1 %) on water demand and compressive strength of concrete mixtures containing 20 % CF as a cement substitute. Statistical analysis yielded polynomial regression models with high adequacy (R² ≈ 0.99), allowing quantitative assessment of each factor and their interactions. The results show that the dominant influence on strength and workability is exerted by the superplasticizer content, which compensates the increase in water demand caused by finer CF. Optimal activation, ensuring an activity index Iₐ = 1.0 (equal strength to reference concrete), is achieved at a CF fineness of 344 m²/kg, thermal treatment at 350 °C, and 1 % polyacrylate superplasticizer. The proposed combination provides a cost-effective and sustainable method for partial cement replacement without loss of strength, enabling efficient recycling of construction and demolition waste in concrete production.