PRESSED MATERIALS BASED ON WASTE CONCRETE POWDER

Abstract

This study investigates the feasibility of using the fine fraction of crushed concrete waste as a component for producing pressed construction composites. It is assumed that powdered concrete waste can harden under pressure through a mixed mechanism involving contact-crystallization and partial hydration, especially if unhydrated cement particles remain in the material. Four types of concrete waste were tested, differing in age, strength, and cement content. Specimens were pressed at 20 MPa, a pressure commonly used in the production of autoclaved materials.

Results showed that samples made from waste with higher cement content and lower age achieved the highest strength – 13.4 MPa at 28 days – and average density of 1.62 g/cm³. Softening coefficients near 0.9 indicate sufficient water resistance. Strength increased steadily over time, peaking at 7–10 days, confirming active structure formation in pressed conditions.

The curing method had a notable effect. Optimal results were obtained under high humidity (in a desiccator), preventing moisture loss. Air-dry curing was less effective due to rapid drying, while water immersion showed slightly reduced strength, likely from excess water disrupting structural development.

A correlation was observed between strength and the mineral composition of the waste, particularly alite and calcium hydroxide content. A pH test of aqueous extracts is suggested as a rapid screening method to evaluate the suitability of waste concrete for pressing.

The findings confirm the potential of recycled concrete fines in producing pressed composites with properties comparable to those of common ceramic materials.