Pressed Cement-Free and Low-Cement Materials Based on Recycled Concrete Powder

The fine powder fraction generated during concrete recycling is often regarded as a low-value by-product or used as a filler in cement-based materials. This study investigates recycled concrete powder (RCP) as the main component of pressed cement-free and low-cement mineral composites. The <0.14 mm fraction was obtained by crushing and sieving concrete waste. Cylindrical specimens were produced by semi-dry pressing at 20 MPa with a forming moisture content of 12-13% and cured under humid-air conditions. Four systems were studied: untreated RCP, thermally activated RCP, RCP with 2.5 wt.% Portland cement, and RCP with 5 wt.% Portland cement. Thermal activation was performed at 600 °C for 2 h. Compressive strength, bulk density, and water resistance coefficient were determined at 3, 7, and 28 days. At 28 days, compressive strength increased from 6.9 MPa for untreated RCP to 11.4 MPa for thermally activated RCP and 12.8 MPa for RCP with 5 wt.% cement, while the water resistance coefficient increased from 0.61 to 0.86. DTA/TGA analysis revealed thermal effects and mass-loss patterns consistent with the presence and evolution of hydrated and carbonate-containing phases. The results demonstrate that RCP can serve as a structure-forming component in pressed cement-free and low-cement materials.