Demonstration of 3D-Printed Concrete Containing Fine Recycled Concrete Aggregates (fCAs) and Recycled Concrete Powder (RCP): Rheology, Early-Age, Shrinkage, Mechanical, and Durability Performance

The paper presents a comprehensive analysis of cement-based composites incorporating both fine recycled concrete aggregates (fRAs) and recycled concrete powder (RCP), which were used for 3D concrete printing. The study evaluates properties ranging from fresh-state behaviour to hardened properties, durability, and microstructural characteristics. In the final stage, a life cycle assessment (LCA) was conducted. A study found that it is feasible to print a composite containing up to 100% fRA as a replacement for natural river sand. Notably, an increase in fRA content enhances the buildability of the mix, as confirmed by green strength tests. However, the open time of mixes containing fRA and RCP was shortened. Incorporation of RCP and fRA led to a decrease in shrinkage within the first 24 h of hydration. Mechanical studies reported a significant reduction in compressive strength (up to 55%) when RCP and fRA were introduced to the mix. Despite the reduction in mechanical properties in specific configurations, all mixes—including 100% fRA and 10% RCP—exhibited compressive strengths above 30 MPa, demonstrating their potential suitability for use in the construction industry. The durability properties of mixes modified with fRA show that there is a statistically significant reduction in flexural strength after 25 and 50 freeze–thaw cycles. In terms of compressive strength, cast specimens did not exhibit any notable reduction in mechanical performance after freezing and thawing cycles. The LCA results demonstrate the high potential for using fRA and RCP derived from concrete waste in the additive manufacturing industry.