Rheological and Environmental Implications of Recycled Concrete Powder as Filler in Concrete 3D Printing

3D printing with concrete has been accounted as a foremost strategy to mitigate low productivity, workforce shortage, and high waste generation in the construction industry. However, substantial environmental impacts related to high cement content in printable mixtures have received minor concern so far. An interesting prospect is the use of recycled concrete powders (RCP) to decrease cement content through their fineness and high specific surface area, which can potentially enhance rheological properties for 3D printing. However, their effects on cementitious mixtures greatly depend on their origin. This research investigated two distinct RCPs to replace 50% of Portland cement in pastes. On cementitious pastes, rotational rheometry, isothermal calorimetry, and a Life Cycle Inventory assessment were conducted. Printability tests on mortars evaluated the effects of RCP on extrudability and buildability. The results showed intensified early hydration for RCP pastes and up to a three-fold increase in static yield stress and higher dynamic yield stresses, regardless of origin. The viscosity of RCP pastes varied in relation to packing density. Extrudability and buildability can be compromised using RCP due to higher yield stress. The LCI assessment indicated a potential decrease of up to 62% in CO2 emissions using RCPs. Therefore, if adequate rheological adjustments are employed in the mix design of RCP mixtures, this material emerges as a feasible strategy to formulate 3D printable mixtures with a lower environmental footprint.