High recovery of anhydrous cement in dried concrete slurry waste for use as supplementary cementitious material in low-CO2 concretes

Abstract

Concrete slurry waste (CSW) is a by-product generated from returned concrete and the mixer truck washing process, accounting for approximately 3–5% of total concrete production. Although various strategies for recycling CSW have been developed, large-scale recovery of its anhydrous cement fraction, such implemented in the present study, has not been previously reported. Moreover, prior studies have focused almost exclusively on the use of low-reactivity CSW as a supplementary cementitious material (SCM). In this study, a filtering and rapid drying procedure was applied to CSW upon its return to the ready-mixed concrete plant (RMCP). The influence of the recovery time and type of concrete waste on the preservation of the anhydrous cement fraction was evaluated. The recovered material was subsequently used to partially replace Portland cement in cementitious paste formulations. The samples were characterized using analytical methods, such X-ray fluorescence, HCl leaching assay, thermogravimetric analysis, isothermal calorimetry, and quantitative X-ray diffraction (QXRD). The results demonstrated that the anhydrous cement content in CSW was significant, approximately 30% by mass, due to the efficiency of the filtering and rapid drying process. Furthermore, the time exposure to water had no notable effect on the preserved anhydrous cement fraction. Three CSW samples were selected and incorporated into cement pastes, replacing 35 %–75% (by mass) of Portland cement. The resulting pastes exhibited mechanical strength values comparable to, or statistically equivalent to, those of the reference paste made with 100 % Portland cement. The recovery methodology has potential for the development of zero-waste ready-mix concrete plants, and the low emission concrete formulation proposed in this study enabled a reduction of up to 55% in specific CO₂ emissions. This approach could reduce Portland cement consumption by approximately 15% (by mass) in ready-mix operations, contributing significantly to sustainability in the concrete industry.