Calcium carbonate and reactive silica recovery from waste cement: The influence of processing parameters on upcycled material properties and carbon intensity

[Display omitted] •The leaching kinetics and reactivity of 1-yr waste hydrated cement paste (HCP) are reported.•The unreacted silica-rich residue is categorized as a potential supplementary cementitious material (SCM)•Precipitated metal oxides (PMOs) are recovered from the leachate via pH swing process.•Pure precipitated calcium carbonates (PCC) are produced from the waste cement paste.•The silica-rich residue profoundly impacted the hydration of new cement admixtures as shown by calorimetry testing. The decarbonization of the cement industry is incredibly challenging due to direct and indirect carbon emissions from clinker production. Use and repurposing of alkaline construction and demolition wastes (C&DW) is an attractive method to potentially decarbonize parts of the built environment through CO2 capture and re-use. Among processes to facilitate this, aqueous two-step dissolution and carbon mineralization may offer distinct opportunities for carbon utilization through the repurposing of C&DW to capture CO2. Furthermore, this allows for the creation of tailored products, such as silica-rich materials, metal oxides, and precipitated calcium carbonates (PCC), which can be reincorporated into new construction materials. In this study, we report the leaching kinetics and reactivity of 1-yr old hydrated waste cement paste produced from neat OPC at various slurry densities and in the presence of internal grinding media to enhance dissolution. At a low slurry density of 1 wt%, we report a single-pass Ca extraction efficiency of 67% which drops to 48% at a slurry density of 10 wt%. A pH-swing process is then utilized to purify the reaction liquor of co-elements, such as Si, Fe, and Al, which are recovered as precipitated metals oxides (PMOs). Both the kinetics of PCC formation and its corresponding purity was analyzed, which was nearly 97% when the pH-swing process was employed. Finally, the undissolved, amorphous high-surface area Si-rich residues recovered during the leaching process were incorporated into new cement admixtures and their reactivity was monitored using calorimetry to study the impact of upcycling these additives within the built environment. Overall, this work motivates the use of ex-situ carbon mineralization as a method to repurpose waste alkaline feedstocks into new build environment low-carbon materials.