Construction and Demolition Waste Stabilization through Biocarbonation of Reactive Magnesia Cement: Effect of Urea Concentration

Biocarbonation of reactive magnesia cement (RMC) is a promising and sustainable biogeotechnology for construction and demolition waste (CDW) stabilization. The concentration of urea is the dominant factor affecting the RMC biocarbonation process and its performance. To better understand the urea concentration effect, three different urea concentrations (2 M, 4 M, and 6 M) were investigated based on a series of CDW stabilization experiments as well as the fresh and mechanical property tests. Two indicators, urea utilization ratio (UUR) and degree of carbonation (DC), were adopted to evaluate the RMC biocarbonation process with different urea concentrations. The results showed that urea concentration significantly influences the CDW stabilization performance and the RMC biocarbonation process. The increasing urea concentration resulted in higher flowability, slower hardening process, and higher bleeding rate of the fresh paste. While the unconfined compressive strength (UCS) decreased as the urea concentration increased, and the failure characteristics changed from brittleness to ductileness. The maximum UCS (1.52 MPa) was obtained at the urea concentration of 2 M, which is the optimal value. It was revealed that the increase in urea concentration limited the RMC hydration and changed the fresh properties. The higher urea concentration also reduced urease activity and inhibited urea hydrolysis, decreasing the amount of available carbonate and the hydrated magnesium carbonates (HMCs) for strength gain. But the excessively low urea concentrations were also detrimental to this process. Based on the obtained results, the effect of the amount of produced HMCs on UCS was discussed, and a linear relationship between DC and UCS was established. The results are important for understanding the underlying influence of urea concentration on the biocarbonation of RMC stabilized CDW and facilitating the optimization of future field applications.