(682d) Wastewater-Derived Struvite As a Green Alternative to Portland Cement: Experimental Insights into Rheological and Mechanical Properties

The construction industry faces challenges with waste management, energy consumption, CO₂ emission reduction, and a decline in raw materials, mainly from cement production. Substituting cement with waste materials simultaneously addresses the waste management and CO₂ emission issues confronting the industry in its bid to achieve a near-zero emission target by 2050. Therefore, this study investigates wastewater-derived struvite as a partial replacement for ordinary Portland cement (OPC). In the experimental design, struvite produced through electrochemical precipitation and commercially available struvite substitute OPC at 3, 5, 10, and 15% rates with a water-to-binder ratio of 0.5. The hydration kinetics, time-dependent rheology, thixotropy, compressive, and flexural tests are performed. The rheological data agree with the thixotropic assessment, indicating that the addition of struvite (with an increasing substitution rate) enhanced the flowability of the paste. The struvite-OPC mix shows an initial increase in equilibrium shear stress and hysteresis loop area, which decrease after 20 min resting time, corroborating struvite’s high early strength. Moreover, the OPC paste has a faster deflocculation rate than the 10% struvite-OPC mixes, which improves after 1 h. Also, the addition of struvite retards the rate and heat of hydration, which is promising for reducing thermal cracking peculiar to OPC. The result further suggests that hydration, formation, deflocculation, and reconstruction of flocs influence the paste’s thixotropy mechanism. Furthermore, the 1-day strength (both compressive and flexural) of 10% struvite is higher than that of OPC, while the 7- and 28-day strength is slightly lower than that of OPC. Generally, substituting OPC with struvite resulted in a reduced rate and heat of hydration, flowability retention, and enhanced early strength. Based on equilibrium shear stress, hysteresis loop area, incipient structural parameter, hydration heat, and reconstruction and deflocculation rates data, a 10% struvite-OPC mix is a promising candidate for maintenance repairs and mass concrete constructions where improved workability and reduced thermal cracking are required. This research contributes foundational knowledge, facilitating the tailored design of struvite-OPC composites for diversified construction applications.