Properties and microstructure of concrete blocks incorporating drinking water treatment sludge exposed to early-age carbonation curing

In recent decades, industrial by-products have gained significant attention for their potential as components of more environmentally friendly construction materials. In this context, this paper presents the manufacture of concrete blocks (CB), in which sand was replaced with oven-dried drinking water treatment sludge (DWTS). In addition, CB with DWTS contents ranging from 0% to 30% by weight were exposed to early-age carbonation curing. Experiments were conducted to determine the compressive strength, splitting tensile strength, and water capillary absorption of CB subjected to normal or early-age carbonation curing. The results indicated that DWTS degraded durability and mechanical properties of CB because of the high-water absorption and high organic content of DWTS. However, early-age carbonation curing was found to improve the overall performance of CB with DWTS, such that the carbonated CB mixtures satisfied the strength requirement of load-bearing blocks. The pH values of CB after carbonation curing increased with increasing content of DWTS. Microstructural analyses showed that the addition of DWTS into CB caused the formation of ettringites because of the presence of alum and gypsum in DWTS, and early-age carbonation curing decomposed these ettringite into calcium carbonates. The results also showed that the production of crystalline CaCO₃ under carbonation had a filling effect that refined the pore sizes. Thus, the findings of this study indicate that utilisation of DWTS in CB offers significant environmental and health benefits via increased greenhouse gas absorption, as well as reduced DWTS disposal in landfills and unsustainable sand extraction.