Utilizing alkaline solid waste for low-carbon construction material via in-situ calcium phase design

Global cement market generates a large amount of greenhouse gases, driving a great interest in developing low-carbon construction materials for climate goals. Although free lime (f-CaO) and low hydration activity limit the applications in construction materials, steel slag, as an alkaline solid waste, is widely regarded as a sustainable alternative to cement. Here, we propose an in-situ calcium phase design strategy of steel slag and develop a high-performance cementitious material through pre-hydration. The pre-hydration effectively reduces the risk of the f-CaO expansion and prevents the occurrence of micro cracks. With the addition of fly ash and alkaline activator, a high elastic modulus Na-rich gel is generated and improves the material's compressive strength by 133.7%. Carbon footprint analysis indicates that the global-warming potential of the high-performance cementitious material (232-265 kg CO₂-eq ton^-1) is only about 34-40% of that of cement, helping to reduce about 2.2-3.0 Gt CO₂-eq from the global cement market. Interestingly, additional energy compensation (heat or microwave) is proven to expeditiously enhance the mechanical properties of the cementitious material and shorten production cycles without bringing excessive CO₂ emissions. This work inspires the strategic utilization of alkaline solid waste in a simple way.