Influence of water glass modulus and alkali content on the properties of alkali-activated thermally activated recycled cement

This study utilizes the synergistic effects of mechanical activation, thermal activation, and chemical activation to prepare recycled cement (RC) and reveals the mechanism by which water glass modulus and alkali content affect the mechanical properties, hydration characteristics, and microstructure of RC, aiming to address the performance deficiencies caused by its low reactivity. Based on preliminary experimental results, this study focuses on cement mortars with a thermal activation temperature of 750 degrees C and an RC replacement rate of 30 %. Through mechanical tests, X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM), we conducted a series of investigations into the influence of alkali activator content on the mechanical properties, hydration characteristics, and microstructure of RC mortars. The test results show that adding an alkali activator does not alter the types of hydration products in RC; it can change the hydration reaction rate and the quantity of final hydration products. As the water glass modulus and alkali content increase, the quartz and gypsum content in the thermally activated RC decrease, the pore structure reduces, the volume of harmless pores increases, and more C-(N)-A-S-H gel is produced. However, when alkali content further increases, the aluminosilicate oligomers in the thermally activated RC dissolve into aluminum and silicate molecules, reducing the oligomer content in the mortar and hindering the polymerization process. The mechanical properties of the alkali-activated thermally activated RC first increase and then decrease with the rise in water glass modulus and alkali content. The optimal mechanical performance of RC is achieved when the water glass modulus is 1.5 and alkali content is 6 %, with a 28-day compressive strength reaching 33.97 MPa, an improvement of 23.04 % compared to RC without alkali activation. At this point, CH and calcite participate in the high-alkali content system reaction, forming C-A-S-H and N-A-S-H gels, leading to a near-complete consumption of CH and the formation of a denser microstructure at early stages. Our study reveals the hydration reaction mechanism and microstructural evolution of alkali-activated RC, optimizing the parameters of water glass modulus and alkali content and providing a theoretical basis for improving the mechanical properties of RC.