Early hydration and liquid phase characteristics of solidified body of clinker-free superfine tailings

To investigate the early hydration and consolidation mechanism of superfine metal tailings powder in the CaO–CaSO₄–H₂O system, a clinker-free consolidation material based on iron tailings powder was prepared by superfine grinding lime, gypsum, and iron tailings. From 3 min to 24 h, the liquid phase of the hydrated slurry was extracted by centrifugation and high-pressure extraction. The changes in ion concentration and conductivity were tested, and the relationship between them was analyzed. The relationship between the formation mechanism of early hydration products of a consolidated body and the change of liquid phase characteristics is studied combined with the hydration exothermic rate curve, field emission scanning electron microscope (SEM), X-ray diffraction analysis (XRD), thermogravimetry-differential thermal analysis (TG–DSC), and other test methods. The results show that lime, gypsum, and amorphous components, which are on the surface of iron tailings powder, dissolve rapidly within a few minutes after solid-liquid mixing. The concentration of each ion in the liquid phase rises sharply, reaching the peak or saturated state successively in 10–30 min, and then decreases rapidly. After 180 min, the decline rate slows down but continues to decline. The liquid conductivity has a very high positive correlation with the total concentration of Ca²⁺, OH^–, and SO₄^2– ions; The first hydration exothermic peak of the consolidated material is concentrated within 0–15 min, which is mainly caused by the wetting and dissolution of soluble components in the consolidated material and the exothermic behavior of lime hydration; The starting and ending time of the second hydration exothermic behavior is 20–180 min, which is mainly caused by the phase change heat generated by the formation of hydration products. Increasing the grinding time significantly prolongs the termination time of the second exothermic behavior and increases its peak value; The phase analysis of hydration products showed that AFt characteristic peak and C–S–H endothermic peak could be seen in the slurry after hydration for 90 min. Research has proved that the amorphous SiO₂ and Al₂O₃ on the surface of superfine iron tailings powder have the characteristics of rapid dissolution in alkaline solutions, and a hydration reaction can occur when lime and gypsum components are encountered. When the solubility product of hydration products is reached, hydration products AFt and C–S–H will be generated. The two hydration products are interspersed and cemented with each other, and the unhydrated iron tailings particles will be consolidated to form a hardened body. Prolonging the grinding time can effectively increase the system’s amorphous SiO₂ and Al₂O₃ content and the proportion of superfine particles in iron tailings, thus improving the slurry hydration rate. While increasing the amount of the hydration product, the filling effect of the micro powder part is further increased, and the strength of the consolidated body is correspondingly improved.