Retarding Mechanism of Nano-Zn-Fe Hydrotalcites for Sulfoaluminate Cement-Based Grouting Materials

A retarder is often required to maintain the workability of sulfoaluminate cement-based grouting materials (SCGMs). Carbohydrates are widely used as cement retarders, but a high carbohydrate content can degrade the late-stage performance of cementitious materials. Nano-Zn–containing compounds have been found to delay cement hydration without affecting mechanical performance after 28 d of curing. Currently, there is no literature reporting the effects of nano-Zn-Fe layered dihydroxyl hydroxide (LDH) on the properties of SCGMs. This study synthesized nano-Zn-Fe LDHs using Zn:Fe molar ratios of 2∶1, 3∶1, and 4∶1 (LDH-2, LDH-3, and LDH-4, respectively). It was found that the higher the Zn:Fe ratio, the slower the hydration heat flow and the lower the compressive strength of SCGMs. LDHs primarily provided nucleation sites for FeOOH, CaZn2(OH)6·2H2O, and Zn(OH)2: an increase in the Zn:Fe ratio caused the primary materials deposited on the surfaces of the Zn-Fe LDHs to gradually change from FeOOH (LDH-2) to CaZn2(OH)6·2H2O (LDH-3), then to Zn(OH)2 (LDH-4). All three LDHs released Zn2+ and Fe3+ into alkaline solutions: the higher the Zn:Fe ratio, the higher the concentration of Zn2+ released, whereas the concentration of Fe3+ remained consistent. Furthermore, the Zn2+ preferentially combined with OH− in the SCGMs, then consumed Ca2+ to form CaZn2(OH)6·2H2O, which inhibited the formation of ettringite; the Fe3+ participated in the hydration of the SCGMs to generate iron-containing ettringite. Thus, the nucleation and discharge of Zn2+ and Fe3+ from the Zn-Fe LDHs influenced the hydration product distribution, pore structure, hydration heat flow, and compressive strength of the SCGMs.