The influence of Na ₂ B ₄ O ₇ on the properties of high-strength recycled sulfoaluminate cement made from concrete waste

Based on preliminary research, recycled sulfoaluminate cement (RSC) has been developed. However, this RSC exhibits a relatively fast hydration rate, which causes it to fail to meet the setting time requirements. Consequently, there is an urgent need to identify the most suitable retarder for this RSC. This study investigated the effects of four retarders (Na2B4O7, C6H11NaO7, (NaPO3)6, and C6H8O7) on the compressive strength, phase composition, functional group composition, hydration heat release, and slump flow of RSC. It also uncovered the mechanism underlying the retarding effect of Na2B4O7 on this cement. The test results demonstrate that among these four retarders, Na2B4O7 exerts the least adverse effect on the strength of RSC. Furthermore, the setting time of the RSC can be effectively controlled by adjusting the dosage of Na2B4O7. Accordingly, Na2B4O7 is the optimal choice among the four retarders. C6H8O7 only slightly prolongs the initial setting time of RSC, but it shortens the final setting time instead. In contrast, (NaPO3)6, C6H11NaO7, and Na2B4O7 exhibit significant retarding effects. When all four materials are compared, the order of retarding effectiveness from strongest to weakest is as follows: Na2B4O7 > C6H11NaO7 > (NaPO3)6 > C6H8O7.