Chemical admixtures for concrete: Effects of accelerators on the volume stability of cement-based materials

In this study, the addition of sodium aluminate type alkali liquid accelerator (AR), aluminum sulfate type alkalifree liquid accelerator (AS), and fluoroaluminate type alkali-free liquid accelerator (AF) were used to test the volume stability, mass loss, and internal humidity change of cement mortar. The impact of various accelerators on the volume stability of cement mortar was examined through a comprehensive analysis utilizing MIP, XRD, IR, SEM, and other characterization techniques. The results show that the shrinkage deformation of the cement mortar mixed with AR type alkali accelerator was more serious than that of the mortar mixed with the other two alkali-free accelerators, and the shrinkage deformation of the cement mortar was more significant with the increase of the dosage of the three accelerators. The most significant reason for the shrinkage and deformation of the AR type alkali accelerator is that the conversion of AFt to AFm reduces the volume of swellable substances and the pore size distribution of 10-50 nm produces more significant capillary negative pressure. The reason for the smaller shrinkage deformation of the mortar mixed with two alkali-free accelerators is that its main component contains a certain amount of SO42-, and the undersulfated reaction will occur relatively less frequently. Furthermore, the 10-50 nm pore size distribution is less than mortar with AR type alkali accelerator, and the capillary negative pressure is lower. Additionally, quantitative analysis of the components was conducted to confirm the factors responsible for the varying levels of shrinkage deformation in the cement mortars with three accelerators.This paper offers valuable guidance for effectively employing accelerators in shotcrete applications.