Effect of super absorbent polymer (SAP) on volume stability and application performance of cement mortar with accelerator

Shotcrete has high cementitious material content, small aggregate particle size, high setting accelerator content, and is prone to shrinkage and deformation, and there are few studies on the volume stability of shotcrete at present. Super absorbent polymer (SAP) is widely used as an internal curing agent, it has low cost and small dosage usage, but it has not been applied to shotcrete to improve volume stability. In this study, super absorbent polymers with different dosages, particle sizes, and extra water were used. The application performance, volume stability and moisture change of different cement mortars were studied under the condition of 4 % of alkali accelerator or 7 % of alkali-free accelerator. The results show that with the increase of super absorbent polymer dosage, the setting time of cement mortar mixed with accelerator is shortened, the 1-day compressive strength is reduced, and the 28-days compressive strength is on average increased. Among them, 0.4SAPAS (addition of 0.4 % SAP) can reduce the initial and final setting time by 101 s and 154 s. Further, the autogenous and drying shrinkage are reduced, and the internal humidity change and mass loss are slowed down. At the age of 180 days, the autogenous and drying shrinkage of 0.4SAPAS were 51 % and 47 % lower than those of AS-7. The XRD, SEM and hydration temperature test showed that the use of super absorbent polymer slightly delayed the hydration process at early age, the hydration temperature peaks of AS-7,0.2SAPAS and 0.6SAPAS are 38.5 °C, 36.7 °C, and 35.1 °C. But at 28 days, the surrounding cement particles were further hydrated due to the release of water from super absorbent polymer.