Effect of surfactants on the viscosity and hydration of magnesium ammonium phosphate cement

Magnesium ammonium phosphate cement (MAPC) is suitable to be used as a repairing material because of its good bonding strength with old concrete. However, the viscosity of MAPC paste is high, which makes it hard to be injected into the cracks when it is used as a crack repairing material. In this paper, tributyl phosphate (TBP), polypropylene glycol 400 (PPG 400) and polyethylene glycol 200 (PEG 200) were used to decrease the viscosity of MAPC paste. Besides, the hydration and performances were also studied. The results demonstrate that, among the three surfactants, TBP is the best in decreasing the viscosity of the MAPC paste. With 3% TBP, the plastic viscosity reduces from 21.4 Pa s to 10.3 Pa s. All the surfactants more or less could decrease the 1 d compressive strength of MAPC. However, the compressive strength is still higher than 65 MPa. PPG 400 and PEG 200 could prolong the setting time of MAPC from 14 min to 17 min. For the MAPC with TBP, PPG 400 and PEG 200, the drying shrinkage decreases from 2.6 × 10−4 to 2.17 × 10−4, 0.88 × 10−4 and 1.52 × 10−4, respectively. The 28 d bonding strength increases from 7.91 MPa to 8.44 MPa, 8.79 MPa and 8.91 MPa, respectively. Therefore, the surfactants can not only reduce the viscosity and drying shrinkage, but also improve the bonding strength between the hardened MAPC paste and old concrete, making it more suitable for the crack repair.