Preparation and accelerating mechanism of aluminum sulfate-based alkali-free liquid flash setting admixture for shotcrete

Abstract

To solve the issues of high rebond losses, low early strength, and the presence of toxic materials in alkali-free liquid flash setting admixture, with setting time and compressive strength as the measurement indexes, a low rebond losses, fluorine-free, and alkali-free liquid flash setting admixture (Named SN) was synthesized under a water bath environment ranging from 60°C to 80°C. The results showed that the optimal mass percentages for the synthesis of SN were: aluminum sulfate octadecahydrate: diethanolamine: magnesium sulfate: pseudo-boehmite: lactic acid: water = 55%: 7.5%: 5%: 2%: 0.4%: 30.1%. When SN was blended with 7% by mass of Portland cement, the cement exhibited initial and final setting time within 4 and 8 min, respectively, and the compressive strength of 16.0 MPa at 24 h. At a blending ratio of 8%, the compressive strength exceeded 1.0 MPa at 6 h. Microscopic observations revealed that SN formed abundant needle-like AFt crystals directly with CH, which led to rapid cementitious consolidation. Additionally, the presence of Mg2+and diethanolamine in SN effectively shortened the induction period of high content tricalcium silicates (C3S) in Portland cement, resulting in rapid strength development at 24 h. After 24 h, SN also promoted the hydration of dicalcium silicates (C2S), making the structure of the later cementitious material more compact and continuously increasing the strength.