Development of a High Early Strength Non-fluorine and Non-alkaline Flash Setting Admixture and Flash Mechanism

To solve the problems of high rebound rate and strength reversion of shotcrete, a non-fluorine and non-alkaline liquid flash setting admixture (FSN) with low rebound and high early strength was synthesized under 60–65 °C water bath environment through orthogonal test design and taking setting time and compressive strength as indicators. The experimental results show that the optimum mass ratio of FSN is aluminum sulfate: diethanolamine:triethanolamine: pseudo-boehmite: lithium carbonate: water=57%: 8%: 0.05%: 2%: 2%: 31%. When FSN is added with 7% of the mass of portland cement, the cement paste can be initially set in 3 min and finally set in 5 min. The compressive strength of mortar is 1.2 MPa at 6 h, 18.0 MPa at 1 d, and more than 100% at 28 days; The microscopic analysis shows that the rapid release of Li⁺, NH⁴⁺, and CO ₃ ²⁻ ions by FSN in the paste solution effectively shortens the induction period of high C₃S content in Portland cement, directly forms early coagulation AFt crystals in FSN and CH dominated by Al₂(SO₄)₃, and forms a large number of C-S-H gels in the later stage, so that the cement can quickly coagulate and harden, and the strength in the later stage is not retracted.