Characteristics and mechanism of efflorescence in fly ash-based geopolymer mortars under quasi-natural condition

The unveiling of the characteristics and mechanism of efflorescence in fly ash-based geopolymer mortars (FGMs) under quasi-natural conditions contributes to further putting forward the method for inhibiting efflorescence. Herein, the characteristics (i.e. pore structure, gel structure, and the mechanical properties) of FGMs under quasi-natural condition was systematically investigated, and the efflorescence mechanism of FGMs was also discussed in-depth by those characteristics, the Na+ on FGMs and conductivity of ultrasonic solutions. It can be found that the drying shrinkage caused by water evaporation increases the porosity by 2.5% at most, which decrease the compressive strength and increase the compressive/flexural strength ratio of FGMs effloresced in quasi-natural condition. Due to water evaporation, Na+ is accumulated on FGMs, increasing the content of Si-O-Al and decreasing the quantity of Si-O-Si bond in the gel framework. Moreover, the accumulation of carbonate in the gel framework causes the fracture and depolymerization of gel chain, increasing the content of Si-O-T out of the framework. Comprehensive analysis of the characteristics of efflorescence, the fluctuant downtrend of concentration of free Na+ and conductivity indicate that the efflorescence process is divided into the early (1st - 3rd cycle), middle (3rd - 7th cycle), and late stage (7th - 14th cycle). In the early stage, the main reaction between Na+ near the surface and CO32- severely decreases the Na+ concentration, then the main migration of Na+ inside FGMs to the surface mitigates the downtrend, finally migrated Na+ reacts with CO32- again.

Automated summary

The study systematically investigated the characteristics of FGMs under quasi-natural conditions, revealing the increase in porosity caused by water evaporation and the accumulation of Na+ and carbonates in the gel framework. The efflorescence process can be divided into early, middle, and late stages based on the fluctuant downtrend of free Na+ concentration and conductivity.