Improvement of early age properties of Portland cement by ternary hardening-accelerating admixture

Abstract

A ternary hardening-accelerating admixture for Portland cement, comprising calcium fluoroaluminate (11CaO.7Al2O3.CaF2), calcium thiocyanate (Ca(SCN)2) and triethanolamine (C6H15NO3), was studied. The cement paste was investigated using compressive strength tests, X-ray diffraction, thermogravimetric analysis, scanning electron microscopy and porous medium surface area analysis. The results showed that the setting time of the cement had been shortened using the ternary admixture; the fluidity and early strength were improved compared with traditional admixtures cured at ambient temperature (20°C). The compressive strength reached 14·5 MPa at 1 d, which was twice that of cement without admixtures and 0·35 times higher than cement with a sodium chloride (NaCl) triethanolamine admixture. Moreover, the total pore volume in the cement with the ternary admixture was much less than the cement with the sodium chloride–triethanolamine admixture. When the ternary admixture was included, not only was the concentration of calcium ions (Ca2+) in the liquid phase increased, but there was also a complexation reaction between the ternary admixture and clinker, which accelerated the cement hydration reaction, and the concentrations of calcium silicate hydrates and ettringite were increased at an early age. The new admixture is a promising non-chloride accelerator and offers more options in hardening-accelerating admixtures for Portland cement systems.