Early activation of high volume fly ash by ternary activator and its activation mechanism

To solve the problems of low early strength and severe plastic cracking caused by high volume fly ash used in cement-based materials. Triethanolamine (TEA), calcium hydroxide (Ca(OH)₂), and sodium silicate (Na₂SiO₃) or sodium sulfate (Na₂SO₄) were selected to conduct a ternary doping test. The compressive strength of samples was measured to determine the best ratio, content, and time effect of the activator, and its action mechanism was studied by various micro test. The quantitative calculation model of main hydration products was established in the fly ash-cement system. Based on the simulation of molecular dynamics, the structure of NASH gel was studied under alkali activation. The results show that the optimal mixing mass ratio of TEA:Ca(OH)₂:Na₂SiO₃ is 2:75:25 and the optimal dosage is 1.02% of the cementitious material. There are a large number of needle-like ettringite, petaloid hydrated calcium aluminate and clusters of hydrated calcium silicate gel in the system, whereas the amount of plate-like CH decreased significantly at hydration for 14 days. The Si/Al is three and aluminium coordination is predominantly tetrahedral, and the order of bonds stability and atoms mobility are Si-O > Al-O > Na-O and Na > O > Al> in the NASH gel, respectively. Under the Na⁺ and alkali environment, the Si(OH)₄ and Al(OH)₄^- formed polycondensation reaction to reform polymers Si-O-Si and Al-O-Si, forming a large amount of NASH gel.