Cementless binder based on high-calcium fly ash with calcium nitrate additive
Fly ash from Berezovskaya Thermal Power Plant, containing a lot of CaO, in combination with silica fume does not expand and exhibits the properties of a binder. However, the strength of this binder is low. The addition of Ca(NO3)2 significantly increases the strength of the binder. The work aims to study the effect of Ca(NO3)2 additive on the strength, heat of hydration, and phase composition of hydration products of the binder based on high-calcium fly ash and silica fume. The results of X-ray diffraction and Differential Thermal Analysis show that the main phases formed during the hydration of binder with the calcium nitrate additive in various dosages are lime, calcium hydroxide, ettringite, CSH(II) type silicates and calcium aluminosilicates, corresponding to such minerals as gismondine, yugavaralite, goosecreekite. When hardening in water, the residual amount of lime after 7 days is sharply reduced. An increase in the dosage of Ca(NO3)2 from 1.5 to 11.8 % leads to a decrease in the CaO content by almost 2 times. With an increase in the dosage of calcium nitrate, the content of portlandite noticeably decreases, and a significant increase in the amount of calcium hydroaluminosilicates, especially the composition of CAS2H4, is observed. A study of the heat release process showed that calcium nitrate greatly accelerates the process in the first 60 minutes of hydration. However, then the composition with the addition of Ca(NO3)2 is inferior in the rate of heat release to binder without the additive. When testing a mortar with polyfractional sand, the addition of Ca(NO3)2 more than doubled the compressive strength of the mortar.