A Kinetic Study of the Pozzolanic Reaction of Fly Ash, CaO, and Na2O in the Preparation of Fly Ash Belite Cement

Abstract

Fly ash belite cement is a kind of low-carbon cement prepared by a two-step process involving hydrothermal synthesis and low-temperature calcination. Pozzolanic reaction pastes, as the precursors of fly ash belite cement prepared by hydrothermal synthesis, are affected mainly by reaction temperature, time, ratios of the mass of fly ash/lime (FA/CA), and the dosage of Na2O. The absorbance rate of CaO with reaction time was tested for all samples, and the reaction kinetic model and parameters of the granule-hydrothermal synthesis method were discussed. A kinetic model for the hydrothermal synthesis in the presence of Na2O was proposed based on the Kondo’s modified Jander equation and Arrhenius equation. The activation energy (Ea) of the process was determined to be 67.76 kJ/mol. In addition, with an increasing dosage of Na2O, the pre-exponential factor A of the Arrhenius equation increased. However, the hydrothermal reaction degree was accurately predicted using the kinetic model characterized by the absorption rate of CaO. The results indicated that Na2O, as an alkali activator, facilitated the diffusion of Ca2+ firstly, then partly dissolved the amorphous phase in the mixtures and, finally, accelerated the formation of poorly crystallized hydrates.

Effects of extended mixing processes on fresh, hardened and durable properties of cement systems incorporating fly ash

“…It is common knowledge that adding fly ash to cement systems can result in a denser microstructure and reduced porosity at later ages. This is because pozzolanic reactions (reaction between CaO and S to form C-S-H products) progress at later ages 47,48 . Golewski 47 mentioned that the homogenous and uniform structure of the portland cement system containing FA was seen after 14-day curing period, which resulted from the transformation of disordered phases into compact and homogenous forms and the filling of porous voids of C-S-H phase.…”

Special Issue: “Ceramics and Construction Materials”

“…Several published articles are focused on binders and concrete [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ], so showing the great scientific interest associated to these materials. Attention has been reserved to the low-carbon cements, i.e., cementing binders produced with low greenhouse emissions [ 1 , 2 ].…”

“…Several published articles are focused on binders and concrete [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ], so showing the great scientific interest associated to these materials. Attention has been reserved to the low-carbon cements, i.e., cementing binders produced with low greenhouse emissions [ 1 , 2 ]. Among these binders are fly ash belite cements and alkali activated materials, the latter representing a potential instrument for the future sustainable development of the construction industry.…”

“…Among these binders are fly ash belite cements and alkali activated materials, the latter representing a potential instrument for the future sustainable development of the construction industry. In study [ 1 ], it has been demonstrated that addition of Na 2 O, as an alkali activator, to fly ash accelerates the formation of poorly crystallized hydrates in the hydrothermal synthesis of the cement. This, in turn, allows the formation of a good pozzolanic material even with the lower temperatures of the process.…”