The fine high-calcium fly ash as the basis of composite cementing material

Building Materials

The high-calcium fly ashes (HCFA) of Krasnoyarsk TPP-2, Russia were studied. The HCFA were selected from each of the 4 fields of the electrostatic precipitator. It was determined that the size distribution, chemical and quantitative phase composition vary significantly from 1st to 4th EF field. The fine high-calcium fly ash (d90 < 10 microns) selected from the fourth field of electrostatic precipitator was the source for high strength specimens. In the composition with a superplasticizer at a water:binder ratio of W/B = 0.25 the specimens were made and then cured from 1 to 120 days, with their compressive strength increasing from 17 to 72 MPa. The strength of these specimens is comparable to the strength of specimens based on Portland cement PC 42.5 N without superplasticizer. The methods of simultaneous thermal analysis (STA) and quantitative X-ray phase analysis (XRD) were used to study phase transformations of high-calcium fly ash in the process of hydration curing. The major newly formed phases are ettringite 3CaO•Al2O3•3CaSO4•32H2O, as well as calcium carboaluminate hydrates Ca4Al2(OH)13(CO3)0.5•4H2O and Ca4Al2(OH)12CO3•5H2O with low crystallinity. The new phases can form a wide range of solid solutions by replacing Al 3+ with Fe 3+. The more the curing age was, the more transformations of calcium silicate amorphous substance contribute to form cryptocrystalline calcium hydrosilicates that increased the initial and long-term strength of the material. The phase transformations and strength indicators allow to use fine high-calcium fly ash of coal-fired power plants as an independent cementing material in modern technologies for producing building materials, in particular, in the technology of self-compacting composite concrete (SCC). The proposed alternative to cement contributes to the solution of a complex environmental problem: (1) in the heat power engineering the accumulation of fine ash particles can be lowered with consequent reduction of the pollution of water, soil and atmosphere with thin dust particles, and (2) in the construction materials industry a part of the cement can be replaced by the fine HCFA, that will save energy and natural resources.