Abstract
Fly ash belite cement (FABC) is predominantly composed of α′L-C2S and C12A7. It is prepared from low-grade fly ashes by hydrothermal synthesis and low-temperature calcination methods. The formation, evolution process, and microstructure of FABC hydration productions were studied in this work, and the ultimate aim is to give a theoretical foundation and technological support for the application of the new cementitious material made of low-quality fly ash. The results showed that the optimal amount of gypsum was about 7% of cement by weight. The 3-day and 28-day compressive strength of cement pastes with 7% gypsum was 13.6 and 60.2 MPa, respectively. Meanwhile, the 28-day flexural and compressive strengths of mortars with 7% gypsum were 4.6 and 25.9 MPa, respectively. The early hydration heat release rate of this low-temperature calcined cement was higher compared with that of high-temperature calcined cement as Portland cement. FABC hydration pastes contained mostly C-S-H, ettringite (AFt), unreacted mullite, and quartz. It was significantly different from Portland cement in that no calcium hydroxide [Ca(OH)2] was observed in the hydration products of different ages because all Ca(OH)2 formed in the hydration reaction could react completely to generate AFt. The ratio of harmful pores (d ≥ 50 nm) reached 55.04% after 3-day hydration. However, it decreased to 6.71%, which was lower than that of Portland cement pastes (35.72%) after 28-day hydration. In the later hydration period from 3 to 28 days, the strength developed rapidly, and a compact microstructure appeared in the hardened paste due to the presence of pores less than 20 nm in diameter.
Highlights
IntroductionA part of it obtained by the dry process has been basically applied in cement and concrete
Nowadays, the annual output of fly ash is approximately 600 million tons in China
The highly active C2 S synthesized at a low temperature was responsible for the relatively high strength of fly ash belite cementitious material
Summary
A part of it obtained by the dry process has been basically applied in cement and concrete. Still a lot of fly ash obtained by the wet process has not been employed in cement and concrete production due to its coarse particles, high carbon content, and low pozzolanic activity [1,2,3,4]. Some recent studies applied ultrafine grinding to improve the activity of stockpiled high-carbon fly ash so as to meet the requirement of fineness and activity index of admixture in cement and concrete. This method could not decrease the content of carbon. The resource utilization of stockpiled high-carbon fly ash has been an urgent need
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