Synthesis Of Sulfate-rich Belite Sulfoaluminate Cement With Phosphogypsum

Phosphogypsum is a byproduct from the production of phosphoric acid by the wet process. Currently, more than 50 million tons of phsophogypsum are generated annually in China. The utilization of phosphogypsum is mainly based on natural gypsum, and many attempts have been made to use phosphogypsum in applications such as cement retarder and gypsum plaster. However, the harmful impurities in phosphogypsum have given rise to many restrictions on the utilization of phosphogypsum, and thus some treatment methods of phosphogypsum have been used to reduce these impurities. The pre-treatment of phosphogypsum can increase the utilization costs, and the properties of pre-treated phosphogypsum are not excellent. Therefore, the reusing proportion of phosphogypsum is low.A calss of belite sulfoaluminate cements has attracted a great deal of interest due to lower energy used and less CO2 emitted during production. Phosphogypsum can be incorporated into the production of belite sulfoaluminate cements to reduce consumption of natural gypsum. The impurities in phosphogypsum can be used as mineralizer which can promote the formation of clinker minerals, and high temperature firing during cement production is beneficial to reduce the adverse effect of impurities. Therefore, the utilization of phosphogypsum for the production of belite sulfoaluminate cement has some advantages. However, the amount of phosphogypsum used in the cement is very low. In order to improve the utilization of phosphogypsum, an excess of phosphogypsum is added to raw materials to produce sulfate-rich belite sulfoaluminate cements. The excessive phosphogypsum can make anhydrite and ternesite phases remain in the produced clinkers. The incorporation of anhydrite and ternesite in the cements leads to significant changes in the mineral compositions. The influence of anhydrite and ternesite on cement properties is a critical problem for the utilization of phosphogypsum.Firstly, the optimum burning conditions for sulfate-rich belite sulfoaluminate cement production and the effect of gypsum source on cement clinkering and performance were studied. And at the same time, the decomposition behavior of phosphogypsum during cement production was also studied. Experimental results indicated that 1270 oC/30 min is chosen as the optimum burning condition, and the clinkers were quenched by applying forced air. The decomposition of phosphogypsum is closely connected to the firing temperature and the mix proportion, and phosphogypsum is slightly easier to decompose than natural gypsum. Phosphogypsum is suitable for the production of sulfate-rich belite sulfoaluminate cement based on mineral composition and cement properties, and the impurities in phosphogypsum do not have negative effect on cement production.Phosphogypsum was fired to simulate the preparation of anhydrite, and the effect of anhydrite on the hydration of ye’elimite was studied. The results indicated that the dissolution rate of the simulated anhydrite is slower than natural gypsum and the simulated anhydrite makes ettringite formation more transformable. On thas basis, cements with different anhydrite content were produced and the influence of anhydrite content on cement properties was researched. The results showed that the setting time can be prolonged when anhydrite content is higher than 10%. The incorporation of anhydrite in the clinkers results in shorter setting time compared with natural gypsum. The presence of anhydrite can significantly improve the strength development, and the suitable addition is between 10% and 20%. The cement containing anhydrite shows much higher compressive strength than that containing natural gypsum. The effect of anhydrite on the dimensional stability of the cements is not significant, and the cements do not show great expansion. The hydration of ye’elimite and the formation of ettringite can be enhanced by the increase of anhydrite content. With the time increasing, anhydrite can promote cement hydration, and play the similar role to natural gypsum. By comparison, it was found that the effects of the simulated anhydrite and the anhydrite formed in the cement are different because of the difference in dissolution rate and hydration reactivity.Ternesite was synthesized by using silica fume and firing at 1200 oC for 8h. It was found that ternesite can hydrate at late ages, and the addition of ye’elimite can promote the hydration of ternesite and gypsum originated from the hydration of ternesite can further promote the hydration of ye’elimite. The effect of ternesite on cement hydration and properties was studied by adding the synthesized ternesite. The results showed that the addition of ternesite can give some increase in later strength development, and reduce the early-age strength. The hydration of ternesite at late ages is beneficial to the formation and stabilization of ettringite. A new protocol for the production of clinkers with ternesite based on two successive firing steps was adopted, and the effect of ternesite on cement properties was also studied. Cements with different content of ternesite can be obtained by controlling the second firing temperature and time. The effects of the formation of ternesite in cements and the addition of ternesite to cement are identical. The presence of ternesite decelerates cement hydration at early ages, but promotes cement hydration at later ages.The effects of mineral compositions on cement properties were analyzed. It was found that the increase of ye’elimite content can promote the early strength, and the suitable content is between 30% and 40%. The increase of ferrite content is not beneficial to early strength, and the suitable content of ferrite is between 10% and 20%. The increase of anhydrite content can reduce the optimum addition of natural gypsum, and when anhydrite content is between 10% and 20%, there is no need to add natural gypsum in cement. The addition of sodium tetraborate to raw materials can promote the stabilization of the high-temperature α’- forms of C2 S, and the optimum addition is 2%. The effect of ternesite in cement is related to the content of anhydrite and ye’elimite in cement, and the optimization of the role of ternesite depends on many factors rather than the specific content range.