Preparation Mechanism And Carbon Dioxide Absorbed Characteristics Of Low Calcium Portland Cement From Decomposied Phosphogypsum

Phosphogypsum is a by-product of the’wet-process phosphoric acid production’process.Ca SO₄·n H₂O is it’s main component,which contains some unreacted calcium fluorophosphate and acidic insoluble substances and other impurities.China’s phosphogypsum storage has reached 830 million tons,a large number of environmental hazards,phosphogypsum reuse problem needs to be solved.With the official proposal of China’s goal of achieving carbon peak in 2030 and achieving carbon neutrality before 2060,preparation of low calcium cement from phosphogypsum partially instead of limestone is one of the ways to achieve the constructive goal of’Dual carbon（Carbon Peaking&Carbon neutrality）’.In order to promote the resource utilization of phosphogypsum to sulfuric acid cogeneration cement,the effect of impurities and non-calcium components on the generation and decomposition of calcium sulfate by-products was investigated and effect of Ca S and C stoichiometric ratios on phosphogypsum desulfurization and effect of maintenance conditions on the performance of low-calcium cements.The by-product Ca S is easy to be produced in the process of acid production and cement production from phosphogypsum.The effects of phosphorus,fluorine impurities and silicon,aluminum and iron doping in non-calcium components of cement raw meal on the formation and decomposition of calcium sulfide were studied by using analytical pure gypsum instead of phosphogypsum.The results show that the addition of iron oxide was used to form Ca₃Fe₄S₃O₆in the reducing atmosphere at 900℃.As the increase of iron oxide conent,Ca S content gradually decreased.The addition of calcium hydrogen phosphate,calcium fluoride,alumina and silicon oxide had no significant effect on the formation of Ca S.The effects of phosphorus,fluorine impurities and non-calcium components on Ca S decomposition in air atmosphere at 1000-1200℃were investigated.The main phase of different impurity samples at different calcination temperatures was Ca SO₄,and the addition of calcium hydrogen phosphate was used to form Ca₂P₂O₇and Ca₃（PO₄）₂.The addition of silica was used to form Ca₆（Si O₄）(Si₃O₁₀)and Ca₂Si O₄.The addition of alumina was used to form Ca Al₄O₇and Ca₄Al₆SO₁₆.Ca₂Fe₂O₅,Ca₂Fe₇O₁₁,Ca₂Fe₂₂O₃₃and other iron oxide and lime phases were formed by adding iron oxide.Ca SO₄and Ca O in the samples were decreased with the increase of calcium hydrogen phosphate,silicon oxide,alumina and iron oxide.The addition of calcium fluoride had no significant effect on Ca S decomposition.The effect of reducing atmosphere on the calcined product of phosphogypsum raw material was investigated at 900°C,the main phases were Ca S and Si O₂.The raw meal after reduction calcination was calcined twice in air atmosphere at 1000-1200°C.It was found that the main phases were Ca SO₄and Ca₂Al（Al Si）O₇at 1000 and 1100℃,and a small amount of Ca O was produced.Ca₂Si O₄and Ca₂Al（Al Si）O₇were formed at 1200℃and Ca SO₄diffraction peak was not observed.The non-calcium component promoted the decomposition of phosphogypsum.In order to explore the effect of by-products Ca S and Ca SO₄on the desulfurization of phosphogypsum.The effects of n（Ca S）/n（Ca SO₄）and calcination temperature on the phase of raw materials with different Ca/Si ratios and the desulfurization rate of phosphogypsum were studied by using analytically pure calcium sulfate instead of phosphogypsum.It was found that the diffraction peak intensity of Ca SO₄was decreased with the increase of n（Ca S）/n（Ca SO₄）and calcination temperature.However,The desulfurization rate of the sample decreased due to the increase of calcium silicon ratio.Using pure gypsum as raw material,the calcium sulfate peak intensity of the simulated raw meal is the lowest under the condition of n（Ca S）/n（Ca SO₄）=3:1,1300°C calcination in air atmosphere for 1h,and the SO₃content of the samples with calcium silicon ratio of 1:1,2:1 and 3:1 is 4.9%,8.54%and11.12%.Respectively.The desulfurization rate of phosphogypsum as raw material sample is better than that of pure gypsum.Under the same conditions,the SO₃content of samples with calcium-silicon ratio of 1:1,2:1 and 3:1 is 0.64%,1.43%and 1.79%.In the process of producing cement with phosphogypsum as raw material,using Ca S as a reducing agent and reducing the calcium-silicon ratio of cement were helpful for phosphogypsum desulfurization.In this chapter,the effects of different types of reducing agents and calcination processes on the phase and properties of low-calcium cement produced by calcining phosphogypsum raw materials were studied.The effect of carbonation curing age on carbon fixation characteristics of low calcium cement was also studied.It is found that when Ca S is used as a reducing agent for the sintering of cement clinker at 1300°C in air atmosphere,the peak intensity of calcium sulfate reaches the lowest when n（Ca S）/n（Ca SO₄）=3;When C is used as a reducing agent for calcination under the same conditions,the peak intensity of calcium sulfate reaches the lowest when the amount of activated carbon is 10%.The desulfurization effect of cement calcined with Ca S as reducing agent is better than that with C as reducing agent.With Ca S as reducing agent and n（Ca S）/n（Ca SO₄）=3,1300℃air atmosphere calcined cement samples for 3,7,28 90 days carbonation curing strength were19.81MPa,27.2MPa,34.09MPa and 39.37MPa;under the same conditions,the carbonation curing strength of 10%wt C reduced cement samples for 3,7 28 and 90 days were 15.49MPa,15.57MPa and 15.64MPa and 19.46MPa.Under the conditions of constant temperature（20℃）,constant humidity（75%）and constant CO₂concentration（20%）,with the increase of carbonation curing age,the compactness of cement samples gradually increased,and the porosity between particles was significantly lower than that of uncarbonized samples.A large number of aragonite and vaterite crystals were generated after carbonization.