Utilization Of Phosphogypsum For The Preparation Of Ternesite-calcium Sulfoaluminate Cement

Phosphogypsum（PG）is a solid waste produced during the production of phosphoric acid by wet process.The main component of PG is dihydrate gypsum.In our country,the annual production of PG exceeds 80 Mt.At present,PG is mainly used in the production of construction materials.However,the soluble impurities in PG lower down the performances.So the treatment is indispensable before recycling.The treatment of phosphogypsum impurities not only increases the utilization cost,but also the impurities can hardly be completely removed.As a result,the utilization rate of PG is low.When PG is used for the preparation of belite-calcium sulfoaluminate（BCSA）cement,the impurities in PG can be used as a mineralizer,promoting the formation of clinker.It makes the impurities in PG become a favorable factor for the preparation of BCSA clinker.But the utilization rate of PG is still very low.In this study,a new type cement of ternesite-calcium sulfoaluminate（TCSA）cement was proposed by introducing ternesite into BCSA cement.As a result,the utilization rate of PG is improved.The study was focused on the following two aspects:（1）Utilization of PG for the preparation of TCSA cement.A method was proposed to promote the coexistence of calcium sulfoaluminate and ternesite.Besides,the calcining process of TCSA clinker and the performance of TCSA cement was studied.（2）Preparation of TCSA cement by the decomposition of PG.The generated CaO after the decomposition of PG could substitute the calcite,which contributes to the maximum utilization of PG in TCSA clinker.Factors affecting the decomposition of PG were studied,which enables the generated CaO and the undecomposed PG satisfy the formation of TCSA clinker.Besides,the influence of impurities concentration on the performance of the cement was studied.Firstly,the synthesis and hydration of ternesite were studied.Twice calcining process was used to synthesis the ternesite of which the purity reaches 94.05%.The hydration of ternesite cured in water with a temperature of 20 and 50 ^oC was studied.The hydration products was analyzed by DSC-TG,XRD and NMR.The results shows that ternesite is a mineral with low hydration activity.It starts to hydrate at 28 d,which proves that it is not a conventionally accepted inert mineral.High temperature water environment curing could accelerate the hydration of ternssite,and its hydration products are dihydrate gypsum,calcium hydroxide and C-S-H gel.The hydration products possess good cementitious property.And then,the effects of impurities on the formation of calcium sulfoaluminate and ternesite were studied by means of phosphogypsum with different impurities and doped impurities.The results demonstrate that both phosphorus and fluorine impurity could promote the formation of calcium sulfoaluminate and lower down the formation temperature.The impurities in PG could promote the sufficient formation of calcium sulfoaluminate in BCSA clinker within the temperature range of 1100-1200 ^oC.Besides,the influence of the impurities and gypsum content on the coexistence of calcium sulfoaluminate and ternesite was studied.Results demonstrates that,by the combined effects of the impurities in PG and an excess amount of gypsum,the coexistence of calcium sulphoaluminate and ternesite could be achieved at a low temperature.The calcining process of TCSA clinker and the amount of gypsum in the raw material were optimized and the aimed clinker was obtained.The results show that the optimum calcining process of TCSA clinker is at 1125-1200 ^oC with a heat preservation time of 15-30 min.To promote the sufficient formation of ternesite in the clinker,mineral composition should be designed with an excess of 5%gypsum.The particle of TCSA clinker is small and the ternesite in the clinker presents an ovoid and stick shape.Due to the loose and porous structure,the clinker shows an excellent grindability.The influences of the amount of added gypsum,impurities and curing temperature on the performance of TCSA cement were studied.The performances were compared with that of BCSA cement.The results show that the hydration of ternesite in the cement could be inhibited by an excess of added gypsum.Phosphogypsum impurities could reduce the hydration activity of calcium sulfoaluminate,but it could improve the hydration activity of ternesite at the later stage,Which contributes to the compressive strength improvement of the cement at the later age.The cement volume stability is not affected by the dihydrate gypsum generated by the hydration at high curing temperature.Compared with BCSA cement containing the same amount of calcium sulfoaluminate,the calcium sulfoaluminate in TCSA cement is relatively higher.The early age strength is obviously improved and the compressive strength at 90 d increases about 17 MPa.Through proportioning,the anticipated decomposition rate of PG for the preparation of TCSA cement by the decomposition of PG was obtained.The factors affecting the decomposition of PG in the raw meal in the reductive environment were studied,which enables the decomposition rate of PG reach the anticipated value and the SO₂ concentration meet the collection requirement.The results shows that the optimum decomposition temperature range of PG is within 1100-1200 ^oC,the optimum heat preservation time is 20 min.The decomposition of PG in the powder raw meal is more homogeneous.When the C/S is 0.7,the decomposition of PG in the raw meal approaches the anticipated value.The concentration of SO₂ is 8.04%,which could be collected within the temperature range of 850-1100 ^oC.In the oxidizing atmosphere,the influence of impurity concentration on the formation of clinker and the optimum calcining process of the clinker were studied.The properties of TCSA clinker was analyzed.The results demonstrates that the concentration of the impurities could promote the formation of the clinker.The optimum calcining process is at 1100-1175 ^oC with a heat preservation time of 20-30min.The concentration of impurities in the clinker lowers down the hydration activity of calcium sulfoalumiante and promotes the hydration of ternesite.Therefore,The compressive strength of TCSA cement lowers down at the early age.But the compressive strength after 28 d increases obviously due to the hydration of ternesite.Meanwhile the pH value of the hydration profucts increases with the prolonging of curing age.It is accepted that the TCSA clinker could be prepared at a low temperature by the combined effects of the phosphorus and fluorine impurities and an excess amount of PG,which makes PG a suitable material for the preparation of TCSA cement.About 0.39 t PG is consumed when 1 t TCSA clinker was produced by PG.About 1.61 t PG is consumed when 1 t TCSA clinker is produced by PG.Therefore,it is an available way for the recycling of PG by the preparation of TCSA cement.