Synthesis Of Iron-Rich Calcium Sulfoaluminate-belite Cement With Steel Slag

Bauxite is the main aluminum material for the production of calcium sulfoaluminate（CSA）cement.With the increasing shortage of aluminum sources,the quality of bauxite is decreasing,resulting a higher cost for the production of CSA cement.It limits the production and application of CSA cement.In this study,for the purpose of reducing the dependence of CSA cement on the natural sources and making CSA cement more competive in price,Fe₂O₃ was introduced to substitute the Al₂O₃ in C₄A₃$,forming C₄A_3-xF_x$.It contributes to reduce the demands of aluminum material and a iron rich belite-calcium sulfoaluminate（BCSA）cement without using bauxite was obtained.The feasibility of the complete substitution of bauxite with steel slag in the BCSA cement raw meal was investigated.Firstly,the optimum calcining process of iron rich BCSA clinker was investigated.The mineral composition and the morphology of the clinker was analyzed.At the same time,the inclusion of iron in the clinker and the decomposion of gypsum during the clinker calcination were studied.The results demonstrates that the optimum calcining process of the clinker is 1175 ^oC for 20 min and iron could lower down the calcining temperature of the clinker.The clinker is mainly composed of calcium sulfoaluminate,belite,iron phase and a slight amount of ternesite.The introduction of an excess amount of iron does not change the mineral composition of the clinker.With the increase of iron content in the clinker,the amount of C₄A_3-xF_x$in the clinker lowers down and more iron phase forms,which proves that the introduced iron is not completely dissolved into the calcium sulphoaluminate mineral,and the remaining iron participates in the reaction to form the iron phase..Under the optimum calcining process,iron does not significantly affect the decomposition of calcium sulfate during clinkering.When x is1.3,the decomposition of calcium sulfate is merely 2.49%.Secondly,the hydration and performance of the obtained cement was investigated.The results showed that the heat flow and cumulative heat release within 24 h decreased with the increase of iron content,which was related to to the decrease of calcium sulfoaluminate content.The cement was prepared with F0.7 clinker which was calcined at the optimum calcining temperature and 7%and 12%natural gypsum was added into the clinker separately.The content of rise of the added gypsum content improves the compressive strength of the cement pastes,achieving a satisfactory compressive strength.The 28 d compressive strength of cement containing 7%added gypsum and12%added gypsum reaches 50.4 MPa and 61.8 MPa separately.After 1 d,the ettringite forms clusters,changing from acicular to columnar.The pH value of iron-rich BCSA cement was also investigated.The results shows that the pH value is within the range of11.0¹1.8,which is similar to that of BCSA cement.The optimum calcining temperature of iron rich BCSA clinker is 1175 ^oC which falls with the temperature range for ternesite formation.The formation of ternesite would consume a slight amount of gypsum,which might affect the formation of calcium sulfoaluminate in the clinker.Therefore,an excess amount of gypsum was introduced into the raw meal of BCSA cement to investigate its influence on the clinker formation of the clinker as well as the hydration and performance of the cement.The results shows that an excess amount of gypsum could promote the formation of calcium sulfoalumianate and ternesite.The more gypsum is introduced into the raw meal,the more ternesite forms.When the content of gypsum is 20%,both anhydrite and belite could be detected in the clinker,proving an insufficient formation of ternesite in the clinker.For the cement raw meal with 10%and 20%gypsum,the 28 d compressive strength reaches 44.8MPa and 53.4MPa,separately.The 90 d compressive strength reaches 53.7MPa and 62.7MPa,separately.It proves that a rise of gypsum content during clinker mineral content design would benefit the compressive strength development of iron rich BCSA cement,especially for the later compressive strength.It is mainly due to the hydration of ternesite at the later age.When the calcium sulfoaluminate content in BCSA clinker is 30%and x is 0.7,steel slag could substitute bauxite completely.The iron rich BCSA clinker could be prepared at 1175 ^oC for 20 min and the cement exhibits good performances.