Influencing Factors On Sintering And Performance Of High Magnesium Recycled Cement Clinker

With the increasing shortage of high-quality limestone resources in our country,efficient and rational use of low-grade high-magnesium limestone and industrial waste to produce qualified cement has gradually become a hot issue in the cement industry.The technology of preparing recycled cement with all-component waste concrete can alleviate the shortage of limestone,clay and other resources faced by cement production.However,the waste concrete has a wide range of sources and complex components,so that the MgO content fluctuates greatly.If the waste concrete uses dolomitic limestone or low-grade high-magnesium limestone as the coarse aggregate,it will lead to high MgO content in the recycled cement.Therefore,from the perspective of safe utilization of high-magnesium limestone resources,expanding the source of waste concrete,and improving the performance of recycled cement,it is necessary to study the influencing factors on sintering and performance of high magnesium recycled cement clinker,and to explore the effective ways to control the negative effects on cement clinker from high-magnesium raw materials.In this paper,using the whole-component waste concrete as the main raw material to burn the high-magnesium recycled cement clinker?setting clinker with MgO content of 10%?,from the three aspects of changing the raw material composition,calcining system and fluoro-sulphur doping scheme,by analyzing the test results of raw material burnability,the amount of different forms of MgO in clinker,clinker mineral composition,cement compressive strength and stability under different test conditions,to study the effective ways to improve the firing and performance of high-magnesium recycled cement clinker,determine the suitable ingredient value,calcining system and fluorine-sulfur doping scheme for high-magnesium recycled cement.The main conclusion of this paper is:When the content of MgO in the recycled cement clinker is high,the high silicon rate SM and the appropriate aluminum rate IM are beneficial to the clinker firing and the strength of the recycled cement.With the increase of SM,C₃S increases,C₂S decreases,the total amount of silicate minerals does not change much;C₃A decreases,C₄AF changes little,total minerals of C₃A and C₄AF decrease;Compared with IM,the effect of SM on the strength of high magnesium recycled cement is more significant,and the 3d strength decreased with the increase of SM,and the highest strength at 28d appeared at SM=3.2.In the high-magnesium recycled cement clinker,the solid solution amount of MgO in the mesophase I-MgO is greater than that in the silicate phase S-MgO,and I-MgO increases with the increase of SM;The impact of IM on I-MgO is more related to SM.,higher SM with higher IM is beneficial to solid solution of MgO in clinker,and low SM is not suitable for higher IM.Due to the high MgO content in the clinker,the steam expansion ratio of the recycled cementWith different raw material composition is greater than the national standard limit.Considering the clinker firing and performance,the C4 group?KH=0.92,SM=3.2,IM=1.8?with high SM and higher IM ingredients can minimize the negative effect of MgO,which is the best choice under the experimental conditions of this paper.,suitable for high magnesium recycled cement dosing scheme.Raising the calcination temperature within a certain range is beneficial to the solid solution of MgO in the high-magnesium recycled cement clinker,reducing the adverse effect on the stability,increasing the amount of C₃S,silicate minerals,and the total of C₃A and C₄AF in the clinker,and improving the strength of each age of cement;At 1350°C,the MgO content in the clinker is the largest,and the cement swelling rate is the lowest?1.76%?,but it is still higher than the national standard limit?0.8%?.The holding time has little effect on the burnability of the high magnesium recycled cement raw material and the clinker performance.Considering the burning of clinker and its properties,effects and energy consumption,the suitable calcination system for the high-magnesium reclaimed cement clinker under the experimental conditions of this paper is determined as follows:calcination temperature1350°C,holding time 30min,and fan cooling.?3?The single-doped SO₃ is unfavorable to improve the burnability,and has no obvious effect on promoting the solid solution of MgO in clinker and reducing the f-MgO content.With the increase of the SO₃ content,the content of C₃S in the clinker is reduced,and the strength increases first,then decreases,and the decrease is greater.Single-doped CaF₂ can significantly improve the burnability of raw meal,increase the content of C₃S and total silicate minerals in clinker,reduce the total amount of C₃A and C₄AF,reduce the content of f-MgO in clinker,promote solid solution of MgO,and significantly increase strength;CaF₂content increased,f-CaO content decreased,clinker mineral composition changed little,strength of recycled cement,especially early strength decreased.Fluorine and sulphur doping can also greatly improve the burnability,increase the content of C₃S in clinker,reduce the content of f-MgO,increase the amount of solid solution of MgO,and have a more significant effect at high CaF₂ content?F=0.8%?.The SO₃ content has no significant effect on the mineral composition of clinker,but has a great influence on the strength of recycled cement,low CaF₂content?F=0.4%?with high fluoro-sulfur ratio?F/SO₃=0.8?,high CaF₂ content?F=0.8%?with a low fluoro-sulphur ratio?F/SO₃=0.53?is more favorable for increasing strength.Considering the burning and properties of clinker,under the experimental conditions of this paper,the suitable fluoro-sulphur doping scheme for high-magnesium recycled cement is:single-dosage of CaF₂?F=0.4%?,low CaF₂ and high fluoro-sulfur ratio?F=0.4%,SO₃=0.5%,F/SO₃=0.8?,high CaF₂ with low fluorosulfur ratio?F=0.8%,SO₃=1.5%,F/SO₃=0.53?.