Synthesis And Application Fundamental Research Of High Belite Sulphoaluminate Cement Based On Comprehensive Disposal Of Various Solid Wastes

With the acceleration of industrialization and urbanization,disordered storage of solid waste,blind mining of natural ores and excessive emission of carbon dioxide have become important factors restricting social development.Resource utilization of solid waste and green development of cement have become two hot issues to be solved.Under the guidance of national policy,the use of industrial solid waste to synthesize new low-carbon cement has become the starting point of common interests for solving the two hot issues.High belite sulphoaluminate cement(HBSAC)is favored in the research and development of new low-carbon cement due to its good performance,low energy consumption,low CO₂emission and low quality requirements for raw materials.At present,solid wastes that can be used to synthesize HBSAC are becoming more and more diverse,such as fly ash,tailings,coal gangue and other aluminum-silica materials,desulfurization gypsum,phosphogypsum,sulfur-fixing ash and other calcium-sulfur materials;however,the utilization of these solid wastes in HBSAC is mostly to replace a single natural raw material,and the utilization rate of solid wastes in cement synthesis is relatively low,usually only 30%to 40%.In order to realize the diversification and reduction of solid waste in the cement industry,a solid waste-based HBSAC(GF-HBSAC)based on multiple solid wastes collaborative disposal technology and without adding gypsum to clinker was developed under the support of National Natural Science Foundation of China(51878366)and Major Basic Research Project of Natural Science Foundation of Shandong(ZR2017ZC0737),and the synthesis theory,hydration performance and performance improvement methods of this cement have been studied systematically by means of macroscopic experimental research and microstructure theoretical analysis.The main research contents and conclusions are as follows:(1)Using petroleum coke desulfurization slag,fly ash,carbide slag and bauxite as raw materials,GF-HBSAC containing Ca SO₄was synthesized at one time by introducing Ca SO₄into the mineral composition of HBSAC clinker of C₄A₃^————S^——-C₂S-C₄AF ternary system.By studying the influence of calcination temperature,holding time and cooling mode on the sintering of GF-HBSAC,the best calcination system of this kind of cement was determined;through qualitative and quantitative analysis of mineral composition of GF-HBSAC,the rationality of raw material ratio and calcination system was verified;by studying the changes of calcination temperature,mineral morphology and grindability under different batching schemes,the influence of mineral composition changes on cement sintering was determined;through the comparative study of the raw material consumption and cost in the synthesis of HBSAC using natural raw materials and solid waste raw materials,the environmental effect and economy of GF-HBSAC were analyzed.The results showed that it is completely feasible to synthesize solid GF-HBSAC containing Ca SO₄with petroleum coke desulfurization slag,fly ash,carbide slag and bauxite at one time;the best calcination system of GF-HBSAC is calcination temperature of 1300?,holding time of 30min and cooling mode of fast cooling;the actual mineral composition of GF-HBSAC is a new ternary system of C₄A_3-xF_x^——S^———-C₂S-Ca SO₄,and Fe element is not combined with Al and Ca element to form the expected mineral C₄AF,but dissolve in C₄A₃^————S^——to form C₄A_3-xF_x^————S^——(x=0.15);there are certain errors between the actual content and the design content of the main chemical composition and mineral composition of GF-HBSAC synthesized under the best calcination system,but the errors are within a reasonable range;the change of mineral composition will not affect the optimal calcination temperature of GF-HBSAC,but as the design content of residual Ca SO₄increases,the sinterability of raw materials becomes worse,the boundary between the clinker mineral crystal particles gradually becomes blurred,the crystal grain size becomes smaller,the adhesion is enhanced,and the grinding of cement becomes more difficult;the utilization of solid waste can reach85%by using various solid wastes such as petroleum coke desulfurization slag to synthesize HBSAC containing residual Ca SO₄,which can save a lot of natural raw materials,especially limestone and gypsum,and has significant environmental effect and good economy.(2)The differences in physical properties,mechanical properties,durability and hydration characteristics between GF-HBSAC and commercial 42.5 grade cement including OPC,SAC and HBSAC were compared and analyzed.The results showed that in terms of physical properties,its water consumption of standard consistency is significantly greater than that of three kinds of commercial cement,and its setting time is close to HBSAC;in terms of mechanical properties,its compressive strength shows significant advantages in the early and late stages of hydration,but its flexural strength does not increase significantly in the later stage of hydration;in terms of durability,its dry shrinkage resistance is excellent,which is basically similar to HBSAC and SAC,and is significantly better than OPC,and its sulfate corrosion resistance is better,close to SAC;in terms of hydration characteristics,its hydration exothermic is later than SAC and HBSAC,and its cumulative heat of 3 days is basically the same as that of OPC and SAC,but it is close to 30%higher than HBSAC,its hydration products are mainly AFt and gel,showing low alkali characteristics.its pore structure of hardened paste is close to that of SAC,which is obviously worse than that of OPC.The application of GF-HBSAC in the field of building materials was discussed,and ultra-light foam concrete with good performance was successfully prepared.(3)By studying the effect of residual gypsum and post-mixed gypsum on the performance of GF-HBSAC,the effect of residual gypsum and post-mixed gypsum was defined,the hydration model of GF-HBSAC containing residual gypsum was established,and the mechanism of residual gypsum was proposed;by studying the effect of residual gypsum content on the performance of GF-HBSAC,the optimal design content of residual gypsum was determined.The results showed that,compared with the post-mixed gypsum,the residual gypsum increases the water consumption of standard consistency,prolongs the setting time,weakens the early strength,improves the late strength,delays the hydration heat release,and increase the cumulative heat of 3days;different from GF-HBSAC containing post-mixed gypsum,the hydration model of GF-HBSAC containing residual gypsum has changed,which is mainly reflected in the position of residual gypsum and hydration products;different from post-mixed gypsum,the action mechanism of residual gypsum has also changed,residual gypsum can participate in hydration in the surface and interior of cement particles,and the hydration reaction in the core of cement particles changes from traditional ion migration control to water migration control,so the hydration degree is higher;the residual gypsum content is an important factor affecting the performance of GF-HBSAC,based on the comprehensive analysis of strength,hydration heat and other properties,the optimal design content of residual gypsum is 15%.(4)Based on GF-HBSAC,three kinds of composite cementitious material systems,i.e.GF-HBSAC-Portland cement,GF-HBSAC-nano-silica and GF-HBSAC-carbide slag,were established.By studying the influence of change in the dosage of Portland cement,nano-silica,and carbide slag on the physical and mechanical properties of the composite cementitious material system,the effects of three modified materials on the performance improvement of GF-HBSAC were evaluated;combined with the analysis of hydration characteristics such as hydration heat and hydration products,the influence mechanism of Portland cement,nano-silica and carbide slag on the hydration of GF-HBSAC were explored.The results showed that the effect of carbide slag is the most significant,followed by nano-silica,and the effect of Portland cement is not good;the influence mechanism of carbide slag on the hydration of GF-HBSAC is to promote the transformation of AH₃to AFt to improve the early strength,and delay the hydration of C₄A₃^———S^———to develop the late strength;the influence mechanism of nano-silica on the hydration of GF-HBSAC lies in the promotion of hydration by nucleation effect,the improvement of pore structure by filling effect and pozzolanic effect;the influence mechanism of Portland cement on the hydration of GF-HBSAC is to reduce the proportion of sulphoaluminate minerals so that weaken the early hydration,change the composition of hydration products so that weaken the joint effect of AFt framework and gel filling,delay the hydration of silicate minerals so that enhance the later hydration.