| Belite-sulfoaluminate cement has many advantages such as early strength,high strength,fast setting and fast hardening,micro-expansion,low drying shrinkage,anti-freeze and anti-seepage,and anti-corrosion.It has a broad development prospect.However,the gypsum as a sulfur source will decompose at a higher calcining temperature,which causes the mass fraction of each component in the clinker formation process to continuously change.Therefore,an excessive amount of gypsum needs to be added when the clinker is batched to ensure that Belite sulphur aluminum Calcium acid can be formed according to a set composition.On the other hand,the SO2 decomposed from gypsum will generate a sulfur cycle in the kiln,and erode the kiln lining,which will also cause environmental pollution.Data show that the cement industry is one of the main sources of air pollution,and 3%to 4%of SO2 in the air comes from the cement industry.Therefore,it is significant to study the low-temperature firing method of Belite calcium sulfoaluminate and study its low-temperature formation process.First of all,the cement clinker with the main minerals of belite(C2S)and calcium sulphoaluminate(C4A3＄)was prepared by hydrothermal synthesis-low temperature calcination in this work.By adjusting the mineral ratio between C4A3$ and C2S,the mechanical properties of BCSA cement were steadily increasing.The influences of calcination temperature on the mineralogical compositions of cement clinker was studied,and the compressive strength development of the hydrated belite-calcium sulphoaluminate cement paste was tested.The phase evolution and microstructure of the hydrated paste was also analyzed using the X-ray powder diffraction(XRD)and scanning electron microscopy(SEM)techniques.Secondly,three kinds of calcium aluminate CA,CA2 and C12A7 were synthesized at 1150℃ using alumina and calcium oxide as raw materials.Then,useslag,calcium oxide,aluminum oxide and calcium sulfate as raw materials,and the synthesized intermediate products are separately added in a certain ratio to prepare-belite sulphoaluminate cement clinker.The effect of calcium aluminate on the formation of cement minerals was investigated by analyzing cement clinker XRD.The hydrated samples(3d,14d,28d)were tested for chemically bound water and Ca(OH)2 content,as well as the microscopic structure,and the effects of calcium silicate on the compressive strength of the belite-sulphoaluminate cement samples were investigatedAt last,slag is selected as the raw material for calcining BCT cement clinker at low temperature.Based on C2S:C4A3$ as 55:45,C5S2＄ accounts for 20 wt.%,15 wt.%,10 wt.%,and 5 wt.%,respectively.The precursor was hydrothermally synthesized at 95℃ and calcined at 1200℃ for 2 h to prepare BCT cement clinker.X-ray diffraction(XRD),scanning electron microscope(SEM)and other testing methods were used to characterize the mineral composition and structure of the calcined samples and hydration products.Physical properties and compressive strength tests were performed on the cement to study the optimal formulation of the BCT cement.The results show that when the precursor is hydrothermally synthesized at 95℃,the belite-calcium sulphoaluminate cement can be fabricated under a low temperature of 1200℃.The belite and calcium sulphoaluminate are identified in the clinker as the main phases.The main hydration product is ettringite.When the contents of C2S and C4A3$ are half and half,the hydrated cement paste exhibits a considerable 3 day and 28 day compressive strengths of 37.34 MPa and 62.94 MPa,respectivelyAdding different types of calcium aluminate(CA,CA2,C12A7)to Belite-calcium sulphoaluminate cement raw materials can promote the formation of calcium sulphoaluminate minerals,which can be fired in one step at 1200～1250℃.Calcium sulphoaluminate cement.XRD analysis of the cement clinker shows that the main minerals of the cement clinker are Belite(C2S)and calcium sulphoaluminate(C4A3＄).The effect of the three intermediate products to promote the production of calcium sulfoaluminate is C12A7>CA>CA2.The more intermediate products,the better the effect of promoting the production of calcium sulfoaluminate,and the better the performance of Belite-calcium sulfoaluminate cement.The compressive strength of the cement sample formed by adding 20%C12A7 is the best.From the SEM image of this group of 14d hydration samples,it can be seen that the calcium sulfoaluminate hydrates to form a large amount of ettringite,which has a dense structure and is staggered.Skeleton,so this group of samples has very high early strength.The clear pulp strength reached 62.7 MPa at 14 days and 74.4 MPa at 28 days.The results show that among the three formulations,the compressive strength of the fired BCT cement clinker of CN-1(C2S:C4A3＄:C5S2$ is 49.5:40.5:10)has the highest compressive strength,up to 101.23 MPa.It can be seen from the hydration XRD and SEM images that the cement hydrates to generate a large amount of AFt and Ca(OH)2.At 28 days of hydration,C4A3$ and C5S2$ have been completely hydrated,and C2S has been partially hydrated... |
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