Effect Of MgO On Formation And Hydration Of Alite In High Magnesium Recycled Cement Clinker

In view of the increasing output of construction waste with waste concrete as the main body and the depletion of natural mineral resources for cement industry production,this research group proposes the technology of making recycled cement from waste concrete without unseparated treatment.Previous studies have found that the content of MgO in recycled cement clinker fluctuates greatly due to the wide source and complex composition of waste concrete;if the coarse aggregate in waste concrete is dolomitic limestone or low-grade and high-magnesium limestone,it is easier to cause the high content of MgO in recycled cement,which not only affects the stability of cement,but also affects other properties of cement.Alite is the most important mineral component in Portland cement.Its formation process,content in clinker,crystal form and hydration characteristics have important influence on cement strength.It is necessary to study the formation and hydration characteristics of Alite in high magnesium recycled cement clinker in order to further expand the source of waste concrete raw materials,clarify the control index of MgO content in waste concrete,give full play to the beneficial influence of MgO on the sintering and properties of recycled cement and improve the strength of recycled cement.The research in this paper can provide useful conclusions for the research,production and application of recycled cement with high content of MgO and other high magnesium cement.In this paper,high-magnesium recycled cement clinker is made from waste concrete with MgO as main raw material.Based on the analysis of the test results of burnability of raw meal,the content of MgO in different forms of clinker,clinker mineral composition,Alite crystal form and compressive strength of cement paste,the effects of the content of MgO on the formation and hydration of Alite in high magnesium recycled cement clinker with different silica rate and the content of SO₃ were studied,and the mechanism of action about effects of the content of MgO on the performance of recycled cement were discussed.The main conclusions of this paper are as follows:?1?The high magnesium recycled cement raw materials with KH=0.92,SM=3.0-3.4,IM=1.8,the content of MgO ranging from 2%to 10%,the content of SO₃ ranging from 0%to1.5%,the content of CaF₂ being 0.82%have good burnability when calcined at 1350?.Excepting for E5 group?SM=3.2,MgO=10%,SO₃=1.0%?,the content of f-CaO is higher slightly?1.44%less than 1.5%of the national standard limit?,and the content of f-CaO in other groups are not more than 0.13%in clinker.Adding SO₃ could improve the compactness and hardness of clinker.?2?When the content of SO₃ is 0%and 1%,the solid solution of MgO in clinker is affected mainly by the content of MgO.SM and SO₃ have no significant effect on the change trend of the content of f-MgO and MgO solid solution.When the content of MgO is 8%,it is the most favorable for the solid solution of MgO.When the content of SO₃ increases to 1.5%,the effect of the content of MgO on the solid solution of MgO in clinker becomes smaller.When the content of MgO is less than 6%,the content of SO₃ has a significant effect on the solid solution of MgO.The high content of SO₃?1.5%?is beneficial to the solid solution of MgO.When the content of MgO is more than 8%,the effect of the content of SO₃ on the solid solution of MgO is related closely to the interaction between SM and the content of MgO.Both high SM?3.4?and high content of MgO?8%,10%?,the addition of SO₃ is not conducive to the solid solution of MgO.For high magnesium recycled cement with high KH and SM,MgO is more likely to be dissolved in silicate phase.?3?The effect of the content of MgO on the composition of silicate minerals with high SM in clinker is greater when SO₃ is not added.When SM=3.2 and 3.4,excepting for C2group?SM=3.4,MgO=4%,SO3=0%?,the content of C₃S and the total amount of C₃S and C₂S in clinker decrease with the increase of the content of MgO,which indicates that MgO is not conducive to the formation of silicate phase when high SM is added;the content of MgO has little effect on the content of C₃A and C₄AF in clinker;and when SM=3.2,the content of C₃S and C₃S/C₂S in clinker are the highest.The content of C₃S is lower and C₂S is higher in each group of clinker with different SM when SO₃ is added,and the total amount of C₃A and C₄AF is decreasing,which indicates that the addition of SO₃ is not conducive to the formation of C₃S.With the increase of the content of SO₃,the influence of SM and the content of MgO on the formation of clinker minerals is weakened.?4?The effect of the content of MgO on crystal form of Alite in high magnesium recycled cement clinker is different under different SM and the content of SO₃.M1,M1+M3and M2+M3 of Alite may appear when the content of MgO is low.With the increase of the content of MgO,the crystal form of Alite has the tendency of stabilizing to M3.When SO₃ is added,Alite can stabilize to M1 type,especially when the content of MgO is low.Influenced by impurity ions in raw material,group B?SO₃=0%?with SM=3.2 appears R-type Alite when the content of MgO is 4%and 8%,and group H?SO₃=1.5%?with SM=3.2 appears M3+R-type Alite when the content of MgO is 4%.?5?Without SO₃,with the increase of the content of MgO,the 3d strength fluctuates irregularly and the 28d strength decreases as a whole.The 3d and 28d strength of A1 group?SM=3.0?with MgO=2%are the highest?40.3MPa,107.2MPa?,and the lowest strength of each age occurs basically when MgO=10%.For high magnesium recycled cement mixed with SO₃,SO₃ and MgO cooperate to strengthen the nucleation process and reduce the influence of the content of MgO on strength of cement;the 3d strength of pure slurry decreases with the increase of the content of SO₃,and the 28d strength is the highest when SO₃=0%.With the increase of SM,the 28d strength growth rate of pure slurry increases with the increase of the content of SO₃,which indicates that the addition of SO₃ has adverse effects on the strength of high magnesium recycled cement at all ages,especially early strength.In this paper,under the experimental conditions,high magnesium recycled cement with high KH and SM should not be mixed with SO₃,especially when the content of MgO is high.