| In this paper,the introduction of B element in light burned magnesia was calcined at 1200°C to reduce the activity of light burned magnesia.The calcined magnesia was used instead of sintered magnesia and retarder as the main raw material for preparing MKPC to reduce the raw material cost of MKPC.In the light burned magnesia,B elements of 0%,0.05%,0.10%,and 0.15% of the content of magnesium oxide were introduced in the forms of boron mud,boric acid,and borax,respectively,calcined at 1100°C,1200°C and 1300°C and held for 3h.The activity of calcined magnesia was determined by water law,and the size of the magnesia grain was observed by SEM to determine the optimal introduction form and amount of B element.Using the B-doped magnesium oxide at different calcination temperatures as raw materials,the reference MKPC test block was prepared with different M/P(mass ratio of magnesium oxide and potassium dihydrogen phosphate).The optimum M/P ratio and the optimum calcination temperature of B-doped magnesium oxide were determined by X-ray diffraction and SEM analysis and the compressive strength of cement blocks of different ages.On the basis of the formula,10%,20%,30% fly ash and 5%,10%,15% zeolite were respectively added,exploring the effects of different proportions of fly ash and zeolite on the water resistance of new MKPC.The results showed that:(1)The activity of magnesium oxide introduced by three ways of introducing B element was reduced.Among them,the activity of magnesium oxide introduced by borax was the lowest,and its activity was reduced from 39.9% to 1.08%.(2)The grain sizes of magnesium oxide after the calcination of B element in light burned magnesia at 1200°C were obviously increased.Among them,the magnesia grain size with 0.15% B element introduced by borax increased the most,increasing from 900 nm to 2.7μm;(3)The reference MKPC prepared by introducing 0.15% B element in the form of borax in magnesium oxide calcined at 1200°C had a higher compressive strength.When M/P was 3,the compressive strength of 7d was about 30 MPa.(4)The setting time of the new reference MKPC prepared by introducing 0.15% B element in the form of borax in magnesium oxide calcined at 1200°C could be extended to about 20 minutes.The addition of 30% fly ash could prolong the setting time of new MKPC to 25 minutes,and the addition of zeolite shortened the setting time of new MKPC.(5)The new reference MKPC prepared by introducing 0.15% B element in the form of borax in magnesium oxide calcined at 1200°C had poor water resistance,and the softening coefficient of 7d was only 0.43.(6)Adding appropriate amount of fly ash and zeolite can increase the compressive strength and softening coefficient of new MKPC.When the addition of fly ash was 10%,the compressive strength of 7d was 35.5MPa,and the softening coefficient was increased to 0.70;When the addition of zeolite is 5%,the 7d compressive strength was 30.1 MPa,and the softening coefficient was increased to 0.68.Therefore,magnesium oxide which was calcined at 1200°C with 0.15% B element in the form of borax can be used as a raw material of MKPC instead of sintered magnesia and retarder.The addition of fly ash and zeolite not only improved the compressive strength of MKPC,but also improved its water resistance. |
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