| Magnesia expansive agent has the characteristics of low hydration water demand,stable hydration product Mg(OH)2 and delayed expansion,which is suitable for compensating the shrinkage of mass concrete with low water cement ratio demand.Magnesia expansive agent is usually prepared from magnesite,dolomite,serpentine and other magnesia rich minerals.Due to the decrease of high-quality magnesite,medium and low-grade magnesite can be utilized by mineral processing technology.However,the tailings discharged from mineral processing directly pollute the environment and cause a great waste of resources.At present,magnesite tailings are roasted into magnesia expansion agent,which can save energy,protect environment and turn waste into treasure.In this paper,magnesite tailings were calcined at 800 ?,900 ?,1000 ?,1100 ?,1200 ?,1300 ?,1400 ?,1500 ? and kept for 1 h to prepare different magnesia expanders.The effects of calcination temperature on the chemical composition,mineral composition and activity of magnesia expanders were analyzed.The effect of magnesia expansive agent on the limited expansion rate of cement paste was analyzed by using magnesia expansive agent prepared at different calcination temperatures as raw materials and mixed with cement paste.The changes of phase composition,hydration process,thermal stability and microstructure of different magnesia expansive agent in cement paste were systematically analyzed by means of X-ray diffraction,scanning electron microscopy,mercury porosimetry and thermal analyzer.The optimum calcination temperature of magnesia expansive agent is determined according to the limited expansion rate of cement samples(M-C)with magnesia expansive agent in different curing ages.The influence of different amount of magnesia expansive agent on the compressive strength and carbonation resistance of cement paste and the effect of magnesia expansive agent on the expansion performance of cement paste under different curing conditions were studied.The results show that the expansion performance of cement paste can be improved by the expansion agent made of magnesite tailings at different calcination temperature.And the expansion performance of cement paste with the increase of expansion rate limited presents the trend of rapid increase in the early stage and slow in the later stage,among which the expansion effect of cement mortar mixed with magnesia expansion agent made of calcination temperature of 1000 ?(M-C-1000)is the best.Under the condition of water curing at 20 ?,the limiting expansion rates of7 days,28 days and 120 days were 0.022%,0.037% and 0.062% respectively;the hydration products of M-C were mainly hydrated calcium silicate and magnesium hydroxide,and there was still non hydrated magnesium oxide in M-C after curing for120 days,and the remaining amount of magnesium oxide in ement mortar mixed with magnesia expansion agent made of calcination temperature of 1400 ?(M-C-1400)was 3.04%;The compressive strength of 10% M-C mortar is lower than 6% M-C.The compressive strength of M-C-1000 cement mortar mixed with 10% and 6% Mg O expansion agent at 1000 ? can reach 45.5 MPa and 47.5 MPa respectively at 28 d,which is 7.06% and 11.76% higher than that of blank cement mortar.With the increase of magnesia expansive agent,the anti-carbonization effect of magnesia expansive agent increases.In the dry curing condition,it is not conducive to compensate the shrinkage of cement mortar;the increase of curing temperature promotes the hydration process of magnesium oxide.In the same age,the expansion of M-C mortar specimen under 40 ? water curing condition is better than that under20 ?.When the water temperature is 40 ? to 120 days,the limited expansion rate of M-C-1000 and M-C-1400 mortar samples increases to 0.113% and 0.062%respectively,which is 82.3% and 55% higher than that of water temperature. |
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