Study On The Properties Of Cement Paste And Mortar With MgO Expansive Admixture

Although magnesia expansive admixture(MEA) has been successfully used in mass concrete, it is difficult to obtain fine quality of MEA due to the poor control condition to calcination temperature in the practical production. Moreover, the particle size of MEA have effect of its hydration and expansive property, and there is no standard specification to its particle size. These factors limit MEA's safe use and promotion. For this, in view of speciality of mass concrete, the influence of particle size of MEA prepared under different calcination temperatures on the expansive property, compressive strength, pore structure and micro-morphology structure of fly ash-cement paste cured at 20 oC and 40 oC was systematically investigated in this paper. And light-burned magnesia(MgO) is tentatively used to compensate the autogenous shrinkage of steam-cured cement paste.In present experiment, X-ray diffraction(XRD), field-emission source scanning electron microscope(FESEM) and isothermal calorimetry were used to study the grain size, lattice distortion, micro-morphology and hydration activity of MEA prepared at the temperature range from 850 oC to 1150 oC respectively. Effects of particle size of MEA prepared at the temperature of 950 oC and 1150 oC on the expansive property of cement and fly ash-cement paste cured at 20 oC and 40 oC respectively were studied by a comparator. Meanwhile, effects of MEA on the compressive strength, pore structure and micro-morphology of paste cure at 40 oC were studied by pressure testing machine, mercury porosimeter and FESEM respectively. TG-DSC method was used to determined the hydration degree of MgO contained in paste. Effect of the addition of fly ash on the hydration of MgO was studied and the essence of fly ash's restrainting the hydration and expansion of MgO was revealed. The hydration and expansion machine of MgO was researched by observing the micro-morphology of hydrates of MgO. Based on above research results, highly reactive MEA prepared by calcining magnesite, was tentatively used to compensate the autogenous shrinkage of steam-cured cement paste. The suitable content of MEA was determined according to effect of MEA on fluidity and compressive strength of cement mortar. The hydration process of MgO in paste and effect of its hydration on the autogenous shrinkage, pore structure and micro-morphology of steam-cured cement paste were studied to determine whether light-burned MEA was suitable to steam curing system.The results indicate that the fineness of MEA prepared at 950 oC has little influence on expansion properties, compressive strength and microstructure of pastes, but that prepared at 1150 oC(M115) has great influence on those of pastes. Replacing 30% cement with fly ash significantly inhibits the hydration of MEA, and reduces the expansion of pastes. However, compared with Portland cement(PC) paste, the fly ash cement(FC) paste would suffer a more noticeable loss of strength, once when 4% M115 has a deleterious effect on the compressive strength of pastes. Because FC paste with fairly dense microstructure has a lower potential expansion space for the hydration products Mg(OH)2. Moreover, Fly ash has a significant inhibiting effect on the hydration of MEA, and more MEA hydrates in later age. Therefore, the calcining temperature of MEA applied in the system of steam curing is selected below 1150 oC. The results of the effect of MEA dosage on the fluidity,compressive strength and microstructure of cement mortar indicate that MEA can be applied in the system of steam curing and the optimum dosage is 4%. Under the condition of w/b of 0.3, at the ages of 28 and 90 days, the autogenous shrinkage of PC paste was decreased by 37% and 31% respectively, and that of FC paste was decreased by 13% and 19% when adding 4% of M85.