Effects Of Nano-MgO And Fly Ash Content On Expansion Of Cement Pastes

Since the 1980 s,light burnt MgO used as an expansive agent for cement and concrete,has been applied on more than thirty dams in China successfully.The light burnt MgO has advantages of needing little water in hydration reaction and its hydration products are stable,and it is mainly used for compensating the thermal shrinkage of dam concrete,simplifying the measures of temperature control,bringing down the project cost and speeding construction schedule.But its disadvantages are that if the concrete contains too much light burnt MgO,the soundness of concrete will be unqualified,which not only can't reach the purpose of compensating the thermal shrinkage,but also reduce the strengths of concrete and damage the structures.So the content of MgO has been controlled in 5 % in the national standards,but the problem with this limit content is that the expansion of concrete is difficult to fully offset the thermal shrinkage.According to studies conducted by scholars,the expansion properties and the soundness content of light burnt MgO in concrete are related to the size of periclase.So in this study we used the nano-MgO as an expansive agent and the size of periclase was nanosclase.The maximum contents of nano-MgO for Portland cement,ordinary Portland cement and each of them containing 30 % fly ash,were studied in this paper.The effects of nano-MgO and fly ash contents on expansion of cement pastes were explored.The strengths of cement mortars varied with nano-MgO were explored.Based on XRD diffraction test,the variation of magnesia content in cement paste was analyzed.The experimental results are shown following:(1)The maximum content of nano-MgO added in ordinary Portland cement paste and ordinary Portland cement paste containing 30% fly ash both reached up to 10 % for soundness;the maximum content of nano-MgO added in Portland cement paste and Portland cement paste containing 30 % fly ash reached up to 10 % and 8 % for soundness,respectively;and the maximum content of light burnt MgO added in Portland cement paste and Portland cement paste containing 30 % fly ash both only reached up to 4 % for soundness.(2)The expansions of Portland cement paste containing light burnt MgO was larger than the expansions of paste containing the same content nano-MgO cured at 40 ? in water at the age of 365 days.(3)Fly ash restrained the expansion of ordinary Portland cement and Portland cement containing nano-MgO.(4)When the cement mortars curing in water at 20 ? and 40?,the expansion due to the hydration of nano-MgO was harmless to mortars' flexural strength and compressive strength at the age of 28 days and 365 days.(5)Based on XRD diffraction test,the hydration rate of nano-MgO curing in water at 20 ? and 40 ? for 365 days was low.The results suggest that nano-MgO was able to be used as an expansive agent for cement and concrete.Added with nano-MgO or mixed nano-MgO with light burnt MgO,the shrinkages of dam concrete may be compensated in more effectively.