The Expansion Performance Of The Cement Paste Varied With The Dosages Of Nano-MgO And Slag

Trapped in energy shortages,environmental degradation and other issues,China established a hydropower priority development strategy.In the process of hydropower development,cement-based materials would still play an important role,but how to solve the problem of crack control in mass concrete was still a diffculty.In order to solve this problem,light burnt MgO expansion agent,developed by China with the advantanges of needing less water in hydration and stable hydration products,was mainly used to compensate the thermal shrinkage of dam concrete,simplify the measures of temperature control,bring down the project cost and speed construction schedule.But its disadvantange was obvious: the dosages of light burnt MgO in cement were limited by the autoclaving method.Within the limit dosages of MgO,the thermal shrinkage of concrete was difficult to be fully offset by the volume expansion.According to the present research results,the expansion performance of concrete and qualified maximum dosages for soundness of MgO were related to the grain size of periclase,preparating temperation,curing conditions and so on.Therefore,the effects of light burnt MgO,nano-MgO and composite admixture of them on the expansion performance and compressive strength of hardened cement pastes were investigated in this paper.Results of the experiments indicate:(1)Based on the Potland cement,the qualified maximum dosages for soundness of light burnt MgO,nano MgO were about 10% and 4% respectively;based on the Potland cement with 50% slag,the qualified maximum dosages for soundness of light burnt MgO,nano MgO were about 8% and 2% respectively;based on the Potland cement with composite admixture of 2%(or 3%)light burnt MgO and 50% slag,the qualified maximum dosages for soundness of nano MgO were about 6%(or 0%);and based on the Potland cement with composite admixture of 4%(or 5%)light burnt MgO and 50% slag,the soundness of hardened cement pastes were already disqualified.(2)Cured in water at 40 ?,the expansion of Potland cement hardened pastes increased with the increase of MgO(light burnt MgO or nano-MgO)dosages and curing time,and the pastes expansion added with nano-MgO tended to be stable easier than that with light burnt MgO.For example,the hardened pastes expansion added with 8% nano-MgO stabilized at the age of 365 d,but the hardened pastes expansion added with 6% light-burnt MgO still increased at the age of 365 d.(3)The slag had inhibitory effects on the expansion and hydration rate of the Potland cement pastes with nano-MgO.(4)Cured in water at 40 ?,the Potland cement hardened pastes expansion added with nano-MgO,50% slag tended to be stable easiest;the pastes expansion added with composite admixture of light burnt MgO,nano-MgO and 50% slag tended to be stable easier;the pastes expansion added with light burnt MgO,50% slag tended to be stable hard.(5)Cured in water at 40 ? for 545 days,the porosity of the Potland cement hardened pastes added with 0%~10% nano-MgO,50% slag decreased and the macropore porosity of the pastes decreased first then increased with the increase of nano-MgO dosages.(6)Cured in water at 40 ?,the expansion of Potland cement hardened pastes added with composite admixture of light burnt MgO,nano-MgO and 50% slag gradually increased with the increase of light burnt MgO dosages in the condition of the total dosages of MgO were fixed.(7)Cured in water at 40 ?,the compressive strength of the Potland cement hardened pastes added with 0%~10% nano-MgO and 50% slag were not decreasing;the compressive strength of the hardened cement pastes added with light burnt MgO and 50% slag increased first and then decreased varied with the light burnt MgO dosages;the compressive strength of the hardened cement pastes added with composite admixture of light burnt MgO,nano-MgO and 50% slag were not decreasing when only if the total dosages of MgO were less than 10% and the dosages of nano-MgO were less than 5%.In summary,nano-MgO can be used as expansion agent in cement or concrete.The method of composite admixture of light burnt MgO and nano-MgO can obtained not only larger and more secure expansion compared to MgO,but also more cost-effective.