Quantitative Determination And Control Of The Periclase In High Magnesia Portland Cement

The high magnesia Portland cement?HMPC?possesses the property of later micro-expansion,which could effectively compensate later shrinkage due to temperature drop in the mass concrete structures with high anti-cracking requirements,like dams.However,no international standard has been established for quantitative determination of periclase in the cement clinkers.Meanwhile,few systematic application researches could be found on the controlling technologies of the content,size and distribution of periclase and preparation of HMPC.In this study,a new technique to quantitatively determine the content of periclase in cement is demonstrated.The relationship between the calcination temperature and the periclase content or the periclase size of HMPC is established,the mechanism of periclase controlling is demonstrated.The relationship between the periclase content,the periclase size and the later expansion increasing rate of HMPC is expounded.Finally,all the technologies have been verified in the HMPC production.The results are shown as follows:?1?A new technique to quantitatively determine the content of periclase in cement is demonstrated,and the problem to distinguish the fused MgO and the periclase has been solved.The technique is established by developing a periclase internal standard substance?PISS?,discussing the test principles of three methods,including the autoclave test,the thermogravimetric analysis,and the chemical analysis to match the test procedures to each other,optimizing the test conditions and mastering the relationship between these methods and the periclase content.In this new technique,an analytical grade reagent such as magnesium carbonate basic?Mg?OH?₂·3MgCO₃·3H₂O?or magnesium hydroxide?Mg?OH?₂?calcined at 1400?is recommended to be the PISS;when the autoclave pressure is 2.2MPa and the autoclave time is 4h,the recovery rate of PISS is up to 99.92%?100.14%,indicating that the technique presents a good recovery effect and high accuracy.?2?The effects of calcination temperature,holding time,and cooling rate on the content,size and distribution of periclase in HMPC are systematically revealed,the relationship between the periclase content or the periclase size of HMPC and calcination temperature is established,and the mechanism of periclase control is demonstrated.Improving the calcination temperature,prolonging the holding time or reducing the cooling rate are beneficial to accelerate the movement and growth of periclase crystals in the liquid phase,which promote the increase of both content and size of periclase.The distribution of periclase in clinker calcined at 1350??1450? is uniform,and that at 1500?is relatively concentrated.During 1350??1500?,there is a linear relationship between the periclase content and calcination temperature,and an exponential function is used to express the relationship between the periclase size and the calcination temperature.The content and size of the periclase could be quantitatively controlled by adjusting the calcination temperature.When the calcination temperature is 1450°C,the HMPC has the highest strength on 471d,which is 86.57MPa.When the calcination temperature is increased,the later expansion increasing rate of HMPC increases.When the holding time is 60min or under the forced-air cooling condition,the HMPC has the highest later expansion increasing rate,which is 106.25%.?3?The relationship of the later expansion increasing rate of HMPC and the periclase content,the periclase size is established.During 1350??1500?,there is a linear relationship between the periclase size and the periclase content,and quadratic polynomial functions are used to express the relationship between the later expansion increasing rate and the periclase content or that between the later expansion increasing rate and the periclase size.When the HMPC clinker is prepared at 1400??1450?,with the holding time of 60min and the forced-air cooling condition,the size of the periclase is from 3?m to 7?m?mainly 5?m?7?m?and the HMPC has high late strength increasing rate and high late expansion increasing rate.?4?The technique to quantitatively determine the periclase content and technologies to control the periclase in HMPC have been applied in HMPC production.The results show that the MgO content of the HMPC is 4.10%,the periclase content is 2.25%,the periclase size is mainly 3?m?7?m,the distribution of periclase in clinker is uniform,and the later expansion increasing rate is up to 93.90%,which also shows the micro-expansion of the HMPC.The autogenous volume deformation of the concrete prepared with the HMPC at 540d is 22.1×10^-6,and other properties are all in good condition.