| Magnesium oxide expansive admixture has been widely used to compensate the shrinkage of mass concrete,especially in marine engineering like dam concrete.Mg O expansive agent has controllable reacting speed,and it needs less water to react to the magnesium hydroxide,which has stable chemical properties and low solubility.These advantages of Mg O make it have good application prospect in civil buildings.Magnesium oxide expansion agent used in dam concrete is mainly used to compensate for temperature shrinkage in the later period.While we hope the magnesium oxide expansion agent used in construction can expand as early as possible to avoid the impact on the long-term performance of the concrete in the later period.Nano-Mg O and micro-Mg O have the same reaction mechanism.But the nano-Mg O has some unique properties from ordinary micro particles because of the nanometer scale.This paper discusses the effect of different Mg O particle sizes(nanoscale/microscale),the dosage of micro-Mg O and nano-Mg O on the cement paste,by studying fluidity,fluidity loss over time,mechanical properties,restrained expansion,dry shrinkage and the microscopic testing methods including hydration heat,thermogravimetry(TG),mercury intrusion porosity(MIP),X-ray diffraction(XRD)and Scanning electron microscope(SEM).The results show that:(1)With the increase of dosages and the decrease of diameter of Mg O expansive agent,the initial fluidity of pastes decline and loss of fluidity over time of cement pastes increase.Nano-Mg O makes large influence on the fluidity and fluidity loss over time because its large specific surface area need more water to wet and will absorb a large amount of polycarboxylate superplasticizer.At the same dosage,the order of influence on fluidity and fluidity loss over time is as follows: paste with nano-Mg Ois larger than paste with both micro-Mg O and nano-Mg O,while paste with micro-Mg O is the smallest.(2)Incorporating Mg O expansion agent will increase the hydration rate and the final heat released of the cement paste.The cement cementing system containing Mg O still conforms to the Krstulovic-Dabic model,and undergoes three reaction stages of NG,I,and D.With the increase of the amount of Mg O,the degree of hydration of the cement pastes increases accordingly.Due to the greater activity and smaller particle size,nano-Mg O prolongs the stage I,which can increases the degree of cement hydration.(3)Incorporation of magnesium oxide expansion agent will adversely affect the mechanical properties of cement mortar.The flexural strength and compressive strength of cement mortars increase first and then decrease.Compared to the pure cement mortar(C),the flexural strength and compressive strength of the cement containing eight percent mortar(M8)are respectively increased 1.05 times and 0.93 times.At the same dosage,cement pastes containing micro-Mg O and nano-Mg O do not have better mechanical properties.(4)The restrained expansion rate of the cement mortar containing Mg O increase with the increase of Mg O dosages,the decrease of particle size and the growth of curing age.The mortars containing micro-Mg O did not show expansion convergence after 42 days,while mortars containing nano-Mg O was restricted due to its small particle size,large specific surface area,and high reactivity.The effect of expansion rate was mainly manifested before 7 days.Before 28 days,the restrained expansion rate of the mortars containing nano-Mg O and the mortars containing micro-and nano-Mg O was larger,but after 28 days,the restrained expansion rate of the mortars containing micro-Mg O was larger.(5)Adding Mg O expansion agent can better compensate the drying shrinkage of cement mortar,and the compensation effect increases with the increase of magnesium oxide content.When the content exceeds 8%,the internal micro-cracks caused by expansion will increase the drying shrinkage of the mortar.The mortars containing nano-Mg O and micro-Mg O have similar compensation effects for drying shrinkage within 28 days. |
