Effect Of Sodium Hexametaphosphate On Dispersion And Hydration Kinetics Of Cement-MEA-PCE Pastes

As a new type of expansive agent,MgO expansive agent（MEA）is more and more widely used in engineering because of its advantages of stable hydration products,adjustable expansion performance and compensation for late shrinkage of concrete.However,MEA will produce many inner holes during the firing process,which will make the fluidity of the slurry worse,which is manifested as the loss of concrete slump over time,especially in summer.In order to improve the adverse effects of MEA on the fluidity of cement paste,sodium hexametaphosphate（SHMP）was used as a fluidity regulator.The effects of SHMP content on the rheological properties,adsorption and dispersion properties,hydration properties,mechanical properties and expansion properties of Cement-MEA-PCE paste were systematically studied.The main conclusions are as follows:（1）The addition of MEA will reduce the fluidity of the plasticized slurry,and the fluidity will be further reduced with the increase of MEA content.Compared with the reference group,the initial fluidity of 6%MEA,12%MEA and 18%MEA decreased by 19.2%,30.8%and42.3%,respectively.PCE will improve the fluidity of Cement-MEA slurry,but MEA will adsorb PCE in the presence of MEA,which will lead to the deterioration of the slump retention performance of the slurry,the increase of PCE dosage and the increase of cost.Especially in the high temperature environment in summer,the effect of PCE will further deteriorate.Among the two kinds of PCE selected in this paper,the water reducing effect of large water reducing agent（PCE1）is better,but the fluidity retention ability is relatively poor.The water reducing effect of comprehensive water reducer（PCE2）is poor,but the fluidity retention ability is relatively good.（2）The addition of SHMP further improved the fluidity of Cement-MEA-PCE paste.The fluidity of paste and mortar at each time point is 1.4%～17.4%higher than that of the reference group.The fitted rheological parameters show that the addition of SHMP reduces the yield stress and consistency of the composite system at each time point,which is consistent with the change rule of fluidity.Due to the competitive adsorption between the strongly polar phosphate group and PCE,the adsorption capacity of PCE decreased after adding SHMP.However,SHMP increases the thickness of the adsorption layer of the composite system.This is because the phosphate is bridged with PCE under the bridging effect of Ca²⁺and Mg²⁺,forming an internal phosphate（multilayer）-external water reducing agent（single layer）structure.This may be the main reason why the addition of SHMP reduces the rheological parameters and improves the fluidity of the slurry.SHMP has obvious dispersion effect on Cement-MEA-PCE slurry,whether MEA is large or engineering dosage,but the improvement of fluidity by SHMP does not double with the increase of its dosage.The test results also found that when a small amount（0.8%）of PCE was added to the slurry,SHMP showed a more pronounced dispersion effect.（3）The Krstulovic-Dabic model can be well applied to the SHMP-Cement-MEA-PCE composite system.The addition of SHMP will not change the hydration mechanism of the composite system.The hydration reaction process will go through three stages:nucleation and crystal growth（NG）,interactions at phase boundaries（I）and diffusion（D）.The addition of SHMP will reduce the hydration heat release rate and heat release of the composite system,reduce the second exothermic peak value,and change the morphology from“high and narrow”to“flat and wide”.With the increase of SHMP content,the hydration kinetic parameters n,K₁’,K₂’and K₃’decrease continuously.When the temperature increases from 20°C to 60°C,the hydration heat release rate and heat release of the composite system increase,the end time of the induction period and the second exothermic peak appear earlier,the peak increases,and the peak width becomes narrower.With the increase of temperature,the hydration kinetic parameters n,K₁’,K₂’and K₃’increase continuously.At 20°C and 40°C,the hydration process of the composite system undergoes three processes of NG-I-D,and at 60°C,only two processes of NG-D are experienced.At 40°C,the time of I process control is greatly shortened,and no I process is experienced until 60°C.（4）The addition of SHMP can improve the compressive and flexural strength of MEA mortar.The greater the amount of SHMP,the greater the compressive and flexural strength.Compared with the reference group,the compressive strength increased by 0.6%～12.7%,and the flexural strength increased by 1.3%～7.4%.Regardless of 9%MEA or 18%MEA,the addition of SHMP reduces the limited expansion rate of the hardened paste of the composite system.According to the TG results,the addition of SHMP inhibits the formation of Mg（OH）₂,resulting in the limited expansion rate of the test group with SHMP.The limited expansion rate of the low MEA content（9%）test block decreased from 6.1×10^-4 to 5.5×10^-4,5.4×10^-4,and the limited expansion rate of the high MEA content（18%）test block decreased from 14.3×10^-4 to12.8×10^-4,12.4×10^-4.In this paper,it is considered that the dosage of SHMP should not be too high.For the selected M-MEA,when the dosage is 0.15%,it can not only improve the fluidity of Cement-MEA-PCE slurry,but also not significantly reduce the limited expansion performance,which is a more suitable dosage.