| Due to its many excellent properties that traditional cement mortars do not have,polymer modified cement mortars are often used as special mortars,such as ceramic tile adhesive,repair mortars,self-leveling mortars,grouting materials and so on.In order to achieve rapid setting and early strengths required by some projects,Portland cement-sulphoaluminate cement composites are usually used as binders in these special polymer modified cement mortars.The physical and mechanical properties of the binders have been widely studied,but the long-term mechanical properties are ignored.During the service of polymer modified cement mortars,environmental temperature has a significant effect on the performances,especially flexural strength,compressive strength,tensile bond strength and volume stability.However,the existing studies mainly focus on the above mechanical strengths within 28 d,and rarely involve the long-term evolution of properties.Hydroxyethyl methylcellulose(HEMC)as a watersoluble polymer is commonly used to improve the workability of cement mortars.HEMC has obvious air entrainment effect and also significantly affects the early mechanical properties of cement mortars.However,there are few studies on the longterm mechanical strengths of cement mortars with HEMC.To manifest the long-term evolutions of properties of cement mortars,a typical HEMC was used in this experiment and the key performances of cement mortars,including flexural strength,compressive strength,tensile bond strength and drying shrinkage,were systematically studied.Firstly,the paper studied the above key performances and hydration characteristics of HEMC modified cement mortars within720 d under 20℃ normal temperature,5℃ low temperature and 40℃ high temperature.Secondly,with controlling the air content,the influence of HEMC on the above key performances and hydration characteristics of cement mortars was studied.Thirdly,the influence of HEMC on the above long-term key performances and hydration characteristics of cement mortars with Portland cement-sulphoaluminate cement(PCCSA)as binders was explored.Finally,the relationship between the above key performances and hydration characteristics of HEMC modified cement mortars was discussed,and the controlling mechanism of different performances were explored.The following conclusions were obtained.The long-term evolution of key performances of HEMC modified cement mortars cured at 20℃ normal temperature,5℃ low temperature and 40℃ high temperature was systematically studied.It was found that at 20℃ normal temperature and 5℃ low temperature cured for 28 d-720 d,and 40℃ high temperature cured for 3 d-90 d,the flexural strength decreased gradually with the content of HEMC increasing from 0% to1%,and reached the lowest value at 1% HEMC.Consequently,with the continuous increase of HEMC content to 3%,the flexural strength gradually increased,still to be lower than that of the control.At 40℃ high temperature cured for 180 d,with the content of HEMC increasing from 0% to 3%,the flexural strengths gradually decreased and the content variations of HEMC presented a limited effect.When the curing ages prolonged to 360 d and 720 d,0.1-0.5% HEMC had a positive effect on the flexural strength development.In terms of compressive strength,with the content of HEMC increasing from 0% to 1%,the compressive strength of cement mortars cured at the above three temperatures for 28 d-720 d gradually decreased,and reached the lowest value at 1% HEMC.Afterwards,with the continuous increase of HEMC content to 3%,the compressive strength gradually increased,still to be lower than that of the control.In terms of tensile bond strength,with the content of HEMC increasing from 0%to 3%,the tensile bond strength of cement mortars cured at 20℃ normal temperature for 3 d-720 d gradually increased.Meanwhile,compared with that cured for 28 d,0.1-0.5% HEMC improved the tensile bond strength of cement mortars cured for 90 d more significantly.Compared with that cured for 90 d,0.5-3% HEMC improved the tensile bond strength of cement mortars cured for 180 d more significantly.The optimum content of HEMC for tensile bond strength of cement mortars cured at 5℃ low temperature for 28 d-720 d was 0.3-0.5%.Cured at 40℃ high temperature for 3 d-90 d,with the content of HEMC increasing from 0% to 3%,the tensile bond strength of cement mortars gradually increased.At 180 d-720 d,the optimum content of HEMC for tensile bond strength was 0.5-1%.In terms of drying shrinkage,at 1 d to 720 d,the drying shrinkage values of cement mortars cured at three different temperatures gradually increased with the HEMC content increasing from 0% to 3%.Meanwhile,the drying shrinkage values of cement mortars cured at 5℃ low temperature presented a slight increase with the content of HEMC increasing from 0 to 0.5% or from 1% to 3%.The drying shrinkage values of cement mortars cured at 40℃ high temperature presented a slight increase with the content of HEMC increasing from 0 to 0.3% or from 1% to 3%.The long-term hydration process,formation of hydrates,pore structure and interface structure of HEMC modified cement mortars cured at these above three temperatures were studied.It was found that HEMC mainly delayed the early hydration process of cement pastes cured at the three temperatures.The greater the content of HEMC,the more obvious the delaying effect.HEMC also delayed the early formation of calcium hydroxide(CH)and the transformation of ettringite(AFt)into monosulfate(AFm).HEMC brought the crystal size of AFt shorter and stronger.With the curing age prolonging up to 720 d,the delaying effect of HEMC on hydration process and the formation of CH gradually weakened.The delaying effect on the transformation of AFt into AFm was also gradually weakened.With the content of HEMC increasing from 0%to 1%,the content of macro pores in cement mortars significantly increased and the content of connected pores gradually decreased.Afterwards,with the content of HEMC increase to 3%,the pore structure was improved,including the decrease of the content of macro pore and the total porosity.Definitely,HEMC improved the interfacial transition zones between cement mortars and concrete substrates,but weakened the interfacial transition zones between cement pastes and aggregates.The evolutions of key performances and hydration characteristics of HEMC modified cement mortars by controlling the air content were revealed.By controlling the same air content(the same with the control,6.5%),0%-1% HEMC had a positive effect on the 7-d and 28-d flexural strengths and tensile bond strengths of cement mortars,but had a negative effect on 3 d-28 d compressive strengths and increased the drying shrinkage values.With the content of HEMC increasing from 0% to 1%,the hydration degree of cement pastes gradually decreased,the total pore volume and porosity of cement mortars presented a slight increase,and the content of connected pores in cement mortars gradually decreased.With the same HEMC content,compared with those of cement mortars without adjusting the air content through adding defoamers,the flexural strength and compressive strength of cement mortars with the air content of 6.5% were significantly improved,the tensile bond strengths were also further improved,and the drying shrinkage values decreased significantly.The hydration degree of cement pastes was basically the same.However,the content of large capillary pores,macro pores and porosity of cement mortars decreased significantly,while the content of connected pores presented a slight increase.By incorporating different contents of defoamers into cement mortars with 0.1%,0.3% and 0.5% HEMC,the air content of the cement mortars decreased in a gradient.Accordingly,the flexural strength and compressive strength increased gradually and the drying shrinkage values presented a slight decrease.The hydration degree of cement pastes presented a slight decrease.But the content of large capillary pores,macro pores and porosity of cement mortars decreased gradually.The long-term evolutions of different HEMC contents on the key performances of cement mortars with three PC-CSA as binders was explored.The PC-CSA cement mortars were 85% PC-15% CSA,70% PC-30% CSA and 50% PC-50% CSA,respectively.Whether incorporating HEMC or not,the early flexural strength and compressive strength of these cement mortars were all high.When the content of CSA in the binders was low,the flexural strength and compressive strength were high at the mid-late age.The effect of HEMC on the strengths was also different with the content variation of CSA in the binders.In 85% PC-15% CSA cement mortars,HEMC significantly decreased the 28 d-720 d flexural and compressive strengths,while HEMC had relatively little effect on the 28 d-720 d strengths of 70% PC-30% CSA and 50%PC-50% CSA cement mortars.In different PC-CSA cement mortars,with the content of HEMC increasing to0.6%,the tensile bond strength all gradually increased.At the same HEMC content,the tensile bond strengths of 85% PC-15% CSA cement mortars were all higher than those of 70% PC-30% CSA.Whether incorporating HEMC or not,with the content of CSA in the binders increasing,the drying shrinkage values decreased gradually within 720 d.The effect of HEMC on the drying shrinkage was also different with the content variation of CSA in the binders.In 85% PC-15% CSA cement mortars,with the content of HEMC increasing,the drying shrinkage values increased gradually,which was similar to that of cement mortar with Portland cement as binders cured at 20℃.In 70%PC-30% CSA and 50% PC-50% CSA cement mortars,HEMC significantly decreased the drying shrinkage values but the content of HEMC presented a slight effect.The effects of different HEMC contents on the hydration characteristics of cement mortars with PC-CSA composites as binders were also studied.It was found that HEMC obviously delayed the hydration heat release in the acceleration and deceleration period,but promoted the hydration heat release in the stable period.With the increase of CSA content in the binders,the retarding effect of HEMC on the hydration heat release in acceleration period and deceleration period decreased gradually,the early hydration heat release gradually increased,but the 72-h hydration heat release gradually decreased.With the increase of CSA content in the binders,the content of AFt increased gradually but the content of CH gradually decreased.HEMC delayed the transformation of AFt to AFm.HEMC significantly delayed the formation of CH within 28-360 d.HEMC significantly increased the content of large capillart pores,the content of macro pores and porosity in these three PC-CSA cement mortars,and significantly reduced the content of connected pores.With the increase of CSA content in the binders,the content of macro pores decreased significantly,and the porosity and the content of connected pores presented a slight decrease.The relationship between flexural strength,compressive strength,tensile bond strength,drying shrinkage values and hydration characteristics of HEMC modified cement mortars was put forward.At above three curing temperatures,the flexural strength and compressive strength had a linear positive correlation with the content of CH,but had a linear negative correlation with the content of macro pores and porosity.The tensile bond strength had a linear positive correlation with the content of CH and the content of large capillary pores.The drying shrinkage value had a linear positive correlation with the content of CH,the content of large capillary pores,the content of macro pores and the porosity.The correlation between the flexural strength and pore structure of cement mortars with the same air content was relatively weak.The compressive strength had a linear negative correlation with the content of large capillary pores and porosity.The tensile bond strength had a positive linear correlation with the content of large capillary pores.The drying shrinkage value had a positive linear correlation with porosity.The flexural strength and compressive strength of cement mortars with different air contents had a linear negative correlation with the content of large capillary pores,the content of macro pores,and porosity.The drying shrinkage value had a linear positive correlation with the content of large capillary pores,the content of macro pores,and porosity.In the PC-CSA cement mortars,the flexural strength and compressive strength had a linear positive correlation with the content of CH,but had a linear negative correlation with the content of macro pores and porosity.The tensile bond strength had a positive linear correlation with the content of CH and the content of large capillary pores.The drying shrinkage value had a positive linear correlation with the content of CH,but a negative linear correlation with the content of medium capillary pores.On the basis of the above researches,the controlling measures of key performances of the cement mortars were preliminarily proposed,including under the different curing conditions of 20℃ normal temperature,5℃ low temperature and 40℃ high temperature,controlling the air content,and the cement mortars with the PC-CSA as binders.In general,under the above conditions,to achieve higher flexural strength,compressive strength and tensile bond strength,the degree of cement hydration should be increased,and the content of macro pores and the porosity should be reduced.To achieve a lower drying shrinkage value,the degree of cement hydration should be decreased,and the content of large capillary pores,the content of macro pores and porosity should be reduced. |
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