Study On The Damage Mechanism Of Calcium Oxide-sodium Carbonate-slag Mortar Under The Effect Of Freeze-thaw And Sulfate

Calcium oxide-sodium carbonate-slag(CNS)cement is a new type of alkali-activated slag cement.Its shrinkage and setting time etc.have been improved,and the cost has also been reduced.Therefore,mortar with strength of 35 MPa(CNS35)and 50 MPa(CNS50)were prepared used the CNS alkali-activated slag cement in this paper,and the normal Portland cement mortar with strength of 35 MPa(PC35)is used as control group.The deterioration of macro-performances(apparent,quality,dynamic elastic modulus,flexural and compressive strength)of CNS mortar under the effect of freeze-thaw cycle(single factor)and freeze-thaw and sulfate(double factors)were studied respectively.Meanwhile,the deterioration mechanism was analyzed by using microhardness,pore structure,XRD,SEM-EDS and so on.Chief laws and results are summarized as follows:1.Under the effect of single factor and double factors,the deterioration of macro-performances of the mortar is slow in early stage(deterioration accumulation period)and accelerate in later stage(deterioration acceleration period)with the increase of freeze-thaw cycle.The sequence of resistance to freeze-thaw is:CNS50>CNS35>PC35.Under the presence of the sulfate,the deterioration of CNS mortar is decreased,but the deterioration of PC mortar is increased.2.Under the effect of single factor and double factors,both the microhardness of paste matrix and interface were reduced with the increase of freeze-thaw cycle.After 300 times of single freeze-thaw,the deterioration of the microhardness of paste matrix and 10μm from the interface of CNS50 mortar decreased 21.8%and 7.1%respectively.CNS35 mortar decreased 34.8%and 22.4%respectively.However,PC35 mortar decreased 44.6%and 55.1%respectively.After 300 times of double factors effect of freeze-thaw and sulfate,the deterioration of the microhardness of paste matrix and 10 μm from the interface of CNS50 mortar decreased 14.7%and 1.7%,which is less than the deterioration under single factor.CNS35 mortar decreased 28.4%and 19.6%,which is also less than the deterioration under single factor.PC35 decreased 47.1%and 60.7%which is also greater than the deterioration under single factor.3.Under the effect of single factor and double factors,with the increase of freeze-thaw cycle,the porosity and total pore volume of mortar increased gradually,the most possible aperture moved towards the macro-pore,the pore structure deteriorated gradually.And the deterioration of pore structure coincided with the macro-performance and microhardness.So the three grade deterioration model between porosity(micro),microhardness deterioration coefficient of paste matrix(fine micro)and flexural deterioration coefficient(macro)was established.It shows that both the microhardness deterioration coefficient of paste matrix and the flexural deterioration coefficient increase with the increase of CNS mortar porosity.They form a space curve which is similar to straight line and there was a good correlation between them.4.The main hydration products of CNS cement are C-S-H(Ⅰ)gel with low Ca/Si ratio,hydrotalcite,hydrotalcite-like,sodium carbonate etc.and no Ca(OH)2 grain.The structure of CNS mortar are more compactness and uniform because of the filling effect of carbonate.So there is no obvious interface transition zone,and the pore structure is more excellent.Thus the deterioration of CNS mortar is less than that of PC mortar under the effect of single factor and double factors.The compactness of the internal structure and pore structure of CNS50 mortar are better than those of CNS35 mortar.Thus the deterioration of CNS50 mortar under the effect of single factor and double factors is less than that of PC mortar under the effect of single factor and double factors.