Effect Of Sulfate Mixing On Hydration Kinetics And C-S-H Microstructure Of FA-C Pastes

In China,sea water and groundwater are using in the preparation of concrete in the South China Sea Islands with lacking of fresh water and the Western Region with the high salinity enviroment.As the formation and evolution mechanism the of microstructure in cementitious pastes under sulfate attack is not yet clear,this paper relies on"973"project"the formation and regulation of the long life of concrete under severe condition of microstructure"?2015CB655101?,²⁹Si NMR,²⁷Al NMR,SEM-EDS and XRD are used to explore the effect of sulfate mixing on the hydration kinetics and formation and evolution of microstructure of C-S-H gel in the FA-C composite paste.On this basis,C-?A?-S-H gels with different Ca/Si and Al/Si were synthesized,and the influence mechanism of sulfate on the thermodynamic stability of C-?A?-S-H was investigated by the thermodynamic simulation software GEMS.The main research results are as follows:The effect of sulfate mixing on the hydration kinetics of FA-C composite paste was studied.With the increase of Na₂SO₄ concentration in the mixing water,the exothermic rate and 3d heat of the pure cement paste and FA-C composite paste are increased,and the hydration degree of 3d to 180d is also increased,and the early hydration degree is higher than that in the later stage of hydration.The hydration process of Na₂SO₄ solution mixing pure cement paste and FA-C composite paste is:NG-I-D.Na₂SO₄ concentration mainly affected hydration and reaction order n of NG process and control time of I process.With the increasing of Na₂SO₄ concentration,hydration and reaction order n of NG process increases,I process control time is shortened,and the reaction rate constant of D process slightly increase.The mechanism of the effect of Na₂SO₄ solution mixing on the microstructure of C-?A?-S-H gel in FA-C composite paste was investigated.With the increase of Na₂SO₄concentration,the degree of polymerization of silicon oxygen tetrahedron,the main chain length?MCL?in pure cement paste increase,Al[4]/Si reduces,and the amplitude decreases with hydration time.Compared with that of the pure cement paste,MCL and Al[4]/Si of C-S-H gel increases with addition of 30%fly ash.With the increase of the mixing concentration of Na₂SO₄ solution,the MCL of C-S-H gel in FA-C composite paste is larger than that in the pure cement paste at the same age.5%Na₂SO₄ mixing solution FA-C composite paste?water cement ratio 0.35?under standard conditions after 7d and 28d curing were immersed into 5%Na₂SO₄ solution for 180d.The MCL of C-S-H gel increases,Ca/Si and Al[4]/Si decrease,and the change amplitude increases with the shortening of curing period.Na₂SO₄ erosion has the effect of decalcification and dealumination on C-S-H gel in FA-C paste.The mechanism of the influence of Na₂SO₄ erosion on thermodynamic stability of hydrothermally synthesized C-?A?-S-H has been investigated.Sulfate has the effect of decalcification on the synthesis of C-?A?-S-H gel,and promote the transformation of two silicon oxygen tetrahedron monomer to polymer gel,resulting in the larger MCL of C-?A?-S-H and the reduction of Ca/Si.However,the decreasing amplitude increases with the increase of Ca/Si.Polymerization degree of C-A-S-H increases with the increase of Al/Si and the decrease of Ca/Si.Sulfate attack stripped Al[4]from the C-A-S-H gel,resulting in a decrease in its Al/Si,promoting the removal of Al[4],Al[5],and Al[6]in TAH and AFm transforming into Al[6]in AFt,and with the Ca/Si decreased,the decrease in Al/Si decreased.The thermodynamic simulation results agree well with the test results.The reduction of Ca/Si and the increase of Al/Si can improve the resist ability of sulfate on the decalcification and dealuminium on C-?A?-S-H gel,and improve its thermodynamic stability.The research results of this paper can provide a reference for the design and preparation of high corrosion resistant concrete in the South China Sea Islands and West region with the high salinity environment.