The Study Of Hydration And Microstructure Of Alkali-activated Slag Cements By Impedance Spectroscopy

Alkali-activated binders have the advantages of much lower environmental impaction, the reducing greenhouse effection and the lower energy consumption. Alkali-activated materials are important environmental binders and have promising prospects. Meanwhile, impedance spectroscopy measurements have close relations with micro structures of cement-based materials and can provide more reliable and more accurate information on pore structures, porosities and pore solutions concentrations. The method is expected to be a sensitive and a non-destructive test method. Hence, impedance spectroscopy measurements with non-destruction can be used to research the hydration characteristics of alkali-activated slag cements in further and promote the application of alkali-activated slag cements.This paper was mainly focused on the influence of modulus of waterglass and alkali content on the hydration and the impedance characteristics of alkali-activated slag cements and analyzed the changes of pore structure, pore solution and circuit elements with the modulus of waterglass and alkali content in the system. This experiment adopted the water/binder ratio of 0.35,modulus of waterglass of 0,0.5,1.0 and 1.5 and the amount of alkali content of 4%,6% and 8% which was based on the mass percent of Na₂O to the total quality of binders.A complete high frequency arc was obtained in the frequency between 40Hz and 35MHz in impedance spectroscopy measurements and the diameter of arcs was decreased with the increase of modulus of waterglass and alkali content. Equivalent circuit model proposed in this paper had a good fitting with the measured Nyquist plots, where R₁ and R₃ represented the resistances of continuous conductive paths and the discontinuous conductive paths respectively, n2 was the constant phase angle index related to the capillary pores in nonconducting solid phase while n3 was the constant phase angle index related to the gel pores in hydration products.Results showed that R₁ and R₃ increased with the prolonged hydration time and R₁ decreased with the increase of modulus of waterglass and alkali content under the influence of early hydration rates and ion concentration of pore solution in alkali activated slag cements. The volume of 5-10nm gel pores to cement pastes was increased with the addition of modulus of waterglass and alkali content. Thus, the number of gel pores was increased and pore structure was more complicated which led to the increase of R₃ with the addition of modulus of waterglass between 0.5 to 1.5 and the alkali content. n2 was increased with the prolonged hydration time and was decreased with the increase of modulus of waterglass and alkali content. n2 was related to the depression of Nyqusit plots below the real axis. Separation the composition of Nyquist plots exactly were contributed to the analysis of the constant phase angle index. n3 was increased in the early hydration time and then was almost unchanged in the range of 0.93-0.99 when the modulus of waterglass were 0.5-1.5 and the alkali content were 4%-8%. n3 was changed between 0.72 and 0.75 in NaOH activated slag cement. The difference of n3 in waterglass activated slag cements and NaOH activated slag cements may be related to the difference of dydration products. The constant phase angle index n2 relating to pore diameters in range of 10-104nm was smaller than the constant phase angle index n3 relating to pore diameters in range of 5-10nm in waterglass activated slag cements no matter what the alkali content were. However, n2 and n3 were almost the same in NaOH activated slag cement.