Research On Service Performance And Pore Solution Of Alkali-activated Slag Concrete Under Sulfate Drying And Wetting Cycles

Alkali-activated cementitious material is considered to be the most promising environment-friendly cementitious material in the 21st century with outstanding performance,economic and ecological advantages.A comprehensive understanding of the durability and characteristics of liquid phase in alkali activated slag concrete is vital important to study the long-term service performance and application.Sulfate attack is an important aspect of durability research.There has not formed a unified conclusion about the sulfate attack mechanism and degradation of alkali activated slag cementitious materials,therefore it is particularly important to study the mechanism of the degradation in order to better evaluate the degree of degradation in practical use.The alkalinity and composition are important liquid phase characteristics in alkali activated slag pore solution,and the variation of alkalinity and composition has important guidance to the research of chloride ion transportation and reinforcement bars corrosion in alkali activated slag.In this paper,the performance degradation mechanism under wetting and drying cycles in sulfate attack environment and pore solution of alkali activated slag concrete has been studied.The sodium hydroxide and water glass were chosen as alkali activators,taking into account the influence of alkali equivalent,modulus of water glass and fly ash,the change of strength,appearance and weight of alkali activated slag concrete under sulfate wet-dry cycle were studied,and the microstructure and hydration products were characterized by SEM?EDS?,XRD and FTIR.The alkalinity and composition of pore solution under different hydration ages and attack conditions were also studied.The main conclusions are summarized in the follow:1.After 150 cycles,the appearance of samples water glass is better than activated by sodium hydroxide,and the incorporation of fly ash leads to more severe degradation.The improvement of the alkali equivalent,from 7%to 9%,can improve the resistance of samples to degradation,whereas the modulus of water glass has no effect.During the whole period of sulfate dry-wet cycling,the compressive strength and dynamic elastic modulus of concrete increased first and then declined.The corrosion coefficient of compressive strength of concrete activated by water glass is higher than that by sodium hydroxide.The addition of fly ash reduces the compressive strength and relative dynamic elastic modulus,and the fly ash is much more difficult to be activated by water glass.The alkali equivalent increased from 7%to 9%,which had little effect on the ultimate compressive strength and relative dynamic elastic modulus of concrete activated by sodium hydroxide,however,it would greatly improve the compressive strength of samples activated by water glass.The samples activated by water glass had better mechanical properties and more excellent resistance to sulfate attack than activated by sodium hydroxide.2.The dry-wet cycling times,alkali equivalent,activator and fly ash have some influence on the sulfate transportation ability.The sulfate diffusion coefficient of samples activated by sodium hydroxide is much higher than activated by water glass.With the dry-wet cycling,the sulfate diffusion coefficient of samples activated by two activators mentioned before.The sulfate diffusion coefficient decreased when the alkali equivalent increased.And the increase of alkali equivalent can significantly reduce the diffusion coefficient of sulfate,however,the increase of modulus of water glass and the incorporation of fly ash lead to the increase of diffusion coefficient.With the dry-wet cycling times increased,the diffusion coefficient of samples activated by sodium hydroxide increased,and the diffusion coefficient of samples activated by water glass increased first and then decreased.3.The hydration products of alkali activated slag did not change after the sulfate wet cycle,and the hydration products were related to the type of activator and the mixing of fly ash.4.Unreacted slag would further hydrate in the wet-dry cycling,filling the pores.The crystallinity of C-S-H in slag activated by sodium hydroxide is higher than activated by water glass,and that is more obvious when fly ash added.After sulfate dry-wet cycling,the existence of ettringite in alkali activated concrete has not being detected,and the chemical deterioration did not occur,and the main reason of deterioration is the crystallization pressure which can destroy the pore wall and cause the specimen surface to be powder.5.The alkalinity of the pore solution in alkali-activated slag decreased with the increase of age,and the alkalinity of samples activated by sodium hydroxide is higher than that of water glass.The addition of fly ash has little effect on the alkalinity of pore solution,and the increase of alkali equivalent increases the alkalinity.The concentration of Na⁺?K⁺?Ca²⁺in pore solution decreased with the increase of curing age,and the concentration of Mg²⁺was lower than the level it can be detected.The concentration of Na⁺in pore solution was always in a high level,whereas K⁺and Ca²⁺was in low level,and the concentration of Na⁺in samples activated by sodium hydroxide is higher than activated by water glass.The addition of fly ash leads to the decrease of Na⁺concentration in samples activated by sodium hydroxide,and the modulus of water glass and the addition of fly ash in the sample activated by water glass have no effect on the Na⁺concentration.6.Under the corrosive conditions,the alkalinity of pore solution in sodium hydroxide activated slag is higher than that of slag activated by water glass.And the content of Cl^-,SO₄^2-in the pore solution activated by sodium hydroxide activated slag is lower than that activated by water glass.In the environment of chloride corrosion,the alkalinity and Cl^-content of the pore solution activated by sodium hydroxide increased with the increase of exposure time,and the SO₄^2-decreased with time expand.The content of Cl^-and SO₄^2-in pore solution was significantly changed with the addition of fly ash.In the composite attack environment,the content of Cl^-,SO₄^2-in the pore solution activated by sodium hydroxide is greatly affected by fly ash.The content of Cl^-,SO₄^2-in the pore solution of the samples was mainly affected by modulus,and the content of Cl^-,SO₄^2-decreased obviously when the modulus of water glass increased.