Study On The Mechanism Of Alkali Dissolution And Degradation Of Alkali Activated Slag Under Sulfate Attack

Research on the mechanism of alkali dissipation and deterioration in sulfate erosion alkali activated slag cementitious system with the increasingly widespread application of concrete in fields such as bridges,tunnels,high-rise buildings,and marine engineering,people’s requirements for concrete have become increasingly high,and its durability has always been a hot topic of concern.In saline alkali areas,oceans,and some underground areas,the content of sulfate is abundant,and concrete is prone to sulfate erosion in these areas.As a new type of cementitious material with high strength,alkali activated slag,as a kind of silicate aluminate activated by strong alkaline solvent,is different from Portland cement-based material in composition,performance and hydration products,so its sulfate erosion characteristics and laws are also different from cement-based materials.Therefore,exploring the laws and internal erosion mechanisms of alkali activated slag cementitious materials under sulfate erosion conditions has become the key to solving the problem of sulfate erosion.This article studies the changes in the apparent morphology,compressive strength,mass loss,and sulfate ion distribution of alkali slag with the age of sulfate corrosion through sulfate corrosion tests,revealing the mechanism of alkali dissolution and degradation of alkali slag.Therefore,this article has conducted exploratory and innovative work in the following aspects:(1)Study on alkali dissipation and degradation process of alkali activated slag under sulfate erosion.By measuring the apparent morphology,compressive strength,mass loss,and sulfate ion distribution of alkali activated slag at different corrosion ages under full immersion conditions of sulfate(sodium sulfate and magnesium sulfate solution),the degradation process and law of alkali activated slag under full immersion sulfate corrosion conditions were studied.The research results indicate that the appearance and morphology of alkali activated slag specimens remain basically intact during the oneyear erosion age,but slight signs of deterioration appear in the later stage of erosion(300d),and cracks appear earlier and more frequently in the magnesium sulfate solution,with a larger range of edge shedding.In terms of compressive strength,the specimens showed a pattern of first increasing and then decreasing,with the mid-term growth rate of the specimens in sodium sulfate solution being more significant during the stage of improving compressive strength;During the decline stage of compressive strength,the decrease rate of the specimen in magnesium sulfate solution is slower.The corrosion resistance coefficient of the alkali activated slag specimen is higher than 8.0,indicating that it has strong resistance to sulfate corrosion.In terms of quality,under different soaking ages,alkali activated slag was removed from its own hydration factors,and the quality of the specimen was significantly improved in sodium sulfate solution.The growth rate of specimen quality in magnesium sulfate solution first increases and then decreases.In terms of sulfate ion distribution,the sulfate ion content inside the alkali activated slag specimen increases with the increase of erosion time and decreases with the increase of sampling depth.(2)Study on surface analysis degradation process of alkali activated slag sulfate erosion under semi-immersion.By measuring the changes in surface morphology,compressive strength,mass loss,and distribution of sulfate ions in different areas of alkali activated slag under sulfate(sodium sulfate and magnesium sulfate solution)semi-immersion conditions,the degradation process and rules of alkali activated slag under sulfate semi immersion erosion were studied.The results show that the surface morphology of alkali activated slag specimens under sodium sulfate erosion is more severely damaged in the dry area and crystallization area than magnesium sulfate,and the magnesium sulfate solution in the immersion area is more significantly damaged.In terms of compressive strength,there is a certain degree of decrease in the dry and crystalline areas of sodium sulfate semi-immersed specimens,while there is a significant increase in the soaking area.However,there is a certain degree of decrease in the dry,crystalline,and soaking areas of magnesium sulfate semi-immersed specimens.The corrosion resistance of different areas of the specimen varies,and the corrosion resistance of the immersed area is stronger than that of the non-immersed area.In terms of quality,under different soaking ages,alkali activated slag can remove its own hydration factors.Whether in sodium sulfate or magnesium sulfate solutions,the quality of alkali activated slag increases in the early stage and decreases in the later stage,and the increase is much smaller than the decrease.In terms of sulfate ion distribution,after soaking in sodium sulfate solution,the barium sulfate content range of the specimens at different depths is 0.8%～4.5%,and after soaking in magnesium sulfate solution,the range is 0.7%～3.6%,indicating that the ions in the sodium sulfate solution immerse into the specimens faster.(3)Study on alkali dissipation degradation mechanism of alkali slag under sulfate attack.By means of X-ray diffraction analysis,scanning electron microscopy,and other detection methods,the microstructure,material composition,and elemental changes of sulfate eroded alkali activated slag under full immersion conditions were studied.The internal product changes of alkali activated slag were analyzed,revealing the mechanism of alkali degradation of alkali activated slag under sulfate erosion.The results show that alkali activated slag generates ettringite and anhydrous mirabilite under the action of sodium sulfate,and the internal alkalinity remains stable;Gypsum,ettringite and brucite are formed under the action of magnesium sulfate,and the decrease of internal alkalinity causes the decomposition of alkali activated slag hydration product C-A-S-H gel.(4)Study on surface degradation mechanism of semi-soaked alkali slag by sulfate attack.By measuring the internal moisture distribution of alkali activated slag specimens through specimen breakage,the internal sulfate transport law of alkali activated slag specimens was studied,and the surface degradation mechanism of alkali activated slag under sulfate semi-immersion erosion conditions was analyzed.The research results indicate that there is an open parabolic wetting line inside the alkali activated slag under sulfate semi-immersion.With the extension of immersion time,the wetting line climbs up along the edge.As the surface moisture evaporates,the sulfate concentration at the edge of the alkali activated slag specimen increases,leading to salt crystallization.The accumulation of internal crystals gradually increases,and the tensile stress on the alkali activated slag exceeds the limit tensile stress,causing expansion cracking.The thesis has 41 diagrams,26 tables and 93 references.