Study On The Anti-carbonation Properties Of Solid Alkali-activated Fly Ash/Slag

Alkali-activated material is a new type of green cementitious material,because of its high strength,fast-setting advantages,alkali-activated material is widely used in various types of engineering construction in recent years.However,the conventional liquid alkali-activated agent is limited by its corrosive,not easy to transport storage problems,which greatly limit the development of the application of alkali-activated cementitious materials,and solid alkali-activated agent can avoid these construction problems.However,the lack of durability is another major disadvantages of alkali-activated cementitious materials.Under the various external environments,alkali-activated materials are easy to carbonization reactions that lead to performance degradation,so first of all,the use of solid activator alkali-activated materials（solid alkali-activated materials）ratio research,through orthogonal experimental method to obtain the best ratio The best ratio was obtained by orthogonal experiments.The carbonation resistance of solid alkali-activated materials under natural exposure and bottom immersion exposure conditions was then investigated,and the early performance,microstructure and internal chemical composition changes of the carbonation products were characterized by XRD,FTIR,SEM,TGA,nitrogen adsorption,etc.The carbonation products of solid alkali-activated materials under various exposure conditions were studied,and compared with the traditional liquid alkali The carbonation products of solid alkali activator materials under various exposure environments were investigated and compared with the conventional liquid alkali activator materials to analyze their carbonation reaction patterns and reaction mechanisms.The main conclusions reached in this paper are as follows:1.The orthogonal test found that the water-cement ratio has the greatest effect on the fluidity and setting time of solid alkali-activated fly ash/slag,and the slag content has the greatest effect on the strength of alkali-activated material.The orthogonal test obtained that when the ratio is 75%slag content（25%fly ash）,modulus is 1.2,alkali content is 6%,water-cement ratio is 0.45,and retarder dosing is 2%,the samples can obtain good fluidity and high early compressive strength at the same time.2.The absorption peaks corresponding to Si-O-T bonds under natural carbonation of solid alkali-activated samples are shifted to higher wave numbers more than those under soaked carbonation,which is caused by the lower Ca/Si in the gel after the natural soaked,indicating that the decalcification of the gel is higher than that under natural carbonation.3.The liquid alkali activator can react with the raw material more fully,so the reaction rate and total exotherm are higher than those of the solid alkali-activated samples,and the raw material participates in the reaction at a greater extent,and more gels are produced,so the porosity of the liquid alkali-activated samples is lower than that of the solid alkali-activated samples after 28d of maintenance,and therefore the strength of the liquid alkali-activated samples is higher than that of the solid alkali-activated samples before carbonation.4.The products after natural carbonation of the solid alkali-activated samples are dominated by sub-stable amorphous carbonates,with the presence of small amounts of calcite.The samples carbonated by immersion showed three different decomposition temperature peaks,and the decomposition temperatures corresponded to amorphous calcium carbonate,vaterite and calcite from high to low,respectively.5.The longer the age of the solid alkali-activated samples exposed to carbonation,the the degree of carbonation will be higher,the pore volume will be larger,and the compressive strength decreased gradually.Among the three different maintenance conditions,the pore volume increased the fastest in the soaked carbonation condition,followed by the natural carbonation group,because water entered the matrix through capillary pressure and evaporated on the surface,and a bottom-up water flow was formed inside,and the water flow carried the metal cations inside the matrix to the surface of the matrix and precipitated by reaction with CO₂,while the ion concentration inside decreased causing the decalcification reaction of the gel,resulting in the volume decreases and more micropores are formed.6.The early alkali-activated carbonation reaction occurs in the pore solution of the matrix,and the calcite and other crystals can fill the pores and improve the compressive strength,but as the carbonation reaction continues,the Ca²⁺ concentration in the pore solution decreases,and the C-S-H gel decalcifies to compensate for the Ca²⁺ concentration in the pore solution,and the volume of the decalcified gel becomes smaller and the pores increase,leading to a decrease in the strength of the long-term carbonated alkali-activated material.Therefore,the compressive strength of alkali-activated materials generally shows the trend of first increasing and then decreasing with exposure time.7.Under natural carbonation conditions,the degree of carbonation of solid activated samples is higher,which is because the degree of carbonation reaction is lower due to the higher degree of alkali activation reaction and more dense pore structure of liquid alkali activated samples;under immersion carbonation conditions,the main carbonation reaction is that the flow of water in the matrix brings the cations in the matrix to the surface and reacts with CO₂ to generate carbonate precipitation,which accelerates the carbonation.The difference in pore structure due to different activator forms has little effect on the carbonation rate of soaked carbonized samples.