The Composition And Microstructure Of Slag-based Cementitious Material Activated By Sodium Hydroxide And Modified Water Glass And Their Influence On Its Carbonation And Drying Shrinkage Performance

Alkali-activated slag?AAS?cementitious material is extensively investigated all over the world due to its high early strength,low permeability,energy saving and low environment load;however,the engineering application of AAS was limited due to its low carbonation resistance and volume stability.In order to solve the above problems,in this paper,the effect of pore structure on the carbonation and drying shrinkage of modified water glass-activated slag?WAS?and Na OH-activated slag?NAS?with the similar composition of reaction products,and the effect of the composition of the reaction products on the carbonation and drying shrinkage of AAS?WAS+NAS?with the similar pore structure were investigated.It was found that when different activator was used,hardened AAS paste presented different composition of reaction products and pore structure.Based on thermodynamics calculation and experiment results,the main reaction products in hardened AAS paste were determined:the main reaction products in NAS were C-S-H gels,hydrotalcite-like compounds,and calcium aluminate hydrate;besides the above reaction products,silica gel also existed in hardened WAS paste.Moreover,the dissolution kinetics and reaction process of slag particles in modified water glass solution and sodium hydroxide solution were investigated by using digital holographic microscope for the first time.The phase evolution of slag particles in alkali-activators was investigated by using XRD and SEM/EDS,and the microstructure characteristics of hardened WAS and NAS pastes were also investigated.Based on the above experiment results,different reaction mechanism for WAS and NAS were revealed.In WAS,reaction products tended to form in solution and the reaction rate was fast,thus leading to a compact microstructure and small pore size of hardened WAS paste.On the contrary,in NAS,reaction products tended to form on the surface or inside of the slag particles and the reaction rate was slow,thus leading to a high porosity and large pore size of hardened NAS paste.Furthermore,the main reaction products in hardened AAS paste were synthesized by chemical precipitation method,and the influence of the chemical compositions,i.e.Ca/Si molar ratio,Mg/Si molar ratio,Al/Si molar ratio and Mg/Al molar ratio,on the structure,carbonation and drying shrinkage of the reaction products were clarified essentially.With a CO₂concentration of 1%,the carbonation depth in all WAS specimens was 0 mm after 90 days,indicating that the carbonation resistance of hardened WAS paste was excellent.The main reasons for the above excellent carbonation resistance were related to the compact microstructure of hardened WAS paste?more than 80%of the pore size was in the range of3-10 nm?and the formation of silica gel film on the surface of hardened WAS paste,which hindered the contact between the CO₂in the air and reaction products.The carbonation depth of NAS specimens was in the range of 4.5-9.1 mm after 90 days,indicating the carbonation resistance of hardened NAS paste was inferior than that of hardened WAS paste.For the hardened NAS paste,when the composition of reaction products was similar,the reaction rate was accelerated and the porosity was reduced with the increase of Na₂O content in the activator,resulting in a slower carbonation rate.When the pore structure was similar,the amount of non-carbonated hydrotalcite-like compounds in the hardened NAS paste was increased by the admixed Mg?OH?₂,resulting in a better carbonation resistance.As the content of Al?OH?₃increased,the amount of calcium aluminate hydrate which is easy to be carbonized and C-A-S-H gels with large interlayer?basal?spacing in the hardened NAS paste was increased,resulting in a poor carbonation resistance.Therefore,the carbonation resistance of hardened NAS paste can be improved by increasing Na₂O content in the activator and Mg/Si ratio of raw materials and reducing Al/Si ratio of raw materials.At the curing temperature of 20±2?and relative humidity of 50±5%,the drying shrinkage was 5.1-11.1 mm/m for hardened WAS paste and 2.0-2.7 mm/m for hardened NAS paste at the curing age of 28 days,indicating the volume stability of NAS was much better compared to WAS.For the hardened WAS paste,when the composition of reaction products was similar,the porosity and drying shrinkage were reduced when the water to solid ratio decreased.When the pore structure of hardened WAS paste was similar,Ca/Si ratio of C-S-H gels was increased and the content of high shrinkable silica gel was reduced when the Ms of activator decreased,resulting in a reduction in drying shrinkage.The amount of hydrotalcite-like compounds with a small drying shrinkage in the hardened WAS paste was increased by the admixed Mg?OH?₂,resulting in a reduction in drying shrinkage.As the content of Al?OH?₃increased,the amount of C-A-S-H with a high drying shrinkage in the hardened WAS past was increased,resulting in an increase in drying shrinkage.Therefore,the drying shrinkage of the hardened WAS paste can be improved by reducing the water to solid ratio and modulus of activator,increasing the Mg/Si ratio of raw materials and reducing the Al/Si ratio of raw materials.Based on the above results,the relationship between the composition/microstructure and properties?carbonation and drying shrinkage?of AAS was established,and the mechanisms related to the influence of the pore structure and the composition of the reaction products on the properties of hardened AAS paste were revealed.Moreover,the control and regulation methods for carbonation and drying shrinkage of hardened AAS paste were proposed,providing theoretical guidance for the application of AAS.Meanwhile,it also revealed that the silica gel,as one of the main reaction products of hardened WAS paste,had a great influence on the carbonation and drying shrinkage.