Investigation Of Mechanical Performance And Relationship Between Mechanical And Microstructure Characrization Of Alkali-activated Slag

As an alternative binder material, Alkali-Activated Slag, AAS, which is known as "green cement", possesses numerous advantages such as environmental protection, low pollution, low energy consumption, simple production process and high performance. Furthermore, alkali-activated slag is characterized as high strength, low hydration heat, high resistance to chloride penetration, high resistance to chemical corrosion and so on. In this project, on the basis of the study of the mixture ratio of alkali-activated concrete, in which three alkali activators, i. e. Na2SO4, NaOH and waterglass, were applied, the correlation between the mechanical properties such as strength, drying shrinkage of the AAS and the type and the concentration of alkali solution were studied. SEM, TG-DSC, MIP, FT-IR, were used to analyze and characterize microstructure and the hydration products of the AAS. The purpose of this study is to provide a fundamental research for popularization and application of AAS cement and concrete. The main results obtained are summarized as follows:(1) Compared with NaOH and Na2SO4, waterglass(M=1.5) presents the best activation when the alkali equivalent was fixed. The early strength of the AAS mortar activated by waterglass is considerably higher than the one activated by NaOH and sodium sulfate with the alkali equivalent content of 6.2%. With the increase of the equivalent content in alkali activators, the strength of the alkali-activated slag mortar increases correspondingly. The 28 d compressive strength of the alkali-activated slag mortar activated by the waterglass is most significant too.(2) With the same alkali equivalent content, during the test period of 0 to 45 d, the drying shrinkage of the slag mortar activated by waterglass is the largest, the one activated by the Na2SO4 ranked the second place, and the one activated by NaOH is minimal. After 45 d ages, the drying shrinkage rate of the slag mortar activated by waterglass and Na2SO4 flatten out gradually, while the one by NaOH increase slightly. With the increase of alkali equivalent content(Na2O, 3.1%-6.2%), the drying shrinkage rate decreases when sodium sulfate as activator. Before 7d ages, the drying shrinkage rate increased with alkali equivalent content when NaOH as activator, and the shrinkage was different for different alkali equivalents content of NaOH, e. g. 3.1% concentrations of NaOH resulting in the maximum shrinkage while 6.2% resulting in the minimum. There’s a value of alkali equivalent from 3.1% to 6.2%, which made the minimum shrinkage when using the water glass as alkaline activator.(3) Waterglass provides the best activation compared with other two activators in terms of microstructure, which is shown as the rapid reaction of hydration and numerous hydration products. With the addition of alkali equivalents of the alkaline activators used in this study and increase of curing ages, the microstructure and the pore structure of samples tended to be more perfect and more reasonable. With 6.2% of the alkali equivalents, the porosity of the samples activated by the alkaline activators were tested at 28 d ages, the porosity of the samples activated by NaOH is 4.54%,the porosity of the samples activated by Na2SO4 is 3.67%, the porosity of the samples activated by waterglass is 0.96% and the percentage of the harmless pore structure in the alkali-activated slag mortar activated by waterglass was 30% higher than activated by Na2SO4 and 57.6% higher than activated by NaOH. It’s found that the samples activated by waterglass had the best pore size gradation. With the increase of curing ages and addition of alkali equivalents, the pore size of the samples tends to be smaller, while the porosity, average pore diameter, the probable aperture and the mean pore size of the alkali-activated slag mortar decreased, while samples activated by waterglass had the largest decline. With respect to different alkaline activators with the same alkali equivalent, the fractal dimension of the waterglass > the fractal dimension of Na2SO4 >the fractal dimension of NaOH. For each alkaline activator, the fractal dimensions increased with the alkali equivalent.(4) The main hydration product of the AAS is C-A-S-H with a low Ca/Si ratio, and no portlandite exist in the hydration product compared to Portland cement. The hydration product of the slag mortar activated by Na2SO4 involves AFt(ettringite). With the increase of curing age and alkali equivalent content, the hydration products of the alkali-activated slag mortars activated by these alkaline activators increased. The TG-DSC analysis of hardened slag activated by NaOH, Na2SO4 and waterglass at 28 d ages in the range of 20-300℃ showed that the corresponding mass loss were 14.15%,14.23%,15.72%, while hardened cement activated by waterglass had the largest mass loss. Hardened slag activated by sodium sulfate has a mass loss less than 1% at 7d age in the range of temperature varied from 300℃ to 700℃, which also indicated that the hydration reaction of the slag activated by sodium sulfate was very slow.