The Modification Of Microstructure And Properties Of Alkali Activated Slag

Alkali-activated slag (AAS) is a new type cementitious material which is mixed by ground granulated blast furnace slag and akaline solutions. AAS as an alternative to Portland cement has been found to have low hydration heat, high rate development of mechanical properties and good durability, etc. However, the application of AAS so far is very limited because of its high drying shrinkage and high rate of carbonation. These drawbacks are closely related to the microstructure of AAS. So in this paper, a study has been conducted to investigate the effects of nano materials, pozzolanic materials and the blended mixture of both on the properties and microstructure of alkali-activated slag pastes(AASP).The effects of total alkali content and modulus on workability and mechanical properties of AASP to identify the optimum mix proportion were studied. The optinum mix proportion was chosen that Ms is 1.2 and total akali content is 4.0%.Firstly, this paper investigated the effect of rutile nano-TiO2, anatase nano-TiO2 and nano-SiO2 on mechanical properties, drying shrinkage and microstructure of AASP based on the optimum mix proportion. Results showed that the addition of rutile nano-TiO2 increased the mechanical properties of AASP, especially the flexural strength. The addition of anatase nano-TiO2 and nano-SiO2 increased the drying shrinkage, and the addition of rutile nano-TiO2 decreased the drying shrinkage by 21.1%. For further understand the impact of nano materials on AASP at micro scale, rutile nano-TiO2 was chosen. SEM, EDS, XRD and FTIR results demonstrated that the addition of rutile nano-TiO2 into AASP accelerates its hydration process which results in more hydration products and denser structure. MTP results showed that the addition of rutile nano-TiO2 reduces the total porosity of AASP to 69.5% and the porosity of 1.25-25 nm mesopores to 59.6% at 28d.Secondly, this paper investigated the effect of metakaolin, fly ash and silica fume by inclusion or addition on mechanical properties, drying shrinkage and microstructure of AASP based on the optimum mix proportion. Results showed that compared to the inclusion and addition of silica fume, the addition of fly ash, the inclusion of metakaolin, the addition of metakaolin substantially increased the mechanical properties and decreased the drying shrinkage. For further understand the impact of pozzolanic materials on AASP at micro scale, metakaolin was chosen. SEM, EDS, XRD and FTIR results demonstrated that the addition of metakaolin into the AASP resulting in more hydration products(C-S-H, C-A-S-H, AFm and zeolite-like products) and denser structure. MIP results showed that the addition of metakaolin reduces the total porosity of AASP to 46.3% and the porosity of 1.25-25 nm mesopores to 43.1% at 28d.Thirdly,based on the above conclusions rutile nano-TiO2 and metakaolin were added together to study the effect of the blended mixture on the propertied and microstructure of AASP. The results showed that the mixture of rutile nano-TiO2 and metakaolin into AASP enhances its mechanical strength, decreases its shrinkage, accelerates its hydration, densifies its microstructure, forms zeolite analog。reduces the percentage of mesopores, and alleviates its micro cracking.To sum up, the rutile nano-TiO2, metakaolin(in addition) and the blended mixture of both could modify the microstructure especially improve the pore structure which finally enhanced the macro-performance of AASP. Therefore, it is feasible to enhance the macro-performance by means of modifing the microstructure of AAS.