Preliminary Study Of Alkali-Slag-Metakaolin Composite Cement

Alkali activated cement has become one of the active investigated field in cemetitiou materials for its outstanding merits like high strength, fast strength development, excellent durability and resource saving superiority and so on. Alakli-slag-metakaolin composite cement (M-AAS), made from slag, metakaolin and alkali, is a cementitious material by mixing geopolymer and alkali activated slag cement. Through strength and shrinkage test, and XRD, DTA and SEM analysis, strength, shrinkage and erosion resistant properties of M-AAS were all preliminarily studied and relative mechanisms were discussed also. The results indicated:The shrinkage of M-AAS was comparatively high. Most of the shrinkage developped at the first fourteen days. The shrinkage increased rapidly within 24 hours, and inclined to stay steadily after 14 days though the shrinkage value was still high comparatively at that time. The shrinkage value of M-AAS associated with the ratio of Ca to Si in the raw materials. The higher the ratio was, the bigger the shrinkage went, and vice versa. The relationship between the ratio of Ca to Si with shrinkage value was needed to be studied further.The strength of M-AAS was comparatively high, but the flexural strength was regressed in the long term. The compressive and flexural strengthes of 3 days were able to surpass 8MPa and 78MPa, respectively, and compressive strength of 90 days were all around 100MPa, but compared to flexural strength of 28 days, the flexural strength of 90 days was regressed by 19.0%. The high strength property of M-AAS associated with its hydration products and Interfacial Transition Zoo (ITZ). The main hydration products of M-AAS were microcline and CSH, no calcium hydroxide was found. The coexistence of microcline and CSH worked as micro-aggregate by each other, and no weak ITZ, both resulted in its high strength. The regression of its strength was related to its high shrinkage property, because the crazing caused by shrinkage would decrease the strength, especially the flexural strength.Sulfate and chloride resistant abilites of M-AAS were outstanding. magnesian resistant ability was not as good as sulfate and chloride resistant properties. However,M-AAS could not resist sulfate and magnesian mixing solution. Erosive properties of M-AAS mainly associated with its hydration products and microstructure of the bulk. There were no calcium hydroxide and aluminate in hydration products, and the bulk was compact, then the bulk were not prone to react with sulfate and chloride solution., so M-AAS was able to resist erosion caused by sulfate and chloride. M-AAS bulk was compact, and the diffusive speed of magnesium is low, but the reaction between CSH and magnesium would destroy the structure, so magnesian resistant ability was not as good as sulfate and chloride resistant abilities of M-AAS. When M-AAS exposed to mixing solutions, calcium hydroxide yielded from reaction between CSH and magnesium would react with sulfate to produce gypsum, and the gypsum would destroy the bulk because of its expansion property, that's why M-AAS was unable to resist erosive sulfate and magnesian mixing solution.