Strength Enhancement And Mechanism Of GH Admixture In Fly Ash Concrete

Fly ash concrete has been widely used in the civil engineering for its low hydration heat,good durability and it is ecologically beneficial. Despite these benefits, practical problems in the application field of fly ash concrete remain. At early ages, the strength of high volumes fly ash concrete is much lower than control concrete. So that how to accelerate the pozzolanic reactivity of fly ash is concentrated on more and more. GH admixture is a good activator for pozzolanic activation of fly ash, and it can improve the early strength of fly ash concrete. It has been researched and used in practical projects for about ten years. Previous research works have focused on the mechanical properties of fly ash concrete with GH and how to use it in the practical project, few studies on the activation and strength enhancement mechanism. In this paper, the activation mechanism and microstructure of the blended cement with fly ash or GH admixture or both were studied by investigated the blended cement pastes and the interfacial zone between aggregates and cement pastes with fly ash. Mortar tests show that: on one hand, the GH admixture can increase the strength of cement mortars; on the other hand, it can increase the strength of blended cement with fly ash.The mechanism of GH admixture on the strength of fly ash concrete was studied by X-ray diffraction analysis (XRD) and Scanning Electro Microscope analysis (SEM). The results show that the role of GH admixture can be attributed to the follows: (1) filling effect, the super fine GH admixture can fill in the interspace between fly ash particles and cement particles, so the pore size and connectivity of the voids was reducted; (2) active fly ash, accelerate the hydration reaction and large amounts of ettringite were formed at early age, lower total porosity than the specimen without GH admixture; (3) reactive with the hydration products of cement, decrease the amounts of Ca(OH)2 products, and increase the products of ettringite and C-S-H, which play important factor in the strength and structure dense. The results of compressive strength measurement correlated nicely with the XRD and SEM analysis results. The test results also demonstrated the existing of microcrack in the concrete. What's more, there are microcrack between fly ash and the products of cement, which is an important factor in affecting the strength and the durability of concrete.