Abstract
Geopolymers are inorganic binders based on mixtures of an aluminosilicate powder with an alkali-silicate solution. Properties of geopolymers are strongly determined by the type of reactive solid, the liquid/solid ratio of paste and, amongst others, the Si/Al ratio of the formed geopolymer network. In this study, fly ash blended metakaolin based geopolymers with varying liquid/solid ratios (l/s), activated by potassium silicate solution, are investigated. Reactivity of metakaolin and fly ash was investigated by powder X-ray diffraction (XRD) and dissolution tests. Reactivity, mechanical properties and microstructure of hardened pastes were analyzed by setting and compressive strength tests, mercury intrusion porosimetry (MIP), capillary water absorption tests, thermogravimetric analysis-differential scanning calorimeter (TGA-DSC), isothermal calorimetry and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS). The results show that substitution of metakaolin by fly ash as well as variation of l/s brings advantages up to a certain degree, but also has a considerable influence on the pore size distribution, mechanical properties, Si/Al ratio of the geopolymer network and the content of bound water.
Highlights
For the synthesis of geopolymers with low to no calcium content, metakaolin is the most used powdery solid material if hardening has to take place at room temperature
The low calcium content is the decisive feature that distinguishes geopolymers from alkali activated binders (AAB) in general, a fact that is based on the definition for geopolymers coined by Joseph Davidovits in 1979 [1], after activating metakaolin with alkali silicate solution [2]
Reactivity of metakaolin and fly ash was measured by performing dissolution tests, where 150 mg of powder was immersed in 150 g of potassium hydroxide solution and continuously stirred with a magnetic stirrer
Summary
For the synthesis of geopolymers with low to no calcium content, metakaolin is the most used powdery solid material if hardening has to take place at room temperature. The low calcium content is the decisive feature that distinguishes geopolymers from alkali activated binders (AAB) in general, a fact that is based on the definition for geopolymers coined by Joseph Davidovits in 1979 [1], after activating metakaolin with alkali silicate solution [2]
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