Reactivity and Microstructure of Metakaolin Based Geopolymers: Effect of Fly Ash and Liquid/Solid Contents

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.

Evaluation of Sulfuric Acid-Induced Degradation of Potassium Silicate Activated Metakaolin Geopolymers by Semi-Quantitative SEM-EDX Analysis

“…The evolution of compressive strength has been presented in a previous publication [ 53 ]. MK54 and MK60 reached 58.2 MPa and 53.6 MPa, respectively, after 28 days.…”

“…This could also lead to a pore-blocking effect. The low intensity of the gypsum peak in the XRD patterns of the deteriorated layers suggests, that even with a very low CaO content of the metakaolin, which is 0.8% [ 53 ], gypsum can be built. Whether this low amount has an influence on the pore-blocking effect or cracking of the deteriorated layer cannot be said with certainty.…”

“…An industrial metakaolin with a density of 2.68 g/cm 3 a high quartz content (39.6%), and an amorphous amount of 46.0% was used to synthesize the two geopolymer mixtures. Chemical and mineralogical composition, specific surface area, and particle size distribution have been presented in a previous publication [ 53 ]. Impurities like quartz reduced the overall reactivity of the metakaolin but could also have a positive effect in the context of the resistance to acid attack, as those particles could partially block the pores and reduce the extent of alkali cations (Na + , K + ) leaching from the geopolymer [ 50 ].…”

“…In addition, higher amounts of quartz reduced the water demand of the powder, which could significantly influence the total porosity and pore size distribution of the hardened binder. Calculation of the amorphous amounts of Si and Al in metakaolin by considering the chemical and the mineralogical composition of metakaolin, as proposed by Vogt et al [ 53 ], resulted in the amorphous molar Si/Al ratio of the metakaolin to be 0.89.…”

“…Information about the mixing procedure of geopolymer paste can be taken from Vogt et al [ 53 ]. Small prisms (80 mm × 20 mm × 20 mm) were cast in polyethylene molds, compacted on a vibration table until no more air bubbles could be seen on the surface, and cured at ambient temperature (21 °C, 50% RH) for 28 days, before exposing the specimens to sulfuric acid (Carl Roth GmbH + Co. KG, Karlsruhe, Germany) at pH 1.…”