Comparison of durability, degradation mechanism and microstructure kinetics of metakaolin-based geopolymers exposed to ambient and washing environments

Abstract

The alkali ions playing the charge-balanced role in the geopolymer backbone may cause durability concerns when the geopolymer is exposed to a washing environment. This work investigated the durability, degradation mechanism, and microstructure kinetics of metakaolin-based geopolymers (MKGs) exposed to a washing environment, in which the MKGs aged in an ambient environment were a control group. Except for compressive strength measurement, the variations in mineralogy, composition, microstructure, and pore structure were characterized by X-ray diffraction (XRD), X-ray Fluorescence Spectrometer (XRF), nuclear magnetic resonance (NMR), and Mercury Intrusion Porosimeter (MIP). In the ambient environment, the efflorescence caused the compressive strength decrease due to the alkali migration from MKGs, resulting in the microstructure degradation where the tetrahedral Al(III) requires the charged-balanced cations. Although the crystallization refined the pore structure of MKGs, the N-A-S-H gel degradation dominated the compressive strength decrease. In the washing environment, MKGs underwent continuous composition leaching. In the early stage, the loss of the unreactive silicate oligomers from the alkaline activator didn't cause N-A-S-H gel degradation. In the later stage, more Na+ loss and substantial Al-rich gel degradation collapsed the pore structure and caused small visible cracks on the MKGs' surface, which deteriorated the compressive strength of the MKGs.