Efflorescence: a critical challenge for geopolymer applications?

Efflorescence is the formation of white salt deposits on or near the surface of concrete. For ordinary Portland cement (OPC) concrete, efflorescence is generally harmless except for the discolouration, and is best described as being 'a skin trouble and not a deep-seated disease'. However, for geopolymers, as they contain much higher soluble alkali content than conventional cement, efflorescence can be a significant issue when the products are exposed to humid air or in contact with water. In this study, the efflorescence phenomenon of geopolymers that synthesised using different activators, solid materials and curing conditions is observed. The efflorescence product is mainly sodium carbonate heptahydrate (Na2CO3·7H2O). The efflorescence potential has been compared via measurements of cation concentrations by atomic absorption spectroscopy (AAS), and determination of pH and electrical conductivity of geopolymer leaching solutions. At the same alkali content (in terms of Na2O), geopolymers synthesised at high temperature (80°C×28 d) exhibit less efflorescence rate than those synthesised at low temperature (20°C×28 d). NaOH activated geopolymers possess slower efflorescence than the sodium silicate solution activated specimens. Adding 20% slag can effectively reduce the initial efflorescence of a fly ash geopolymer. From a long term view, however, the efflorescence potential of such samples could be equivalent to the activated 100% fly ash when considering the alkali leaching results. Further investigations to prevent efflorescence, or at least to reduce its rate, are urgently required for wider applications of fly ash-based geopolymers.