Peracetic acid as a novel blowing agent in the direct foaming of alkali-activated materials

In this study, peracetic acid (PAA) was investigated as a novel blowing agent for the preparation of porous alkali-activated materials from metakaolin or blast furnace slag by the direct foaming method. PAA is an organic peroxide with lower stability of the OO bond in comparison to H2O2. It also introduces acetate anions to the system, which can chelate cations to potentially increase the extent of precursor dissolution and decrease the surface tension of the gas-liquid interfaces prompting open porosity formation. PAA was compared with H2O2 to prepare porous alkali-activated materials by characterizing setting time, compressive strength, reaction kinetics (by isothermal calorimetry), porosity (by mercury intrusion porosimetry and helium gas pycnometry), and chemical composition (by electron probe microanalyzer, EPMA). The most significant finding was that the use of PAA caused 92%–94% and 72%–80% lower volume increase compared to H2O2 upon curing for metakaolin and blast furnace slag-based foams, respectively. The enhanced dissolution of the precursors when using PAA were observed from the EPMA analysis, which promoted higher compressive strength for PAA-based foams (451–537% compared to H2O2-based foams). There was also an indication that the use of PAA promoted the open porosity formation. As a practical implication, PAA could be well suited for applications in which the volume increase during the foam setting must be minimized.