Indirect information about the geopolymer’s stability can be obtained by measuring both pH and ionic conductivity (IC) over time. To this aim, Milli-Q water (1:10 w:v) was added to the ground and sifted (d < 125 µm) samples. After shaking, waiting for the sedimentation, and filtering (d < 0.45 µm) under vacuum the solutions, the values of pH and ionic conductivity of the eluate were collected after 1 h [3 (link),4 (link)]. IC measurements were performed with a Crison GLP31 (conductivity cell 50 72, made up of glass and platinum, measuring range from 0 to 50,000 µS/cm, temperature range from −35 to 85 °C), whereas pH measurements were performed with a Crison GLP21 (pH measuring range 0–14, reference element of Ag/AgCl with Ag+ ion barrier, operating temperature range −10 to 100 °C). Crison GLP21 and 31 instruments were from Hach Lange Spain, S.L.U, Barcelona, Spain.
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