The buffering capacity of the materials was determined by a pH
titration test (CEN/TS, 2006). First, the initial pH of each sample
was measured in a 1:100 sample/deionised water ratio. Next, consecutive
volumes of 200 lL of 1 M HNO3 or 1 M NaOH were added,
and the suspension was stirred. The resulting pH of the suspension
was measured 20 min after each addition of the acid or base. The
acid and base additions were repeated until the pH range of
2–12 was covered.