The monolith leaching test used in this study was a modified version of the conventional NEN 7345 leaching test (Dutch Standardization Institute 1995).

The conventional NEN 7345 test consists of immersing the test specimen in a closed polypropylene reactor containing deionized water as a leachant (initially acidified to pH 4.0 by nitric acid addition). A liquid-to-solid volume ratio (L/S) of

5.0 was used. During the test, there was no agitation of the leaching solution and the pH was free to change according to the acid or basic characteristics of the com­pounds released from the test specimen. The leachate was periodically replaced with an equal volume of leachant, after cumulative leach times of 0.25,1,2.25,4, 9, 16, 36, and 64 days. The eight leachates were filtered through a 0.45-p. m membrane filter and, after pH and electric conductivity measurements had been conducted, they were acidified to pH 2.0 with 5 M nitric acid. Finally, each leachate was analyzed for the concentrations of selected heavy metals (in this study, Cd, Cu, Cr, Ni, Pb, and Zn) by using an atomic absorption spectrophotometer equipped with a graphite furnace.

The modified leaching test used in this study differed from the conventional NEN 7345 test only for the continuous and moderate agitation of the leaching solution within the reactor (realized by a magnetic stirrer) and, especially, for the pH evolution throughout the test. In particular, the pH was kept constant to a predetermined value by means of an automatic titrator. The pH was monitored constantly, and the titrator compensated for alterations of pH by small additions of 0.5 M HNO3 from a burette. A pH of 6.0, which is characteristic for most natural waters, was selected for the modified NEN 7345 test. The addition of nitric acid was motivated by the necessity of compensating for the pH rise due to the release of alkaline compounds (alkalies and calcium hydroxide) from the cemen­titious test specimen, without altering the leachability of heavy metals through complexation reactions. Indeed, the nitrate ion is known as a poor complexing agent toward most heavy metals. Figure 10.1 illustrates the apparatus used for the monolith leaching test.

Three replicate leaching tests were performed and the test results were averaged. In each test, one cubic specimen (64 cm3 in volume) was used and was moistened with 320 cm3 of deionized water (L/S = 5.0; eight leachant renewals).