Senior research scientist, Industrial Research Limited, Lower Hutt,
The best means of selective dissolution of ordinary portland cement (OPC) from ground granulated blast-furnace slag (BFS) cements has been, until now, a sodium-based alkaline aqueous solution of ethylenediamine-tetraacetate with chelating properties supplemented by the presence of 6% triethanolamine (TEA).
Selectivity of the solvent is substantially improved by replacing sodium with tetramethylammonium (TMA). This large cation has a selective decelerating effect on the release of aluminum from aluminosilicate glasses. Thus, its introduction slows down dissolution of BFS and slightly accelerates the dissolution of OPC. This improvement removes the need for careful buffering at pH 11.6. A higher pH (for example, 12.5) is used, which avoids the formation of siliceous precipitates.
Further, it was found that the use of lithium as cation makes the EDTA-TEA solution a good solvent of BFS that allows separation from aggregates based on aluminosilicate rocks. Consecutive application of the two versions of the same chelating system thus allows good separation and quantitative dissolution of OPC and BFS from the unhydrated slag-cement.
Determination of the degree of hydration of BFS could be derived from measurements of Ca and Si in the leach rather than the measurement of weight losses during leaching. This technique reduces errors from the formation of solvent resistant Mg-Al-hydroxide phases (for example, hydrotalcite) and extends the range of application.
Paper ID: CCA10342J