Clay mineralogy of modified marginal aggregates

In 2003-04, a joint engineering and geological study was carried out to determine the changes that occur when lime based additives are mixed with roading aggregate, and which of the three additives most commonly used in the Auckland region was the most suitable for particular rock types. Samples of fresh, partly weathered and weathered rock were obtained from greywacke, basalt and andesite quarries. The properties of the samples were determined using a variety of geological and geochemical analytical techniques. Similar techniques were used to establish the changes that occurred when Portland cement, hydrated lime and Durabind were added. All the samples contained some swelling clay minerals, some more than others. Chemical reactions of the additives with the swelling clays include cation exchange and, over a prolonged period, the production of new hydrated calcium aluminosilicate minerals, such as hydrogrossular, through pozzolanic reactions. The permanence of the process was also examined and it was noted that under weak acidic conditions the stabilising effect of the additives could be reversed. The study showed that some additives are more suitable for use with particular rock types and that the risk of failure may depend on the process used. (a)