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This study, undertaken in 2003–2004, aimed to:

1. study changes produced when additives are mixed with roading aggregates containing swelling clay minerals;
2. identify the most suitable chemical to use for a particular rock type and the quantity required to achieve particular effects, namely:

• • conversion of the clay to a better form,
  • establishment of bonds between particles,
  • strength that is insensitive to water content,
  • minimal drying shrinkage
  • permanence.

Samples of fresh, partly weathered and weathered rocks of three rock types were taken from four quarries. Cement, lime or Durabind was added to the partly weathered samples at two levels.

The untreated and treated samples were analysed. Proctor Needle and CBR testing was performed on the partly weathered samples.

Volcanic rocks contained a higher proportion of more readily weathered minerals than greywacke. Drury greywacke was more intensively sheared than Otaika greywacke. All partly weathered materials treated with 3% of additive had satisfactory CBR values. Methylene blue tests and X-ray diffraction analysis confirmed that 3% additive almost eliminated swelling tendencies.

Geochemical studies identified that additives reversed the trends of changing fluid composition caused by weathering.

Cement stability was investigated by suspending crushed treated samples in water. Swelling tendency increased over time, but in alkaline conditions, the aggregates returned to 30–70% of their original values after four days.

Keywords: Adesite, aggregate, analysis, basalt, CBR, clay minerals, Durabind, greywacke, marginal aggregate, modification, Proctor Needle test, roads, rocks, stabilisation, treatment, weathering