The presence of clay either within the aggregate particles or as dust coatings can have a significant effect on asphalt mix properties. Again, the type of clay present will govern whether the effect is detrimental and leads to premature failure. Figure 7 shows two laboratory prepared samples of 30/14 hot rolled asphalt. The one on the left has been made using weathered unsound basalt whereas the sample on the right has been made using sound basalt. The samples were sliced using a diamond saw and subjected to five cycles of immersion in magnesium sulphate solution followed by oven drying