CONCRETE.

Hokitika Guardian, 20 January 1925, Page 3

CONCRETE.

IMITATES NATURAL STONE. COLOURED WITH OXIDES. Concrete casts, also work laid “in situ.” can be coloured to imitate any natural stone. I his is effected by mixing mineral’ oxides of the required color with the cement used for the surface coat. The color coat should not exceed 1-S inch in thickness, as exides are too expensive to use for the entire thickness of the cast. The quantity ol oxide to he added to the cement depends upon the strength of the oxide. Some are much stronger than others. Five per cent of a strong oxide will impart a close resemblance of the desired color to the concrete, hut a weak oxide will require from 10 to 15 per cent., and even 2(1 per cent,, to obtain the same color. Some of the red oxides range in color from scarlet, gradually deepening to chocolate. and some contain 95 per cent, ol pure ferric oxide, which is made from sii 1] ill ato of iron. This is a byproduct . and is frequently evolved froni waste liquors at tinplate works. This kind of oxide is far more suitable for colouring concrete than ochres and most of the earthy oxides. Earthy colours, like Venetian red, soon fade, and have a sickly appearance. The oxides should he intimately mixed with the foment in a dry state before it is gauged, and is generally done by hand, though better results will he obtained by the use oT a grinding machine. It is a safe plan to try various proportions ol color and cement, and gauge small parts, and when set and dry, select those most suitable for the purpose.