Artificial Cements.

When I began to investigate the subject of native cements and limes, I was under the impression that we had no stone capable of furnishing hydraulic limes, consequently some little time was devoted to the consideration of providing an artificial substitute; but the existence of natural cementing ingredients of a high character having been fully established, the necessity for adopting the latter expedient is removed, the subject will Wherefore be dismissed in a few words.

As you are probably aware, English Portland cement is made from two of the most common and abundant raw materials in the country—chalk and clay—and the manufacture is equally simple. The materials are mixed in the proportion of seven of the former to three of the latter, then burned in a kiln and pulverized as already described. In Germany, where there is no chalk, a substitute is found in hard limestone. This entails extra labour in pulverizing the raw material as well as the cement, but the result is practically the same.

Ordinary yellow clay does not make good cement; that in common use page 124 is a dark blue unctuous variety found in tidal estuaries and swamps. Blue clays, supposed to be suitable for the purpose, are abundant throughout the province. A sample from the railway cutting at Caversham was analyzed by Professor Black with the following results, which are shown alongside an English type:—

	Otago Clay.	English Clay.
Silica	65.28	68.45
Alumina	23.18	11.64
Iron	3.20	14.80
Lime and magnesia	2.58	0.75
Alkalies	1.04	4.00
Water	5.19	—
	100.47	99.64

These figures are not near enough to prove that this clay is good for making cement, but they are sufficient to show that there is every chance of getting the proper kind if required.

The last item neutralizes the good qualities of the others, so we pass it into the category of unsuitable materials.

The idea of utilising the rich limes induced me three years ago to mate an examination of volcanic clays to ascertain if they contained any of the properties of the Pozzuolanas of the old world that have been used from time immemorial to mix with lime in hydraulic works. About 40 specimens of all shades of colour imaginable were collected and tested by being made into mortar with an equal proportion of lime, then kept in water for two months. Four or five samples of drab and neutral tints gave indications of being feebly hydraulic, so possibly a more complete investigation would lead to the discovery of a material of considerable utility. The great objec- page 125 tion to Pozzuolanas is that, like the Moeraki boulders, uniformity of composition cannot be ensured.