Aggregates.

Except in the case of the higher hydraulic limes and cements, where the maximum strength is obtained by using them in a pure state, as much depends on the aggregate as on the cementing material, notwithstanding which there is no article used in construction that commands so little attention. The main essentials of a good aggregate are sharpness and freedom from earth or other impurities of a similar nature. The proper size and hardness vary with the quality of the cementing material;—rich lime takes a coarse soft sand, and cement a fine hard one.

As no attempt had been made to determine the relative merits of the Otago sands, I collected a number in the vicinity of Dunedin and experimented on them in the following manner, and with the results given in Table V. Each kind of sand was made into mortar with Waihola lime in the proportion of one of lime to two of sand. The lime had been air-slaked, and was sifted through a gold-dust sieve before being used. The ingredients were measured in the most exact manner, and carefully mixed with the smallest quantity of water that would give plasticity. The mortar was then used to cement ordinary bricks placed crosswise, which gave a bearing surface of about 18 square inches. After being kept in the open air for 100 days the bricks were pulled asunder with weights increased gradually to the breaking point. It will be seen from the table that the highest results were obtained from Anderson Bay sand, which broke with a strain of 226 pounds. About 1½ square niches of the mortar in the inside was not quite hard. Assuming that this only supported half as much as the other portion, we make the cohesive strength 13 pounds on the square inch. Two samples of each kind of sand were tested. Taking only the highest in each pair, we find that, out of a total of 27, four broke with strains ranging from 226 to 150 pounds, nine from 150 to 100, six from 100 to 75, and six from 75 to 47, while two did not stand any measurable strain. I regret to add that many of the last three classes are constantly used in Dunedin.