The strength of Portland cement varies, as has already been mentioned, according to its original composition as regards the percentage of chalk and clay used in its manufacture, with the degree of burning to which it has been subjected, and according to the fineness to which it has been ground.

The strength of gauged Portland cement rapidly increases with age, the breaking weight on a sectional area of 21/4 square inches being as shown in the second column of the following Table for neat cement weighing 112 lbs. per bushel.1

Fig. 87.

1 In some moist climates abroad cement would be deadened by this treatment, so that when sending cement to such climates it should be thoroughly cooled before it is packed into the casks.

The strength of Portland cement mortar decreases as the proportion of sand is greater, which will be seen by the remaining columns of the Table.

The following Table2 shows the strength of Portland cement at different periods of time with different degrees of fineness and with different proportions of sand.

Table 2. Showing The Cement Coarsely Ground And Sifted Through A Fine Sieve Neat And Mixed With Sand

The German mould used = 5 square centimetres in section.