From the nature of natural cements a much greater variation in strength among different brands, and even among different samples of the same brand, is to be expected. With Portland cements made in accordance with ordinary methods, the variations in strength among ten or twenty brands will usually be comparatively small. One of them may possibly prove unsound, and one or two others may give inferior strength, but the variations in strength among three-fourths of the samples will not generally exceed 20 per cent. With the same number of brands of natural cements, variations of 50 to 200 per cent, may be expected.

Table 41. Interpretation Of Short Time Tests Of Natural Cement, Several Brands

In Table 41 six brands of natural cement are compared by tests at seven days, twenty-eight days and two years. These six brands have been arranged in the table according to their value as shown by the two year tests, and it is seen that the first three, Jn, Hn and Bn, are especially poor, while the last three, Mn, Nn and Kn, are exceptionally good. In the short time tests of briquets maintained at ordinary temperature, Jn and Bn gave low results and Nn and Kn gave fairly high results, in harmony with the long time tests; but Hn, which proved to be one of the poorest samples, gave in every case the highest, or next to the highest, result in seven and twenty-eight day cold tests. In this table we find again that the results of the briquets maintained at 80° C. for seven days gave, in a general way, the best indication of the relative values of the six brands.