Page:Popular Science Monthly Volume 38.djvu/683

Rh plaster of Paris may be taken as the type, water simply combines with some constituent of the cement already present. In others, of which Portland cement is the most important example, certain chemical reactions must first take place. These reactions give rise to substances which, as soon as formed, combine with water and constitute the true cementaceous material. Portland cement contains as chief, sometimes as almost sole constituent, a lime peridote, and in addition a tricalcium aluminate, Ca2Al2O6 soluble in 3,000 parts of water, and a dark-brown fusible substance, Ca3Al2Fe909. In the act of setting, the tricalcium aluminate first dissolves in water and then begins to separate again as a mass of felted needles consisting of calcium aluminum hydrate, which extend in every direction and are directly the cause of the first setting of the cement. At the same time an action begins which requires a much longer time for its completion, and which probably consists in a combination of the first formed aluminum hydrate with the calcium peridote and the water, forming a mineral belonging to the zeolite class and possessing very probably the composition H10CaAl2Si4O17. This zeolite crystallizes out as it forms, and this continues, for long periods subsequent to the first setting of the cement, to add to its solidity and tenacity."

Following the reasoning of Prof. Griffin, we are unable to understand the meaning of "pure lime cement," as the two terms "pure lime" and "cement," when used in an engineering sense, are incompatible. The effect of the presence of magnesia upon the quality of cement is not perfectly understood; but that an increased hardening in cement for a long period of time is due alone to its presence, is not so, as cements that contain no magnesia have been known to improve constantly during a period of two years.

To quote from Prof. Griffin:

"So a Portland cement will develop its full strength in a few months, while our natural cements will not for years, and, so long as it (this chemical action) continues, the structure improves."

Unless Prof. Griffin classes Portland cement as a "pure lime cement" (which it is not), he has advanced no proof of the above quotation; and furthermore this statement itself is incorrect. It is a fact well known to all engineers and builders that as a class Portland cements are slower setting than the natural cements; and also that natural cements attain their full strength within a comparatively short time (within the first year as a maximum limit), and that, after the full strength has been attained, this strength may decrease, as time goes on, in some natural cements. There has, however, been found no limit of time beyond which Portland cement deteriorated, and for two or three years at least it improves its strength.