55. Microscopical examinations are of some interest and value to those who are thoroughly versed in the chemistry of the burning and hardening of cements, as an aid in determining the part played by each compound in the hardening.
Examinations may be made either of the dry powder, or of thin sections of hardened cement, or clinker. Dry powder of Portland cement appears to be made up of scaly particles, many of which are clearly defined and semi-transparent, while natural cement particles are more nearly opaque and less angular. Thin sections of Portland cement clinker have been found to exhibit colorless crystals somewhat cubical in structure, which are thought to form the essential hardening constituent; thin sections of hardened Portland cement show a clear crystalline structure. Prof. Hayter Lewis found that the particles in good Portland cement were angular in form, consisting of scales and splinters, while the particles of cement of poor quality were rounded or nodular.
Microscopic examinations have no place at present in ordinary tests of quality.
The color of cement is chiefly derived from its impurities, such as oxides of iron and manganese, rather than from its essential ingredients, and the color is therefore of minor importance. Other things being equal, a hard burned Portland cement will be darker in color than an underburned product. An excess of lime may be indicated by a bluish cast, and excess of clay or underburning may give a brownish shade. Gray or greenish gray is usually considered to be indicative of a good Portland.
57. The colors of natural cements have a wide range, varying from a light yellow to a very dark brown, without reference to quality. Owing to a popular idea that dark color indicated strength, some manufacturers have been said to add coloring matter to their product, but although this may have been true at one time, the correction of this false idea has doubtless rendered such a practice quite unnecessary now. Variations in shade in different samples of the same brand of natural cement may indicate differences in burning or in the composition of the rock; but the interpretation of color for any given brand must be the result of close study, for some cements become lighter on burning and others become darker, while in some cases no variation in shade can be detected for different degrees of burning.