CHEMICAL ANALYSIS. Chemical analysis may render valu able service in the detection of adulteration of cement with consider able amounts of inert material, such as slag or ground limestone. It is of use, also, in determining whether certain constituents, believed to be harmful when in excess of a certain percentage, as magnesia and sulphuric anhydride, are present in inadmissible proportions.
The determination of the principal constituents of cement— silica, alumina, iron oxide and lime—is not conclusive as an indica tion of quality. Faulty character of cement results more frequently from imperfect preparation of the raw material or defective burning than from incorrect proportions of the constituents. Cement made from very finely-ground material, and thoroughly burned, may contain much more lime than the amount usually present and still be perfectly sound. On the other hand, cements low in lime may, on account of careless preparation of the raw material, be of dan gerous character, Further, the ash of the fuel used in burning may so greatly modify the composition of the product as largely to destroy the significance of the results of analysis.
"As a method to be followed for the analysis of cement, that proposed by the Committee on Uniformity in the Analysis of Mater ials for the Portland Cement Industry, of the New York Section of the Society for Chemical Industry, and published in the Journal of the Society for January 15, 1902, is recommended."* The following simple chemical tests may at times be valuable for testing the purity of a portland cement.
To test for the presence of limestone or sand, "put into a test tube as much cement as can be taken on a nickel five-cent piece, moisten it with half a teaspoonful of water, and cover with clear muriatic acid poured slowly upon the cement while stirring it with the glass rod. Pure portland cement will effervesce slightly and will give off some pungent gas, and will gradually form a bright yellow jelly without any sediment. Powdered limestone or powdered cement-rock mixed with the cement will cause a violent effervescence, the acid boiling and giving off strong fumes until all of the carbonate of lime has been consumed, when the bright yellow jelly will form.
Powdered sand or quartz or silica mixed with the cement will produce no other effect than to remain undissolved as a sediment at the bottom of the yellow jelly. Reject cement which has either of these adulterants." To test for the presence of slag, " add benzine to methylene iodide which has a specific gravity of 3.29, until the specific gravity of the mixture is 2.95. Put a half inch of the dry cement into a test tube, and pour in a little of the mixture, stirring to a thin grout. Then cork the tube and let it stand. If slag is present, it will remain at the top while the cement will settle to the bottom. The separa tion can not be seen if coloring matter is present." "Coloring matter in any cement will show itself in the acid test by giving a black or gray color to the resultant jelly which would otherwise be yellow. The coloring matter may, or may not, be injurious in itself; but its presence shows that the manufacturer wished to disguise the cement, which should be rejected, because there are plenty of good cements which need no disguise." When cement powder is mixed with water to a plastic condition and allowed to stand, the cement chemically com bines with the water and the entire mass gradually becomes firm and hard. • This process of solidifying is called setting. Cements differ very widely in their rate and manner of setting. Some occupy but a few minutes in the operation, while others require several hours. Some begin to set comparatively early and take considerable time to complete the process; while others stand considerable time without apparent change, and then set very quickly.
A knowledge of the activity of a cement is of importance both in testing and in using a cement, since its strength is seriously im paired if the mortar is disturbed after it has begun to set. Ordinarily the moderately slow-setting cements are preferable, since they need not be handled so rapidly and may be mixed in larger quantities; but in some cases it is necessary to use a rapid-setting cement, as for example when an inflow of water is to be prevented.