The color of the natural cements used in this country vary with the locality in which they are found. Most of the cements mentioned in Section III are brown in color, in light or dark shades. "In Rosendale cement a light color generally indicates an inferior, underburnt rock.
"The weight of good Rosendale cement varies from 49 to 56 pounds per cubic foot, or 60 to 70 pounds per bushel, according to its fineness and the density of packing. The harder-burned varieties are also heavier than those that are underburned. The weight per barrel of Ulster County Rosendale cement averages 300 pounds net; Akron, Milwaukee, Utica and Louisville cements weigh 265 pounds per barrel net."
Testing Rosendale Cement. - The value of cements for making mortar varies greatly with their physical properties, and since one lot is liable to differ very much from another lot of the same brand, it is very necessary to be able to test the character of any particular cement.
Brand. - Any particular brand of cement will generally average about the same strength and quality, and the architect should ascertain what brands of cement are giving the best satisfaction and specify those brands. For ordinary building purposes it will only be necessary for the superintendent to examine the casks to see that they bear the brand specified and to see that the cement has not been injured by dampness. If the cement is found to have become hard or crusty, it has absorbed moisture and should not be used in making the mortar.
If the superintendent has any doubts of the quality of the cement, let him take two handfuls of cement and mix with as little water as possible into two cakes; put one in water and leave the other in air. If the air cake dries of a light color without any particular well-defined cracks, and the water cake sets with a darker color and without cracks, the cement is probably good. If the cement cracks badly in setting, or if it becomes contorted (sometimes called blowing), it is positively poor and should be rejected.
Another simple test of the soundness of cement, which is the property of not expanding or contracting, or checking or cracking in setting, is to place some mortar in a glass tube (a swelled lamp chimney is excellent for this purpose) and pour water on top. If the tube breaks the cement is unfit for use in damp places. Any natural cements that give satisfactory results with these simple tests will answer for making mortar for any ordinary building construction.
Where great strength is required in the mortar it is better to use Portland cement, but if for any reason Portland cement cannot be obtained, or its price prohibits its use, then the strength of the natural cement should be carefully tested, in the manner described in Sections 117-123 for testing Portland cement.
Clear Rosendale cement one week old in water should have a tensile strength per square inch of at least 60 pounds, and the best brands should average 100 pounds.
Storing. - It is very essential that cements of all kinds should be stored in a dry place, where there is no danger of its absorbing moisture, until it can be used. A very little moisture will cause the cement to set, and any cement that has commenced to set should be rejected.
Weight and Strength. - "Good Roman cement should not weigh more than 75 pounds per bushel, and should set very quickly (within about fifteen minutes of being gauged into paste)." A heavier cement than this is likely to be overburnt or else injured by the absorption of carbonic acid from the air.
Neat Roman cement seven days old in water should possess a tensile strength of from 50 to 80 pounds per square inch.
Storing. - Roman cement is sold in a ground state and is put up in casks, which must be kept carefully closed and dry, otherwise the cement will absorb carbonic acid and become inert.
Uses. - The strength of Roman cement diminishes rapidly when mixed with sand, and not more than 1 or 1½ parts of sand to 1 of cement should be used in mixing the mortar. Roman cement mortar should be mixed in very small quantities and used at once, and on no account beaten up again after the setting has commenced.