Seasoning or aging improves cement in that the lime and the magnesia are slaked by the absorption of moisture from the air. The effect of lime and magnesia seems to be more serious in water than in air, and greater in sea-water than in fresh water.
The action of sulphur in a cement is extremely variable, depending upon the state in which it may exist and upon the nature of the cement. Sulphur may occur naturally in the cement or may be added in the form of sulphate of lime (plaster of paris) to retard the time of set (§ 154). Under certain conditions the sulphur may form sulphides, which on exposure to the air oxidize and form sul phates and cause the mortar to decrease in strength. Many, if not all, of the slag cements contain an excess of sulphides, and are there fore unfit for use in the air, particularly a very dry atmosphere, although under water they may give satisfactory results and compare favorably with portland cement.
Pat Test. The ordinary or "normal" method of testing soundness is to make small cakes or pats of neat mortar 3 or 4 inches in diameter, about half an inch thick and having thin edges, upon a clean sheet of glass; and expose one in the air at ordinary tempera ture and immerse the other in water at about the temperature of the air, and examine both from day to day for 28 days if possible, to see if they show any cracks or signs of distortion. The German standard specifications require the cake to be kept 24 hours in a closed box or under a damp cloth, and then stored in water. The French, to make sure that the pats do not get dry before immersion, recommend that the cakes be immersed immediately after mixing without wait ing for the mortar to set. Some really sound natural cements will disintegrate if immersed before setting has begun.
The amount of water used in mixing the mortar, within a reason able limit, seems to have no material effect on the results. However, as it costs but little more time and trouble, it is wise to use mortar of standard consistency (see § 161).
To examine the pat, note the following: 1. Is it loose from the glass? For the very best results the pat should remain attached to the glass; but being loose, in either the air or the water, is not considered indicative of serious unsoundness.
2. Is the side next to the glass flat or curved? The pats are more likely to curl in air than in water, and a moderate curvature in air is not regarded as very serious.
3. Is the glass cracked? Really good cements frequently crack the glass of the water pats.
4. Are there any radial cracks? Neither pats should show any radial cracks, wide at the thin edge of the pat and narrowing as they go toward the center. These cracks should not be confused with irregular hair-like shrinkage cracks, which appear over the entire surface when the pats are made too wet or dry out too much while setting.
5. Are there any blotches on the surface? There should be no blotches, as they usually indicate an unsafe amount of sulphides. The presence of sulphides will also be revealed by a greenish color of the interior of the pat exposed under water.
If there are any considerable indications of sulphides, before accepting the cement a chemical analysis should be made to determine the amount of sulphur and the probable ultimate action of the cement (see § 148).
Another excellent method of examining for the presence of sul phides is, in making the test for tensile strength (§ 157-79), to store part of the briquettes in air and part in water. Any material differ ence in strength between the two lots is sufficient ground for rejecting the cement for use in a dry place. Of course due consideration should be given to the possible effect of evaporation of water from the briquettes stored in air.
Accelerated Tests. The normal or cold-pat test, extending over a reasonable period, sometimes fails to detect unsoundness; and many efforts have been made to utilize heat to accelerate the action, with a view of determining from the effect of heat during a short time what would be the action in a longer period under normal conditions. Some of these tests have been fairly successful, but none have been extensively employed. It is difficult to interpret the tests, as the results vary with the per cent of lime, magnesia, sul phides, etc., present, and with their proportions relative to each other and to the whole. There is a great diversity as to the value of accelerated tests. Many natural cements which go all to pieces in the accelerated tests, particularly the boiling test, still stand well in actual service. This is a strong argument against drawing adverse conclusions from accelerated tests when applied to portland cement.