108. The quantity of water used in gaging has a most important influence on the test for time of setting, an increased quantity of water retarding the setting. This may be seen from Table 20.

Table 20. Effect Of Consistency Of Mortar On The Time Of Setting

As might be supposed, this influence varies with different samples, and M. H. LeChatelier2 has given the following table which illustrates this point.

Table 21. Effect Of Consistency Of Mortar On Time Of Setting

1 "Masonry Construction," I. O. Baker, p. 98.

2 "Tests of Hydr. Materials," p. 33.

109. It is necessary, then, in writing specifications and in making tests, where the time of setting is at all carefully considered, to note the consistency of the paste used in the test. Practically, it is preferable to use a paste rather thinner than that usually employed for briquets.

The consistency is sometimes defined by M. Vicat's apparatus of a rod 1 cm. in diameter, or by M. LeChatelier's modification of the same mentioned above, or by the requirement that it shall be at the point of ceasing to adhere to the trowel. Another definition is that it shall, when placed on a glass plate, flow toward the edges only on repeated jarring of the plate. This last is a very fair approximate method, though giving a rather thin paste.

That mortars set more slowly than neat cement paste is largely due to the increased amount of water present in the former, this excess of water being required to moisten the grains of sand. The relation between the time of setting of mortars and neat cement paste is not definite. M. Candlot found the time of setting of one-to-three mortars to be from two to twenty times as great as that of the paste of neat cement of normal composition.

110. The temperature of the cement and water also has an important bearing on the observed time of setting. As the temperature of the materials is increased, the time of setting diminishes in about the same proportion. The following table gives a few of the results obtained by M. Candlot1 with Portland cements.

Table 22. Effect Of Temperature Of Materials On Time Of Setting

Table 23 gives the results of similar tests made under the author's direction. The temperatures of cement and water were varied while the temperature of the room in which the tests were made remained nearly constant, or from 63° to 67° Fahr.

1 "Ciments et Chaux Hydrauliques," par E. Candlot.

111. Amount Of Gaging

If a cement paste containing a moderate amount of water be insufficiently gaged, it will appear dry, when a more thorough working might make it plastic. Thus an insufficient gaging may make a cement appear quicker setting. It is also the case that when a cement is regaged after having begun to set, the second setting will take place more slowly; this, however, is a somewhat different matter.

112. The temperature and character of the medium in which the pat is kept during the setting process will have a decided influence on the rate of setting.

This is clearly shown by the following table, given by M. Paul Alexandre,1 from which it appears that different samples are affected in very different degrees. It is seen that the higher the temperature, the more rapid the setting.

Table 24. Time Of Setting As Affected By Temperature Of The Water And Of The Medium In Which Cement Sets

TABLE 23. Effect of Temperatures of Cement and water on the Time of Setting of Paste.

113. At temperatures below 32° F. (0° C), setting seems to be entirely suspended. If a cement paste, which has been submitted to such low temperatures since gaging, is brought into a warm room, the setting process begins as though the mortar had just been gaged. It must not be concluded, however, that freezing has no evil effect on mortars. (See Art. 50).

114. Setting In Air And Water

A cement paste sets much quicker in air than in water. This is due to the percolation of water to the interior of the pat, when it is immersed as soon as made, being analogous to using an excess of water in gaging. When a pat sets in dry air, the evaporation of water from the surface hastens the hardening of that portion. If immersed directly after it has set in air, it re-softens, and this is also true of some briquets immersed when twenty-four hours old. The time of setting of cements that are to be deposited under water may well be tested in that medium, when they should be protected by a mold of some form to retain their shape. Ordinarily the time of setting should be tested in moist air.

Cements are said to set more quickly in compressed air than in free air; this may be partially due to the higher temperature usually existing in the former.

115. Requirements As To Time Of Setting

What is desirable as to time of setting will, of course, depend on the work in hand; certain purposes requiring that the cement shall be able to retain its shape soon after deposition, while in other cases ability to mix large quantities at a time, without fear of the cement setting before it is in place, may be very convenient. An extremely quick setting cement should be regarded with suspicion until it has proved itself of good quality. It is sometimes stated that where a quick setting mortar is desired, natural cement must be used, but this is not true; either Portland or natural may be found with almost any rate of setting desired. As a general rule, however, among cements that have been stored several months, the Portlands are slower setting.

1 "Recherches Experimentales sur les Mortiers Hydrauliques," par. Paul Alexandre.

Portland cement will ordinarily begin to set in from twenty minutes to six hours, and natural cement in from ten minutes to two hours, though there are many cements the time of setting of which is outside of these limits.