123. Mixing the Mortar

Cements should be tested, both neat and mixed with sand. Briquettes made entirely of cement are more convenient for testing, as they may be tested sooner, and there can be less variation in the mixture. But in building, cement is rarely used without an admixture of sand, and the most valuable comparative test of different brands submitted would be to make the briquettes of cement and sand such as is to be used in the mortar, as while one cement might give a greater strength when used without sand, when mixed with the sand it might show a less result than another brand, owing to the comparative fineness of the two.

Practically the benefit to be obtained in testing the strength of cements for building purposes will be to determine which of the brands that are available are the most desirable, considering both the cost and the strength, although where a certain strength is specified the cement submitted by the contractor should be tested to see if it meets the requirements of the specifications.

In comparing different brands of cement great care should be used to see that the same kind and quality of sand is used in each case, as difference in the sand might cause as much difference in the results as there would be between the cements. It is recommended that an average of at least five briquettes of each brand of cement be taken as the strength of the cement.

The manner of making the briquettes should be as follows: On a thick glass plate lay sheets of blotting paper soaked in water, and on top of each sheet place a mould wetted with water. Before mixing the mortar an experimental batch should be made to determine the exact amount of water required to mix the cement to the proper consistency. If the cement is mixed with sand, both the cement and the dry sand should be carefully weighed to get the desired proportions, 1 to 1 or 1 to 3, as desired, and the sand and cement thoroughly mixed dry in a tight box. All the water required for mixing should be added at once, and the whole mass thoroughly and rapidly mixed for five minutes. "With the mortar so obtained the moulds should be at once filled with one filling, so high as to be rounded on top, the mortar being well pressed in. The projecting mortar should then be pounded with a trowel, first gently and from the side, then harder into the moulds, until the mortar grows elastic and water flushes to the surface. A pounding of at least one minute is essential. The mass projecting over the mould should now be cut off with a knife and the surface smoothed." The briquettes should be removed from the moulds as soon as they are hard enough to stand it without breaking, and should be placed in a box lined with zinc and provided with a cover. The briquettes should remain in the box twenty-four hours, after which they should be placed under water, to remain until tested. They should be constantly covered with water until tested, which should be done as soon as they are taken from the water.

Age of Briquette for Testing. - Half of the briquettes are generally tested at the end of seven days, and the remainder at the end of twenty-eight days. If it is impracticable to wait twenty-eight days they may be tested at the end of one and seven days respectively, and the ultimate strength of the cement judged by the increase in strength between the two dates. When sand is used in making the briquettes it is desirable to wait until the briquettes are twenty-eight days old.