Oost of Mortar

Frequently, in practice, cement mortar which has taken an initial set, is re-mixed and used. Masons generally claim that re-tempering, i.e., adding water and re-mixing, is beneficial; while engineers and architects usually specify that mortar which has taken an initial set shall not be used.

Re-tempering makes the mortar slightly less "short" or "brash," that is, a little more plastic and easy to handle. Re-tempering also increases the time of set, the increase being very different for different cements, But on the other hand, re-tempering usually weakens a cement mortar. A quick-setting natural cement sometimes loses 30 or 40 per cent of its strength by being re-tempered after s' an ling 20 minutes, and 70 or 80 per cent by being re-tempered after standing 1 hour. With slow-setting cements, particularly portlands, the loss by re-tempering immediately after initial set is not material. A mortar which has been insufficiently worked is sometimes made appreciably stronger by re-tempering, the additional labor in re mixing more than compensating for the loss caused by breaking the set.

The Loss of strength by re-tempering is greater for quick-setting than for slow-setting cement, and greater for neat than for sand mortar, and greater with fine sand than with coarse. The loss increases with the amount of set. If mortar is to stand a consider able time, the injury will be less if it is re-tempered several times during the interval than if it is allowed to stand undisturbed to the end of the time and is then re-mixed. Re-tempered mortar shrinks less in setting than fresh mortar, which is an advantage in joining new concrete to old (see § 345).

The only safe rule for practical work is to require the mortar to be thoroughly mixed, and then not permit any to be used which has taken an initial set. This rule should be more strenuously insisted upon with natural than with portland cements, and more with quick-setting than with slow-setting varieties.

Matz WITH CEMENT.

Cement mortar before it begins to set has no cohesive or adhesive properties, and is what the mason calls "poor," "short," "brash"; and consequently is difficult to use, since it will not stick to the edge of the brick already laid suffi ciently to give mortar with which to strike the joint. The addition of a small per cent of lime makes the mortar "fat" or "rich," and causes it to work better under the trowel.

The lime should be slaked before being mixed with the cement. Formerly it was necessary to use lime paste for this purpose; but now either lime paste or dry hydrated lime may be used. Dry hydrated lime made from pure limestone contains 32 per cent of water, but lime made from either an impure or a magnesian lime. stone will contain less, depending upon the amount of impurities or of magnesium present. Lime paste usually weighs about. 21 times as much as the unslaked lime.

The effect of the lime upon the strength of the mortar will vary with the character of the cement, the fineness of the sand, and the proportions of cement to sand. The addition of unslaked lime equal to 5 to 10 per cent of the cement does not materially decrease the strength of a 1:3 or a 1 :4 mortar, and frequently slightly increases it. In all cases the addition of 5 to 10 per cent of lime decreases the cost more rapidly than the strength and hence is economical; but the substitution of more than about 10 per cent decreases the strength more rapidly than the cost, and hence is not economical. The economy of using lime with cement is, of course, greater with portland than with natural cement owing to the greater cost of the former. One large manufacturer of natural cement grinds 15 per cent of hydrated lime with the cement and sells the mixture as " bricklayer's cement." The addition of lime as above to a 1 : 3 or a 1 : 4 cement mortar makes it more dense, and hence more nearly waterproof; and also increases its adhesive strength more than its cohesive strength.* The addition of lime to cement mortar does not materially affect the time of set, and usually slightly increases it.

The discussion in the preceding section refers to the addition of a comparatively small portion of lime to a cement mortar; but it is also common to add a small per cent of cement to a lime mortar when a mortar of greater strength or greater activity is desired than can be obtained with lime alone. The cement adds to the strength of the mortar, but not proportionally to the increase in cost. When a stronger or quicker-setting mortar is desired than can be obtained with lime alone, it would be cheaper to use a lean cement mortar, if such a mortar were not so difficult to handle with a trowel.