90. Portland Cement Mortar. A barrel of Portland cement will contain 370 to 3S0 pounds, net, of cement. Its capacity averages about 3.3 cubic feet, although with some brands the ca pacity may reach 3.75. The expansion when the cement is thrown loosely in a pile or into a measuring box, varies from 10 to 40 per cent. The subject will be discussed further under the head of "Concrete." 91. Lime in Cement Mortar. Lime is frequently employed in the cement mortar used for buildings, for a combination of reasons: (a) It is unquestionably more economical; but if the percentage added (or that which replaces the cement) is more than about 5 per cent, the strength of the mortar is sacrificed. The percentage of loss of strength depends on the richness of the mortar.
(b) When used with a mortar leaner than 1: 2, the substitution of about 10 per cent of lime for an equal weight of cement will render concrete more water-tight, although at some sacrifice in strength.
(c) It always makes the mortar work more easily and smoothly. In fact, a rich cement mortar is very brash; it will not stick to the bricks or stones when striking a joint. It actually increases the output of the masons to use a mortar which is rendered smoother by the addition of lime.
The substitution of more than 20 per cent of lime decreases the strength faster than the decrease in cost, and therefore should not be permitted unless strength is a secondary consideration and the combination is considered more as an addition of cement to a lime mortar in order to render it hydraulic.
92. Effect of Re-gauging or Re-mixing Mortar. Specifications and textbooks have repeatedly copied from one another a requirement that all mortar which is not used immediately after being mixed and before it has taken an initial set must be rejected and thrown away. This specification is evidently based on the idea that after the initial set has been disturbed and destroyed, the cement no longer has the power of hardening, or at least that such power is very materially and seriously reduced. Repeated experiments, however, have shown that under some conditions the ultimate strength of the mortar (or concrete) is actually increased, and that it is not seriously injured even when the mortar is re-gauged several hours after being originally mixed with water.
Such a specification against re-mixing is never applied to lime paste, since it is well known that a lime paste is considerably im proved by being left for several days (or even months) before being used. This is evidently due to the fact that even during such a period the carbonic acid of the atmosphere cannot penetrate appreci ably into the mass of the paste, while the greater length of time merely insures a more perfect slaking of the lime. The presence of free, unslaked lime in either lime or cement mortar is always injurious, because it generally results in expansion and disruption and possibly in injurious chemical reaction.
Tests with Portland cement have shown that if it is re-mixed two hours after being combined with water, its strength, both ten sile and compressive, is greater 'after six months' hardening, although it will be less after seven days' hardening, than in similar specimens which are moulded immediately after mixing. It is also found that
the re-mixing makes the cement much slower in its setting. The adhesion, mdreover, is reduced by re-mixing, which is an important consideration in the use of reinforced concrete.
The effects of tests with natural cement are somewhat contra dictory, and this is perhaps the reason for the original writing of such a specifications The result of an elaborate series of tests made by Mr. Thomas F. Richardson showed that quick-setting cements which had been re-mixed showed a considerable falling off in strength in specimens broken after 7 days and 2S clays of hardening, yet the ultimate strength after six months of hardening was invariably increased. It is also found that for both Portland and natural cements there is a very considerable increase in the strength of the mortar when it is worked continuously for two hours before mould ing or placing in the masonry. Such an increase is probably due to the more perfect mixing of the constituents of the mortar.
The conclusion of the whole matter appears to be, that when it is desirable that considerable strength shall be attained within a few clays or weeks (as is generally the case, and especially so with rein forced-concrete work), the specification against re-mixing should be rigidly enforced. For the comparatively few• cases where a slow acquirement of the ultimate strength is permissible, re-mixing might be tolerated, although there is still the question whether the ex pected gain in ultimate strength would pay for the extra work. It would be seldom, if ever, that this claimed property of cement mortar could be relied on to save a batch of mortar which would otherwise be rejected because it had been allowed to stand after being' mixed until it had taken an initial set.
93. Proportions of Materials for Mortar. (1) Lime Mortar. As previously stated in section 88, a barrel of unslaked lime should be mixed with about S1 cubic feet of water. This will make about 9 cubic feet of lime paste. Mixing this with a cubic yard of sand will make about 1 cubic yard of 1 :3 lime mortar. This means approxi mately 1 volume of unslakcd lime to S volumes of sand.
(2) Cement Mortars. The volume of cement depends very largely on whether it is loosely dropped in a pile, shaken together, or packed. The practical commercial methods of obtaining a mix ture of definite proportions will be given later under "Concrete," section 94. Natural cement mortars arc usually mixed in the 1:2 ratio, although a 1: 1 mixture would probably be used for tunnel work or bridge abutments where natural cement would be used at all. Portland cement will be used to make 1: 3 mortar for ordinary work, and 1: 2 mortar for very high-grade work. As previously stated, a small percentage of lime is sometimes substituted for an equal volume of cement in order to make the mortar work better.