This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
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 improved 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 appreciably 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 tensile 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, moreover, 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 contradictory, and this is perhaps the reason for the original writing of such a specificatiom 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 28 days 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 days 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 expected 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.