MORTAR. The cementing or binding materials used in building operations, which have a lime base, in one or other of Its forms, are known technically by the name of Montana; and they are either com posed of the ordinary varieties of limes and sand ; or of phuiter, or of cement, mixed with the same material; or again, in certain eases, of limes mixed with peculiar volcanic productions, such as PUZZUOLANOS and 'Dines. The tern mortar is therefore a generic one, though its use is almost confined in practice to the mixtures of limo and sand employed for the purpose of building.
In the article LIME, reference has been made to the theoretical con ditions affecting the proportions of amid it may bo advisable to mix with the respective varieties of that material; but it may be hero stated that precisely in the ratio of the expansion of the lime during its absorption of water is it able to support a larger volume of rand for the purpose of mortar making. It thence follows that, as the hydraulic limes, or those which set under water, do not expand much under those circumstances, they support a smaller quantity of sand than the rich, or pure, limes can do. In fact, the best hydraulic limes will not carry, to use a workman's phrase, more than about 14 to 2 proportions of sand to 1 of the lime ; both being measured in bulk, and after the slaking of the lime ; whilst the rich limes will carry as much as 3 proportions of sand to 1 of lime, when made into mortar. Thus the better a lime is fitted for the purposes of external masonry, the smaller will be the proportion of mend it will bear ; and to trate this priociple by the practice of our London, or southern, builders, it may suffice to state, that the chalk lime is usually mixed with 3 portions of sand to 1 of lime ; the semi-hydraulic graystone lime, from the chalk-marl, with 21 sand to 1 lime; whilst the blue-lias lime is mixed with proportions of 1/ sand to 1 lime.
The operation of ordinary mortar making depends for its success upon the energy and the stability of the crystallisation of the lime ; kir the sand simply acts the part of furnishing a nucleus around which the crystals may arrange themselves, and unless it should happen to contain a notable proportion of silica in the soluble state it will remain chemi cally inert. Now the pure carbonates of lime yield a caustic lime, which is very easily soluble in water, and which does not crystallise at all when kept in large masses ; and the only lime compounds able to solidify under water—whether salt or fresh—are those which contain the various proportions of silicate of alumina, described in the article LIME. It is on this account that in countries, such as Holland, or
Central Italy, where the natural limestones are remarkably pure car bonates, it is customary to introduce into the mortars to be employed in hydraulic building the tram and puzzuolanos obtained in the same districts, because those materials are composed of the dchydrised silicate of alumina, which when ground fine will combine chemically with the pure bydrated carbonate of lime. It was supposed that any ilehydrised silicate of alumina would produce this result, and the French and German engineers, following the occasional practice of the ancient I tomans, actually substituted underburnt,brick dust for the trans or puzzuolano, in some marine walling, and some canal works. But it would appear that peculiar conditions of the silicate of alumina are requisite, at least in the case of mortars intended to resist the action of sea-water ; because in almost every instance the mortars composed of pounded bricks, lime, and sand, have disintegrated after a short ex posure to the sea ; those which were only exposed to the action of fresh water have resisted more successfully, and it is notorious that the Roman mortars possess a very extraordinary hardness and durability, even at the present day. Without dwelling upon these still rather obscure points in the theory of the action of limes, it may suffice here to observe that the above considerations point out the nature of the mortars required for particular positions, or for particular works. Thus, the mortars required for hydraulic works in sea-Water must be composed of the most energetic cements, and the latter must even he used without any mixture of sand, if the mortars are intended to be at once exposed to the action of the sea ; if, however, the mortars can be protected for a sufficient length of time to allow the crystallisation to take place effectually, the cements may be mixed with sand, or even the best hydraulic limes may be used. For river work, in non-tidal rivers especially, the best hydraulic limes should be used ; and for all exposed walling, the common descriptions of hydraulic lime may be resorted to ; whilst the purer limes should be reserved for internal works, and for surface renderings of small thickness.