Properties of Cementing Materials.
Before proceeding to treat in detail the native productions, it is necessary to consider the properties of limes and cements generally. In doing so, I should begin by stating that the terms “Lime” and “Cement,” although always used to denote different and distinct articles, are applicable
to either of them. The cementing ingredients are the same in both cases, the only difference being in the proportions in which they occur. Pure lime is practically worthless for building purposes; it never acquires the necessary cohesive strength in any situation, and never hardens at all in a damp place. In order to make good mortar the limestone must contain a mixture of clay—the proportion varies from 8 per cent. in ordinary building lime to 35 per cent. in strong hydraulic cement. If it were necessary to have a clear dividing line between limes and cements, the best place to strike it would be at the neutral point where the adhesive and cohesive forces are equal. The particles of rich and moderately hydraulic limes adhere more readily to a foreign substance than to each other, but the conditions are reversed with strong hydraulic limes and cements. This gives the only tangible difference I can imagine between the two articles, but as it does not admit of a practical application, the distinction would only be valuable from a scientific point of view.
Limes and cements are usually divided into four classes, according to their properties and strength—
The Common or Rich Limes that contain less than 10 per cent. of clay or other impurities.
Poor Limes, in which the impurities consist of from 10 to 25 per cent. of sand or other insoluble ingredients that will not enter into chemical combination with the lime.
Hydraulic Limes, such as contain from 10 to 30 per cent. of alumina and soluble silica.
Hydraulic Cements, containing from 30 to 40 per cent. of alumina, soluble silica, and other impurities.
In addition to the ingredients named, each of the above classes frequently contains small quantities of iron, manganese, magnesia, potash or soda, with sulphuric and other acids, which do not seem to have an injurious effect on the cementitious properties of the article; on the contrary, some of them, such as iron and soda and some of the acids, are always present in the best cements;—the quantity, however, of all foreign substances, except silica and alumina, seldom exceeds 5 per cent. We may therefore assume, shortly, that our mortars are simply lime and clay in varying proportions.
As already stated, the common or rich limes are comparatively useless where strength is required, and absolutely worthless in a damp situation. They are easily burned and slaked, swell to a great extent in slaking, shrink in drying, and are soluble in water when set. Their adhesive properties are stronger than their cohesive ones, consequently they cannot be used without a large admixture of sand. It is common to hear the expression that
mortar is injured by too much sand, but the chances are that its bad qualities are the result of the opposite condition, and, above all that, the sand and lime are not properly mixed. If greater care was exercised in this behoof, so that an approach could be made to the theoretical maximum of an atom of sand between each atom of lime, the result would be an immediate doubling of the strength of rich lime mortar.
Poor Limes possess all the bad qualities of the rich ones, and have an additional drawback in irregularity of consistency through not slaking so readily, which necessitates grinding.
Hydraulic Limes are frequently sub-divided into three or four sections, ranging from slightly to eminently hydraulic, the former being practically a rich lime, and the latter a cement. These limes do not slake readily, nor do they expand much in the process. The higher kinds slake so slowly and so imperfectly that they are always pulverized by grinding. Hydraulic limes set under water in from three to fourteen days, according to the strength of the sample.
Hydraulic Cements cannot be slaked by water in the usual way; they are, properly speaking, not calcined, but vitrified. The produce of the kilns resembles slag from a blast furnace, and it requires the aid of stone-breakers, iron rollers, and French burr mill-stones to convert it into the cement of commerce. In common with the higher kinds of hydraulic lime, cement does not require any admixture of sand to make it into mortar; the maximum strength is obtained by using it in a pure state. Some hydraulic cements set under water in a few minutes, but the best kinds take a few hours. In seven days the latter attain a tensile strength of 250 pounds to the square inch.
The quality of limestones cannot be determined by a knowledge of the geological formation in which they occur, nor by their general appearance. Hydraulic limes are perhaps more plentiful in what may be called the mediæval rocks—cretaceous to carboniferous—than in any others; but, as they are frequently met with in the formations above and below those named, we may give them an almost universal range of locality. The character of the stone seems to be determined chiefly by its immediate surroundings—the outer beds are argillaceous or silicious, according as the adjoining stratum is clay or sand, and the whole rock is influenced by the manner in which it was deposited, and the subsequent changes to which it was subjected. If the lime had been deposited in still, clean water on a rocky bottom, and had attained a considerable degree of hardness before being disturbed by convulsions from below, or pressure from above, we might expect it to be comparatively pure; but if deposited in an estuary where the water is muddy and the bottom soft, and where floods leave
occasional beds of silt and sand, the stone cannot fail to contain impurities. Even after the deposit has taken place the stone may be altered by mechanical and chemical agencies, there being a peculiar relationship between the lime and clay in hydraulic limestone that seems to be easily affected by external causes. A good illustration of the influence of its surroundings on the character of the stone is found at Mr. Macdonald's quarries, Otago Peninsula. The rock is much shattered, and divided into large blocks by “backs” running through it in all directions. The blocks in one of the beds produce two distinct varieties of stone, the analyses of which are given in. Nos. 13 and 15, Table III. The light-coloured stone occupes from two to three feet of the outside of the block, and gradually merges into the dark one which composes the heart;—they vary little in consistency, but, as will be seen from the table, there is a great difference in their composition. Assuming the block was originally homogeneous, of which there can be little doubt, we find that the crust has lost 4 per cent. of carbonate of lime and 2 ¼ per cent. of carbonate of magnesia, while, on the other hand, it has gained 2 per cent. of iron in addition to the increased percentage of silica and alumina due to the abstraction of the lime and magnesia.
Hydraulic limestones are generally compact in texture and dark in colour, grey, blue, drab, or brown being the prevailing colours; white indicates pure lime. It does not, however, follow that all the dark-coloured limestones are hydraulic, for they may contain sand and other insoluble matters that neutralize the effect of the clay; and the darkest of all limestones—black marble—is almost pure carbonate of lime. Still, a rule may be established in a negative manner by saying that no white limestones produce hydraulic lime.
Notwithstanding the advance made in all the practical sciences within the last few years, there is still a doubt as to the causes that produce the setting and hardening of lime and cement mortar. The old theory was that all mortars hardened by the absorption of carbonic acid from the atmosphere, and it was supposed that in time the quantity absorbed would equal that expelled in burning, so that the mortar would revert to its original carbonate, and become again a limestone. It is true that rich limes will not set without carbonic acid. The mortar in the inside of a bastion at Strasbourg was found, after 160 years, to be quite soft, and the same thing was observed in a masonry pillar, nine feet in diameter, at St. Peter's, Berlin, its age being 80 years. It has also been found by experiment that a mortar of rich lime will not set in the exhausted receiver of an air pump. But carbonic acid alone will not perfect the hardening process, consequently it is supposed to be assisted by the crystallization of the carbonate of lime between and around the particles of the aggregates.
Without such extraneous aid it is difficult to account for the hardening of mortar in thick walls. The operation proceeds from the outside, consequently every advance made is a barrier to the next in so much that it excludes the air from the softer mortar inside. There is every reason to believe that hydrate of lime, when exposed to the atmosphere, will revert to its original carbonate; but the process is such a slow one that it may be almost classed with the geological epochs. The oldest mortar in the world, that from a Phœnecian temple in Cyprus, is still far short of the ingredients it possessed when a limestone.
The induration of hydraulic mortars is attributable in a small degree to the same causes as affect the rich ones, but principally to the formation and crystallization of complex silicates of lime and alumina, the precise nature of which is imperfectly understood. It is quite evident that the absorption of carbonic acid has very little to do with the setting of hydraulic mortar, for against its slow action already noticed we have the fact that large blocks of cement concrete harden uniformly to the consistency of stone in a few months under water, which proves that the setting property is inherent, and not the result of external influences.
The treatment they receive in burning has a considerable effect on the quality of limes and cements of all kinds, but more particularly on those that are only moderately hydraulic. Under burning has been known to impart a spurious hydraulicity to rich limes, and over burning occasionally destroys that property in cement, but as a rule there is little trouble in obtaining maximum results with these extreme classes. The burning of hydraulic limes is a much more delicate operation, the niceties of which can only be acquired by long experience in the art generally, and considerable practice with the actual materials that are to be operated on.
As already indicated, the ordinary aggregates are essential to the induration of rich limes, but in the higher hydraulic varieties and in cement they are simply diluents. As rich limes do not possess the faculty of expelling any excess of moisture with which they are in contact, it is advisable to employ a porous aggregate, such as the sands produced by aluminous and calcareous rock, but when the aggregate is only employed to weaken the mortar by making it go further, there is little danger in using hard silicious sand, provided it is free from earthy impurities; indeed, this is an indispensible condition in all aggregates. It has been ascertained by experiment that the best proportion of sand for rich limes is 2 ½ to 1, and for ordinary hydraulic limes 1 ¾ to 1. This explains the partiality of builders to rich limes. It will, in their own phraseology, “carry more sand,” which means that strength and comfort are sacrificed for an insignificant saving in cost.
In order to institute a comparison between the various articles under discussion, I give the following table of tensile strength per square inch in pounds on limes, cement, and mortar one year old.