so, it is not necessary to suppose that the sand exercises a chemical action on the lime: because we know, for example, that in the preparation of rockcandy, and of verdigris or acetate of copper, &c. it is only necessary to stretch threads, or place sticks of wood, in the solutions, to determine the crystals to group around them like clusters of grapes, while none show themselves any where else. Sands in this, do no more than aid the tendency of hydraulic limes to take cohesion; and their influence is null when this tendency does not exist, as for example in fat lime. If the experiments with this last lime be attentively examined, it will be seen that they harden better alone than when mixed with saud: that the particles of hydro-carbo nate have greater cohesion amongst themselves than adhesion to sand; whence it naturally follows that sands which divide the mass most thoroughly, that is to say, the finest sands, are the worst, because in an equal section the extent of surfaces in contact is the greatest. The contrary takes place in hydraulic limes, according to Mr. Vicat. If some experiments of General Treussart do not accord with these ideas, it seems to be owing to the too great quantity of sand used in his mortars.

§ 55. On the solidification of deteriorated hydraulic limes.

We have seen, at the commencement of this article, that hydraulic limes slaked spontaneously in the air, and not used for some time, lost their energy, more and more. We will add an analogous fact that we have had occasion to observe; it is that the hydraulic lime of Doué preserved for two years in flagons, well stoppered, so as to prevent any change of air, or the absorption of humidity, remained in fragments it is true; but these fragments had lost the power of slaking, and also of setting under water, after having been tempered like plaster. What has occurred in this case? This we have to explain.

It is well known that the limestone affording hydraulic lime, when treated, before calcination, with muriatic acid, leaves generally an insoluble residue composed of silex and alumine; but that immediately after calcination, the same stone, now become quick lime, is completely soluble, which shows the existence of a chemical combination: this being premised—we have taken hydraulic lime which had been slaked in air for two months; treated it with muriatic acid in excess, and it gave a considerable gelatinous residue: we have treated hydraulic lime preserved for two years in the same way, and the gelatinous residue was so abundant that we were forced to believe there could be no silex dissolved with the lime. But however that might be, is it not evident that the silicate of lime is decomposed, at least in part? that, therefore, in the case of the deterioration of hydraulic limes, the silex having taken cohesion, is only to be considered as being mixed intimately with the lime, and consequently can do no more than perform the office of puzzolana mixed in small doses with fat lime. Hence the great inferiority of the results, and the necessity of remedying them by the method of General Treussart.

§ 36. On the solidification of Puzzolana Mortars.

Let us pass now to the case of puzzolana mortars. According to what has been said before, an indispensable condition to the solidification

of lime under water, is, that a small portion of this same lime be first rendered insoluble-each of the insoluble particles becoming the centre of attraction with respect to the surrounding layers of hydrate. Is this condition fulfilled on mixing fat lime with puzzolana? This is put beyond doubt by Mr. Vicat showing that puzzolanas have the property of precipitating lime from its solution in water, and that their energy is proportionate to the quantity of lime water they can thus precipitate. Whatever may be the cause of this property, whether it does, or does not, belong to a chemical combination between the lime and puzzolana-a combination difficult to conceive on account of the state of cohesion of the silex-a cohesion assuredly much greater than in deteriorated hydraulic lime, it is not less true that this fact suffices to establish a satisfactory analogy between the solidification of puzzolana mortars, and those made of hydraulic lime. It may be inferred from thence that the mixture of puzzolana with slightly hydraulic lime will give good results, because in such limes the proportion of hydrate would not be superabundant; with limes very hydraulic, it may be inferred, on the contrary, that this mixture would become injurious, because the proportion of lime would be too small, and there would result a kind of plastic cement, at the term of complete calcination.

The puzzolanas are mixed like sands, in the proportion of one and a half to two and a half in volume, to one of lime in paste. This proportion supposes, no doubt, much more silex, than is required to constitute hydraulic limes; but it must be remarked that in these limes, all the particles act in the most favourable circumstances possible, while in the above mixtures, they are collected in grains of some size, acting only by their surfaces, and their action being weakened by the cohesion that they already possess in a high degree.

§ 37. On the solidification of Plastic Cements.

Let us, in the last place, see to what degree the principles previously admitted will serve to explain the solidification of plastic cements, in the most general case; that is to say, as was explained at the end of chapter XVIII, section 25.

1st. In fat limes imperfectly calcined or super-calcined, the particles carbonated or super-calcined, requiring to be worked to make a paste with water, and being convertible into hydrate only with difficulty, it may be conceived to be not impossible that they play the same part as lime rendered insoluble by silex or by puzzolana, and that thus the commencement of hydraulic quality is obtained.

2d. In hydraulic limes imperfectly calcined, the particles of carbonate act in the same manner, but with this advantage that their influence on the solidification is increased by that of the silicate of lime which is present. In the same lime stones carried to the second maximum of energy, the particles of carbonate are replaced by the equally insoluble super-calcined particles.

3d. In the septaria furnishing the ordinary plastic cement, the clay is too abundant to leave, at the term of complete calcination, the proportion of hydrate of lime necessary to a good solidification: the imperfect calcination, then, has for object to render the silex only partially soluble in acids so that it may the more resemble the mode of action of puzzolanas.

Lastly, at the term of super-calcination, the clay of these same lime

stones passes to a state of less energetic puzzolana than at the term of complete calcination, and thereby leaves a greater proportion of lime susceptible of conversion into hydrate.

The developments into which we have entered relative to the solidification of mortars in general, are far, no doubt, from exhausting the subject; and require to be sustained by numerous experiments. We are fully aware of this; but our object not being to present a treatise on mortars, we have restricted ourselves to so much as is useful in understanding the operations of lime burning, which we have described.

Sect. IV. SOME RESEARCHES RELATIVE TO LIME AND

MORTARS.

BY M. COURTOIS,
Engineer of Roads and Bridges.*

(From the Annales des Ponts et Chaussées, Paris, 1834.)

The art of making good mortars is every day becoming better appreciated, from the influence it exercises in the economy and on the duration of constructions. If, in every great work, the constructors would make known the researches they have instituted, and the conclusions to which they have been led, we should soon know the resources presented by each locality, and be every where able, progressively, to improve the manufacture of

mortars.

Having had occasion at various times to make numerous essays on lime and hydraulic mortars, and on the means of making them economically, I think I am fulfilling a duty in presenting a summary of the principal results.

My investigations have had particularly in view the qualities of the mixtures, or combinations of lime and clay, comprised between hydraulic lime and cement; but in order not to leave any void in the scale of various proportions to be tried, I thought it best to try all combinations possible with 100 parts of the mixture.

I shall examine successively,

1st. The elements of these combinations, viz. limestones, clayey earth, and marl; I shall give the analysis of each, and examine their respective properties.

2nd. The combinations made with the most simple proportions, and which, for greater clearness in the tables, I shall call combinations of the first order: these will be mixtures of 1, 2, 3, 4, &c., parts of clay, with 9, 8, 7, 6, &c., parts of lime.

3rd. The combinations of the second order, or hydraulic pastes, result

*The object of these researches was to give to fat lime the hydraulic quality necessary, and to mortars a determinate resistance, at a small expense.

ing from the preceding compounds, mixed, in their turn, with various proportions of fat lime.

4th. Combinations of the third order, or mortars resulting from the mixture of the preceding hydraulic pastes with twice their volume of sand.

I shall give, moreover, the experiments made with each hydraulic paste, and each mortar, to determine the hardness, after various periods of immersion, and to determine at the same time the resistance to rupture of each of the combinations.

I shall afterward examine the natural substances analogous to these sev eral artificial combinations: indicating how they may be known, and showing the abundance of these hydraulic substances in nature.

Following my observations on the resistance of artificial hydraulic pastes, I shall give analogous experiments applied to the comparison of a great number of natural hydraulic pastes, and mortars made of these pastes.

it

Lime.

CHAPTER XX.

Artificial combinations of Lime and Clay.

Without going into details as to the chemical properties of lime, may suffice to say that this substance forms the base of limestones, and gypsum, or plaster of Paris; and that it combines, at a high temperature, with silex, forming what is called a silicate of lime. In the humid way, it appears to combine with clay, either crude or calcined, affording a hydrosilicate with a base of lime and alumine.

Lime, as it is used in making mortars, is obtained by the calcination of limestones, which are carbonates or subcarbonates more or less pure, found in all the formations; it is obtained also from some animal productions, as shells of oysters, and other shells.

The effect of calcination is to drive out the water and carbonic acid. By breaking the stone into pieces about one and a half inch square and passing a current of steam through the ignited mass, a cubic metre (35.32 cubic feet) of lime stone may be converted into lime, in a kiln with a continual fire, by burning about seven cubic feet of sea-coal:* in cold and moist seasons the consumption of coal is augmented; and amounts, sometimes, to a third of the volume of the stone.

If the limestone be argillaceous, the fire should be managed cautiously, and be less violent than to calcine a nearly pure carbonate of lime; for when the fire is too high, the lime and clay fuse, and give a vitreous matter inert, or without causticity, which is a double silicate of lime and alumine.

Whenever a piece of limestone is cooled before the calcination is completed, it becomes necessary to wet it before putting it again in the kiln;

*On the canal of Ardennes, where I caused large quantities of lime to be burned, the lime burners threw into the kiln one measure of sea-coal to five, six and even seven measures of limestone; it should be observed, however, that the stone was a chlorite-chalk, easier to convert into lime than limestones generally.

At Theil in the department of Ardèche, whence the best hydraulic lime that I know of is derived, they consume but one sixth of a measure of sea-coal in burning one measure of lime.

AUTHOR.