Mortar of common lime should not be employed in heavy masonry or in (lamp situations. Where the mass of masonry is large, the lime mortar will become hardened with great difficulty, and after a long time. The penetration of the final induration due to the absorption of carbonic acid is very slow. The observations of M. Vicat showed that carbonization extended only a few millimeters the first year and afterward more slowly. The induration of the lime along the surfaces of contact with a harder material is usually more rapid than in the interior of the mass of lime, and the strength of adhesion to stone or brick is often greater than that of cohesion between the particles of mortar.
9. Hydraulic Lime.—Hydraulic lime is obtained by burning limestone containing silica and alumina in sufficient quantities to impart the ability to harden under water. The hydraulic elements are present in such quantities that they combine with a portion of the lime, forming silicates and aluminates of lime, leaving the remainder as free lime in an uncombined state.
The hydraulic activity of a lime or cement, that is, its ability to harden under water, depends primarily upon the relative propor tions of the hydraulic ingredients and of lime. Silica and alumina are considered to be the effective hydraulic ingredients, and it is common to designate the ratio of the sum of the weights of silica and alumina to that of lime in the material its hydraulic index. The hydraulic index gives, therefore, within certain limits, a measure of the hydraulicity of the various classes of limes. It is to be remem bered, however, that there are other factors to be considered in judging of the action of lime than this simple proportion. The other ingredients may by their combinations withdraw portions of the active elements so as to modify the effective ratio between them, while the activity of the lime depends largely upon the state of combination in which the active elements exist. This is not shown by analysis, and may he greatly modified by the manipulation given the material during manufacture.
Limes with hydraulic index less than possess little if any hydraulic properties, and are known as common limes. When the hydraulic index is between 10/100 and 20/100 the lime is feebly hydraulic, and may require from twelve to twenty days to set under water. Hydraulic lime proper includes that of index from about 20/100 to 40/100. These may harden in from two to eight or ten days.
The quantity of free lime in the material is dependent upon the degree of burning, as well as upon the amount of lime contained by the stone. If the stone be underburned, the combination of the hydraulic elements with the lime is not complete, and more of the lime remains in a free state. For this reason, a stone of high hydraulic index may, when underburned, yield a lime, but burned at a high temperature becomes unslakable. The best limes are usually those which can he burned at a high temperature to complete the chemical combinations. It is necessary that sufficient free lime be present to cause the lime to slake properly, but it is also desirable that the quantity of uncombined lime be as small as possible, as the setting properties are due to the silicates and alu minates, while the hydrated lime remains inert during the initial hardening of the mortar.
According to Professor LeChatclier, limestone for hydraulic lime should contain but little alumina, as the aluminates are hydrated during the slaking of the lime, while the silicates are not affected, the heat of the slaking preventing their hydration.
The following is given as an average analysis of the best French hydraulic lime: Silica 22 Alumina 2 Oxide of iron 1 Lime 63 Magnesia 1.5 Sulphuric acid. 0.5 Water 10 100 It is important that the slaking be very thorough, as the pres ence of unhydrated free lime in the mortar while hardening is an element of danger to the work. Any lime becoming hydrated after the setting of the mortar may, by its swelling, cause distortion and perhaps disintegration of the mortar.
After the lime has been reduced to powder by slaking, it is forced through sieves which permit the passage of all pulverized particles but hold those of appreciable size, including the underburned rock and the overburned parts which refuse to slake. The residue left from the sifting of hydraulic lime is known as grapplers. This material is mainly composed of hard material more rich in silica and alumina than the other portions of the lime. The grapplers are frequently ground and sold as cement, and when properly handled may form cement of fairly good quality.
10. Hydrated Lime.—When quicklime is slaked with the quantity of water necessary completely to hydrate it, and the resulting mate rial is bolted to remove all unslaked particles, the result is a very fine white powder, commercially known as hydrated lime. This lime is sold on the market in barrels or bags, and it is in convenient form for use. Lime in this form may be kept for considerable periods without deterioration, provided it is protected from con tact with moisture.
Hydrated lime ordinarily weighs about 40 pounds per cubic foot, and contains approximately 75 per cent of quicklime. By mixing with about an equal weight of water, it may be reduced to lime paste, or lime putty, as it is commonly called in building oper ations. Lime paste occupies a slightly greater volume than the hydrated lime from which it is prepared.
The use of hydrated lime for mixing with cement mortar in ordinary masonry construction is rapidly increasing. It is also frequently used in small proportions in Portland cement concrete to make the concrete flow more smoothly, and sometimes to decrease the permeability of the mortar. (Sec Art. 23.) 11. Specifications for Lime.—In ordinary building operations lime is commonly employed in the form of quicklime and slaked where used. Usually the quality of the lime has been judged by its activity in slaking and no particular tests are specified. Tests of composition by chemical analysis and of completeness of slaking by washing through sieves are, however, frequently employed.
Hydrated lime is now largely used for mixing with cement mortar and for plastering work, and this use is rapidly extending. The tests employed for hydrated lime include chemical analysis, fine ness, and permanence of volume or soundness.
The American Society for Testing Materials has adopted standard specifications giving methods for making these tests. These speci fications are given in the Book of Standards of the Society or may be obtained in pamphlet form from the Secretary of the Society. As they are now undergoing revision they are subject to change and will not be given here.