HYDRAULIC CEMENT 827 



its pores, it may become harder, or it may after a time partially break up, with or 

 without sensible warmth. If a piece of fresh pure lime be similarly treated, in a few 

 minutes it begins to heat and swell, and the violence of the action is such as to throw 

 it abroad in a bulky impalpable powder. In the latter case one affinity only comes 

 into play, that of the lime for the water ; but with the cement, a second affinity is in 

 operation. By the action of the heat in the process of burning, the affinity of the 

 lime for the silicic acid of the clay is brought into play, and modifies the violence of 

 its affinity for water, and when the temperature is sufficiently high, the lime and the 

 silicic acid enter into chemical combination, and the compounds so formed, on being 

 moistened with water, pass into hydrates in a gradual and quiet manner, without any 

 of the tumultuous action which is so remarkable in the combination of pure lime with 

 water. 



The hydrated silicates resulting are not afterwards appreciably affected by water, 

 and so much of the lime as the silicic acid can take up passes from a soluble to an 

 insoluble condition, thus affording a cementing material suitable for hydraulic pur- 

 poses. When the heat of the kiln is too far urged, vitrification ensues from the 

 complete formation of an anhydrous silicate, and a comparatively inert substance is 

 produced, which undergoes no change on being mixed with water, or will only unite 

 with it after considerable periods of time. 



The hydraulic limes are intermediate between the cements and the pure limes, and 

 partake of the character and mode of action of each class of substances. The pro- 

 portion of clay in them is generally smaller than in the cements, and the lime 

 sufficiently predominates to produce by its violent affinity for water the degree of 

 heat, expansion, and amount of vapour requisite to throw them into powder. When 

 the quantity of clay, as compared with the lime, is small, the violence of the slaking 

 is scarcely less than that of the pure limes, but in proportion as the clay is relatively 

 larger in amount, the heat generated in combining with the water is less, and the 

 lime slakes to a powder more imperfectly and with less increase of volume ; until at 

 last, on reaching the boundary of the cements, the slaking must be urged on by con- 

 fining the heat generated, and the lime allowed to lie longer to ensure the completion 

 of the action before it is mixed with sand and made into mortar. 



In the ordinary kiln-burning, some portions of the hydraulic limes, by too great 

 heat of the charge, are brought into that condition when the silicates formed combine 

 very slowly with water. The silicic acid having undergone a great modification, the 

 powders of such limes can unite and form new hydrated compounds, but by a process 

 so slow that it may be measured by weeks, months, or years, instead of minutes, hours, 

 and days, thereby, in badly-mixed mortar, doing great injury ; expansion taking place 

 after the mortar has been laid in the work, it has been known to throw down arches, 

 and lift heavy masses of masonry. 



If hydraulic limes be allowed to dry too rapidly, they will not properly harden. 



With regard to cements, some portions of a charge are often spoiled by lowness of 

 temperature in burning, and thus prevent the desired rapidity of forming silicates, 

 and retard the solidification of the mortar without the assistance of water. 



Besides the proportion of clay and lime, and the temperature of burning, the com- 

 position of the clay has much to do with the rapid setting of hydraulic limes and 

 cements. The larger the amount of iron and alumina present, the more readily will 

 the lime and clay pass through the different stages to the formation of a perfect 

 cement, though temperature in burning has much to do with its character, for from 

 the same bed of chalk-marl have been made a hydraulic lime, a quick setting cement, 

 and a cement resembling the Portland cement. See LIME. 



As the cements may, and practically always do, partake of the slow solidifying 

 action of the hydraulic limes, so the hydraulic limes partake, when exposed to the air, 

 of the hardening action of the carbonic acid gas of the atmosphere, to which the pure 

 limes owe the external hard crust. 



The addition of puzzolana, trass, &c., often improves the hydraulic limes. The 

 action of these substances is similar to that of clay found naturally mingled with 

 the ^carbonate of lime in hydraulic limestones. These have been calcined by vol- 

 canic heat, and though burnt clay may produce a similar effect, it is rarely so perfect. 



Preparation of this Cement. The limestone to be employed in the manufacture of the 

 cement is burnt to quick-lime by any of the ordinary processes, avoiding over-burning, 

 as usual. Limes burned until they assume a blue or greenish tinge require peculiar 

 and very careful treatment to fit them for the purpose. 



The quick-lime is either wheeled in barrows or carried in baskets into an oven, 

 and is laid on the perforated arches forming the floor, to the depth of 1 foot 6 inches 

 on the crown, and 2 feet over the haimches. With a large-sized lime this depth 

 may increase a few inches, and when the lime is very small it must be somewhat 

 decreased ; or, in other words, the lime must not be so closely and deeply packed as 



