MORTAR- MAKING, 895 



or lesser degree this property of solidifying without the assist- 

 ance of the atmosphere. From this property, which enables 

 them to harden under water, they are called hydraulic limes, 

 or hydrauHc cements. 



As pure lime mortar must combine with carbonic acid that 

 it may harden or set, and as in this combination it must part 

 with the Avater contained in it, it follows that hydrate of pui-e 

 lime in a state of paste, if kept moist, will renniin for an in- 

 definite period without absorption of carbonic acid and con- 

 sequently unfit for use as a cement, whilst if exposed to the dry 

 air without pressure, a small quantity of carbonic acid gas is 

 gradually absorbed from the atmosphei'e; but the lime 

 assumes the form of powdered chalk or marble, which is 

 wholly useless as a cement, no longer forming paste with water. 



Hydraulic lime is now being most extensively used in 

 America, and is generally recommended by professional men 

 in that country for its superior and uniform properties and 

 rehability. In a very interesting pai)er entitled "High Class 

 Mortars," read at a meeting of the Engineers' Club at 

 Philadeljjhia, the writer spoke particularly of the growing 

 demand for high-class mortars, and said : — " When immense 

 buildings of eight, ten, and twelve stories high are run up in 

 a single season the demand for better mortar is making itself 

 felt. 



In the olden times, when buildings like the Strasbourg 

 Cathedral took centuries to build, the defective qualities of 

 ordinary lime mortar were not so noticeable, as the weight 

 was super-imposed gradually, and allowed the mortar time to 

 harden." This is no longer the case, and the ordinary lime 

 mortar inside the thick walls, nearly deprived of air, sets 

 slower than the mortar on the outside where the air reaches, 

 and as the weight is piled on a settling takes place in the 

 wall where the mortar is still soft. This is due to the fact 

 that ordinary lime mortar sets mainly through the aid of an 

 external element, viz., carbonic acid gas, which it takes up 

 fi-om the atmosphere slowly. Deprive it of air, and the 

 setting never takes place. To meet this emergency ex- 

 perienced architects and engineers are looking to mortar 

 having hydraulic properties or in other words cement, not 

 only for use in damp foundations but for use in entire brick 

 and stonework of large buildings where speedy work is desii'ed. 



Cement (i.e., hydi'aulic lime) mortar difiers from ordiruiry 

 lime (in its setthig) in that it sets within itself without the 

 aid of external elements. The great rule in mixing mortars 



