6 



only at points where the mortar is exposed to the air, the ma- 

 terial in the middle of thick walls never becomes recarbonated. 

 In order to counteract the shrinkage which \vould otherwise 

 take place during the drying of the mortar, sand is invariably 

 added in the preparation of lime mortars, and as noted above, 

 it is probable that certain reactions take place between the lime 

 and the sand. Such reactions, however, though possibly con- 

 tributing somewhat to the hardness of old mortars, are only in- 

 cidental and subsidiary to the principal cause of" setting, recar- 

 bonation. The presence of impurities in the original limestone 

 affects the character and value of the lime produced. Of these 

 impurities, the presence of silica and alumina in sufficient quan- 

 tities will give hydraulic properties to the resulting limes ; such 

 materials will be discussed in the next group as Hydraulic 

 Limes and Natural Cements. 



Magnesian Limes. The presence of any considerable 

 amount of magnesium carbonate in the limestone from which a 

 lime is obtained has a noticeable effect upon the character of the 

 product. If burned at the temperature usual for a pure lime- 

 stone, maignesian limestones give a lime which slakes slowly 

 without evolving much heat, expands less in slaking, and sets 

 more rapidly than pure lime. To this class belongs the well 

 known and much used limes of Canaan (Conn.) ; Tuckahoe, 

 Pleasantville and Ossining, (N. Y.) ; various localities in New 

 Jersey and Ohio; Cedar Hollow (Penn.), and Chewacla (Ala.) 

 Under certain conditions of burning, pure magnesian limestone 

 yields hydraulic products, but in this case, as in the case of the 

 product obtained by burning pure magnesite, the set seems to 

 be due to the formation of a hydroxide rather than of a carbon- 

 ate. Magnesian limestones carrying sufficient silica and alum- 

 ina will give, on burning, a hydraulic cement falling in the next 

 group under the head of Natural Cements. 



GROUP II--COMPLEX CEMENTS. 



The cementing materials grouped here as Silicate or Hydrau- 

 lic Cements, include all those materials whose setting proper- 

 ties are due to the formation of new compounds, during manu- 

 facture or use, and not to the mere reassumption of the original 

 composition of the material from which the cement was made. 

 These new compounds may be formed either bv chemical change 



