INTRODUCTION. 7 



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

 it is possible that certain reactions take place between the lime and the 

 sand. Such reactions, however, though possibly contributing some- 

 what to the hardness of old mortars, are only incidental and subsidiary 

 to the principal cause of setting recarbonation. 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 quantities 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 limestone, magnesian 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 

 belong the well-known and much-used limes of Canaan (Conn.) ; Tuck- 

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

 and Ohio; and Cedar Hollow (Penn.). Under certain conditions of 

 burning, pure magnesian limestones yield hydraulic products, but in 

 this case, as in the case of the product obtained by burning pure mag- 

 nesite, the set seems to be due in part at least to the formation of a 

 hydroxide rather than of a carbonate. Magnesian limestones carrying 

 sufficient silica and alumina will give, on burning, a hydraulic cement 

 falling in the next group under the head of Natural Cements. 



Group II. Complex Cementing Materials. 



The cementing materials grouped here as Complex Cements include 

 all those materials whose setting properties are due to the formation of 

 new compounds, during manufacture or use, and not to the mere reas- 

 sumption of the original composition of the material from which the 

 cement was made. These new compounds may be formed either by 

 chemical change during manufacture or by chemical interaction, in 

 use, of materials which have merely been mechanically mixed during 

 manufacture. 



Subgroup II a. Silicate cementing materials : hydraulic cements. 

 In the class of silicate cements are included all the materials commonly 

 known as cements by the engineer (natural cements, Portland cement, 

 puzzolan cements), together with the hydraulic limes. 



Though differing widely in raw materials, methods of manufacture, 



