PHYSICAL AND CHEMICAL PROPERTIES. PARTS IV, V. 147 
commercial importance of this class of cement consists principally in the 
fact that the addition of small quantities to Portland cement increases the 
resistance of the latter to the deteriorating action of sea and other impure 
waters. 
Hydraulic lime.—Hydraulic limes and natural cements depend largely 
upon the same reaction for their hydraulic properties as pozzuolane 
cement; but here the burning process to which the raw materials are sub- 
jected increases the activity of the anhydrous silicic acid which increases 
the rate of hardening. 
Hydraulic limes are used quite commonly in England, Europe, and in 
certain parts of the Orient where underburned set-kiln Portland cements 
are slaked, dried, and screened, and sold as hydraulic lime. 
Natural (or Roman) cement—Natural cements are largely used in 
America because of their cheapness. They harden more rapidly in air or 
water than hydrated lime, but generally speaking, they lack uniformity in 
strength, setting properties, and constancy of volume to a much greater 
extent than Portland cements. 
Here in the Philippines much consideration has been given to the 
question of the possibility and the commercial feasibility of the local 
manufacture of cement.” The present and near future resources and the 
commercial and economic conditions of these Islands might favor the 
manufacture of what may be called an artificial Roman cement. Albert 
Victor Bleininger “ makes the following summary in regard to the burning 
of natural or Roman cements: 
“1. For Roman cements, high in lime, approaching the tri-calcium 
silicate and low in magnesia, the best burning temperature is 1000°. Such 
stone can be made into Portland cement by vitrification. This cannot 
be done successfully, however, with a Roman cement equipment, but requires 
more elaborate and expensive machinery and kilns. 
“2. Roman cements, lower in lime than (1), and free from magnesia, 
ean only be burnt at 1000°. 
“3. Roman cements, high in magnesia, should only be burnt at 950°, as 
the great strength produced by making a magnesia Portland cement is 
counterbalanced by the danger of inconstancy in the volume due to the 
two rates of hydration of lime and magnesia, the latter hydrating only 
after the first has assumed a stony hardness, thus breaking the structure 
of the cement. In very sandy mortars and some kinds of work they might 
be used, but at this stage of our knowledge concerning cements they 
cannot be considered a safe proposition. 
“A, Cements, dolomitic, less basic than the subsilicates, to be burnt only 
at about 950°.” ‘ 
Underburning tends to decrease the strength of these cements for the 
reason that more or less calcium or magnesium carbonate remains unde- 
composed and consequently inert. 
™ Cox, Alvin J., This Journal, Sec. A (1908), 3, 391 to 407; (1909), 4, 
211 to 231; Min. Resources P. I., Bur. Sci., Div. Min. (1909); Pratt, W. 
E., ibid. (1912). 
*® Bull. Geol. Survey Ohio (1904), IV, 186. 
