146 REIBLING AND REYES. 
work of E. D. Campbell “* and others shows that the nearer we 
approach a thoroughly combined and fused clinker, the less the 
expansion of the resulting cement. 
Keiseman’s work shows the manner in which these compounds 
set and harden without an appreciable change in volume. 
The fact that the hardening process of hydraulic, fused, cal- 
cium compounds is not limited to exposed surfaces, but takes 
place throughout the mass, is one of the main reasons that good 
Portland cements give the most constant and reliable develop- 
ment of strength in actual construction work. 
THE EFFECTS OF FREE AND COMBINED LIME IN HYDRAULIC 
CEMENTS. 
From an engineering standpoint, hydraulic cements may be classified 
in the following order of general usefulness and merit: 
1. Portland cements. 
2. Natural or Roman cements. 
8. Pozzuolane cements made from blast furnace slag and slaked 
lime. 
4. Hydraulic limes. 
5. Pozzuolane cements made from natural voleanic matter and 
slaked lime. 
The physical and chemical properties and the manufacturing require- 
ments of the most efficient of these five classes of cements offer a remark- 
able demonstration of the properties of lime which have been discussed in 
the preceding pages. Typical cements of these five classes are known, the 
ultimate chemical analyses of which vary so slightly that they are within 
the limits of different Portland cements of standard quality. However, 
from the natural pozzuolane cements with their low tensile strength, low 
specific gravity, and slow, unreliable hardening properties, there is a rise 
in efficiency and usefulness through hydraulic lime and slag and natural 
cements to the great, reliable development of strength and the high specific 
gravity of good Portland cement. The great difference in these five classes 
of hydraulic cements is due almost entirely to the condition of the calcium 
oxide characteristic of each. 
Natural pozzuolane cement.—Starting with the poorest, we have a raw 
mixture of hydrated lime and anhydrous silicious material. The latter 
reacts with the lime, and to this reaction hydraulicity is due. The wonder- 
ful durability of the pozzuolane cement used by the Romans is due largely 
to ideal climatic conditions and the porous, open-grained structure of the 
volcanic tufa used, as well as to the quantity of available silicic acid. 
These conditions are impossible or difficult to obtain elsewhere, and even 
were it not so, the extremely slow hardening process involved is so poorly 
adapted to meet the demands of modern construction work that the present 
*° Amer. Chem. Soc. (1904), 26, 1278. 
