144 REIBLING AND REYES. 
hydrous silicic acid. The process of hardening in this case is also slow 
and tedious and more or less confined to the superficial surfaces of the 
particles, and for permanency in air requires constant or continuous 
moisture. 
Under certain conditions the presence of slaked lime in cement 
may influence the strength mechanically, inasmuch as it affects 
its troweling properties and in certain conditions increases the 
impermeability of the mortar. However, to increase the imper- 
meability, it can only be used to advantage with mortars leaner 
than 1: 2, as it appears impossible to replace with this material 
the water which separates the grains of cement, or cement and 
sand, in neat pastes and 1:1 or 1: 2 mortars, respectively. 
THE EFFECTS OF COMBINED CALCIUM OXIDE. 
The exact nature of the compounds formed and the chemical 
action involved in the hardening process of cement is more or 
less imperfectly understood, but recent years have contributed 
much to our knowledge of the physical factors. The influence 
of lime, which has entered into combination with silica, alumina, 
and iron oxide, on the strength has been the subject of much 
investigation.”* 
Schott fused various mixtures of nonfused ingredients con- 
taining calcium oxide and silica, alumina, and iron oxide in suffi- 
ciently large quantites to determine the soundness and tensile 
strength of the compounds produced. We have abstracted these 
tests in Table XX XVIII. 
From these results it is apparent that pulverized fused com-~ 
pounds of lime and silica; lime and alumina (or iron oxide) ; 
and lime, silica, and alumina (or iron oxide) possess the property 
of hardening under water without an appreciable change in vol- 
ume and without the necessity of preliminary curing unless free 
lime is present. 
The extraordinary strength of the fused mixture obtained by 
adding from 5 to 8.5 per cent of alumina to the weight of the 
other ingredient is especially noteworthy. Within limits, the 
strength of the silicates increases as the lime increases, while 
with the aluminates the opposite is true. Portland cement is 
also usually manufactured from’ nonfused ingredients, but the 
raw materials are only heated to the point of incipient fusion 
* Schott. The calcium silicates and calcium aluminates contained in 
Portland cement (translated by Dr. W. Michaelis, jr.), Cement & Eng. 
News (1910), 22, Nos. 9 to 12; Keiseman, The hardening and constitu- 
tion of Portland cement, Cement & Eng. News (1911), 23, Nos. 1 to 4. 
