eckel.] DEFINITIONS OF CEMENTS. 19 
the engineer. The relationship of the various cementing materials 05 
can be concisely expressed as in the following diagram: 
Nonhydraulic cements. 
Hydraulic cements 
Plaster of Paris, cement plaster, Keene's cement, etc. 
Common lime. 
Hydraulic lime. 
Natural cements. 
Portland cements. 
Puzzolan cements. 
Nonhydraulic cements. — Nonhydraulic cements do not have the 
property of "setting" or hardening under water. They are made by 
burning, at a comparatively low temperature, either gypsum or pure 
limestone. The products obtained by burning gypsum are marketed 
as "plaster of Paris," "cement plaster," "Keene's cement," etc., 
according to details in the process of manufacture. The product of 
burning limestone is common lime. The plasters and limes will not 
be further discussed in the present bulletin. 
Hydraulic cements. — The hydraulic cements are those which set 
when used under water, though the different kinds differ greatly in 
the extent to which they possess this property, which is due to the 
formation during manufacture of compounds of lime with silica, 
alumina, and iron oxide. 
On heating a pure limestone (CaCOJ containing less than, sa}^, 
10 per cent of silica, alumina, and iron oxide together, its carbon 
dioxide (C0 3 ) is driven off, leaving more or less pure calcium oxide 
(CaO) (quicklime or common lime). If the limestone contains much 
silica, alumina, or iron oxide, the result is quite different. 
Natural cements. — Natural cements are produced by burning a nat- 
urally impure limestone, containing from 15 to 10 per cent of silica, 
alumina, and iron oxide, at a comparatively low temperature, about 
that of ordinary lime burning. The operation can therefore be car- 
ried on in a kiln closely resembling an ordinary lime kiln. During 
the burning the carbon dioxide of the limestone is almost entirely 
driven off, and the lime combines with the silica, alumina,, and iron 
oxide, forming a mass containing silicates, aluminates, and ferrites of 
lime. If the original limestone contained much magnesium carbonate 
the burned rock will contain a corresponding amount of magnesia. 
The burned mass will not slack if water be added. It is necessary, 
therefore, to grind it rather fine. After grinding, if the resulting 
powder (natural cement) be mixed with water it will harden rapidly. 
This hardening or setting will also take place under water. Natural 
cements differ from ordinary limes in two noticeable ways: 
(1) The burned mass does not slack on the addition of water. 
a For a more detailed discussion see Municipal Engineering, vol. 24, 1903, pp. 335-336, and Am. 
Geologist, vol. 29, 1902, pp. 146-154. 
