PHYSICAL AND CHEMICAL PROPERTIES. PARTS IV, V. 
149 
composition and the degree of burning could be uniformly regulated. It 
might not be a feasible method in countries where the cost of the produc- 
tion of Portland cement is low, but the high cost of imported coal and 
Portland cement in the Philippines would overcome this objection, and 
especially since local coals could be utilized for burning the natural cement. 
Slag cement—The manufacture of slag cement has not yet developed 
its maximum efficiency, although it will undoubtedly be perfected. They 
show their best strength when submerged constantly in water. The pres- 
ence of calcium sulphide decreases their efficiency in works exposed to 
air and tends to cause a white efflorescence. The calcium hydrate which 
they contain also makes them liable to shrink and crack during the setting 
stage. 
It has been thought that the sudden cooling brought about by quenching 
the molten slag converted it into true pozzuolane and that slag cements 
depend on the same reaction as pozzuolane for their hydraulic properties. 
This belief has little scientific foundation. If, as is generally assumed, 
slow cooling tends to break down or dissociate complex mineral compounds 
into simpler ones, it is evident that the quenching of molten slag would 
tend to prevent rather than to aid the formation of pozzuolane material. 
On the other hand, there appears to be every reason to believe that the 
severely criticised work of Zulkowski™ leads to the correct conclusion that 
the hardening properties of basic slag and Portland cement are analogous; 
that is, due to the colloidal properties of the calcium silicates and alumi- 
nates. The liquid condition of the slag and its low lime and high silica 
and alumina contents make rapid cooling essential for the preservation 
of high-limed silicates and low-limed aluminates. Take, for example, a 
Chicago slag, the analysis” of which is given in Table XXXIX together 
with the composition of the silicates and aluminates formed at a high and 
at a low fusion temperature. 
TABLE XXXIX.—An interpretation of a Chicago slag. 
— 
Available 
eon atitdente Fused at a high temperature. 
Fused at a low temperature. 
1.1CaO remains for the formation of 
higher limed aluminates. 
CALCULATION: 
32.20 SiO02 as 1CaO, 1SiO2 requires 30.0 
Per cent. 
SiOz —82.20 | The calcium oxide is sufficient to combine | The calcium oxide available is not suf- 
AlzOs—15. 50 with all of the silica as 1CaO, 1SiO2, and ficient to conyert more than about 
CaO —48.14 all of the alumina as 2CaO, Al2O3. Also one-third of the silica into 2Ca0, 
1Si02. 
CALCULATION: 
$2.2 SiO2 as 1CaO, 1SiO2 requires 30.0 
CaO. : 
15.5 Al2z03 as 1CaO, 1Al203 requires 
CaO. 8.5 CaO. 
15.5 AlzOs as 2CaO, 1Al203 requires 17.0 | CaO accounted for = 38.5 
CaO. CaO available = 48.1 
CaO SOBER for = 47.0 @aO\hydifferencel = nolG 
CaOlavailable bel 9.6 CaO is capable of converting 10.3 
@aO by difference = 1.1 Si02 from 1CaO, 1S8iO2 to 2 CaO, 
1Si02. 
‘Chem. Ind., Berlin (1889), 12, 69. 
” Eckel, H. C., Mineral Resources of the U. S. (1901). 
