148 REIBLING AND REYES. 
The evil effects of overburning which long has been an established fact 
are very apparent from the curves of strength shown in fig. 24. 
Gilmore” obtained these results by burning Ulster County, New York, 
cement in four stages represented by 1, 2, 3, and 4, respectively, where 
1 stands for underburning, 2 for proper burning, 3 for overburning without 
vitrification, and 4 for vitrification. Gaetschenberger™ obtained similar 
results with dolomitic marl from near Heidelberg. 
The exact cause of the evil effect of overburning is but imperfectly 
understood. Bleininger suggests that the hydraulicity of natural cements 
is pozzuolane in character and that up to a certain temperature these 
calearious mixtures are but pozzuolane-like, simple silicates, which on 
further application of heat become chemically more complex and non-, 
or but slightly, hydraulic. Bleininger failed to consider the alterations in 
the properties of the free lime at the various temperatures. 
An overburned natural cement is essentially an underburned Portland 
CURVE No-1=!:2 MORTAR, AGE 95 
29 » 2° NEAT ,, 
” 3= 4» 60 
4= 1:1 60 
Fic. 24.—A diagram showing the evil effects of over-burning a Roman cement. 
cement. In both instances the heat has been high enough to destroy the 
quick-slaking properties of the free lime, but not high enough to enable 
this calcium oxide to enter into combination. The manufacturers of 
natural cement well realize the evil effects of either under- or overburning 
their raw material. However, owing to the present universal practise 
of burning cement rock in set kilns and the variable chemical composition 
of the raw material, at times both under- and overburning are unavoidable, 
which explains the failure of these manufacturers to produce a uniform 
product. 
To produce a more desirable cement of this class in the Philippines, we 
advocate the method of producing an artificial Roman cement by blending 
clay and limestone in the proper proportion and then burning the mixture 
at a low temperature in a rotary kiln. By this method the chemical 
* Limes, Hydraulic Cements and Mortars (1870), 158. 
“ Quoted by Bleininger, loc. cit. 
lila a te 
