STATE GEOLOGIST. 360 
increase in volume and thus destroys the mortar. A number of other 
investigators have attacked the problem like Chatelier, Debray, Ileret, 
Deval and others. The results may be summarized by  say- 
ing that the alumina and gypsum are the weak points of the cement while 
the ferric oxide is beneficial. The cements should be’as silicious and 
as high in ferric oxide as possible. 
Addition of Pozzuolane.—Michaelis, Sr., has also shown _ that 
the resistance of Portland cement to sea water may be increased 
enormously by the addition of pozzuolanic material, like trass. If trass 
or a similar material 1s not available, there is no reason whatever why 
well burnt clay may not be used; in fact, the writer has found that just 
as natural cements are improved by the addition of slaked lime,the reaction 
may be reversed in the case of the basic Portland cements by adding 
pozzuolanic material or ground burnt clay. Thus we increase the re- 
sulting strength by the addition of a secondary hydraulic reaction and 
obtain as the resulting strength the initial strength of the Portland 
cement plus the strength derived from the secondary, pozzuolanic action. 
In addition, by virtue of the secondary reaction, the resistance of the 
cement to all kinds of influence is increased by the more silicious char- 
acter of the mortar. This idea has been carried out in the production 
of the so-called sand cement, which is simply ordinary Portland cement 
ground together with trass or sand, and is sold for sea water work. The 
writer has seen such a plant at Staten Island and another one is reported 
to be at Chicago. In one of these plants, however, the chemical principle 
underlying their process was not understood, for in place of the sand, 
limestone was used because it was easier to grind. This evidently could 
not improve the cement in the desired direction, though an increase in ten- 
sile strength might be observed, independent of any pozzuolanic action, 
due simply to the second grinding of the cement. 
| It would be interesting to observe cement work in which the. cement 
was ground together with well burnt clay under adverse conditions, that 
is, exposed to water containing carbonic acid or any water of acid reac- 
tion. Experiments conducted by the writer on a small scale in which 
cement was ground together in a ball-mill with burnt clay in the pro- 
portion: Of 10:1 showed a imiich ereater’ resistance to the action of 
acidulated water (hydrochloric) than cement mortar not so treated. But 
the laboratory results of such experiments are, as a rule, not comparable 
with the practical results. If, however, practice should confirm this 
suggestion, the durability of exposed cement surfaces, of reservoir walls, 
sea walls, piers, cement blocks would thus be improved cheaply and sat- 
isfactorily. It should be the task of Portland cement manufacturers to 
study these questions and to prepare for the market cements especially 
suitable for exposed work. Much of the lack of confidence which has 
existed in certain quarters in years past in the use of cement would thus 
be eliminated. 
