STATE GEOLOGIST. d+] 
exposed to the sun frequently becomes quicker in its action. ‘This is due 
to the breaking down of the coarser particles to dust, a tendency shown 
by practically all cements, for even coarse clinker will in time break down 
to a powder. 
It has been known for a long time that small amounts of certain 
salts added to Portland cement affect the time of setting in a most decided 
manner lhus) the emect of different chlorides added to a Portland 
cement, whose setting time was 12 minutes, was as follows: 
Name of Substance. Fi per cent.) 2 per cent.|3 per cent.) 4 per cent. 5 per cent. \6 per cent. 
| : | _ | 
ss = | ‘ | 
| | | | | 
| Minutes | Minutes |} Minutes) Minutes) Minutes | Minutes 
Magnesium Chloride......... e290 35 55 | gH | NO | 260 
anim Ohlondes 564 984 AS) > HOO), BLO | 465 | 580 
Sodium Chloride............ BD GE 100 | 150 270 330 
Ammonium Chloride........ 31 45 85 100 200 270 
Some contradictory facts are, however, noticed in this connection, 
Thus a dilute solution of calcium chloride retards the setting, a concen- 
trated solution accelerates it. Soda and potash solutions accelerate the 
setting. Ae 
The best work on this subject has been probably done by Dr. 
Rohland, who ascribes the action of various small amounts of salts to 
catalysis and gives expression to the following statement: 
Where the velocity of reaction setting is very slow the catalytic 
agent produces a greater effect than when the velocity is greater. Posi- 
tive (accelerating) and negative (retarding) reagents may neutralize 
each other. The quantities may be very small; I per cent and less is 
often sufficient to produce great changes. He makes a list of catalytic 
agents including: 
Calcium sulphate, 
Aluminum chloride, 
Sodium carbonate 
Calcium chloride 
Barium chloride 
Potassium bichromate 
Sodium chloride 
cro 
He also brings out the fact that aluminates in cement may decompose 
. on storing and thus bring about changes in the rate of setting, so that 
sometimes in spite of curing the cement becomes quicker setting. The 
