PHYSICAL AND CHEMICAL PROPERTIES. PARTS Iv, Vv. 181 
(6) The set takes place so quickly that the cements become 
regauged by any ordinary process of manipulation. 
(c) The immediate generation of heat when water is added is 
due partly to the hydration of free lime and partly to 
the setting of the cement. 
B. The effects of plaster on nonseasoned cement: 
(a) As the amount of plaster used is increased, the initial set 
takes place earlier, then later, and finally again earlier. 
The amount of water required for normal consistency at 
first decidedly decreases, and then slightly increases. 
The same quantity of plaster does not affect the set and 
plasticity of different cements to the same extent. 
(6) Small amounts of plaster have no appreciable effect upon 
the slaking of ignited lime, and do not prevent the Boner 
tion of heat due to the slaking of the free lime. 
(c) The effect of minute quantities of plaster is to quicken the 
set of a manipulated cement until the amount added is 
sufficient to prevent regauging. 
(d) Further additions of plaster retard the set, and from 1.5 
to 8 per cent produces a maximum effect. Beyond this 
amount the natural tendency of plaster of Paris to set 
quickly manifests itself in the combined results obtained, 
and the time of initial set again approaches a minimum. 
C. The effects of various methods of seasoning the nonplastered, 
ground cement or clinker: 
(a) As soon as the free lime has become thoroughly hydrated, 
less water and less plaster are required to produce a 
normal paste of standard consistency and set. 
(6) The efficiency of the different methods of seasoning depends 
entirely upon the relative amounts of calcium hydrate 
produced and maintained. 
(c) The conversion into carbonate decreases the plasticity and 
the retarding influence of the sulphate. 
D. The effects produced by seasoning plastered cements showed that: 
(a) No radical difference is manifested if plaster is added before 
or after the cement has seasoned. 
(6) The constituents of the atmosphere may alter the condition 
of free lime without affecting the compounds which cause 
cements to set. 
(c) The substitution of calcium carbonate for slaked lime reduces 
the plasticity and decreases the efficiency of plaster or 
gypsum as a retarder. 
(d) Cement itself may finally become practically inert after 
prolonged exposure. 
EK. Effect of free lime on retarders: 
(a) When free calcium oxide is present, the heat of hydration 
when water is added increases the natural rate of set. 
In different cements the normal rate of the setting reac- 
tions vary and are not affected to the same extent by 
heat or retarders. 
(6) A preliminary hydration of the free lime adds to the 
efficiency of the retarder and increases the plasticity. 
