PHYSICAL AND CHEMICAL PROPERTIES. PARTS IV, V. 143 
quently, if a hardened cement contains anhydrous free lime the 
strength which it otherwise is capable of developing and main- 
taining is certain, sooner or later, to be affected when this lime 
is permitted to slake. 
THE EFFECTS OF SLAKED LIME. 
The influence of slaked*? lime on the strength of Portland 
cement is also affected by the nature of the cement and the man- 
ner in which it is used. 
This is made apparent by a consideration of the properties of slaked 
lime which concern its efficiency as a mortar constituent. Water combines 
with lime to form calcium hydroxide, an absolutely dry powdery substance. 
When water is added to this powder, the bulk is much reduced and it may 
be worked into an extremely rich and unctuous paste. This paste when 
sealed in an air-tight receptacle undergoes no change and possesses no 
inherent power of solidification. When permitted to dry it shrinks and 
forms a porous mass of no great hardness. Because of the great shrinkage 
of lime paste which accompanies its drying, it can not be used neat, but 
must always be mixed with several times its volume of sand. 
It is necessary to distinguish between this so-called “set,’’ which is due 
to the mere drying out of the lime paste and a slight deposit of calcium 
hydroxide crystals, and the actual induration which is due to the gradual 
absorption of carbon dioxide. If the slaked lime is exposed to the air, 
these three processes may all go on more or less simultaneously. However, 
the presence of too little or too much water may wholly prevent the actual 
induration of slaked-lime mortars. According to Mr. Clifford Richardson,” 
if the mortar contains more than 5 per cent of water, the absorption of 
carbon dioxide does not go on. While the mortar contains as much as 
0.7 per cent, the absorption continues. His tests also show that the mere 
drying out of the mortar is sufficient to enable it to resist the pressure 
of masonry, while further hardening furnishes the necessary bond. 
The actual induration of lime mortars takes place very slowly and it is 
confined more or less to exposed surfaces and porous places where the air 
is able to penetrate. In certain thick walls and massive impermeable 
mortars, the slaked lime has failed thoroughly to harden after ages. Cox™ 
attributes the extreme hardness of the mortar used in the old buildings 
in the Philippines to the porosity of the stone employed. 
Pure slaked lime does not possess the ability to harden under water, 
but hydraulic properties may be imparted to it by the addition of pul- 
verized materials, such as volcanic tufa, granulated blast-furnace slag, 
clay, brick dust, and in fact any substance which will furnish soluble or 
® Slaked lime, which is amorphous, should not be confused with crystal- 
line calcium hydroxide. It is known that some pure limes burned at very 
high temperatures can be hydrated but not slaked, showing hydraulic 
properties (see also Part III, page 234). 
“Taylor and Thompson, Concrete, Plain and Reinforced. New York 
(1907), 58. 
“This Journal, Sec. A (1908), 3, 410, 411. 
