164 REIBLING AND REYES. 
effect of which is apparent from their results some of which are plotted in 
the form of expansion curves in fig. 25. 
As pointed out by Campbell and White, bar 77A, which had been im- 
mersed in water for five years, showed the gradual expansion and the 
slight drop in the curve at about six months usual with good cements. 
Bar 78A of the same cement, to which had been added about 2.8 per cent 
of hard-burned free lime, expanded 0.220 per cent in water during the first 
week. It continued to expand at a comparatively rapid but diminishing 
rate until after two months, when the rate of expansion had settled down to 
practically a normal rate, showing that the free lime had become completely 
hydrated by two months’ immersion in water. 
The beneficial effects of aging even a hard-burned, sound cement is shown 
by the much greater constancy in volume as compared with the fresh 
cement. However, the aged cements show the customary expansion after 
three months. 
The effect of free lime on cement bars kept in air was not perceptible 
during the first seven days. The bar of cement containing 2.8 per cent 
of hard-burned free lime contracted at a normal rate during the first seven 
days in air, but expansion due to hydration of the lime was noticeable 
after that date and continued to increase at an accelerated rate until at 
the end of nine months the bar had expanded 1.17 per cent and had cracked 
so badly that observations were discontinued. 
In case free lime is present in cement used in structures above ground, 
or where it is usually dry, the expansion will be gradual but several times 
greater in volume than when the material is under water. The hydration 
of the hard-burned lime in air is so slow that there may be continuous 
expansion up to five years or more. Aging diminishes the expansion some- 
what, but a cement aged seven weeks when ground still showed the influence 
of free lime. 
In a similar manner the influence of free magnesia was determined. It 
was found that the rate of hydration of hard-burned magnesia is almost im- 
perceptibly slow, so slow, in fact, that aging does not seem to diminish its 
deleterious effects and the 24-hour boiling test does not detect it even 
when as much as 4 per cent hard-burned free magnesia is present. Owing 
to this slow rate of hydration, hard-burned free magnesia has no appreciable 
effect in cement used above ground where it is continually dry. Even when 
continuously immersed in water the expansion due to this constituent is not 
appreciable until after two months and only becomes distinctly evident 
after a year. The expansion then continues at an increasingly rapid rate 
for at least five years. 
Campbell and White’s observation that any cement worked up fresh will 
change in volume more than if properly aged and will show a noticeable 
drop in the curve representing change of volume after about three months, a 
period which corresponds to that in which briquettes show a falling off in 
tensile strength, suggested that the fluctuations in strength might be due to 
the slow hydration of free lime. 
In order to gain further information ahout the slaking of 
free lime in hard-burned rotary clinker, expansion bars were 
made of neat mortar using the cements described in Table LII. 
i 
