alk or limestone is "burned," i.e. heated in a kiln until
its carbonic acid has been driven off, it yields pure lime. This slakes
violently with water, giving slaked lime, which can be made into a
smooth paste with water and mixed with sand to form common mortar. The
setting of the mortar is due to the drying of the lime (a purely
physical phenomenon, no chemical action occurring between the lime and
the sand). The function of the sand is simply that of a diluent to
prevent undue shrinkage and cracking in drying. Subsequent hardening of
the mortar is caused by the gradual absorption of carbonic acid from the
air by the lime, a skin of carbonate of lime being formed; but the
action is superficial. Mortar made from pure or "fat" lime cannot
withstand the action of water, and is only used for work done above
water-level. If, however, such "fat" lime is mixed in the presence of
water, not with sand but with silica in an active form, i.e. amorphous
and (generally) hydrated, or with a silicate containing silica in an
active condition, it will unite with the silica and form a silicate of
lime capable of resisting the action of water. The mixture of the lime
and active silica or silicate is a pozzuolanic cement. The simplest of
all pozzuolanic cements would be a mixture of pure lime and hydrated
silica, but though the latter is prepared artificially for various
purposes, it is too expensive to be used as a cement material. A similar
obstacle lies in the way of using a certain native form of active
silica, viz. kieselguhr, for it is too valuable as an absorbent of
nitroglycerine, for the manufacture of dynamite, to be available for
making pozzuolanic cement. There are, however, many silicious
substances occurring abundantly in nature which can thus be used. They
are mostly of volcanic origin, and include pumice, tufa, santorin earth,
trass and pozzuolana itself. The following analyses show their general
composition:--

+-----------------------------+-----------+-----------+-----------+
| |Neapolitan | Roman | |
| | Pozzuo- | Pozzuo- | Trass |
| | lana | lana |(per cent) |
| |(per cent) |(per cent) | |
+-----------------------------+-----------+-----------+-----------|
| Soluble silica (SiO2) | 27.80 | 32.64 | 19.32 |
| Insoluble silicious resid