CHEMISTRY AND PHYSICS. 43 
water and mixed with sand, constitutes lime mortar, the proportioning of the 
ingredients and mode of manipulating them being varied to suit the required uses. 
The qualities of a good mortar are that it will ‘‘set’’ or harden into a sub- 
stance of great strength; that it will adhere firmly to the surfaces which it joins 
together; and that it will preserve its hardness and strength under atmospheric 
influences. The setting of both lime and cement mortars is due to chemical 
changes in the mass of the mortar; it is, however, of an entirely different nature 
in the case of lime and cement mortars. In the case of lime mortar, the setting is 
due to the absorption from the atmosphere of carbonic-acid gas, which is ab- 
sorbed by and combines chemically with the calcium of the hydrate of lime, 
forming carbonate of lime, and at the same time giving off water. This setting 
of lime mortar is dependent upon access to the atmosphere, from which it obtains 
carbonic acid. The hardening, therefore, must be from the outside and extend 
inwards at diminishing rates; this of necessity precludes the use of lime mortar 
for masonry under water. The giving off of water in the setting of lime mortar 
explains why walls built with lime mortar remain damp for such a length of time, 
rendering newly-built houses unhealthy. 
Cement possessing hydraulic properties is manufactured from limestone or 
marl containing about one-fifth part of clay; this clay consists of alumina and 
silica. 
There are two kinds of cement, natural and artificial (or Portland). Since 
these cements will not slake in water, they will require to be ground. While both 
these cements will set under water and harden throughout their whole mass, 
they possess distinct characteristics, both chemical and physical. Natural 
cements are those in which the burning has not been carried to the point of vitri- 
fication, while artificial (or Portland) cements are those in which this has been 
done. The quality of the cement is also affected by the proportions of the in- 
gredients and the mode of manipulation. The underburning of the natural 
cements merely drives off the carbonic acid from the limestone, and changes the 
clay to a substance akin to brick, the result being in the nature of a mechanical 
mixture, corresponding to that of lime with pozzuolana, forming the hydraulic 
lime of the Romans and of the early English engineers. The burning to vitrifica- 
tion of a properly proportioned mechanical mixture of carbonate of lime and 
clay effects a chemical combination of the constituents forming calcium aluminates 
and calcium silicates, the quick setting being due to the aluminates and the 
strength to the silicates. 
Examined under a microscope, the physical appearance of natural cement is 
a mass of nodular particles analogous to that of sand, while Portland cement ap- 
pears to be a mass of overlapping and interlacing scales. 
Mortar consists of an aggregate of sand mixed with a cementing substance, as 
lime or cement, with suitable quantity of water to bring it to the desired degree 
of consistency, as the voids between the sand particles are nearly one-half the 
volume of the sand. Absolute density will only be obtained when that quantity 
of cement is used, or the mortar is cement 1,sand 2. With lime as the cement 
material, the case is somewhat different, as the water adds considerable to the 
volume of the hydrate of lime; hence, the filling of the voids will be attained 
with less than that quantity of lime. In practice, less than one-half volume of 
cement will also serve; the usual practice for good work is to make mortar in 
proportion of 1 to 3. 
Portland cement is ground to a certain prescribed fineness: (1) Because the 
coarser particles do not set, but remain inert,asso much sand. (2) Because the 
finer the particles the greater surface of sand they will cover. 
