A N N U A L liE TOUT-—M INlOKA'L 1ND USTIIIES. 43 
heat, pressure and moisture, and result in the formation 
of hydro-silicates of calcium and magnesium. 
The sand used in the manufacture of sand-lime brick 
should be comparatively pure and preferably with some 
variation in the size of the grains. The mixture of lime, 
sand, and water, is cut out in the form of bricks and 
conveyed to a hardening cylinder. Necessary heat and 
pressure is obtained in the hardening cylinder adapted for 
the purpose. The sand-lime bricks are placed in this cyl¬ 
inder and subjected to a pressure and temperature which 
varies according to the method of treatment. With a 
pressure of about two atmospheres and a temperature of 
125 degrees C. the duration of exposure in the cylinder 
should be about seventy-two hours. With a steam pres¬ 
sure of seven to ten atmospheres and 170 to 175 C, six 
to ten hours’ exposure is usually sufficient. 
CONCRETE BUILDING BLOCKS, HOLLOW BLOCKS, OR 
ARTIFICIAL STONE. 
The materials for the manufacture of concrete building 
blocks are sand, gravel, or crushed stone, and cement. 
The sand intended for use in making cement blocks should 
be sharp and angular. Coarse grained sand is preferred 
to fine grained. A mixture of coarse and fine grained 
sand, however, gives excellent results, and requires less 
cement, since the relative proportion of voids in the sand 
is reduced by the presence of the fine grains of sand among 
the coarse. It has been commonly supposed that a clean 
sand must be used, but recent experiments seem to indi¬ 
cate that a small proportion of loam or clay is not detri¬ 
mental. Tests carried on at the Ohio State University 
appear to indicate that clay or loam up to 15 per cent of 
the weight of the sand adds strength to a mortar.* 
Concrete building blocks have been, as a rule, favorably 
received throughout the State. The ease of manufacture, 
together with the relatively small cost of equipment and 
^Bulletin No. 2, 4th Series, Ohio Geol. Survey. P. 33, 1904. 
