4 The Colorado Experiment Station. 
materials found in Natural cement are mixed by nature in approxi¬ 
mately the correct proportions and when burned does not always 
make a cement of uniform strength. Portland cement is mechan¬ 
ically mixed in chemically correct proportions. Portland cement 
makes a uniformly stronger mixture than the Natural cement and 
is always used where great strength is required. 
In cement fence post construction, it is desirable that the post 
be made as light and as strong as possible, and thus it is practical 
to use nothing but the best grade of Portland cement. 
Sand. —Clean, sharp sand with grains varying in size from 
small to large makes the best mixture. By clean sand is meant that 
which is free from clay, loam, or foreign materials. These tend to 
retard the proper setting of the cement and destroy its adhesive 
quality. In many sections mica is found mixed with sand in large 
enough quantity to seriously interfere with the strength of a mix¬ 
ture made from it. 
Sharp sand is composed of sharp, angular grains of all sizes 
and makes better mixture than that which is smooth and round, or 
“river worn. ,, 
A sand composed of fine and coarse grains mixed, is to be 
preferred, because less cement will be required to fill the voids than 
either used by itself. 
Leaves, sticks, stones or gravel should be removed by screening. 
Gravel .—The same general rules used in the selection of a good 
grade of sand will apply to gravel. It should be composed of clean, 
sharp pebbles of all sizes. For post construction, the pebbles must 
not be too large, as they will interfere with the proper placement of 
reinforcement. 
Broken Stone .—Broken stone used for post construction must 
contain no large pieces as they will interfere with the placement 
of the reinforcement. It is necessary to use some sand with the 
stone to fill voids and thus save cement. It is not desirable to use 
soft sandstone, soft limestone, slates, or shales. Granites, hard 
limestones, and coarse gravel which has been crushed, is considered 
best. 
Water .—The water used in making a cement or concrete mix¬ 
ture should be clean and free from alkali. Satisfactory experiments 
have not been conducted to show the effects of alkali water used 
in making a mixture of this kind, but enough is known as to its 
effect on cured cement constructions to justify not using it in the 
mixture. 
PROPORTIONS. 
On account of a difference in the total open space or voids in 
sands or gravel composed of different sized particles and also that 
more cement is required in some conditions than in others, it is of- 
