26 ANNUAL REPORT 
NATURAL VERSUS PORTLAND CEMENT. 
The question of whether to use natural or Portland cement mortar 
depends upon the strength required, upon the time which can be allowed 
for the mortar to gain sufficient strength for the immediate requirements 
of the work and upon the relative price of the two cements. , 
It may be that a rapid setting mortar is needed without great ultimate 
strength ; if so, a natural cement will fulfill the requirements at a less cost 
than Portland cements. The time of setting of both natural and Portland 
cements varies so widely with different brands that only general limits may 
be stated. Natural cement usually begins to set in from five to forty min- 
utes, and attains its permanent set in from twenty minutes to two and one- 
half hours, while Portland cement begins to set in from three-fourths of an 
hour to three hours and attains its final set in from two and one-half to 
eight hours. Some experimental Portlands, however, have been known 
to begin setting within three minutes and to have attained hard set in 
fifteen minutes. 
To illustrate the question of relative cost and strength of the two 
cements, suppose the specifications for a certain structure call for a cement 
which shall develop an ultimate tensile strength in the work, of 200 
pounds per square inch. Most of the natural cements in mortars of 1 
cement to 2 sand will develop that strength in three months, and greater 
strength in six months or a year; while Portland cements will give similar 
strengths if mixed in the proportion of I cement to 4 or 5 sand. Assum- 
ing sand at $1.25 per cubic yard, natural cement at 90 cents per barrel 
of 265 pounds, and Portland cement at $2.50 per barrel of 380 pounds, 
the material for a cubic yard of mortar will cost: 
TABLE 2. 
For Portland Cement For Natural Cement 
Proportions | to 5. Proportions 1 to 2. 
0.92 cu. yds. of sand at $1.25 $1.15 | 0.81 cu. yds. sand at $1.25 $1.01 
1.12 bbls. cement at 2.50 2.80 | 2.49 bbls cement at .90 2.24 
$3.95 $3.25 
From this comparison it is seen that a natural cement mortar of I 
cement, 2 sand, is 70 cents cheaper per cubic yard than a Portland cement 
mortar of 1 cement to 5 sand, both mixtures having about the same ten- 
sile strength. These comparisons, however, can only be made under 
known conditions, the proportions being dependent upon the weight of 
the cement and sand per cubic foot, the voids in the sand and the amount 
of water used in the mixing, while the amount saved is greatly influ- 
enced by the relative cost of the two cements and the cost of the sand. 
