Mortars containing small amounts of sand are often stronger than neat cement mortars. Especially is this true of most natural cements. Some of these will give as high strengths when mixed with two parts sand by weight as when neat, and usually the one-to-one mortars are stronger than the neat mortars. These remarks refer to tensile tests where a good quality of sand is used and the mortars are three months old or more. The neat cement mortars gain their strength more rapidly, short time tests usually not showing the results mentioned, Portland cements of good quality usually give about the same tensile strength neat and with one part sand by weight. Tests showing the rate of decrease of strength with added sand are discussed in §§363 to 365.

Table 59. Comparison Of Proportions By Weight And Volume

 Parts Dry Sand to One Cement byWeight. Equivalent Parts Sand, Proportions Stated by volume. Portland Cement. Natural Cement. Parts Loose Dry Sand to One of Packed Cement. Parts Loose Damp Sand to One of Packed Cement. Parts Loose Dry Sand toOne of Loose Cement. Parts Loose Damp Sand toOne of Loose Cement. PartsLoose Dry Sand to One of Packed Cement. Parts Loose Damp Sand toOne of Packed Cement. Parts Loose Dry Sand to One of Loose Cement. Parts Loose Damp Sand toOne of Loose Cement. 1 1.10 1.38 0.85 1.06 0.75 0.94 0.60 0.75 2 2.20 2.75 1.70 2.12 1.50 1.88 1.20 1.50 3 3.30 4 12 2.55 3.19 2.25 2.81 1 80 2.25 4 4.40 5.50 3.40 4.25 3.00 3.75 2.40 3.00 5 5.50 6.88 4.25 5.31 3.75 4.69 3.00 3.75 6 6.60 8.25 5.10 6.38 4.50 5.62 3.60 4.50

In preparing the above table the following assumptions are made :

 Weight in aBarrel. volume of aBarrel. Packed. Loose Dry. Loose Damp. Portland cement . Natural cement . Sand .... 380 280 3.45 cu. ft. 3.75 cu. ft. 11075 85 60 100 80

Portland cements are usually mixed with from one to three parts sand by weight, and natural cements are mixed with from one to four parts by weight (or three-fourths part to three parts by measure). For certain special purposes poorer mortars are sometimes employed. To arrive at the proper proportion to use in mortar for a given purpose, the tables of strength given in Chapter XV will be of value.