In considering the proportion of sand to be mixed with different limes and cements it is necessary to bear in mind that the strength of the joint formed by the mortar will have an influence upon that of the wall.

The following Table shows how different limes and cements are weakened by the addition of various proportions of sand : -

1 M.P.I.C.E, vol. lxii. p. 212.

## Table Showing The Effect Of Different Proportions Of Sand In Mortars Made From Various Cements

 Age when tried. Proportion or Cement or Lime and Sand. 1 c. Neat. lc.Is. 1 c.2 s. lc. 3 s. lc.4 s. lc. 5s.1 Breaking Weight in lbs. upon Area of 10 Inches. 11 days Bricks broke first 504 433 303 420 238 Medina ..... ,, 400 352 278 201 149 83 Roman ..... ,, 400 279 178 154 149 73 Atkinson's.. ,, ... 385 175 79 49 Scott's Cement.. ,, 292 286 308 328 281 194 Lias Lime.. ,, 119 80 124 29 37 42

The above figures are from experiments made for General Scott by tearing asunder bricks united by the different kinds of mortar, and set in air. The sectional area torn asunder being 4 x 2 1/2 = 10 inches in each case.

The Table at page 168 gives fuller particulars as to the loss of strength caused by adding sand to Portland cement.

The proportion of the ingredients in mortar is generally specified thus: - "1 quicklime to 2 (or more) of sand," meaning that 1 measure of quicklime in lump 2 is to be mixed with 2 measures (or more) of sand.

Now, the quantities of sand put at different times into a measure vary a little, according to the amount of moisture the material contains; but so little that practically it makes no difference, and this mode of measuring sand is very convenient and sufficiently accurate.

With the lime, however, many conditions have to be fulfilled in order to make it certain that the same quantity always fills the same measure.

The specific gravity of the calcined stone, the size of the lumps, the nature of the burning, the freshness of the lime, all cause the actual quantity contained in a given measure to differ considerably.

1 Portland cement mortar made with 8 parts of sand to 1 of cement may advantageously be used in preference to lime mortar (see p. 208).

2 The pieces of calcined stone are called "lump-lime," or in the North "lime-shells."

In order to avoid this uncertainty it has been proposed that the weight of lime for a given quantity of sand should be specified.

Practically, however, this has not been carried out to any great extent, and the bulk of lime to be used is generally specified as well as that of the sand.

The following proportions are given by General Scott for mortar in brickwork built with ordinary London stock bricks.

 Parts by Measure. Quicklime. Sand, 1 3 Feebly hydraulic limes ... 1 2 1/2 Hydraulic limes (such as Lias) . 1 2 Roman cement.. 1 lor l 1/2 Medina „.. 1 2 Atkinson's ,,... 1 2 Portland „.. 1 5 Scott's „ ... 1 4 Selenitic „.. (see p. 179).

"The proportions here recommended apply only to works above the surface of the ground, or free from the action of a body of water."

"For hydraulic purposes and foundations 1 sand to 1 quicklime is as much as should be admitted. With cement mortar 2 sand may be used with 1 cement, unless actually in contact with water, when 1 part of sand should be the limit allowed." l