The importance of good mortar can scarcely be over-estimated. If the mortar is bad. the wall is bad. Bad mortar allows wind and rain to penetrate, favours vegetation, easily cracks, and rapidly crumbles away, exposing the arrises of the bricks and stones to atmospheric action, and thus leading to their decay. When the face of a brick decays, it will usually be found that the mortar has first been eaten from the joints; a good mortar-joint not only makes a wall drier and stronger, but also more durable. In vain are the bricks and stones selected if the mortar also is not carefully prepared, and, be it added, used in sufficient quantity to fill the joints.

Unfortunately mortar is easily stamped, and so are mortar-joints, and as long as these matters are left in the hands of jerry-builders and unscrupulous contractors, such will be the case. It is a good plan for the building owner to provide all lime, cement, and sand; then, and then only, may he hope to have them mixed in proper proportions and used in sufficient quantity to flush the joints, and even then he will be disappointed if constant supervision be not exercised, for the ordinary bricklayer can scarcely be compelled to make solid vertical joint: he scrapes his trowel on one arris of the brick, and leaves three-fourths of the joint absolutely devoid of mortar. Still, the temptation to do this is less when he knows that his master will not grumble at the number of hodfuls which he uses.

Mortar ought to serve at least three purposes: it ought to form a soft but gradually hardening bed to receive the various building-materials, so that these shall obtain an uniform bearing notwithstanding the irregularity of their surfaces; in the second place, it ought to prevent the passage of wind and rain through the joints of the walling; and, lastly, it ought to have adhesive and cohesive strength enough to bind the component parts of the wall into one solid mass. Jerry-builder's mortar seldom does more than partially serve the first and second purposes. Only the best Portland-cement mortar will thoroughly fulfil the three.

The by-laws of the London County Council relating to mortar are as follows: - All brick and stone work shall be put together with good mortar or good center The mortar to be used, must be composed of freshly -burned lime and clean sharp sand or grit, without earthy matter, in the proportions of one of lime to three of sand or grit, the cement to be used must be Portland cement, or other cement of aqual quality to be the District Surveyor, mixed with clean sharp sand or grit, in the proportions of one of cement to four of sand of grit. Burnt ballast or broken brick may be substituted for sand or grit, provided such material be properly mixed with lime in a mortar mill".

As far as they go, these regulations are satisfactory; but they do not go far enough, as they do not say what is meant by "lime" and "Portland cement". The lime best suited for agricultural purposes is the least adapted for mortar, and yet in many districts the same lime is used in both cases. Indeed, "lime" may mean anything from the fattest of fat limes or the poorest of poor limes to the best ground lias lime, while "Portland cement" may be anything from very bad to very good. Certainly the by laws are explicit enough to render penal the substitution of "gas-lime" (i.e. lime which has been used for the purification of coal-gas) for "freshly-burned" lime, and of filthy street-scrapings and mud for "clean sharp sand or grit", - both substitutions not unknown in the building-trade. It will be noticed also that mortar containing ashes or furnace-clinkers in lieu of sand does not comply with the regulations.

Careful experiments have been made by Mr. Charles Colson1 to ascertain the relative values of mortars containing gray lime, Portland cement, and mixed lime and cement, the briquettes being kept in air. The results are summarized in Table II.,2and a column is added showing the relative cost per unit of strength.

In these experiments, three sample- of gray lime were used, and were found to vary greatly in strength. The fractured briquettes of the lime-mortar "showed that induration . . . had penetrated only to the extent of from one-eighth to three-sixteenths of an inch, but in the majority of instances to only one-eighth of an inch. The remainder of the area, although dry and moderately hard, had become so mainly from the evaporation of the moisture originally contained in the mass, and in no sense from the absorption of carbonic acid. It was possible, moreover, to crush it in the hand without any great exertion of force".

The loam used in the tests was "yellow, fresh-dug, and rather damp". The quantity of water includes that required for slaking the lime.

The Portland-cement mortars (Nos. 4, 5, and 6) were so raw and harsh " that it would be practically impossible to use them in a satisfactory manner". In order to render them "more plastic and tenacious", lime or loam was added in the remaining tests, to the extent of one-twelfth of the volume of the sand, this being the least quantity that would render the mortars convenient for working. Both these ingredients act injuriously on the mortars, and materially enhance the cost per unit of strength. Loam, however, is much the worse of the. two. If we .compare tests 5 and 11, we find that the addition of the small quantitv of loam lessens the value of the mortar more than 50 per cent. The economy, therefore, of using clean sand - artificially washed if necessary - is dent

1 Proc. Inst. C. E. voL liv. (1877-78, part iv.).

2 Reproduced from the author's work on "Concrete".

Table II. Tensile Strength Of Gray Lime And Portland Cement Mortars, Etc, At The Age Of Six Months

Composition of volumes

No.

Portland cement.

Gray Lime.

Loam

Sand.

Water.

No.of Tests.

Average strength in lbs. per sq. inch.

Ratios of strength.

Coat per cub. yd. of mortar.

Relative cost per unit of strength.

1

-

1

-

2

1. 33

17

27.13

36.88

s.

2

-

1

-

2

1.33

27

47.09

1

11.83

100

3

-

1

-

2

1.33

27

36.44

2.81

35

4

1

-

-

6

1.25

15

103.79

11.56

5

1

-

-

8

1.66

20

68.8

1.86

9.93

45

6

1

-

-

10

2

35

50.16

1.36

8.88

55 52

7

1

.5

-

6

1.5

70

73.47

2

12.2

8

1

.66

-

8

2

74

58.94

1.6

10.72

57

9

1

.83

-

10

2.5

85

42.34

1.14

9.75

72

10

1

-

.5

6

1

21

60.8

1.64

11.44

59

11

1

-

•66

8

1.33

25

38.43

1.04

9.82

80

12

1

-

.83

10

2

19

28.66

0.77

8.76

96

It will be noticed that a mortar composed of one part of Portland cement, one-half part of gray lime, and six parts of sand, - a mortar, be it said, which is sufficiently plastic for the bricklayer's purpose, - is, at the age of six months, exactly twice as strong as a mortar composed of one part of gray lime and two parts of sand, while the cost per cubic yard is practically identical. As far, therefore, as convenience in working, strength, cost, and, I may add, durability, are concered. the advantage is on the whole greatly in favour of the cement-mortar, but it must not be forgotten that a mortar containing such a large proportion of sand is far from impervious. In any volume of sand, the interstices between the grains constitute from one-third to one-half of the bulk; it follows, therefore, that if cement to the amount of only one-sixth of the volume of sand be added, a large proportion of voids will still remain, and the mortar cannot fail to be somewhat porous.