HYDRAULIC MORTAR.
The following remarks on hydraulic mortar, made in 1811 to the Commissioners of Montrose Bridge, are interesting as showing the detail which he brought to bear on all his works:—
“The best mortar for water work is a mixture of Pozzolano earth with lime and sand, but the late interrupted state of commercial intercourse with the Mediterranean has for years past rendered Pozzolano so scarce an article as hardly to be procured on any terms. Your reporter has therefore been induced to make various experiments with preparations of lime and Roman cement, and finds that a mixture may be made which will set under water and answer every purpose. For this mortar the lime ought to be well burned, and put into casks when drawn from the kiln. It should be brought to the work as recently after being burnt as possible. This will be most readily attained by taking the lime from Boddam kilns. English lime is in general stronger and cleaner, but some of it brought for the purpose of agriculture is not so suitable for buildings as Lord Elgin’s lime. These limes, however, cannot be had very newly burnt, and it will be preferable to take lime from some of the kilns in the neighbourhood which are of good character. When brought to the bridge the lime should be kept under cover, opening only one barrel at a time; the shells must be pounded to a state of powder, and immediately before mixing it with the other ingredients it will be proper to sprinkle a little water upon it to dissolve any gritty particles that may remain amongst it.
“The sand for this work, though fine, must nevertheless be sharp; it must also be passed through a sieve, and cleaned of all impurities by washing, if found necessary. For ramming the joints and pointing under water, let equal parts of lime in its powdered state and of Roman cement be used, with one fourth part of prepared sand, but for the upper works the quantity of Roman cement in the mortar may be reduced to one third part.
“The mortar must be mixed in small quantities and quickly beaten up into a consistency suitable for the work. All white specks, which are apt to swell and spoil the joints, must be carefully rejected from the mortar.”