It has before been pointed out that the addition of sand improves the setting of fat limes -

1. Because the porous structure caused by the presence of the sand enables the carbonic acid of the air to penetrate farther, and thus to reconvert a greater depth of the lime into carbonate.

1 Calcium carbonate.

2. Because the particles of lime adhere more firmly to particles of sand than to one another.

It has been stated also that pure silica in the shape of sand acts merely mechanically, and enters into no chemical combination with the lime.

For all practical purposes this is true, but experiments have shown that in the course of several years some such action does take place to a very slight degree.

In Petzholt's experiments (described by General Gilmore) he found -

1. That in mortar one hundred years old there was more soluble silica than in the original lime.

2. That in mortar three hundred years old there was three times as much soluble silica as in the mortar one hundred years old.

Now, it is a well-known chemical fact that silica does dissolve in alkaline water, though with extreme slowness; and in this case, no doubt, in the course of a hundred years, a small portion of silica had been so dissolved, and thus enabled to attack the lime.

On the other hand, General Scott mentions a case in which fat lime mortar from a wall five years old was found to be set only on the exterior, and to be in a friable pulpy state inside. Also another case of fat lime mortar fifty years old, which was so soft that it could be beaten up with a trowel.

On the whole we may allow that in fat lime mortar made with siliceous sand a minute proportion of silicate is formed in the course of many years. However, the time required to develop this action is so very long that the fact is of no practical importance to the engineer or builder.

The hardening of lime does not depend merely upon the chemical effect of the combinations which result in the formation of the carbonate, and to a slight degree of the silicate.

It is also caused partly by the crystallisation of the hydrate of lime. The water in fresh mortar contains lime in solution. As the mortar dries the water evaporates, and leaves crystals of lime deposited upon the adjacent particles of lime or sand. These crystals attach themselves firmly to the particles, and will withstand a considerable tensile force.

In the same way, wherever the air can penetrate the carbonic acid contained in it combines with the lime, and deposits crystals of carbonate of lime.

As before mentioned, the formation of silicates in a fat lime mortar, if it ever occurs, is so slow as to be of no practical value.

The uselessness of fat lime mortar for good work is shown by the following extracts from some of the greatest authorities on the subject: -

Sir Charles Pasley says that "chalk-lime mortar when wet is a pulp or paste, and when dry it is little better than dust."

Vicat says, "Their use should for ever be prohibited in works of any importance."

General Freussart says, "Where good hydraulic lime is to be had, no other kind should be used for any purpose whatever."

General Scott, in a paper on the subject, says "In the foregoing remarks the worthlessness of pure or fat lime mortars for all constructions, and especially for such as involve the use of heavy masonry, or which will remain damp for any length of time, has been insisted on; and it has been explained that their unfitness for thick and damp walls results from their not containing within themselves any property by which solidification can be brought about."