HYDRAULIC OR NON HYDRAULIC LIME

Introduction to Hydraulic and Non Hydraulic Limes

Limes when used in building can be categorised into two types, either hydraulic (Natural Hydraulic Limes) or non-hydraulic (lime putty).

Hydraulic limes set by hydrolysis, a reaction caused by water.  It causes a faster and harder set, therefore these limes are more often used for exterior work, especially in exposed or damp conditions.  Hydraulic limes are available as a bagged powder and in differing degrees of strength.

Non Hydraulic Limes are putties and set by carbonation.  This causes a much slower set and the lime remains softer and more breathable.  Our non-hydraulic lime putties are matured on site and used to produce our medium, coarse and fine set plasters and mortars.

Hydraulic Lime

NATURAL HYDRAULIC LIME (NHL) is produced by burning and slaking limestone which is quarried with a proportion of silica and trace elements.

It is the silica and trace elements that when burnt, become reactive with water and will cause an initial hydraulic set of the mortar when mixed with water.

It is the initial hydraulic set that makes the hydraulic lime mortars easier to use for the contact work and for conservation work where time scales and ease of use are important. The work still needs protection from wind, rain, frost and drying, but for much shorter periods of time than with lime putty mortars.

Lime Putty

Lime putty or non-hydraulic lime is produced by slaking fresh Quicklime (lump lime) in an excess of water which is then left to mature for at least three months. The resulting fat lime putty hardens as a result of exposure to the air /re-carbonation and does not set under water. This lime is often regarded as the most appropriate lime to use in the conservation of old buildings where maximum permeability is required.

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Lime, Hydraulic or Non-Hydraulic?

Hydraulic lime sets by hydrolysis whereas non-hydraulic lime sets bycarbonation. The hydraulic lime can set underwater, that's because hydrolysis is a reaction caused by water. The non-hydraulic lime needs air to carbonate and thus set.

In physical terms, the two materials are very different to work with. Hydraulic lime is available as a bagged powder whereas the non-hydraulic lime is a putty, hence the name fat lime putty. This makes working with hydraulic lime much easier for most builders as it's practically the same as working with cement.

Non-hydraulic lime is softer and sets much more slowly. The carbonation process is very slow and the material remains soft and flexible. This of course, can be extremely advantageous if that's what is required. The hydraulic lime is available in degrees of strength, is faster setting and more durable.

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Lime

Common lime, sometimes called quicklime or caustic lime, is produced by the calcination (or heating to redness) oflimestonesof varyingcomposition. This is done by burning thestonein a kiln with an oviod vertical section and circular horizontal section. The broken stone and fuel (generally coal) are put in in layers, thefirelighted at the bottom, and as the lime drops to the bottom new layers of stone and coal are put in at the top, so that the kiln may be kept burning for weeks at atime. The limestones from which limes and cements are produced differ greatly in their composition, ranging from pure carbonate of lime, such aswhitechalk ormarble, to stones containing 10 per cent, or more of impurities, such as silica, alumina (clay), magnesia, oxide of manganese and traces of the alkalies. The quality of the lime will consequently depend much upon the percentage of impurities contained in the stone from which it is made. Lime is manufactured in nearly every State in the Union, each locality generally producing its own supply.

There is considerable difference, however, in the limes of different localities, and before using a new lime the architect should make careful inquiries regarding its quality, and if it has not been much used it would be better to procure a lime of known quality, at least forplasteringpurposes; for common mortar it is not necessary to be so particular.

In most parts of New England lime is sold by the barrel, but in many parts of the country it is sold in bulk, either by the bushel or byweight

Characteristics of Good Lime

Good lime should possess the followingcharacteristics: 1. Freedom from cinders and clinkers, with not more than 10 per cent, of other impurities. 2. It should be inhardlumps, with but little dust. 3. It should slake readily in water, forming a very fine, smooth paste, without any residue. 4. It should dissolve in soft water.

There are some limes which leave a residue consisting of small stones and silica and alumina in the mortar box, after the lime is drained off. Such limes may answer for making mortar for building purposes, but should not be used for plastering if a better quality of lime can be procured.

Slaking and Making into Mortar

The first step in the manufacture of lime mortar consists in the slaking of the lime. This is generally done by putting the lime in a water-tight box and adding water either through a hose or by pails, the amount of water depending upon the quality of the lime. Lime such as is sold in New England requires a volume of water equal to two and one-half to three times the volume of the lime. The water is rapidly absorbed by the lime, causing a great elevation of temperature, the evolution of hot and slightly caustic vapor, and the bursting of the lime into pieces, and finally the lime is reduced to a powder, the volume of which is from two and a half to three and a half times the volume of the original lime. In this condition the lime is said, to be slaked and is ready for making into mortar. The Thomaston and Rockland (Maine) lime, as also most other limes sold in New England, slake without leaving a residue, and the mortar is made by mixing clean, sharp sand with the slaked lime in theproportionof 1 part of lime to about 5 of sand by volume. Practically theproportion of sandis seldom, if ever, measured, but the sand is added till the person mixing the mortar thinks it is of the proper proportion. Forbrickworkover a certainproportionof sand cannot well be added, for if there is too much sand in the mortar it will stick to the trowel and will not work easily. With stonework the temptation is always to add too much sand, as sand is generally cheaper than lime. The architect or superintendent should take pains to make himself familiar with the appearance of good mortar, so that he can readily tell at a glance if it has too much sand. Mortar that contains a large proportion of lime is said to be rich; if it has a large proportion of sand and works hard it is said to be stiff, and to make it work more readily it is tempered by the addition of water. Tempered mortar looks much richer than stiff mortar, though it may not be so. If the mortar slides readily from the trowel it is of good quality, but if the mortar sticks to the trowel there is too much sand in proportion to the lime. Thecolorof the mortar depends much upon the kind and color of the sand used.

Many of the limes used in the Western States when slaked leave a residue of stones, lumps andgravel, so that instead of mixing the mortar in the same box in which the lime is slaked, a larger proportion of water is added, and the slaked lime and water (about as thick as cream) is run off through a fine sieve into another box, in which the mortar ismixed. Such lime does not make as good mortar as that which leaves no impurities, but it does very well for use in brick andstone work.

The general custom in making lime mortar is to mix the sand with the lime as soon as the latter is slaked and letting it stand until required for use. Much stronger and better mortar would be obtained, however, if the sand were not mixed with the slaked lime until the mortar was needed

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