What makes hydraulic lime hydraulic

Indeed, the process may actually result in a better set ultimately, as the lime is more mature. A similar effect can be achieved by adding pozzolanic additives to non-hydraulic lime as these additives contain highly reactive silica and alumina.

However, the performance of a hybrid mortar was called into question by English Heritage following a number of spectacular failures, after which it banned the use of these mixtures on grant-aided work. The results of a study by the Building Research Establishment and English Heritage, which are now being prepared for publication, show that the addition of a small amount of non-hydraulic lime per cent improves workability but anything above this level significantly impairs durability.

Mixes containing and hydraulic lime:non-hydraulic lime:sand actually performed less well than a standard non-hydraulic lime:sand mix in their tests. This is to ensure that the new mortar performs in the same manner as the old and is similar in appearance. The original mix is best determined by analysis. Several companies offer mortar analysis services - see The Building Conservation Directory or the Directory pages of this website for details. Common aggregates include local river sand and particles of brick which may not have any pozzolanic effect , stone and old mortar, as well as extraneous material from the firing process in particular, such as specs of coal dust.

The choice of aggregate has a significant effect on the performance and the appearance of lime mortar. In particular, any aggregate used should be well washed and graded, free from sulphates this tends to rule out the addition of coal dust even if found in the original mortar , clinker and alkalis such as sodium and potassium hydroxide. Other factors which have a significant effect on performance include particle size and shape. The correct specification of the mortar for pointing or rendering old buildings is vital.

Bear in mind that some proprietary mixes may contain cement, and that a mortar which is too hard or too impervious may cause extensive damage to historic masonry and other structures. Further information. This article is reproduced from Historic Churches , Some limes require a more methodical working practice as they can take longer to set than their synthetic alternatives.

This longer setting time is negated by working methodically. Sometimes, builders can be wary of switching to using lime products. They can dismiss them as too difficult. However, modern lime materials can be as easy to use as cementitious products which offer none of the long-term benefits of lime.

Any lime mortar must always be softer and more porous than the main building material. This is why issues such as spalling can occur when using OPC mortar to repair masonry of soft brick. The more exposed to the elements a lime mortar or lime render is, the greater the need for a faster set and greater durability, to cope with harsher freeze thaw cycles.

The less hydraulic a lime is, the more it will flex and move with a building. Timber structures therefore need a more flexible and breathable lime.

Lime putty or hemp lime mixes should be used with caution in houses where damp is a problem, if a wall is permanently very damp, a putty mix may never set. Sand or aggregate — a key component for the strength and durability of the mix. Sand also prevents the material from shrinking as it dries out. A binder — acting to hold the mix together. The three key ingredients to any lime mix are : lime, water and sand. When mixed, these ingredients set to form a substance similar to man-made limestone.

Although there are many different kinds of lime which differ in both chemistry and strength, all limes are made when limestone is heated in a lime kiln. The heating process produces quicklime to which water is then added to form slaked lime. Non-hydraulic limes set as they absorb CO2 from the atmosphere. This can mean they have a slower setting time than other limes as very specific environmental conditions must be met for this absorption to take place.

Hydraulic limes initially set when water is added, or even while underwater, and then harden over time as CO2 is absorbed, giving a harder mix which is more weather resilient. Although made in the same way as non-hydraulic limes, these are burnt from a different limestone. Subscribe to the Ecomerchant mailing list to receive updates on new arrivals, special offers and other discount information.

Email us: info ecomerchant. Non-hydraulic lime is made from a pure limestone, pure calcium carbonate, and tends to be in the form of a putty. Non-hydraulic lime sets by carbonation re-absorbing carbon dioxide from the air. It is softer and sets much more slowly than hydraulic lime and remains softer for longer as the carbonation process is very slow.

The fatty nature of lime putty lends itself especially well to plasters and renders and its flexibility allows for the subtle movement common to older buildings constructed with little or no foundations. A pozzolan is a term for a mineral additive which, when mixed with non-hydraulic lime mortars, renders and plasters, brings about a slight hydraulic set, achieving a harder, faster set.

Pozzolans have been in use for millennia and the name comes from the use by the Romans of volcanic pumices and tuffs found at Pozzuoli, at the foot of Mount Vesuvius.

Pozzolans continue to be used to this day and are a tried and tested means of increasing strength and speeding the set in non-hydraulic lime mixes to improve their performance. Breathaplasta is comprised of a blend of limes and so has characteristics of both types of construction lime.

This makes Breathaplasta a very versatile product and well suited to a wide range of applications across several different building types and ages — from heritage to new build.

Breathaplasta is predominantly a high quality, ultra-fine non-hydraulic building lime blended with a small amount of moderately hydraulic lime NHL 3. Supplied pre-mixed in powder form, Breathaplasta simply requires the addition of water to create a smooth blended plaster mix. As Breathaplasta is mostly comprised of a soft, non-hydraulic lime it sets by carbonation and absorbs carbon dioxide from the air.

Combined with a fine organic bio material, Breathaplasta has a highly open microporous structure that is ultra-breathable and allows moisture to freely dissipate through its surface.

This makes Breathaplasta well suited to heritage and listed properties that require consideration for breathable materials that wick away moisture. In addition to the high vapour permeability breathability of Breathaplasta, key features of the product are its quick setting time and ease of use, and this is due to the small amount of moderately hydraulic lime NHL 3.