Development of pre-dried and blended lime mortars for the ready-mix market

Mortar is a mixture of sand, a binder such as cement, and water. This mixture is used in masonry construction to fill the gaps between the bricks and blocks used in construction. It is applied as a paste during construction, and then later sets hard, holding the blocks, or bricks together firmly. It can also be used to fix parts of a construction where its original mortar may have been washed away. Currently, mortar can be either mixed on site, or factory-produced ready-to-use mortar can be bought. (Allen, 2003)

Ready-to-use mortars are replacing on-site mixed mortars, due to their advantages over on-site mixed mortar. They are made in factories under tightly-controlled conditions and delivered to site, ready to use. They have guaranteed mix proportions and overcome any potential problems relating to site mixing. They contain accurate cement ratio in the mixture. This is because the cement content is controlled in the production factory, where accurate measuring techniques are put into use.

The factory-made mortars also offer the advantage of consistence in quality, since the mixing procedures and contents are constant in the factories. This also results to consistence in strength and color. Since all the mixing is done in the factory, using these mortars reduces mixing and labor costs, and also reduces wastage of materials that is experienced during mixing of mortar on site. Health and safety on site is improved, as worker’s direct contact with the mortar is reduced.

Wet ready-to-use mortars are stored in tubs on site and require no further mixing. They have a retarding agent, which makes them fully usable for a certain period of time, normally around 36 hours.

Dry ready-to-use mortars are stored in silos or bags. Silos are delivered to site complete with integral mixers and requiring only power and water supplies to be connected.

The factory-produced silo mortars offer a range of mix proportions and overcome many potential problems experienced in on site mixing. The silo is delivered to site, complete with integral mixer. Once power and water supplies are connected, mortar can be produced as required. The rate at which water is added can be controlled to achieve the required consistency. The silos can either be single-compartment or two-compartment.

(http://www.mortar.org.uk/)

Since the factory produced silo mortars are of guaranteed composition, and are thoroughly mixed, they will therefore provide satisfactory durability. However, the designer still has the

responsibility to specify the correct mortar designation for the type of structure, exposure

conditions and type of masonry units. In hot conditions some stiffening may occur which may be corrected by the addition of a small amount of water followed by trowel mixing on the spot board in the traditional manner. Once the initial set has started the mortar must not be reconstituted in a mechanical mixer or by any other method.

The setting of cement is affected by weather and will proceed more slowly when it is colder.

This factor is taken into account when the mortars are manufactured but subsequent

significant reductions in temperature may increase the retardation period and extend the

working life and will have no adverse effect on the masonry. It is inadvisable to proceed with the construction of masonry whilst the temperature is below 3ºC and falling. If the mortar freezes any frozen material or crust should be discarded.

Currently Used Binders

Portland cement mortar is made by mixing Portland cement, which in this case is the binder, with sand and water. This type of mortar was invented in the mid-nineteenth century, as part of scientific efforts to develop stronger mortars than those that existed at the time. It was popularized during the late nineteenth century.  One of its main advantages is that it sets hard and quickly, increasing the speed of construction.

However, it is not advisable for it to be used for the repair of older buildings constructed in lime mortar, because this type of binder lacks the flexibility, softness and breathability of lime that is required for such functions for proper function. Sulfate-resisting Portland cement may be used to replace ordinary Portland cement in cement:sand, cement:lime:sand and cement:sand with plasticizer mortars to reduce sulfate attack, in cases where wet conditions might be prolonged in the presence of soluble sulfates either in the ground or in clay bricks.

Polymers are other types of binders, which are used in conjunction with cement hydrate binders of conventional cement mortar. The polymeric binders include latexes or emulsions, redispersible polymer powders, water-soluble polymers, liquid resins and monomers. The mortar made using this combination of binders has low permeability, and it reduces cases of drying shrinkage cracking, and its mainly designed for purposes of repairing concrete structures. An example of polymeric binders is MagneLine (Allen, 2003).

Masonry cement itself is a pre-mixed, complete binder. Normally masonry cement contains roughly ¾ of ordinary Portland cement, ¼ of an inert fine mineral filler, and then a powdered air-entraining component is added. Because of this make-up of the material and the air-entrainment, masonry cement mortars are made up with proportions differing from the other mortar types.

Polymer cement mortars (PCM) are the materials which are made by partially replacing the cement hydrate binders of conventional cement mortar with polymers. The polymeric admixtures include latexes or emulsions, redispersible polymer powders, water-soluble polymers, liquid resins and monomers. It has low permeability, and it reduces the incidence of drying shrinkage cracking, mainly designed for repairing concrete structures. Example: MagneLine

Another binder is pozzolana. Pozzolana is a fine, sandy volcanic ash, originally discovered and dug in Italy, but later at a number of other sites.  It is found in various colours: black, white, grey and red. When finely ground and mixed with lime, it acts as Portland cement and makes a strong mortar. The mortar made using this composition has the advantage that will also set under water.

Non-hydraulic or semi-hydraulic limes are other types of binders. These, however, have insufficient setting and hardening strength, hence they are not considered as suitable total binders, but may be added as a binder constituent to produce a cement: lime: sand mortar.

Using Lime

Lime mortar is created by mixing sand, slaked lime (the binder) and water. In making lime mortar, Limestone is burnt in a kiln to form quicklime, which is then slaked (mixed with water) to form slaked lime, either in the form of lime putty or of hydrated lime powder. Sand and water are then added to form the mortar. This kind of lime mortar, known as non-hydraulic, sets very slowly through the process of reaction with the carbon dioxide in the air (Boynton,1980).

The speed of set can be increased by using impure limestones in the kiln, to form a hydraulic lime that will set on contact with water. Another alternative is the use of  a pozzolanic material, such as calcined clay or brick dust, which is added to the mortar. This will have the same effect of making the mortar set reasonably quickly by reaction with the water in the mortar.

Lime mortar is considered breathable, because it will allow moisture to freely move through it, and then evaporate from its surface. It remains slightly flexible, even when it is set, and it will let walls move without cracking too much. Lime mortars also repair fine cracks themselves as rainwater slowly deposits fresh calcium carbonate taken into solution from the surrounding lime mortar, hence the fine cracks end up repairing themselves.

Lime mortar is not mixed in the same way as cement mortars, which are simply turned over and over,with occasional chopping. In order to make a strong lime mortar, it is essential to coat each particle of aggregate with lime paste, hence the  pile of mortar, after initially mixing in the aggregates with a shovel, must be beaten with pick axe handles in addition to chopping and turning. The longer mixing and beating can be prolonged the better and more efficient the mortar will be. It is also of importance that only the smallest amount of extra water is added during the mixing stage. One must not pour lots of water in an attempt to make mixing easier. Adding the extra water will severely weaken the mix, causing a lot of shrinkage and cracking during drying.

Its also of utmost importance that the required ratio of aggregate to binder be maintained. After the mixing process, the mortar should be stored for as long as possible before putting it into use. The purpose of this is to enable the lime to completely coat every particle of aggregate, forming an efficient and well bound mortar. On the contrary, if this does not take place the lime mortar will not completely bond with the aggregate. (Boynton,1980).

During this storage period, the lime particles become smaller as they mature and develop closer contact with the aggregate.

The basic manufacturing process of non hydraulic lime for constructional purposes involves the following; quarrying calcium carbonate (CaCO3), commonly in the form of limestone, chalk or shell. This is then heated in a kiln at around 700-900°C, at which temperature, carbon dioxide is pressurized and disassociates itself from the raw material, forming calcium oxide (CaO), or quicklime.

When the calcium oxide (quicklime) is combined with water (referred to as the slaking process), it reacts violently, breaking down to form calcium hydroxide (Ca(OH)2) (slaked lime). At this stage two forms of material are achievable, a hydrated lime (a dry powder slaked with a minimum of water) or a lime putty (a wet material slaked with an excess of water). To make a basic mortar, the calcium hydroxide is then mixed with sand (and water if made from a dry hydrated lime). The final stage in this process is the setting of the lime in which the carbon dioxide is reabsorbed back into the calcium hydroxide to form calcium carbonate, generally in the form of calcite. This process is known as carbonation.

When we use a fresh lime mortar for mass masonry construction the amount of calcite present depends upon the degree to which the carbonation of calcium hydroxide has occurred. In walls that are relatively thick the ability of CO2 to diffuse through the fabric is often reduced. In addition an increase in the hydraulicity in the hydraulic lime can also inhibit the diffusion of CO2 into the wall core due to higher quantities of relatively dense calcium silicate hydrates (C-S-H) and calcium aluminate hydrates (C-A-H). It is clear that in this situation we would have both lime in the form of calcium hydroxide and calcite forming the mass of the mortar with the calcium hydroxide being noted in regions in which CO2 cannot effectively penetrate.

Generally speaking calcium hydroxide is considered to be free lime, which Allen (2003) define as:

Lime in a mortar or hydraulic lime which remains as calcium hydroxide and has not yet converted or combined with a pozzolan or other minerals or compounds. It is more soluble than calcium carbonate and can be transported within the pore solution and is available for deposition to heal fine cracks – autogenous healing. Not to be confused with free present as calcium oxide in Portland cement, which is much lower quantity and is a measure of efficiency of burning, i.e. low free lime equals well burnt Portland cement.

It is clear that calcium hydroxide (Ca(OH)2)/free lime is highly soluble when compared with other components within the mortar. It is however, important to emphasize that calcium carbonate (CaCO3) is also soluble in water, although 100 times less soluble than calcium hydroxide. Although the solubility of CaCO3 is low it does contribute to the overall quantity of lime that can enter into solution and cannot therefore be discounted from this study.

The ability of both calcium carbonate (CaCO3) and calcium hydroxide (Ca(OH)2) to dissolve is a function of the water temperature and both decrease with an increase in temperature.

Boynton (1980) further emphasises that “solubility decreases steadily as temperature is raised above 0°C”. It is clear that the conversion of calcium hydroxide into calcite is extremely important in preventing dissolution and potential migration especially in saturated masonry conditions.

It is the author’s view that both calcium hydroxide and calcium carbonate components of the mortar can be viewed as high and low solubility, with low being associated with calcite (CaCO3) and other polymorphs, such as aragonite and vaterite, and high with calcium hydroxide (Ca(OH)2). However, it must be emphasised that different solubility levels may also be associated with different polymorphs of CaCO3 i.e. calcite and aragonite, and may be explained by the different relative surface areas of the crystallites.

In addition, the solubility of both CaCO3 and Ca(OH)2 can be seen as a function of not only the temperature of the water but also the concentration of CO2 dissolved into the water and the presence of other solutes such as calcium chloride.

The relationship between temperature and solubility illustrates that those buildings exposed to high levels of cold rainfall may be more vulnerable than their counterparts in warmer climates.

Why Select lime?

The use of lime as the binder has a number of advantages over Portland cement. Mortars and plasters made with no lime and a low percentage of Portland cement will end up having low workability (the correct combination of flow, water retention and cohesiveness), they will also be porous and will not be so durable. If an attempt is made to overcome this by increasing cement, other problems such as harshness, brittleness and shrinkage will in turn tend to occur.

Lime is a much better binder than cement in plasterwork. Although its setting will be slow, the result will look better and the softer surface will be less prone to cracking. Lime mortars have a high degree of workability which is highly desirable in mortars and plasters. Due to the slow setting property of lime mortars, they allow mixing in large quantities without fear of going off before use.

In harsh climatic conditions, lime mortars and plasters may not be very durable but this can easily be overcome by the use of hydraulic limes or the addition of a small percentage of Portland cement into the mix. Siliceous materials, known as pozzolanas (eg volcanic ash and rice husk ash), can also be mixed with lime to improve its strength and durability.

Architects are increasingly becoming aware of the problems of Portland cement mortars and many now specify blended lime-Portland cement mortars. Recently, lime has played a leading role, worldwide, in the conservation of old buildings, most of which were built in the ‘pre-Portland cement’ era. It is well recognized that successful preservation of ancient monuments, such as churches, castles and other historic sites, necessitates the application of the same binding systems as were used originally. Attempts in the past at patching up these buildings with ordinary Portland cement-based mixes have invariably led to even greater problems of decay occurring at a later stage. (http://environment.uwe.ac.uk/video/cd_new_demo/conweb/walls/mortars, retrieved 29th sept 2008)

References

Allen, G. (2003), Hydraulic Lime Mortar for Stone, Brick and Block Masonry, Donhead, Shaftesbury,

Boynton, R.S. (1980), Chemistry and Technology of Lime and Limestone, 2nd ed., John Wiley & Son, New York, NY, .

Beckman, P. (1995), Structural Aspects of Building Conservation, McGraw-Hill, New York, NY

http://www.mortar.org.uk/ , retrieved 29th sep. 2008

http://environment.uwe.ac.uk/video/cd_new_demo/conweb/walls/mortars, retrieved 29th sept 2008