2. Materials

 

2.1 Mortars

2.1.1 What mortars do in a wall

2.1.2 Different binders

2.1.2a Lime and lime mortars
2.1.2b Cement : lime : sand mortars – gauged mortars
2.1.2c A digression on hydraulic cements
2.1.2d Masonry cement mortars
2.1.2e Ordinary Portland cement mortars with plasticizers

2.1.3 Sands for Mortars

2.1.3a Grading of sand
2.1.3b Particle shape
2.1.3c Other physical properties
2.1.3d Chemical properties
2.1.3.e The bulking of sand

2.1.4 Admixtures

2.1.4a Accelerators and anti-freeze
2.1.4b Retarder
2.1.4c Waterproofers
2.1.4d Pigments

2.1.5 Water

2.2 Bricks and Ceramics

2.2.1 Fired clay bricks

2.2.2 Calcium silicate bricks

2.2.3 Terra cotta and faience

2.3 Natural and Artificial Stone

 

2. MATERIALS

2.1 Mortars

2.1.1 What Mortars do in a Wall

Mortars consist of “aggregates” (sand) which provide the bulk of the volume together with a “binder” which, as the name suggests, sticks the aggregate together. The third ingredient, water, gives the mortar is “fluidity” to enable it to be placed as required and, where portland cements are used, enables the chemical reaction to take place which results in the cement powder becoming a hard mass.

These three are the main constituents of mortar; other materials are sometimes added to modify the behaviour of the mortar – these are discussed later on.

In masonry walls, the primary function of your mortar is to provide a continuous bed between bricks.

In addition the mortar in an external wall will seal all the joints between the units and prevent penetration by wind and/or rain, snow, etc. Recall from the previous chapter how important weather resistance is in walling and the different ways in which this can be achieved.

In the longer term, the mortar joints protect the walling material itself (stone, brick, and so on) from attack by weather. This may be either by frost attack or by the dissolving of the wall material, particularly where industrial pollution of the atmosphere gives rise to mildly acidic rain (mainly with stones). The mortar itself is subject to deterioration from either (or both) of these causes and if the joints are allowed to disintegrate will allow progressively deeper penetration of the wall by the weather and possibly attack on the wall materials.

2.1.2. Different Binders

As mentioned above, a mortar consists of binder, aggregate and water. For thousands of years the binder used all over the world was lime. In this country it is only really within this century, perhaps since the 1st World War that lime and sand mortars have been replaced by mortars in which portland cements are used as binders.

2.1.2a Lime and Lime Mortars

Lime is manufactured from either chalk or limestone. The manufacturing process is very simple, consisting of heating the crushed chalk or limestone in a kiln to drive off carbon dioxide. This ‘quick lime’ is very caustic (will burn the skin easily) and must be ‘slaked’ with water to give hydrated lime (or slaked lime) which is the substance used in mortar. Quick lime reacts violently with water expanding and even exploding; this would be too unstable apart from anything else to use as a binder in mortar. Slaking is the controlled addition of quick lime to water so that the usable hydrated lime is obtained.

Slaking in this country is now usually carried out at the Lime works and the Lime is supplied as a fine powder in 25kg bags (note Lime has about half the density of cement).

In the days when quick lime was slaked in shallow pits on site, the lime was run to a putty and left with water standing on top of the putty to mature. This maturing of the lime putty produces a mortar which is more “fatty” (easier to spread) than a mortar made by mixing sand with hydrated lime.

A lime mortar gains strength fairly slowly by a process known as ‘carbonation’ (that is reaction with carbon dioxide – from the atmosphere). Thus eventually the lime mortar returns to the chalk or limestone from which completely different from (and much slower than) the ‘hydration’ process (reaction with water which is characteristic of portland cements and other ‘hydraulic’ cements). The water in a lime mortar is there solely to vie the mortar the fluidity necessary to allow it to be spread, eventually it all leaves the mortar by evaporation.

There are three problems with lime mortar.

(1) Low strength when fully matured.
(2) Water solubility and deterioration in continuously damp conditions.
(3) Slow rate of strength gain and vulnerability to frost attack during construction.

Before leaving lime it should be mentioned that the solution to some of the problems above has always been available in the form of ‘hydraulic lime’. These are limes made from limestone which contains certain clay impurities which make the resulting lime behave like a portland cement and react with the water to produce a strong binder. These hydraulic limes in fact pointed early inventors in the direction of discovering portland cement. The distinction is often made between ‘hydraulic limes’ and ‘non-hydraulic limes’. Hydraulic limes harden by hydration and carbonation, non-hydraulic limes harden by carbonation only. High calcium limes are a particular group of the latter which are particularly “white”.

Finally, before moving on to portland cement bound mortars, another intermediate type should be mentioned, the lime – pozzolana mortar. Pozzolanas are natural or man made powders, rich in silica, which, when mixed with normal limes, give them hydraulic properties, that is greater strength, low solubility in water and a hydration reaction. The Romans used pozzolanas widely, the name deriving from Pozzuoli, a town north of Naples, where deposits of volcanic earth were mined (and still are). Pozzolanas are also used with Portland cements as a cement substitute when they are cheaper than portland cement.

Apart from natural pozzolanas there are a number of ‘man-made’ materials which react with lime to produce a hydraulic material. The Romans also used crushed burnt clay brick (or tile) dust for much of their masonry work. In recent years fine ash from coal fired power stations has also been found to have pozzzolanic properties. This is known as ‘fly ash’ or ‘pulverized fuel ash’ (pfa) and is obtainable from the electricity generating board. Not all power stations produce pfa that is suitable for use in a mortar : a lower sulphate content (less than 1% is important and specialist advice should be sought if these materials are to be used.

2.1.2b Cement : Lime : Sand Mortars – Gauged Mortars

Once portland cements become accepted as reliable materials after the turn of the century, builders began to use them to overcome some of the shortcomings of lime : sand mortars.

Straight portland cement : sand mortars were not satisfactory for the bricklayer principally because the mortar lacked cohesion and was very crumbly when spread on the wall. Thus lime mortars were gauged with cements, rather like pozzolanas, to give a higher ultimate strength, a more rapid rate of strength increase (thus reducing risk of frost attack during construction) and resistance to water.

When masonry cements (see later section) became popular in say, the 1950’s, gauged mortars became less and less used because of the inconvenience of mixing lime and cement on site (one bag was preferred to two).

More recently they have become popular again where a pre-mixed lime : sand “coarse stuff” is delivered by lorry to the site from a mortar plant, which can operate fairly good quality control procedures. The coarse stuff is gauged in the site mixer with the appropriate quantity of cement. There are particular advantages in using this procedure where coloured mortars are specified since the mortar plant is capable of producing more consistent colour in the mortar as compared with on site mixing using pigments.

For repointing work on its own the use of pre-mixed lime : sand coarse stuff may not be economically justified since the quantities of mortar needed are likely to be small and to attract a part load surcharge on the basic price.

2.1.2c A Digression on Hydraulic Cements

Portland cements are the most widely used of hydraulic cements (that is those that harden by reacting with water). Without going into details Portland cements are made from a mixture of chalk (or limestone) and clay which is fired in a rotary kiln to produce clinker, this is subsequently crushed to form cement powder. Gypsum (as used for plastering) is added to modify the early reaction of the cement with water.

The characteristics of portland cements can be varied by the cement maker mainly by control of the raw materials or of the grinding process. The normal cement is known as ‘ordinary portland cement’ (o.p.c.) and is used for most jobs. It is the cheapest of the cements.

‘Rapid hardening portland cement (r.h.p.c.), (Blue Circle, ‘Ferrocrete’) is chemically the same as o.p.c but is more finely ground thus allowing the water to react more quickly with the cement powder (the hydration process, also called the ‘hardening’ of the cement). We do not think that there are likely to be many situations in which you would need to use this type of cement.

Here we should distinguish between terms used – ‘hardening’ describes the process of strength increase in cement, ‘setting’ is used to describe the change in the physical nature of cement and water when it changes from behaving as a liquid to behaving as a semi-liquid or ‘gel’.

The ‘initial set’ of cement is an important stage and once it has occurred then the mortar containing the cement should not be disturbed. After a time, mortars will become stiff and unworkable, in this condition they should not be “knocked back” with further water to make them more workable. Now we must return to the various kinds of portland cements that are likely to be used in pointing.

‘Sulphate resisting portland cement’ (s.r.p.c) (“Sulfacrete”) may be specified for pointing in exposed elements of a building (exposed in the ‘weather’ sense of the word described in chapter 1.

Mortars containing Portland cements are liable to deteriorate when sulphate salts, dissolved in water, are absorbed by the mortar and react with the cement in the mortar. The product of the reaction occupies a greater volume than the original cement, so that it swells and disrupts the physical structure of the mortar. Two things are necessary, sulphates water.

Sulphates occur naturally in clay soils and are not eliminated in the brick manufacturing process; some makes of brick can contain quite high amounts of sulphates. Present Codes of Practice recommend that these bricks with high sulphate content should not be used in positions of severe exposure to the weather (for example, chimney stacks, parapets and boundary walls) : bricks with moderate sulphate content should be bedded in mortars made with s.r.p.c (see also later notes on building defects and remedial treatments).

White portland cement (‘Snowcrete’) is chemically the same as o.p.c in its active constituents; it does, however, contain far less of the impurities which give o.p.c its typical grey colour. White portland cement is produced from china clay and very pure chalk through a more rigorously controlled manufacturing process. It is thus considerably more expensive than o.p.c. In pointing its use may be required in producing pale or pigmented mortars (in the latter case in conjunction with pigments, it should however be noted that because there is very much more sand in the mortar, and colour tends to be the dominant factor in determining mortar colour.

Masonry cements are modified portland cements and are discussed further below (2.1.2d).

Before ending this digression on hydraulic cements, we should mention ‘High Alumina Cement’ (h.a.c), ‘Fondu Cement or ‘Ciment Fondu’. This is not a Portland cement and is manufactured from bauxite and limestone which are fused together at very high temperatives and again crushed. Mortars (and concretes) made with h.a.c differ from previous mortars in three particular characteristics.

(1) Good resistance to sulphates.
(2) Very rapid hydration (full strength in 24 hours or so).
(3) Resistance to high temperatures (kilns, furnaces etc).

Unfortunately h.a.c concretes were misused in structural applications in the 60’s and early 70’s and after a period of frantic checking, propping and strengthening, virtually all references to them were withdrawn from codes of practice and building regulations.

When used with portland cements a “flash set” can occur depending upon the ratio of the two cements used. This is generally considered undesirable but can be useful in situations (eg plugging leaks) where rapid setting is wanted.

2.2.2d Masonry Cement Mortars

You will recall that straight portland cement and sand mortars are not very satisfactory from the bricklayer’s point of view. One way round this is to gauge a lime mortar with cement to produce the desired combination of properties. Another way is to use a masonry cement, such as ‘Walcrete’, as the binder.

Masonry cements are portland cements, o.p.c, modified by the addition of a filler which helps to retain water in the mix and a “plasticizer” which makes the mix workable for spreading by trowel. Thus with one bag on site instead of two (lime and o.p.c) a suitable mortar can be produced.

Plasticizers work by dispersing minute bubbles throughout the mix which allow the sand grains to slide more easily over each other and thus produce a cohesive mix that is easily spread. The introduction of minute bubbles into the mortar will reduce the strength of the mortar (though this is not often in itself very important) but seems to give improved resistance to frost attack.

Where masonry cements are used plasticizers should not be added at the mixer.

2.1.2e Ordinary Portland Cement Mortars with Plasticizers

Masonry cements, because of their lower cement content as compared with o.p.c etc, should not be used for making concrete. In order to reduce variety of cements kept on site many builders prefer to stick with o.p.c and use this for concrete and mortars. In order to obtain the required cohesiveness in the mortar, plasticizers (usually liquid) are added at the mixer.

As mentioned in the last section these generate minute bubbles which lubricate the aggregate particles. In masonry cements the quantity of plasticizer (and, therefore, the percentage of air entrained into the mortar) is controlled carefully at the cement works. Site addition of plasticizers has given rise to problems with over-dosed mortars due to careless batching at the mixer or failure to appreciate the importance of not exceeding the manufacturer’s recommended dosage of plasticizers.

Control of dosage on site is probably the major problem with this type of mortar.

2.1.2 Sands for Mortars

At first glance all sands may seem to be the same. On further reflection, you might agree that there can be quite a variation in colour or again that there might be differences in the “feel” of a handful of one sand as compared with another. These differences between sands can have quite a marked effect on the mortar produced, especially bearing in mind that perhaps two thirds of the mortar consists of sand. We now look at the properties of sand and see how these can affect the mortar.

2.1.3a Grading of Sand

This is perhaps the most important physical property of the sand. The term ‘grading’ is used to describe the way in which differing proportions of small, medium or large individual grains are mixed together to produce a sand.

A coarse sand will contain a high proportion of large sand particles and will look and feel gritty. Sands of this type are known as “sharp sands” or “concreting sands”. At the other extreme are “soft sands” (also called “building sands” or “bricklayer’s sands”) which contain a high proportion of fine particles. Sieve analysis of the sand allows the grading to be measured in a more precise way than the descriptions above – “coarse” or “soft”, etc.

The practical importance of grading lies in its effect on the “workability” of the mortar. This property is significant mainly in the early ‘wet’ stages of the mortar in its application to the wall. The mortar should be cohesive (stick together), if it is crumbly and friable it will be difficult to apply.

Coarse sands will have relatively large voids between the individual sand particles and the normal one-third proportion of binder may not fill the voids adequately and hold together the sand particles. At the other extreme a sand with a high proportion of fine (silt) material, whilst presenting a smaller volume of voids (between the particles) will at the same time present a larger total surface area, more water will be required in the mortar in order to wet the surface of the aggregate. The additional water will tend to weaken the mix (by increasing the water : cement ratio). Alternatively, if further cement is now added to compensate for the increased water content the high cement content will lead to greater shrinkage stressed in the mortar as it sets and hardens and this could give rise to cracking in the mortar or between the mortar and the walling units (bricks, stone etc).

The sands discussed so far have shown a continuous grading through all particle sizes though in one there was an emphasis on coarse and in the other on fine particles.

Sands may also occur in which there is a deficiency to a greater or lesser extent of particles say, over the middle and lower range of sizes – single sized sands. These behave rather like coarse sands only to a greater extent since there is now virtually no fine material at all to occupy the voids between the large particles; extremely unworkable mixes are likely to result in more results.

2.1.3b Particle Shape

Grading of sand (as discussed in the last section) is the main factor in determining workability. The shape of the individual sand grains also has some effect on the workability achieved.

Natural sand derives from solid rock by a process of weathering, frost attack, transportation in streams and rivers and, over millions of years, grains become fairly well rounded. At the other extreme in certain parts of the country, natural sands do not occur at all and sand has to be manufactured by crushing suitable hard rock. The aggregates produced by crushing tend to be angular in shape since they have not been subjected to millions of years of rolling about in a stream or in the sea.

For a given amount of solid material the shape which has the least surface area is the sphere. Thus the more nearly a sand approaches the sphere in its particle shape, the smaller for a given bulk volume is the surface area to be wetted by the binder/water. Also from the geometry of sphere packing (like a greengrocer piling apples on a stall) the voids ratio with spheres is about 30%, which ties up very well with our binder : aggregate ratio of 1:3.

In contrast angular aggregates present:-

(i) a greater surface area for a given volume, and

(ii) in general a greater voids ratio since the particles do not pack as closely together.

There are other particle shapes intermediate between the two extremes described above but these are more important in considering coarse aggregates for concrete work.

2.1.3c Other Physical Properties

The strength of the aggregate itself is not usually significant and its determination is difficult.

The specific gravity of aggregates can vary over a large range, say from about 0.6 up to 3.5-4. For mortars again this is not a significant property – natural sands or crushed rock fine aggregate will have a specific gravity of about 2.5.

2.1.3d Chemical Properties

The most important consideration here is the need to ensure that the aggregate used does not contain contaminants that will adversely affect the hydration of the cement. The sand should be supplied washed and should be stored on site preferably on a hard standing or boards to avoid contamination with top soil or other organic matter (leaf and plant debris). Where material is stored on site for a longish period, cats urine etc, accumulating in the sand can make it useless.

In coastal areas sea dredged sands may be available. These should be thoroughly washed to remove salts, since although these do not adversely affect the hydration of the cement (they will possibly have a mild accelerating affect), they could cause problems after the completion of the work with efflorescence.

We have already talked about sulphate attack on mortar (sulphates being a particular type of salt). In general salts present in either the bricks or the mortar will dissolve in absorbed water when the wall becomes wet, particularly in winter when the wall will be very wet. When the bricks/mortar dry out the water evaporates from the surface leaving behind on the surface the salts that had been dissolved from the body of the wall. This salt deposit has a white fluffy appearance and is known as “efflorescence” (literally ‘a flowering out’) : it is most noticeable in the first dry spells of spring and early summer.

‘Efflorescence’ normally does no permanent harm to the brickwork, but clients will often be unhappy about the ‘blotchy’ appearance. If “remedial” action is necessary the salt deposits should be brushed off with a soft brush, not a wire brush. On no account should the wall be washed since this merely transports the salt back into the wall. Acid treatments are of no benefit. The brushing may need repeating at intervals until all soluble salts have come to the surface.

2.1.3e The Bulking of Sand

The expression, bulking of sand is used of a sand to refer to the peculiar way in which the volume of the sand varies according to the amount of water contained (mainly contained in the space between the sand particles). In most sands the spaces between particles are small and water absorbed into these spaces can force apart the sand particles, thus “puffing” up the original volume. Beyond a critical moisture content (usually about 7% by weight), the system collapses and the volume occupied by the wet sand is the same as that occupied by dry sand. The critical feature here is that the bulk volume of wet sand can increase by 20-30% over the bulk volume of the same sand in a dry condition. The proportion of mortars (see later) is usually expressed as a ratio based upon volumes of dry material. Account should be taken therefore of ‘bulking’ in the measuring out of the sand for a mortar, both in order to ensure that the specified proportions are obtained and, perhaps more importantly, to ensure that successive batches of mortar mixed, say in drying weather, do not vary excessively in their mix proportions.

Ideally measuring should be done by weight since the bulk weight of the sand will only increase by about 5% between dry and saturated states. Realistically, for the small batch quantities likely to be used in repointing, careful volume batching with gauge boxes should satisfy most architects, surveyors and contractors (see section later on mixing).

2.1.3 Admixtures

(Note: This section only applies to mortars containing portland cements.)

‘Admixture’ is the correct term for substances other than binder/aggregate and water which may be added to a mix on site. The term ‘additive’ is also used loosely in this sense but should really only be used for substances added to a cement during manufacture.

We have already come across pozzolanas and plasticizers and seen the way in which they modify the properties of lime or cement. In this section we shall be looking at other chemical admixtures which you may wish to use. Before going into detail I should explain why there is a reluctance to allow admixtures on the part of some architects; at the very least nowadays a specification will include a clause to the effect that – “Admixtures may not be used except with the express consent of the architect in writing”. Hopefully you will remember that two golden rules if you are using plastiscizers on site with o.p.c are:

(i) To measure accurately (not in half milk bottles nor in sprinkles or handfuls), and

(ii) On no account to use more than the manufacturer states.

It should also be said that only admixtures from reputable manufacturers should be used: you will find bricklayers using Fairy Liquid (5 squirts or whatever) as a plasticizer. It works of course, but it is hardly worthwhile laying yourself open to all kinds of “incompetence”, “amateurish” accusations is problems arise on a job.

Excessive use of plasticizer will entrain large numbers of air bubbles into the mortar : when this occurs in a mortar in which careless batching has meant that the binder content is very low, rapid deterioration of the mortar by frost action and general weathering occurs. This has happened sufficiently often for people to be wary about using plasticizers added on site.

Another major problem area with admixtures concerns disintegration of concrete in reinforced concrete structures where accelerators have been used to speed up hydration so that, in winter, early strengths are higher when compared with those that could be obtained with unaccelerated concrete. Unfortunately the accelerator used is calcium chloride, a hygroscopic (ie draws moisture to itself) salt. Again overdosage is often at the bottom of the problem : the chloride salt remains in the concrete, close to the steel reinforcement, and attracts moisture to itself. The combination of salt, moisture and mild steel leads to rusting of the latter. The volume of the rust is greater than the original volume of the steel so that expansion takes place which the surrounding concrete cannot resist and the concrete falls away exposing the steel directly to the atmosphere so that corrosion can then proceed more rapidly.

Note the way in which embedded steel disrupts the surrounding material/structure by the expansion that occurs on rusting : we shall return to it again much later when diagnosing defects in walls. We shall also very soon be coming back to accelerators in mortars.

Both the problems outlined above are due to human failures rather than to defective technology, particularly in that the man responsible for batching the mix has never been told the effects of incorrect proportioning. On a Murhphy’s law basis architects, surveyors and even main contractors want to have things done in the simplest way and eliminate the certainty that some time, somewhere the wrong thing will be done.

2.1.4a Accelerators and Anti-Freeze

Frost is the major cause for your having repointing work to do. Frost is also one of the major enemies in your being able to work continuously during winter and produce satisfactory, durable work. Frost attack in its first aspect will be looked at later in diagnosis of defects; here we are concerned with frost and its effect on fresh mortar during perhaps the first few weeks of its life.

Firstly, as is well known, when water freezes its volume increases. This expansion is the force that leads to disruption of materials and the problems of frost attack. In making a mortar we have to add more water than as needed for hydration to occur in order to make the mix workable. This excess water occupies the spaces between the aggregate particles and eventually evaporates.

Until it can evaporate the water exists as free water within the body of the mortar (some water is of course busy combining with the cement). If the temperature of the mortar falls below zero then the free water will freeze and may disrupt the physical structure of the mortar : when the water thaws the disrupted mortar tends to crumble into lumps.

Apart from water freezing the other factor is the strength of the mortar. In a fully matured mortar, the expansion caused by the freezing of water is resisted by the (low) tensile strength of the mortar. Until the mortar has matured there is going to be even less strength available to resist the expansion of the ice. The sooner the mortar reaches an adequate strength the better is it able to resist freezing. This increase in strength is one way of looking at hydration. The rate at which hydration (and strength increase) takes place depends upon temperature : the higher the temperature (up to about 25-30C) the more rapidly hydration progresses. At zero, hydration ceases (and of course, near zero proceeds at snail’s pace). So the problem is now compounded – free water freezes in a mortar whose binder is weak and not increasing in strength. There is a third factor, we tend to measure hydration in terms of strength increase. However, the reaction between cement and water generates heat (chemists call it an ‘exothermic’ reaction, which merely means ‘gives out heat’). For a given quantity of cement there is a fixed amount of heat “locked in” which is released when water reacts with the cement. That fixed amount can be released slowly or rapidly depending upon the rate at which hydration is taking place. When the air temperature drives down the mortar temperature, the rate of hydration drops, the internal heat generation in the mortar also drops (for practical purposes ceases) and again in a bad situation worsens.

Now enter the accelerator : it speeds up hydration : we gain strength more rapidly : we generate internal heat more rapidly and, added bonus, the accelerator is a salt and salt water freezes at a lower temperature than fresh water. However, we need to be careful. The mortar joints in a brick wall may occupy a small proportion of the total volume (maybe about 15%). In long spells of low temperatures, the mass of brickwork is going to be chilled and we will have a “storage heater in reverse” effect. The heat that is being generated in the mortar is being drawn away into the cold bricks probably more rapidly than it can be generated. After a while therefore despite the accelerator the mortar temperature is going to start to drop. The accelerator will probably provide ‘anti-freeze’ properties where there is going to be an overnight frost (not too severe) with daytime temperatures rising to perhaps 5-10C. The lowering of the freezing point of the water to about –1 to –2C is going to help here.

In periods of continuous frost (day and night) or when severe frosts occur overnight, extreme caution is needed in relying on an accelerator to protect the mortar against freezing.

One final twist to this tale is the need to appreciate that salts are being introduced into the construction. For repointing of external clay brickwork, there is a probability that efflorescence (see notes on sands above) will occur on the joints. Apart from the need to clean it off to keep the client happy, no permanent damage should result. However, certain non-clay materials may deteriorate due to the presence of the salts (see below – calcium silicate bricks and natural stone). Where potentially vulnerable materials are present it may be unwise to use a calcium chloride accelerator.

In the wake of the calcium chloride/reinforced concrete panic, the Building Research Station at Garston developed accelerators based on formats and these are now available : they appear to avoid most of the difficulties suggested above, although bear in mind that a small amount of mortar (especially pointing on the outer face of the wall) is not going to generate much heat itself even if accelerated.

Generally speaking, accelerators are not recommended for repointing works.

2.1.4b Retarder

Retarders have an effect opposite to that of accelerators, hydration is slowed down for a period of time dependent upon the amount of retarder used. This slowing down (retardation) is seen in the delay in the initial set of the mortar occurring (recall that “initital set” is the word used for the physical change that occurs when the cement and water cease to be liquid and become a ‘gel’).

Again careful dosage is important since hydration can be killed by an excess of retarder. You should also note that there are two ways in which retarders are produced for the building work – ‘an integral retarder’ to be mixed in throughout the mortar (this is the kind that we are dealing with here) and ‘a surface retarder’ which is of paste like consistency and to be applied to the face of moulds where exposed aggregate finishes are wanted on concrete.

You are most likely to think of using a retarder in very hot weather when initial set is taking place very quickly. The rapid application by the pointing gun (as compared with repointing by hand) is unlikely to give rise to difficulties. However, it may be that the tooling of the joints is having to be carried out sooner than is desired, a retarder could then be used to keep the mix open for a longer period giving more flexibility in organising the work between gunning in the joint and tooling it up.

The other use of these admixtures that you are likely to come across is in retarded ready mixed mortars. These are a relatively new product and often prove more economical and more reliable than site prepared mortars for new brickwork. The mortar is supplied in large plastic containers for handling on site by crane or forklift and generally must be used within a day of delivery (before the retarding effect wears off). It is likely that for most repointing jobs the quantities of mortar used per day would not justify the buying of mortar in this form.

2.1.4.c Waterproofers

“Waterproofing” admixtures are designed to fill the pores of the mortar with a water repellent substance. To be effective the mortar itself must be proportioned to produce as dense a mix as possible.

It is difficult to think of a situation in which their use would be beneficial. For example in situations where one brick thick walls are damp due to rain penetration, one would be cautious in recommending repointing in waterproof mortar as a means of curing the rain penetration. Again in retaining walls, where possibly joints and bricks or stonework are eroded due to water percolating under pressure from the back of the wall, repointing with a dense mortar of low water permeability may only worsen the effects on the bricks themselves (or stone).

Water repellents for application to the wall surface are dealt with later in Chapter 5.

2.1.4.d Pigments

The ‘normal’ colour of a mortar depends mainly upon the type (colour) of sand used and to a lesser extent upon the type of cement used (in particular o.p.c., white cement and sulphate resisting cement). However, the range of colouring thus produced is limited and on occasion you may be asked to produce a mortar with either a contrasting or complimenting colour to the bricks. You should remember that in most cases walls are seen from a distance such that the jointing is not distinct from the brickwork. The overall colour of the wall as seen is therefore a combination of the brick colour and the mortar colour. The number of combinations is quite large but perhaps the most surprising result is the lightening of the colour of a wall of dark bricks by use of pale coloured mortars. Alternatively the joint between the bricks can be made to “disappear” by use of mortar of the same colour as the bricks.

Pigments can be mixed into the mortar to produce these colour effect. Pigments used should comply with BS 1014 to ensure compatibility with cement and quantities added should not exceed 10% of the weight of the cement (in the case of carbon black the amount should not exceed 5% of the weight of cement). In order to produce consistent colouration for the mortar particular attention must be paid to the batching of materials including the effect of bulking of the sand.

Alternatives to the use of pigments are:-

(1) The use of Cullamix/Tilcon or other approved (see 2.1.2d above) (though here you are limited to the colours that Blue Circle manufacture).

(2) The use of ready mixed coarse stuff or ready mixed retarded mortars in each case with the pigment gauged in bulk at the mortar plant.

2.1.4 Water

The general rule is that if you can drink the water, it is suitable for making mortar. In the majority of cases mains water is likely to be used and no problems should arise. If for some reason mixing water is taken from a rain water butt, say, or an old tank or cistern as long as the water is clear it can be used. If there is scum or algae or living organisms in the water then it may adversely affect the mortar. Sea water or brackish water will accelerate slightly hydration of the mortar (some reduction in setting time is possible), but the major problem is likely to be that of efflorescence (see 2.1.3d above).

 

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2.2 Bricks and Ceramics

Bricks have been used for walling in this country since the Middle Ages. In the days of hand manufacture, brickmakers travelled from site to site, the first job for any building project being to make the bricks on site. Variations in size, colour, strength and durability were very large. Later brick making no longer went from job to job but was carried out in a fixed spot where suitable clay was available and the bricks were transported from the brickfield to the site of the proposed building. Right through to Victorian times transport problems and costs ensured that local bricks, in general, did not travel far. Since then modern transport systems have simplified distribution and reduced it s costs. The result, as we see it today, has been elimination of the local brickworks and the domination of brickmaking by relatively few manufacturers. In dealing with older brick buildings one of the major headaches is in matching old bricks in repairs.

In repointing and repairing the elevations of a building, the bricks will probably be of the variety known as “facing bricks”, having been selected for colour and/or shape and surface texture. For clay bricks the colours range from pale yellow through reds, browns and purples to very dark blues, this variation depending upon the type of clay (particularly oxides that may be present) and the method of firing in the kiln. In the south of England, in particular, a number of multi-colour stock bricks have been and are made in which there is a considerable variation in the colouring of bricks of the same type.

The surface texture of the brick also plays an important part in determining the appearance of a wall. Hand made bricks are usually sand faced and the face may be irregular. Many types of machine made bricks have sand facing applied to them to imitate hand made bricks. When a brick is fired at a high temperature the clay starts to melt and form a glass. This glassy surface again gives a characteristic texture to the brickwork. In Victorian times in the Oxford area there was a vogue for Flemish bond to be built with dark blue partly vitrified headers and red stretchers. It is difficult to match the headers nowadays though luckily they are less likely to deteriorate because of the reduced absorption through the partial nitrification of the brick.

Glazed bricks proper are produced by applying a potter’s glaze to the green brick and firing this in a cikln.

Engineering bricks are characterised by very high strength and low water absorption. The latter in particular, makes these bricks particularly durable when used in situations of severe exposure (for example, retaining walls, bridges and, in buildings, cills and copings). Engineering bricks tend to be uniform in colour (blue or red) and of regular shape and precise dimension. This enables them to be laid with a thinner joint (6mm) than that often necessary for one of the more irregular facing bricks (10mm).

Engineering bricks are classified ‘A’ or ‘B’ according to their average compressive strength and minimum absorption (class ‘B’ less and more than class ‘A’).

Semi-engineering bricks (in the south typically, Warnhams) do not meet the full requirements of the British Standard Specification for engineering bricks but are superior to commons.

‘Commons’ as the name suggests, are bricks which are used for ordinary work, mainly internal, where particular properties of strength, weather resistance or appearance are not acquired. The most widely used common brick now is the ‘Fletton’, a superior version of this is the Fletton facing brick.

Most bricks are produced nowadays to a nominal work size of 215 x 102 x 65mm (length x width x height) on which there are manufacturing tolerances. In the past there has been much wider variation on the size of bricks particularly in the height. Prior to metrication (late 60’s) bricks in the South of England were made 2 5/8” deep (67mm) and in the north 2 ¾” deep (70mm). In the Midlands a mixture of both sizes is likely. The present 65mm brick matches the 2 5/8” brick reasonably well in repairs but with the 2 ¾” brick, the bed joints will have to be deeper than those in the original brickwork; this should be discussed with the client and/or his architect in order to avoid possible complaints later on (unjustified) about sloppy workmanship.

There has also been a demand for bricks that are narrower on the bed than normal – 2” facing bricks have been and still are available.

The other variation that occurs in some (modern) brickwork is the use of “modular bricks”. These are bricks whose dimensions are multiples of 100mm typically 200 or 300mm long x 100 wide x 100 high. Demand has always been limited and difficulties again might be experienced in matching these bricks in repairs.

Apart from rectangular bricks there are ‘standard special shapes’ made for some types of brick (see fig2.3). Unfortunately the range made now does not match that which has been made in the past and difficulties can occur in trying to find a reasonable match to an existing special. Some brickmakers will manufacture special shapes to order but this of course tends to be very expensive and manufacturing times are likely to be long.

With some bricks it is possible to cut a rectangular brick with a carborundum disc to produce the required shape. The bricks suitable for cutting should be frogless and unperforated and of uniform colour through the body of the brick where cutting is going to expose the inside of the brick. Squints are probably the easiest shape to produce by cutting : cants and plinths are possible but their stops will be difficult. Bullnoses and their variants would be impossible.

2.2.2. Calcium Silicate Bricks

These are also known as ‘sand lime’ bricks and are made from a mixture of siliceous sand and lime, mixed together, pressed in a mould to form the brick shape and then subjected to the action of steam under pressure in an autoclave (a sort of giant pressure cooker). The steam pressure treatment causes chemical changes in the mixture, producing a synthetic sand stone – grains of sand bound with calcium silicate. Where crushed flint is used, the bricks are known as “flint-lime” bricks. Pigments are often included in the mix to give the required colour.

The bricks are supplied in ‘classes’ (class 1 to class 7) based upon average compressive strength. Class 1, the weakest, should only be used internally, external cavity walls, etc should be in class 2 bricks. For the more severely exposed situations (parapets, boundary walls, cills and copings) classes 3 or 4 should be used. In any patching of sand lime brickwork it is important to ensure that the bricks used for any of the locations just mentioned are at least of the quality noted.

The colour of sand lime bricks is more uniform than that of clay bricks and apart from white a number of pastel coloured bricks are made. White sand lime bricks have been used in recent years to provide high daylight reflection in light wells and internal courtyards and have provided a more reasonably priced alternative tot he more traditional white glazed bricks. The shape is very regular and tolerance on size much less than that for a clay brick.

One important difference between sand-lime bricks and clay bricks is the difference in the “moisture movement” or each type. For any material, this term is used to describe the change in length that occurs when water is absorbed (length increases as compared with the length of the dry brick) when the absorbed water dries out, the brick length reduces again, a process that will continue. In clay bricks moisture movement is not very high (except when the bricks are first drawn from the kiln, when they take up moisture, usually from the air, and expand). Calcium silicate bricks move more and cracking can occur in brickwork built with them, particularly where there is limited freedom to expand. Generally the limitation will arise either because the mortar used is too strong or because the brickwork is tied to, say, a structural concrete frame or to perhaps clay brickwork (eg. One and a half brick wall faced in sand limes and backed with flettons).

Current practice is to include movement and separation joints, but many older buildings have been built without a full appreciation of this point.

One other peculiarity of sand lime bricks is the gradual breakdown of the calcium silicate when chloride salts are present and the brickwork becomes wet. This is most likely to occur in buildings fairly near the sea where salt spray can be blown onto the building.

2.2.4 Terra Cotta and Faience

Both terms come from Italy as no doubt originally did the techniques for which they are used. ‘Terra Cotta’ is merely Italian for ‘cooked earth’ and is used to describe a ceramic facing, often elaborately moulded or shaped, made from a fine clay, fired but not glazed. The panels or tiles were often fixed to facades using wire cramps (as for marbles and polished granite) with joints pointed up. Much terra cotta is a characteristic browny red but buff and yellow examples are also found. Chimney pots and air bricks are the commonest examples of terra cotta..

Faience (derived from the town of Faenza in northern Italy) is used to describe what is essentially a glazed terra cotta (ie large glazed tile facing).

2.3 Natural and Artificial Stones

Natural stone is used to describe a stone that has formed naturally over millions of years in the ground and has been quarried to produce blocks of material for building. You will recall from the introduction that for walling purposes we distinguish between ashlar work and rubble stone work, purely depending upon the amount of work done to shape the quarried stone and in no way reflecting on the quality or type of stone.

Artificial stone (also reconstructed stone and cast stone) is not really stone at all, but a fine concrete made with aggregates (often of crushed stone) and cement with pigments so as to imitate the colour and appearance of natural stone. Thus artificial stone can be cast in blocks and then built into a wall exactly as though it were a natural ashlar : it can be cast in mould to give the appearance of rubble stone work (eg Bradstone) : it can be cast in slabs which are then hydraulically split to give a less obviously man made texture and appearance.

Natural stones are generally classified as sedimentary, metamorphic or igneous for building purposes.

The sedimentary group is probably the most important, certainly in the south and covers limestones and sandstones. These stones are relatively soft (as compared with say granite), easily worked and used extensively in ashlar work and in rubble stone work (across the whole of the cotswolds for instance). Stones are named from the general quarrying area and sometimes also the individual quarry or even level from which the stone is dug. For example all Portland stone comes from the Isle of Portland in Dorset but may be further described as “whit bed” or “shelly bed” etc. Few stones in this group polish and they are generally left with a slightly grainy texture.

With much stonework, the quarry from which stone originally came is often no longer being worked and an acceptable substitute must be found. In the case of minor ‘patching’ repairs to rubble stone work, sound second hand stone (already weathered and proven) is probably as good a replacement as any.

Chalk is also a sedimentary rock though generally too soft and likely to soften in the rain for use as external walling. At certain points on the ‘chalk belt’ harder chalks have been worked in the past (‘clunch’ in the Cambridge area).

Flints (non crystalline siliceous material) are found in bands in chalk and are widely used in and near the chalk belt for walling. In particular flints can be knapped to expose the glassy surface within the rind and are then roughly squared on the face before being built into a wall. Brick lacing courses and quoins are often used to stabilise the flints.

Metamorphic stones will probably have started life as a sedimentary type and then been changed by volcanic or other violent movement which subject the stone to great heat and/or pressure changing the stone physically and chemically. In this group slate and marble are widely used in building work (the former of course not solely restricted to thin sheets in roof covering). Marble because of its cost is generally used as a thin facing veneer, probably ¾” to 1” thick, fixed to the background wall with wire cramps; joints between the pieces of marble are pointed often in a putty or mastic.

The igneous rocks are those produced by volcanic action; the best known of these is granite. In granite areas (Cornwall, parts of Scotland, for example), granite will be widely used in every type of building. Elsewhere in the country it is occasionally used for public buildings particularly where resistance to atmospheric pollution was felt to be important.

 

The above information should not be taken as recommendations for any individual contract/project and are guidelines only. Consult your local licencee for advice on the projects in your area.

 

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