Units: brick or block-sized pieces of stone, fired clay, concrete or calcium silicate bonded aggregate that are assembled to make masonry. Usually, but not invariably, these are in the form of rectangular prisms, but many special shapes are manufactured and stones are often used either as found or shaped to be partly squared, faced or split faced. Manufactured units are produced in the following standard forms: solid, frogged, cellular, perforated, hollow, key-faced, fair-faced and split faced, as illustrated in Fig. 1.
They are also available as ‘standard specials’ with a range of curves, non-right-angled corners, plinths, cappings, etc. These are described and specified in BS 4729 (2005).
Mortar: a material that is plastic and can flow when fresh but sets hard over a period of hours to days. Its purpose is to fill the gaps caused by variations in the size and shape of units such that the masonry is stable and resists the flow of air and water. Mortar is compounded from a binder (e.g. cement) and a filler/aggregate (usually sand).
Binder: a finely ground material which when mixed with water reacts chemically and then sets hard and binds aggregates into solid masses to form either units or mortars. Non-hydraulic binders such as resins are occasionally used.
Work size: the size of a masonry unit specified for its manufacture, to which its actual size should conform within specified permissible deviations. As a rough guide for the following sections, bricks are considered to be units with face dimensions of up to 337.5 mm long by 112.5 mm high with a maximum depth of 225 mm, while blocks are larger units, with face dimensions of up to 1500 mm by 500 mm.
UK units are usually smaller than these limits. A standard UK brick is 215 mm long by 65 mm high, with a depth of 102.5 mm. There is no standard block size but the commonest size is 440 mm long by 215 mm high, with a depth of 100 mm.
Co-ordinating size: the size of a co-ordinating space allocated to a masonry unit, including allowances for joints and tolerances. The coordination grid into which they fit is usually around 10 mm larger for each dimension, as illustrated by Fig. 2.
Fair-faced: masonry, within the variability of individual units, precisely flat on the visible face. This is normally only possible on one side of solid walls.
Other definitions can be found in BS 6100-6 (2008).
Materials Used for Manufacture of Units and Mortars
Rocks, Sand and Fillers
Rock (or stone): The main types of rock used for masonry in the UK are:
- sedimentary rocks, formed from compressed sediments on the bottom of ancient seas e.g. limestones and sandstones
- metamorphic rocks, formed by the action of pressure and high temperature on other rock deposits, e.g. marbles and slates
- igneous rocks, formed by melting of rock during volcanic activity, e.g. granites and basalts.
Both the strength and durability of rocks are very variable and will depend on the porosity and the distribution of the pores. Generally strength increases and porosity decreases from sedimentary through metamorphic to igneous. Most sedimentary rocks have a layer structure and will be significantly stronger normal to the bedding plane than in the other two directions (i.e. they are anisotropic). Igneous rocks and some fine-grained sedimentary and metamorphic rocks are fairly isotropic, i.e. they have similar properties in all directions. Such rocks are termed ‘freestones’ because they can be cut in any plane and are usually suitable for carving to elaborate shapes and polishing. Rock or stone is used in three main ways – as thin sheet cladding, as solid building units and in the form of crushed aggregate to make concretes and mortars.
Sand – nature and composition: Sand is used widely as a constituent of masonry in mortar, in concrete units and sandlime units, and in grouts and renders. It is a mixture of rock particles of different sizes from about 10 mm diameter down to 75 µm diameter. Most sand is a naturally occurring rock powder derived from recent naturally occurring alluvial deposits such as the beds of rivers and sea beaches or from older deposits formed by alluvial or glacial action. In some areas it may be derived from dunes or by crushing quarried rocks. The chemical and geological composition will reflect the area from which it is derived.
The commonest sands are those based on silica (SiO2), partly because of its wide distribution in rocks such as sandstones and the flint in limestones, and partly because silica is hard and chemically resistant. Other likely constituents are clay, derived from the decomposition of feldspars, calcium carbonate (CaCO3), in the form of chalk or limestone from shells in some marine sands, and micas in sands from weathered granites. Crushed rocks such as crushed basalts and granites will reflect their origins.
Sands should be mostly free of particles of clay (with a size of between 75 and 30 micrometres), which causes unsatisfactorily high shrinkage characteristics and chemical interactions with binders. Most of the constituents of sand are relatively chemically inert to environmental agents, but chalk or limestone particles will be dissolved slowly by mild acids and clays may react in time with acids or alkalis.
Most sand constituents are also fairly hard and are resistant, in themselves, to mechanical abrasion and erosion by windblown dust or waterborne particles.
Mortar and rendering sands: Mortar sand must not contain particles with a diameter greater than about half the thinnest joint thickness, e.g. around 5 mm for masonry with 10 mm joints.
It should also have a good range of particle sizes from the largest to the smallest (an even grading) since this leads to good packing of the particles to give a dense, strong mass resistant to erosion, permeation and chemical attack.
Many naturally occurring alluvial deposits fall naturally into the required grading and may be used as dug or just with a few coarser particles screened off. These are usually termed pit sands. Sands that are outside the normal range must be sieved to remove coarse fractions and washed to remove excess clay particles.
The shape of the particles is also important for mortar sands. Very flaky materials such as slates and micas are not very suitable as it is difficult to make them workable. Very porous absorbent materials are also unsatisfactory for dry-mixed mortars since they cause rapid falls in workability during use by absorbing the mixing water.
They may be suitable for mixes based on wet premixed lime-sand ‘coarse stuff’. Sand may be sieved into fractions and regraded, but this is rarely done for a mortar sand.
Figure 3 shows the grading curves for the sands allowed under the previous standard BS 1200 (1976). This gives two allowed grades, S for structural use and G, with slightly wider limits, for general purposes. The current standard BS EN 13139 (2002) gives similar grading limits and tolerances plus limits on sulphates, chlorides and materials that modify settingrate.
As it is fairly complex there is an associated guidance document PD6682-3 (2003). Rendering mixes require sands with broadly similar characteristics to those of mortars, but a good grading is even more important to avoid shrinkage cracking and spalling and to give good bond to the substrate.
Concreting sands: The requirement for sands for concrete units have been discussed in former articles.
Ground sand: Finely ground silica sand is used particularly in the manufacture of aircrete (AAC) materials and as an inert filler.
Fly ash (pulverised fuel ash; pfa): Fly ash, the main by-product of modern coal-fired electricity generating stations, is a chemically active filler often used in concrete units.
Chalk (calcium carbonate; CaCO3): In a finely ground state chalk is used as a filler and plasticity aid in masonry cement and some grouts.
Clay is a very widely distributed material that is produced by weathering and decomposition of acid alumino-silicate rocks such as the feldspars, granites and gneisses. Typical broad types are the kaolin group, of which kaolinite has a composition Al2O3.2SiO2.2H2O, the montmorillonite group, of which montmorillonite itself has the composition Al2O3.4SiO2.nH2O, and the clay micas, which typically have a composition K2O.MgO.4Al2O3. 7SiO2.2H2O.
They will frequently contain iron and other transition metals, which can substitute for the aluminium. The clays used for clay brick manufacture normally comprise only partly clay minerals, which impart plasticity when wetted, the balance being made up of other minerals.
Brick earths, shales, marls, etc. mostly contain finely divided silica, lime and other materials associated with the particular deposit, e.g. carbon in coal-measure shales. Most brick clays contain iron compounds, which give the red, yellow, and blue colours to fired bricks.
Table 1 gives the compositions of some typical clays in terms of their content of oxides and organic matter (coal, oil, etc.). The properties of clays result from their layer structure, which comprises SiO4 tetrahedra bonded via oxygen to aluminium atoms, which are also bonded to hydroxyl groups to balance the charge.
The layers form loosely bound flat sheet-like structures that are easily parted and can adsorb and bond lightly to varying amounts of water between the sheets. As more water is adsorbed the clay swells and the inter-sheet bonds become weaker, i.e. the clay becomes more plastic and allows various shaping techniques to be used.
Manufactured lightweight aggregates, including sintered pfa, expanded clay and foamed slag have been described a formar article. Other lightweight aggregates used particularly for unit manufacture are:
- Furnace clinker, a partially fused ash from the bottom of solid fuelled industrial furnaces.
- Furnace bottom ash. Most large modern furnaces, especially those used to raise steam in power stations, burn finely ground coal dust as a dust/ air mixture. A proportion of the fine ash particles suspended in the gas stream sinter together to form larger particles which fall to the base of the furnace.
- Perlite, volcanic ash that is deposited as a fine glassy dust that can be converted to a lightweight aggregate by hot sintering.
- Pumice, a light foamed rock formed when volcanic lava cools. It is normally imported from volcanic regions such as Italy.
The binder is the component that binds together mixtures of sands, aggregates, fillers, plasticisers, pigments, etc. used to make mortars, concrete units, sandlime units and grouts. Widely used binders are based on one of:
- hydraulic cements, which react chemically with water at normal factory/site temperatures
- lime–silica mixtures, which react only in the presence of high-pressure steam
- lime–pozzolan mixtures, which set slowly at ambient temperatures, or pure lime which sets slowly in air by carbonation.
Portland cement: Currently, the most popular binder for general purposes is a CEM I Portland cement or a sulphateresisting Portland cement.
Masonry cement: This is a factory prepared mixture of Portland cement with a fine inert filler/plasticiser (around 20%) and an air-entraining agent to give additional plasticity. It is intended solely for mixing with sand and water to make bedding mortars. When the fine powder is lime the appropriate standard is BS EN 197-1 Notation CEM1 (2000) while for any other filler, e.g. ground chalk or pfa etc., the relevant standard is BS EN 413-1, Class MC (2004).
Lime and hydraulic lime: Lime (CaO) is widely used as an ingredient in mortars, plasters and masonry units. The pure oxide form, called quicklime, was used widely in the past for mortars for stonework. It is produced by heating pure limestone to a high temperature and then ‘slaking’ with water to produce hydrated lime, Ca(OH)2.
Since it does not have any setting action in the short term it may be kept wet for days or weeks provided it is covered and prevented from drying out. The wet mix with sand is termed ‘coarse stuff’. Contemporary lime mortar may be made from pre-hydrated lime but is otherwise similar.
The initial setting action of this mortar depends only on dewatering by contact with the units so it is not suitable for the construction of slender structures that require rapid development of flexural strength. Over periods of months or years the lime in this mortar carbonates and hardens to form calcium carbonate as in equation (1), but it is never as hard or durable as properly specified hydraulic cement mortars:
Ca(OH)2 + CO2 → CaCO3 + H2O ……(1)
Hydraulic lime was widely used in the past and is frequently specified for repairing historic buildings to match the original mortar. It is basically a quicklime – calcium oxide – produced by heating impure limestone to a high temperature. The impurities, usually siliceous or clay, lead to the formation of a proportion of hydraulically active compounds such as calcium silicates or aluminates.
The binder is made by partial hydrolysis (slaking) of the lime with water. The high temperatures and steam caused by the reaction help to break down the mass to a powder. The mortar is made as normal by gauging (mixing in prescribed proportions) the finely ground binder with sand and water. The classic reference works are Vicat (1837) and Cowper (1927).
More recent information is given by Ashurst (1983) and the BRE Good Building Guide 66 (2005). Most of the hydraulically active cements and limes may be blended with pure hydrated lime in various proportions to make hybrid binders, which give mortars with a lower strength and rigidity but still maintain the plasticity of the 1:3 binder:sand ratio. This leads to mortars that are more tolerant of movement and more economical.
Sandlime: The binder used for sandlime bricks and aircrete (autoclaved aerated concrete – AAC) blocks is lime (calcium hydroxide, Ca(OH)2), which reacts with silica during autoclaving to produce calcium silicate hydrates. The reaction, in a simplified form, is:
Ca(OH)2 + SiO2 → CaSiO3 + H2O …….(2)
The lime is usually added directly as hydrated calcined limestone or may be derived in part from Portland cement incorporated in small quantities to give early age strength to the unit.
Other Constituents and Additives
Many organic compounds improve the plasticity, or workability, of mortars, rendering mortars, infilling grouts and concrete used for the manufacture of units. All the classic mortar plasticisers operate by causing air to be entrained as small bubbles. These bubbles fill the spaces between the sand grains and induce plasticity.
Typical materials are based on Vinsol resin, a by-product of cellulose pulp manufacture, or other naturally available or synthetic detergents. They are surfactants and alter surface tension and other properties. Mortar plasticisers should conform to BS EN 934-3.
Superplasticisers, used only for concrete and grout mixes, plasticise by a different mechanism that does not cause air entrainment.
A number of synthetic copolymer plastics may be produced in the form of a ‘latex’, a finely divided dispersion of the plastic in water usually stabilised by a surfactant. Generally the solids content is around 50% of the dispersion. At a temperature known as the film-forming temperature, they dehydrate to form a continuous polymer solid. When combined with hydraulic cement mixes these materials have a number of beneficial effects: they increase adhesion of mortar to all substrates; increase the tensile strength and durability; and reduce the stiffness and permeability.
Because of these effects they are widely used in flooring screeds and renders but are also used to formulate high-bond mortars and waterproof mortars. The better polymers are based on copolymerised mixtures of butadiene, styrene and acrylics.
Polyvinylidene dichloride (PVDC) has also been marketed for this application but it can give off chlorine, which can attack buried metals. Polyvinyl acetate (PVA) is only suitable for use in dry conditions as it is unstable in moist conditions.
Polyvinyl propionate has been found to give less satisfactory flow properties than the acrylic copolymers. These materials should never be used with sands containing more than 2% of clay or silt particles. Dosage is usually in the range of 5–20% of the cement weight. Table 2 gives some properties of common types (de Vekey, 1975; de Vekey and Majumdar, 1977).
Through-coloured units and mortars of particular colours may be manufactured either by selecting suitably coloured ingredients or by adding pigments. Units may also be coloured by applying surface layers but this is more common for fired clay than for concrete or calcium silicate units. Pigments are in the form of inert coloured powders of a similar fineness to the binder, so they tend to dilute the mix and reduce strength.
Most pigments should be limited to a maximum of 10% by weight of the binder in mortars and carbon black to 3%. Some typical pigments, from information in ASTM task group C09.03.08.05 (1980), are synthetic red iron oxide, Fe2O3; yellow iron oxide; black iron oxide, FeO (or Fe3O4) and brown iron oxide, Fe2O3.xH2O; natural brown iron oxide, Fe2O3.xH2O; chromium oxide green, Cr2O3; carbon black (concrete grade); cobalt blue; ultramarine blue; copper phthalocyanine; and dalamar (hansa) yellow.
Only pigments resistant to alkali attack and wettable under test mix conditions are included. All but the last two are not faded by light. Pigments for mortar should conform to BS EN 12878 (2005).
Retarders are used to delay the initial set of hydraulic cement mortars. They are generally polyhydroxycarbon compounds. Typical examples are sugar, lignosulphonates and hydroxycarboxylic acids.
Accelerators based on calcium chloride (CaCl2) have, in the past, been used in small amounts in concrete block manufacture and for mortars. All current codes of practice and standards do not permit the addition of chlorides because they are corrosion accelerators for embedded steel fixings.
Alternatives such as calcium formate (Ca(CHO2)2) may be satisfactory. Accelerators are not effective when building with mortar in frosty weather and are no substitute for proper protection of the work.