Bricks: Fired / unfired clay, reclaimed & calcium silicate

History

The use of bricks in the Modern period stems from a revival of brick making in the late 13th – early 14th centuries in response to a combination of a shortage of local stone and the influence of Europe where brick was used extensively. By the middle of the 16th century, brick making had become a distinct industry competing with stone as a structural material.

As the industry grew, bricks became cheaper – leading to its travelling downwards through the social spectrum. With the introduction of the railways in the 19th century, significant consignments of brick could for the first time be transported from the brickfields, such as those in Bedfordshire, to the conurbations of London, the Midlands and the industrial North where they were used to build terraces for housing a rapidly expanding working class.

In the 20th century, mechanisation largely replaced making bricks by hand and this with other innovations helped fuel the building booms of the inter-war years and again in the 1960s and 70s following the rise in post war population.



Brick is a traditional building material. Heed is still paid to its almost unique quality of conveying a ‘genius locii’ upon any building built from local clay. Brick construction itself continues to be regarded and taught as one of the fundamental construction types of contemporary building, and the industry itself continues to flourish.

If the last few decades have brought opportunities through technological development, so too have they brought a new scrutiny in which fired clay bricks are examined against their environmental impact. Within the current debate concerning sustainable materials, brick is lined-up against a range of traditional and new materials. The brick industry will be hoping to match its strong credentials of durability with tradition against alternative forms of construction offering, particularly, reduced embodied energy.

Types of brick

Reclaimed bricks

With an estimated 2.5bn bricks¹ resulting from demolition each year, it is not surprising that there is a healthy market in reclaimed bricks. More of a surprise might be in the knowledge that only 5% of the 2.5bn are actually reclaimed – 50% are crushed and used in hardcore and fill.

The Demolition Protocol states that bricks have a recovery potential of 10% - rising to 100% in some buildings.

But what restricts the current recovery of usable bricks is complicated, though two factors are salient: the uncompetitive pricing of reclaimed bricks compared with new units but also the (diminishing) quantity of bricks available from pre-1940s buildings which include lime rather than the harder modern cement mortars which are more difficult to remove from the brick.

However, there continues to be high levels of availability, with even large builders’ merchants now listed alongside the traditional specialist suppliers.

The quality of new bricks is governed by BS EN 771-1 applied to the manufacturing process, but this, as might be expected, is a standard unavailable to reclaimed bricks. Samples from a batch of reclaimed bricks can be tested, but the results cannot be extended to establish the overall quality of the consignment. However, suppliers can go far in providing assurances and this combined with modest assumptions about the likely performance of the bricks can result in successful use.

Clay facing and common bricks

Clay facing and common bricks represent by far the greater output from manufacturers. Clay bricks can come in a variety of forms, but one notable difference is that between perforated and solid where the former is both lighter and slightly more resource efficient.

Manufacturing

The manufacturing process can be loosely divided into 4 stages.

Extraction (or Clay ‘Winning’)

Clay is removed from quarries and transported to the factory (though traditionally factories were usually adjacent to the quarries). Once it has reached the factory the clay is ground down using rollers into fine powder before being mixed with water.

Forming

Bricks can be formed by one of two basic processes:

Extrusion – Clay is forced through an extruder and out through a die into a continuous brick-shaped column. The column is cut into single bricks ready for the dryers. Extruded bricks are generally perforated but cannot be frogged.

Soft mud moulding –Clay is ‘thrown’ into a mould which has been pre-lined with a releasing agent such as sand, oil or water. The excess clay is removed from the top and the brick released from the mould. Prior to mechanisation, this was all undertaken by hand – but the labour-intensive nature of the process and its consequential expense means that in modern time ‘hand made’ bricks tend to be reserved for niche applications and ‘specials’.

Drying

To prevent moisture from causing bricks to explode in the kilns, they are first dried before being fired. Drying takes place in conditions of between 80-120ºC, lasts for between 18 – 40 hours and can cause shrinkage of up to 10% on each dimension.

Firing

The dried clay is fired to fuse clay particles and impurities (‘vitrification’) to produce the hard brick in its completed form and livery. Bricks can be fired in either small batches in ‘Intermittant’ kilns or the more energy – efficient and larger capacity ‘Continuous kilns’. On completion of firing the bricks are selected and packaged – a process that can be either manual or automated.

Thermal conductivity²

- Density 1200 kgm³: 0.36 W/mK (Protected); 0.36 W/mK (Unprotected);
- Density 1600 kgm³: 0.52 W/mK (Protected); 0.71 W/mK (Unprotected);
- Density 2000 kgm³: 0.70 W/mK (Protected); 0.96 W/mK (Unprotected);

Embodied energy

- General bricks: 3 (+/-1) MJ / kg ⁽³⁾ or 2.67 MJ / kg (excluding transport to site) ⁽⁷⁾
- Facing bricks: 8.2 MJ / kg (‘very small sample size’)⁽³⁾

Calcium Silicate bricks

Despite the method of using steam under pressure to cure sand and lime being patented in England in 1886, much of the subsequent development and eventual use of calcium silicate bricks has prospered more in Europe than the UK. Notable uses of the brick in London include Battersea Power Station and the RIBA building in Portland Place.

Calcium silicate (‘sandlime’ or ‘flintlime’) bricks are made by mixing quicklime or hydrated lime with silica sand together with enough water to allow the mixture to be moulded. The mixture is left until the lime is completely hydrated when it is pressed into moulds and cured in a high-pressure autoclave for two to three hours. In this process the lime reacts with silica to form hydrated calcium silicates, producing a durable strong brick. The finished bricks are very accurate and uniform, although the sharp arrises need careful handling to avoid damage to brick.

Through use of less energy and without the air pollutants associated with firing clay, calcium silicate bricks are considered to render significantly less impact on the environment than clay bricks.

Thermal conductivity²:

- Density 1700 kgm³: 1.04 W/mK (Protected); 1.12 W/mK (Unprotected);

- Density 2000 kgm³: 1.16 W/mK (Protected); 1.58 W/mK (Unprotected);

- Density 2200 kgm³: 1.51 W/mK (Protected); 2.06 W/mK (Unprotected)

Embodied energy³:

8.2 MJ / kg

Unfired Clay bricks (generally non-load bearing)

Unfired clay is one of civilisation’s oldest form of building material with origins located as far back as 14000 BC around the Lower Nile.

Following in the wake of the widespread use of unfired clay in, particularly, Germany, UK architects are increasingly attracted to the use of unfired clay in construction because of its perceived benefits to indoor air quality as well as its very low environmental impact.

Commercially available unfired clay bricks are commonly made of an extruded mixture of clay, sand and water with sawdust added as a binder, which is then air-dried.

Thermal conductivity⁵

approx. 0.95 W/mK (compressed) – 0.21 W/mK (uncompressed)

Embodied energy⁴

0.44 MJ / kg

The environmental impact of bricks

Fired clay bricks are responsible for the greater of environmental impacts amongst bricks. The firing of clay consumes large amounts of energy produced largely from fossil fuels – causing release of CO₂. The primary source of air pollution is the firing kiln. Emissions are from the combustion of fuel and gaseous emissions driven off as the clay is fired, including sulfur dioxide, hydrogen fluoride and hydrogen chloride. Factors that may affect emissions include raw material composition and moisture content, kiln
fuel type, kiln operating parameters, and plant design.

The other major impact is the degradation of the landscape resulting from the extraction of raw materials. Clay pits are a familiar sight in the UK, in some cases appearing to dominate the landscape and often replete with retired chimneys and decaying infrastructure.

Mitigation of environmental impact

The brick industry in the UK has worked hard over the last decade to reduce its environmental impact. For example, figures supplied by the BDA in 2009 show that energy consumed per tonne of output fell from 5,100,130 MWh in 2001 to 4,193,104 Mwh in 2007 (though there was a modest rise in the following year attributed to an economic downcycle). During the same period plant investment went up from £119M to £167M in 2008 as new more efficient plant was brought online and economies were made in the use of potable water, landfill gas from exhausted pits, reduction of waste to landfill and an increased efficiency of road transport.

Effort too goes into rehabilitating many landscapes previously blighted by brick extraction. One of the most successful techniques is to created water-based nature reserves.

Movies

* A useful account of popular brick textures in video provided by a leading brick supplier

Standards

Both the old British Standard Specification for clay bricks, BS 3921 and the Standard Specification for Clay and Calcium Silicate Modular Bricks have been replaced by the European CEN Standard Specification for clay masonry units, BS EN 771-1.

Unique national standards are support by the National Annex which is published as part of BS EN 771-1. The UK Annex provides guidance to specifiers regarding features such as dimensions, density, compressive strength and water absorption.

The CE mark denoting the EU's conformity with supporting declarations will be feature backed by some manufacturers, but it is by no means compulsory.

Further information

• Brick Development Association (BDA) - (www.brick.org.uk)

References:

1: BioRegional
2: CIBSE Guide A 1999
3: Inventory of Carbon & Energy (ICE) – Version 1.6a – Hammond & Jones, Univ Bath
4: arc Architects
5: Claytec
6: Sustainability Strategy For The Brick Industry – Brick Development Association, 2009
7: Brick Development Association

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