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Concrete blocks and green alternatives

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Sandy Patience looks at products less harmful to the environment than traditional concrete blocks

Thanks to their versatility, concrete blocks have been a feature of UK building sites for nearly a century. Though still almost ubiquitous, a combination of demanding thermal standards, the association of cement production with global warming and the damage caused by the extraction of aggregates has prompted new thinking.

One obvious solution is to use less damaging substitutes such as ‘recycled aggregates’ - materials previously used in construction like recycled concrete; as well as ‘secondary aggregates’ that are by-products of industrial or natural processes. Examples include pulverised fuel ash from coal, used as a partial cement substitute and ‘stent’, a waste rock by-product of china clay workings.

There are alternatives to concrete, particularly in conjunction with less traditional forms of construction. In Europe, where cavity walls never really took hold, honeycomb fired clay blocks have been used for years, usually as a structural background to render on the one side and plaster on the other. ‘Ziegel’ blocks are light, easy-to-use, boast lower embodied energy, and have good thermal characteristics.

Closer to home, technology has been developed to exploit the ‘carbon sequestration’ (absorption of carbon dioxide during growth) properties of crops. Hemp in particular shows promise. Tested in its spray-on format on several high-profile buildings, hemp combined with a lime-cement binder is now available in a load bearing block.

Though not load bearing themselves, wood fibre-cement blocks are an alternative to polystyrene/foam permanent shuttering. The blocks are assembled, complete with integral insulation in preparation for the concrete. The system is increasingly popular with builders attracted by speed of assembly, with 26 storeys achievable.




Produced in the Cotswolds,Bekstone is reconstituted stone walling manufactured using reclaimed aggregates, recycled water and renewable energy. The manufacturer has been awarded the Carbon Trust Standard.

Manufacturer Bekstone
Supplier Bekstone
Component materials Cement, reclaimed limestone aggregate, recycled water, pigments
Density 2,050kg/m3
Compressive strength 20N/mm²
Thermal conductivity 1.25 W/mK (protected), 1.35 W/mK (unprotected)
Thickness of block 102mm
Country of manufacture UK




Aerated concrete blocks contain up to 80 per cent pulverised fuel ash, a by-product of the coalburning power industry. Volume for volume the blocks can achieve greatly reduced amounts of embodied energy compared with conventional concrete blocks.

Manufacturer Hanson Thermalite
Supplier Builders merchants
Component materials Lime, cement, pulverised fuel ash
Density 470kg/m3
Loadbearing strength 2.9N/mm²
Thermal conductivity 0.11W/mK
Thicknesses of block 100, 115, 125, 130, 140, 150, 190, 200, 215, 265, 300mm
Country of manufacture UK




Featuring high thermal performance and relatively low embodied energy, clay honeycomb blocks can be used in many of the same applications as concrete blocks. Blocks are assembled through dry vertical interlocking and thin-mortar horizontal jointing.

Manufacturer Ibstock/ZWK
Supplier Natural Building Technologies
Component materials Clay, recycled paper, recycled PU bead
Density 650kg/m3
Loadbearing strength 7N/mm²
Thermal conductivity 0.09W/mK
Thicknesses of block 300, 365, 425mm
Countries of manufacture UK and Germany




Looks and performance of Enviroblock are as for a conventional concrete block, but conventional virgin materials are substituted with recycled concrete aggregate (RCA)
or stent, a secondary aggregate.

Manufacturer Masterblock
Supplier Masterblock
Component materials Cement and minimum 80% RCA or secondary aggregate
Density 1,450kg/m3 (lightweight) and 1,950kg/m3 (dense)
Loadbearing strength 7.3N/mm² and 10.4N/mm²
Thermal conductivity 1.27W/mK@3% m/c to 1.37W/mK@5% m/c (dense)
Thicknesses 100, 140mm
Country of manufacture UK




Hemp blocks are manufactured using hemp blended with a lime and cement binder. Because of the bio-composite nature of the material, hemp blocks’ unique environmental benefit is the carbon sequestration that can be offset against other areas of a building’s construction.

Manufacturer Lime Technology
Supplier Lime Technology
Component materials Hemp with lime and cement binder
Density 1,100-1,200kg/m3
Loadbearing strength 3N/mm²
Thermal conductivity 0.36W/mK
Thicknesses of block 100mm
Country of manufacture UK




The ICF blocks consist of moulded cement-bound wood chip. The system is stacked and staggered in dry-bond to create the formwork. Poured concrete around steel reinforcement inside the block cavities stabilises the wall structure. An integral slab of mineral wool provides additional insulation.

Manufacturer Durisol UK
Supplier Durisol UK
Component materials Recycled and treated timber, cement and PFA
Loadbearing strength Determined by the concrete core
Thermal conductivity 0.19W/mK (mineral wool) and 0.15W/mK (phenolic foam)
Thicknesses of block 170, 250, 300, 365mm
Country of manufacture UK

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