High alumina cement

High Alumina Cement (HAC), sometimes known as calcium aluminate cement (CAC) or aluminous cement, is composed of calcium aluminates, unlike Portland cement which is composed of calcium silicates. It is manufactured from limestone or chalk and bauxite (Special clay having extremely high alumina content).

HAC was first developed by Lafarge, the cement producer, and became available in the UK in 1925. It was used in particular for marine applications where it was considered to be resistant to chemical attack. It became popular in the 1950s, 60s and 70s, as it developed strength rapidly and so was relatively fast to manufacture. It was widely used in structural concrete such as pre-cast beams.

However, HAC was prone to a crystalline re-arrangement (or ‘conversion’), which could result in reduced strength and also vulnerability to chemical attack when exposed to water for long periods (perhaps as a result of poor detailing or poor manufacturing). This resulted in five high-profile structural failures of roof beams (where the presence of water is more likely) during the 1970s.

In 1975, MP for Sutton and Cheam, Neil Macfarlane, said “Those words—or the abbreviation "HAC"—are rapidly and relentlessly becoming a combination of misery, apprehension, worry and fear for thousands of people in the United Kingdom.”

HAC is no longer used in structural concrete in the UK, although it is still prevalent in buildings constructed in the 50s and 60s, and continues to be used for non-structural uses under the name Calcium Aluminate Cement (CAC).

In 1975, The Department of the Environment (DOE) Building Regulations Advisory Committee (BRAC) published guidance for design-check procedures for High Alumina Cement. Commonly known as the BRAC rules, this guidance remains the best advice available and continues to be used to assess the structural performance of buildings containing pre-cast HAC concrete beams.

It should be noted that many buildings that contain HAC components are entirely problem free, and the problems that have occurred have been traced back to manufacturing faults. However, If the presence of HAC is suspected, testing should be carried out, and if it is confirmed, HAC components should be assessed for strength and long-term durability. This is likely to require expert advice.

[edit] Related articles on Designing Buildings Wiki

• Aggregate.
• Asbestos.
• Brick.
• Cement.
• Coal ash.
• Concrete v steel.
• Defects in brickwork.
• Deleterious materials.
• Demolition.
• Ferro-cement.
• Hempcrete.
• How scientists solved the riddle of cement’s structure.
• Lime mortar.
• Portland cement.
• Pre-construction information.
• Research on novel cements to reduce CO2 emissions
• Screed.
• Sustainable materials.
• Types of cement.

[edit] External references

• Building Regulations Advisory Committee (BRAC) rules.

Comments