Cement and Lime
Our work at Tunstead
The Tunstead site is home to one of the largest limestone quarries in Europe, producing over six million tonnes of stone a year.
The process is highly efficient as the extraction of limestone, production of lime and cement are all intrinsically linked and over 99% of quarried material is converted into saleable products and the remainder is used on site for restoration purposes.
The Tunstead site is split into the Tunstead Quarry, the original quarry which was first opened in 1929, and Old Moor Quarry which started life in 1985.
At present, computer-designed blasting techniques are used to dislodge approximately 30,000 tonnes of stone per blast. The stone is then transported to the primary crusher by tipper trucks that are able to carry up to 100 tonnes per trip.
The primary crusher processes up to 3,000 tonnes per hour. Conveyor belts then carry the partly crushed rock to a secondary crushing unit where a wide range of sizes are produced, from 125mm down to dust.
The stone can be washed at this point to remove any clay deposits (this ‘washing’ is then used in cement manufacture, ensuring no waste) or stored in silos and stockpiles to be despatched by rail, road or to be used in production processes on site.
To read more about the geology of Tunstead, visit the environment section.
The washed, high purity stone is then transferred by conveyors to the lime kilns. The limestone is burnt at very high temperatures (1,100- 1,300°C) to induce a chemical reaction that produces quicklime (calcium oxide).
Tunstead has a number of shaft kilns which can process limestone from 90mm up to 125mm in size. A medium reactivity quicklime is produced which is commonly used in iron and steel manufacturing and for the production of aerated concrete blocks. This quicklime can also be used to make hydrated lime which is used in a range of applications including:
- Chemical manufacturing
- Drinking water treatment
- Effluent neutralisation
The site originally utilised two rotary kilns, but in 2009 £14 million was spent on a Maerz kiln to replace them. The new kiln burns less fossil fuels than its predecessors and has led to a reduction in sulphur emissions from the site. The Maerz kiln has two inter-connected, vertical shafts which are fired in sequence to achieve excellent energy efficiency. The final product is a high reactivity, high purity quicklime which is used in industrial effluent treatment, steelmaking and soil stabilisation.
Facilities to produce ground-lime and also milk of lime are installed at Tunstead. Ground lime products are a fine powder sold under the name Calbux and milk of lime is a liquid called Kalic. More information on Lime products is here.
Cement is the most versatile building material in the world and is the second most used resource in the world, after water. Cement is used to create the built environment around us, from homes, offices and hospitals, to roads, airports, bridges and power stations.
Cement is essentially a binder and it can be mixed with aggregates, water and other ingredients so that it makes a concrete which sets and hardens as it dries to bind the other materials together.
The production process is now fuelled by coal, petroleum coke and over 50% waste derived fuels. To find out more about these sustainable fuels, go to the environment section.
Inside the kiln a complex chemical reaction takes place to form granules of ‘clinker’ which are then combined with gypsum and milled into the finished cement product.
Tunstead has a long-established reputation for high-quality products and we place special emphasis on maintaining quality assurance.
Our policy is to use rail transport whenever practical to reduce the environmental impact of distributing lime, roadstone and cement to our bulk customers. Approximately 40% of output travels by rail.
Regulation and Governance
Our operation is strictly regulated by the Environment Agency (EA) through our environmental permit. We are required to meet UK legislation on air and water emission limits, and European legislation (the Waste Incineration Directive or WID) for our use of waste-derived fuels.
We have made significant improvements to our environmental performance by reducing levels of these emissions.
We have high-tech systems in place to monitor all aspects of our operation in great detail and in line with the requirements of our operating permit. With some of our emissions we have continuous (24-hour) monitors established and with others we are required to gather data ‘extractively’ on a regular basis as specified by EA.
At Tarmac we combine the knowledge and expertise of two of the construction industry’s most iconic brands: Tarmac, the pioneers of the modern road and Blue Circle the company that patented Portland Cement.
With over 150 years’ experience, we have many innovative achievements to our name: the invention of asphalt, a product that remains synonymous with our brand; the construction of the first motorway; the introduction of next-generation concretes and cements.
These breakthroughs all helped to shape the world we live in. Now we are working to help reshape it. Across our business, the wider supply chain and our research partners, we are looking at new ways to enhance productively, reduce carbon and value engineer efficiencies – whilst driving more sustainable construction.