Devanshu Mudgal at Haddonstone explains why the durability of cast stone enables it to play an environmentally-responsible role in a wide range of construction projects
For anyone who has stood and marvelled at the beautifully preserved floors within the 4th century Roman villa at Chedworth in the Cotswolds, it is worth remembering that these extensive areas of mosaic, set in mortar, are among the later examples of the artform to be found across Europe. Some date back over 2,000 years, having survived everything from being buried in volcanic ash to submersion in aggressive groundwater.
The durability and longevity of the mosaics’ structure are important to consider, because they shed light on the true environmental impact of many cement-based products. Manufacturers of cast stone products can rightly argue that the energy required to convert limestone and clay into the ubiquitous grey powder employed across the construction industry should be viewed in the context of their whole life sustainability credentials.
Contemporary cast stone as a construction material
Contemporary cast stone products such as cills, lintels and string courses may not be destined to survive for millennia, but they bear very favourable comparison to common construction materials including PVC extrusions or energy intense aluminium components.
While some cast stone manufacturers use just a pigmented sand and cement mix for their production runs, finely graded limestone, such as Bath stone, is recognised as offering the best outcomes. Semi-dry cast stone incorporates 60% crushed limestone and a 2% water content to achieve a minimum design strength of 35N. Waterproofing admixtures reduce moisture penetration and, along with vapour curing, help protect against frost while also accelerating the gain of strength to enable products such as quoin stones to offer excellent durability and general resistance to weathering.
The so-called wet cast process also features a 2% water content, with the increased slump and workability being created using ‘super plasticisers,’ making the mixes self-compacting. In addition, the target compressive strength rises to 45N, without any need for vapour curing, further enhancing the likely lifespan of cast stone products.
In fact, cast stone steps and flooring manufactured today can be expected to offer a maintenance-free life of many decades, while items such as copings – which are permanently exposed to the elements – should provide solid service for as long as the walls that they sit atop remain standing.
A sustainable option
Research has further shown that the extended working life offered by cast stone means that its embodied energy becomes insignificant, while the natural process of carbonation – where carbon dioxide is absorbed from the atmosphere and changes state – constitutes a form of carbon capture. Then from a local perspective, the fact that most projects here make use of products manufactured in the UK means the energy involved in transportation is also minimised. In addition, production standards and supply chain monitoring are higher than in many other countries from where products are imported.
There is an industry-wide consensus on the need to improve environmental performance, underlined by the UK Cast Stone Association announcing the launch in May this year of its Sustainability Charter – whereby the membership has committed to adopting a series of measures, including cutting waste, pollution and energy usage. The UKCSA will also work constructively with other organisations to deliver sustainable policies and practices.
It is one of the responsibilities of the building industry’s heavy-side manufacturers to look to cut wastage of energy and resources, both within their supply chain and their distribution networks. In the case of cast stone specialists, this means ensuring their sources for limestone as well as other aggregates are properly assessed for their own environmental performance though it is important to realise that, unlike cut masonry, cast stone presents an ideal use for much of the smaller quarried material which would otherwise go to spoil heaps.
Reducing energy use within the factory is key with many manufacturers investing in new generation machinery and renewable energy sources such as solar panels. Considerable efforts are also being made to find ways to recycle production waste – including when mixers have to be purged of leftover material for a change in colour.
Then, from the factory gate to the end customer, there has been a real focus on using cleaner transport, as well as transport management strategies and software which optimise the use of lorries and other vehicles. This includes employing national carriers for small consignments, switching to reusable packaging where possible, and dispatching products on wooden pallets.
Investing in research
Returning to the subject of cement and other constituent raw materials, there are research projects currently in progress involving leading members of the UKCSA and universities, including University College London (UCL) and Cranfield University. It is hoped that these academic-industry alliances will lead to the development of viable cement replacement products and alternative, carbon neutral aggregates.
Cranfield University is also overseeing a PhD project that aims, among other goals, to define the carbon footprint of cast stone and reduce it. This has the potential to deliver further carbon reductions and give specifiers confidence that cast stone can play an environmentally-responsible part in many types of construction projects.
Devanshu Mudgal is senior systems engineer and sustainability adviser at Haddonstone