The Gypsum Trench

Cement is a key ingredient in any stabilisation or groundworks activity but with the growing urgency to reduce carbon emissions the hunt is on to find ways of making it greener. One way of doing this is to create low CO₂ blending powders from industrial waste to add to the mix. Dr Mark Tyrer, principal research fellow at University College, London, explains how one such gypsum waste-derived material is showing promise as trench fill.

Cement is mankind’s largest manufactured product. It eclipses the entire world production of metals, ceramics and polymers combined, and at around 2,000million tonnes a year it is hard to visualise. Well let me help.

We produce over a cubic kilometre of cement a year. This is mixed into over a cubic mile of concrete. Cement manufacture, and the construction industry that relies on it, employs over 111million people worldwide.

Of course, this activity is not without some penalty and cement manufacture – after power and transport – is the third largest source of carbon dioxide emissions, accounting for around 2,000million tonnes a year of CO₂.

There has been a long-recognised need for the industry to increase its efficiency. The World Business Council for Sustainable Development reported that inadequate R&D investment was a weakness of the industry and Action No.6 recommends a major collaborative R&D effort focused on long-term CO₂ reduction.

However, it is not an easy task as cement poses a two-fold problem. Creating cement clinker is a high temperature process at around 1450°C. This is when limestone or chalk – the source of calcium – and clay or shale – the source of silica and alumina – are calcined at a sufficiently high temperature to form cement clinker minerals.

The energy levels required to do this are huge but have been reduced considerably by modern kiln designs. However, there is a second problem. There is an inherent release of CO₂ associated with the calcium carbonate.

The blended mineral ‘meal’ is heated in its journey through the kiln, first losing water and then, as the temperature approaches 900°C, the limestone decomposes to form free lime (CaO) and carbon dioxide (CO₂). This step cannot be avoided as there is no other large-scale source of calcium available for cement manufacture.

So the industry is faced with seeking new solutions to this problem. One solution is “belitic” cements. Belite is the mineral name for dicalcium silicate while tricalcium silicate dominates Portland cement. Using these means belitic cements require less calcium to create and so release less CO₂ during manufacture.

Research into these and other low energy cements, such as sulphoaluminates, continues to grow. However, the best bet for cement replacement materials in the near future are slags and ashes. Used in combination with conventional cement, they effectively dilute its CO₂ impact per tonne of concrete.

Blended cements have been with us for a long time and the technology is mature. The Humber bridge is probably the best known UK example of Portland blended with blast furnace slag (BFS) cement, while the new runway pavements of East Midlands Airport are made from Portland blended with pulverised fuel ash (PFA) concrete.

These two pozzolanic materials (BFS and PFA) dominate the blended cement concrete manufactured world wide, yet there are many other materials which are suitable for use as industrial pozzolans.

A pozzolanic cement requires a source of reactive (usually glassy) silica and something which will chemically react with it to form a new compound. This is usually a poorly crystalline calcium silicate hydrate, but may contain other ions – especially sulphate – that reflect the chemistry of the activator.

An example of a sulphate-activated pozzolan is supersulphated cement (SSC), once a common product across Europe but now largely replaced by sulphate resisting cements used in foundation engineering.

SSC is a blend of blast furnace slag and calcium sulphate, which, unlike Portland cement, is tolerant of sulphate in groundwater. Unfortunately, its storage life is short as it absorbs moisture and so has largely been replaced with modern sulphate resisting cements.

However, a group of UK researchers, along with industrial partners Lafarge Plasterboard and Huntsman Tioxide Europe, have developed materials based on these ideas using by-product gypsum and industrial slags to produce novel, low energy binders for massive concrete applications.

They have also completed several medium-sized pours to examine the possibility of using the material produced from the cement as an engineering trench fill and as a backfill for closing mine workings and other excavations.

The target of the research was to develop materials that will divert gypsum from landfill, using other industrial by-products to produce carbon neutral cements at very low cost. And the results are very encouraging.

By-product gypsum is not in short supply. It is a ubiquitous product in an industrialised society, originating from both neutralisation of waste acids by addition of limestone and from desulphurisation of flue gasses.

While much of it is re-used, a large fraction is landfilled as it is too highly coloured to be attractive to potential customers. As a result, the UK gypsum industry is a curious creature. It imports both natural and white by-product gypsum to supplement domestic natural gypsum, yet landfill part of the by-product gypsum we produce.

Researchers at Imperial College and the University of Birmingham have been developing ways to increase the re-use of by-product gypsum in a project lead by Peter Claisse, Professor of Civil Engineering at Coventry University.

The work has been wide ranging, comparing the measured and predicted reactivity of many industrial minerals towards gypsum in the presence of alkaline activators. One particular material was identified through an interest in transport Peter shares with his colleague at Birmingham University, Dr Gurmel Ghataora.

Both engineers were aware of a problem with steel converter slags, which they knew as railway ballast. When this material was crushed to form an aggregate for concrete production, it was known to be unsound – the aggregate swelled and cracked the concrete.

It transpired that tiny crystals of free lime were the problem. Water diffused along the grain boundaries of the slag, until it found the lime inclusions, these reacted to form calcium hydroxide (slaked lime) and expanded.

As a result, the market for this material has been as unbound fill – largely railway ballast – as the material is very dense. During processing to produce this ballast, the slag produces a highly alkaline dust, which is classified as a leachable waste, posing an increasingly expensive disposal problem.

However, initial experiments using this material as a source of alkalinity in blended cements have been very encouraging and formulations have been developed that largely comprise by-product gypsum and steel slag crusher fines.

It is thought that the alkalinity released from the free lime kick-starts the hydration reaction, dissolving the glassy phase of the slag and fly ash additions, giving it an early strength. Strength development continues through sulphate activation, where the gyspum reacts to produce ettringite, closing internal porosity in the hardened material.

Under testing as a pour, the material has shown itself to be a highly flowable concrete (see pictures above), suitable for emplacement with minimal working. This shows obvious applications in filling surface excavations. One of the trials conducted by Birmingham University has examined repeated cyclical loading of the hydrated fill to demonstrate its suitability for highway applications (see pictures on page 22).

Coventry University has demonstrated larger scale pours, including rapid hardening materials which will support a vehicle within 36 hours of emplacement. This latter application is particularly exciting as it is usually uneconomic to close mine workings with conventional or foamed concrete.

The production of pozzolanic concrete at a similar price to the disposal costs of its constituents opens up new opportunities to reclaim land that is currently subject to mining subsidence.

From the Lothian coal field in the north, to the Kent coal field in the south, there are areas of land where mine-workings are dangerously close to the surface.

In short, industrial pozzolanic cements such as these offer a resource-efficient route to re-using waste industrial minerals by putting them to practical use.

The work here was co-ordinated by the Resource Efficiency Knowledge Transfer Network. The Minerals Industry Research Organisation (MIRO) is building a consortium of industrial partners to take the project to the next phase. To get involved email Mark at [email protected].