New research may have discovered a dramatic new use for a problem waste material unsuitable for landfill dumping - as a concrete aggregate.
Diarmaid Fleming reports from Watford.
Concrete's potential contribution to a more sustainable world includes the possibility of using by-products from other industries that would otherwise pose awkward disposal problems. A new opportunity comes from pioneering work by scientists at the Building Research Establishment, who have been researching the possible uses in concrete of slags and other byproducts from non-ferrous metal production. This could eventually mean an impressive double whammy, saving tax for the metal and construction industries, while helping the environment.
Producers of such metals as zinc and aluminium currently pay millions to stockpile or send slag and unwanted by-products to landfill. If these were to prove viable as an aggregate, concrete manufacturers would then have a source of recycled material, exempt from the aggregates tax introduced earlier this year, while metal producers would no longer need to dump the material and pay landfill taxes and charges. The studies are led by Dr Andrew Dunster, principal consultant at the BRE Centre for Concrete Construction, working with Scott Wilson Pavement Engineering.
'Around a year ago, we did a feasibility study of waste products from the non-ferrous metals industry. We homed in on two products because they looked promising for use in construction: slag from Imperial Smelting Furnace (ISF) zinc production, and spent 'pot linings' used in aluminium manufacture, ' says Dunster.
He adds: 'ISF zinc slag is a glassy material, hard and dense with low porosity, so it could possibly replace sand in concrete or bitumen. The UK produces around 80,000t of ISF slag each year. Because it contains zinc and lead, landfill is not a preferred option for its disposal and around 2.5Mt of it is currently stockpiled.'
Spent pot linings (SPL) are a by-product of the production of aluminium metal by the electrolytic reduction of bauxite in Hall Cells or 'aluminium pots'.
The carbon-based lining, which acts as the cathode during the refining process, eventually becomes contaminated and unstable, and has to be replaced, along with its backing of refractory bricks.
'The refractory bricks appeared promising as they are basically alumino-silicate with properties similar to natural aggregates, ' says Dunster. 'But some contaminants arise during the smelting process which are more of an issue.'
Availability is not a worry. It is estimated that 500,000 tonnes of SPL are produced worldwide, with between 4,000 and 7,000 tonnes produced in the UK annually with no use for it identified.
The research project, which is partially funded by the Department of Trade and Industry, not surprisingly, has attracted the support of several major non-ferrous metals producers. Studies so far have found that, for use in concrete, ISF slag needed virtually no processing work such as removal of contaminants compared with SPL materials, so investigations with slag are more advanced. Composition is also more uniform.
'There is a little bit of variation between ISF slag samples, but we are just using untreated waste products. If it was to be commercially used, it could be processed to even out variations, but at the moment, these are not significant, ' says Dunster. Its physical properties - granular like coarse sand - also make it suitable.
A full-scale test was needed, so a 30m stretch of concrete roadway was to be built inside Britannia Zinc Ltd's works near Avonmouth, taking the busy and heavy traffic to and from the plant. Half the surface would be made from ordinary concrete, the remainder from a design mix using ISF, with the performance of each surface monitored.
Interesting behaviour began to emerge from the two ISF slag mixes, one in which 50% of the fines were replaced with slag, the other with 75%. The design mix was to produce C40 concrete, with a 50-80mm slump and an air content of 5-6%. CEM 1 - better known as Ordinary Portland Cement - was used with an air entraining agent for frost resistance and a water reducer to aid workability.
'We found that the concrete cubes with 75% ISF slag tended to crumble after 24 hours and were fairly fragile, ' Dunster says. 'But within seven days, they had reached a similar compressive strength to the control mix. The concrete with only 50% slag was hard at 24 hours.'
Strength gain rates were slower, due to the presence of lead and zinc in the ISF. Previous studies have shown that zinc and lead ions within cementitous mixes form a gelatinous layer around cement grains. These prevent water in the mix accessing the cement, inhibiting hydration. But as the water diffuses through the gelatinous layer, hydration occurs as normal.
The lead and zinc content in ISF slag also raised concerns that significant leaching of these elements from the concrete would pose a serious environmental hazard, but laboratory tests have so far shown that the quantities are negligible. Traces of arsenic have been found but are well within safe limits.
On site, the new slab forms part of a conventional highway surface, with the concrete placed on sub-base material over a capping layer on the subgrade.
The road is located on good quality made ground as the team wanted solely to monitor concrete performance and did not want any undesirable effects from the underlying soil which could cloud the results.
No reinforcement was used either, to enable pure concrete core samples to be taken without mesh present. A curing membrane was used to prevent shrinkage cracking.
Visual inspections, testing of cores and microscopy will assess strength development, carbonation and leaching of zinc, lead and arsenic, and examine any deleterious processes under way, although so far the results are promising, says Dunster.
'We would like to obtain as many of the concrete's key physical features by measuring as many sensible properties as possible, ' he notes, adding that with more 'exotic' features such as creep, tensile strength and shrinkage may be examined in the coming months.
More funding is needed however to verify longer-term properties of the concrete. 'Because it is a glassy aggregate, this could give rise to fears of alkali-silica reaction (ASR). Aggregates susceptible to ASR tend to be disordered crystalline structures i. e. glassy, ' says Dunster.
ASR, or 'concrete cancer' as it was commonly called after it was first recognised as a problem in the 1980s, spelt ruin for some concrete structures afflicted by the phenomenon.
'We will be hoping for follow-on projects to see what its performance would be in the longer term: we have no proof of how it will perform in 10 or 20 years.'
Ironically, the zinc content could give rise to extra protection of reinforcement. 'It would be interesting to look at whether it might lead to some kind of galvanising effect providing additional protection of reinforcement bars from corrosion, ' Dunster adds.
Other features of the material are the greater concrete density of 2.48t/m 3compared to the control mix without ISF, which was 2.35kg/m 3. While this would mitigate against structural use, it could prove beneficial where denser concrete is needed, such as in some foundation applications or marine defence works.
The news with the other materials from aluminium production is not immediately so encouraging, Dunster reports. 'Early indications are that refractory bricks could be promising, but they would need to be crushed and the contaminants removed, ' he says.
'We are not so sure about spent pot linings. They have quite a high carbon content and contain fluorides and cyanides and there are issues as to whether these would become fully immobilised within the concrete, and whether they could affect setting or react with cement. So there are more unanswered questions.'
The current tests are due to run until next September, with Scott Wilson due to perform similar tests for bitumen and a demonstration roadway to run from next May until the following July.