THE UTILISATION OF RECYCLED AGGREGATES GENERATED FROM HIGHWAY ARISINGS AND STEEL SLAG FINES. Summary project report submitted to.

Transcription

1 THE UNIVERSITY OF BIRMINGHAM THE UTILISATION OF RECYCLED AGGREGATES GENERATED FROM HIGHWAY ARISINGS AND STEEL SLAG FINES Summary project report submitted to Global Environmental Management Trust Ltd. (Project no 27) Project funded by Tarmac s Environmental Trust Fund By Gurmel S Ghataora Richard J Freer-Hewish James Jessic March 2004 Department of Civil Engineering School of Engineering The University of Birmingham Birmingham B15 2TT UK

2 CONTENTS Page 1 Introduction 1 2 Aims of the research 1 3 Source material Recycled asphalt Steel slag 2 4 Potential applications Recycled asphalt Steel slag Applications investigated Reclaimed asphalt utilised as pathway surfacing Steel slag fines used for colliery spoil remediation Field trial using steel slag to ameliorate acid generation in colliery spoil 6 Conclusions Conclusions regarding recycled aggregates from highways arisings 6.2 Conclusions regarding steel slag fines 7 7 Acknowledgements 7 7 References List of Tables Table 1 Properties of recycled asphalt Table 2 Properties of steel slag Table 3 Chemical constituents of steel slag Table 4 Main uses of recycled aggregates Table 5 Main uses of steel slag Table 6 Results of tests on eluate and EA contamination threshold limits.

3 1 Introduction There is an increasing demand to reutilize materials and this pressure is perhaps large on recycling plants that are located in urban locations, where available space is likely to be a major constraint on operations. Highway arisings from three recycling plants were investigated for this study. In the United Kingdom about 1 million tonnes of basic oxygen steel (BOS) slag is produced annually and about 10 million tonnes of BOS slag is held in stockpiles undergoing weathering to allow for hydration of free lime. In the United Kingdom this slag is currently under utilised. A parallel investigation at the University of Birmingham (Poh et al., 2005) showed that there was slow release of alkalinity from steel slag and it was therefore conjectured that it may be possible to use it for the remediation of acidity arising in run-off water from colliery spoil mounds. A report on a laboratory scale investigation and a small field trial on the use of colliery spoil to remediate acidity is also presented herein. This research project on the possible uses of reclaimed asphalt and steel slag fines was undertaken for Global Environmental Management Trust Ltd. and it was funded by Tarmac s Environmental Trust Fund. The investigation was undertaken between November 2000 and October Aims of the Research Th aims of the research were to characterise both recycled aggregates and steel slag fines and then to find potential applications for their reuse in an economic manner. 3 Source Material 3.1 Recycled Aggregates Recycled materials were obtained from Tarmac s recycling plants located in Wolverhampton, Birmingham and Nottingham in the UK. All these materials ranged in particle size between 45 µm and 32 mm, with about 70% of the material in the 6mm to 32mm size range. These materials had a density of about 2300kg/m 3. Other properties measured are summarised in Table 1. Scanning electron micrographs were used to identify the chemical elements present in the recycled materials. The elements found in all the samples obtained from the three sites are listed below: Aluminium Calcium Carbon Iron Magnesium Manganese Oxygen Potassium Silicon Sodium Sulphur 1

4 Property Measured Site Birmingham Nottingham Wolverhampton ph Loss on ignition (%) Coarse Fraction (2.36 to 32mm) Fine fraction (75µm to 2.36 mm Filler fraction (sub 75µm) Porosity (%) Carbonate content (% by dissolved mass) Paramagnetic minerals (%) Loss from acid leaching (%) Loss from thermal treatment (%) Loss from magnetic separation (%) Remaining inert material (%) Leaching tests ph Electrical conductivity (ms/cm) Calcium ions (ppm) Ferrous ions (ppm) Table 1 Properties of Recycled Asphalt Steel Slag Steel slag from both Teesport and Port Talbot was used for this investigation. The particle size for both these slags was similar and it ranged between about 60µm to 30 mm, with a coefficient of uniformity of about 20. Both were well graded and exhibited density of about 3360 kg/m 3. Other key properties of the slag are listed in Table 2 below. Property measured Value ph 12.1 Porosity (%) 31 to 45 Leaching tests ph 12.8 Electrical conductivity (ms/cm) 3.70 Calcium ions (ppm) 321 Ferrous ions (ppm) 0 Table 2 Properties of steel slag 2

5 The chemical analysis of the slag used is listed in Table 3 Determinand Percentage of Total Sample Determinand Percentage of Total Sample Slag source Port Talbot Teesport Slag source Port Talbot Teesport Silicon as SiO Potassium as K Calcium as CaO Total Sulphur Aluminium as Al 2 O Total Iron as Fe Magnesium as MgO Metallic Iron as Fe Titanium as TiO Ferrous Iron as FeO Phosphorus as P 2 O Ferric Iron as Fe 2 O Manganese as MnO Free Lime as CaO Sodium as Na 2 O < Table 3 Chemical constituents of steel slag 4 Potential Applications Based on the mineralogical and chemical properties of the materials the following possible uses were considered. 4.1 Recycled Asphalt The variability of the materials and their chemical properties of the reclaimed asphalt materials limit their uses. Some of the main uses are listed in Table 4. Application Absorbency Aggregate Pathway material Comments Visually recycled asphalt aggregate looked similar to commercially absorbency aggregate used for clearing up spillages of oil. However, the porosity and absorption of recycled material was not high enough. Recycled aggregates were already used in a commercially available product known as Toptrek. However, if a suitable cheap dye could be found then there was potential for further development. Table 4 Main uses of Recycled Aggregates 4.2 Steel Slag There are numerous uses of steel slag. However, the main ones are listed in Table 5 below. 3

6 Application Remineralisation Slag Binder Colliery spoil remediation Comments For use on agricultural land This had potential and was under investigation Literature and current research at the University of Birmingham suggested that slag released lime slowly over a long period. Thus, it had potentials for remediating acidic colliery spoil. This area was also investigated Table 5 Main uses of Steel Slag 5 Applications Investigated 5.1 Recycled Aggregates Utilised as Pathway Surfacing The use of reclaimed asphalt as pathway material was investigated as unbound and bound material obtained from the Wolverhampton Recycling Centre only. Ordinary Portland cement was used as a binder. Five pigments (black, green, red, yellow and brown) were used in the surfacing layers. The pigment concentrations ranging between 1% and 15% were investigated. After laboratory based study, small sections, measuring about 30cm x 30 cm x 8 cm thick, were laid as test patches in an untrafficked section of a car park at the University of Birmingham. The research and development work showed that aggregates from highway arisings, when mixed with 10% by mass addition of cement and 2% by mass addition of pigment, performed well as a pathway material and showed little sign of fading or deterioration. After initial compaction and curing the formulation provided a well-compacted and stable body, with a durable, competent and brightly coloured surface. After subjecting the pathway test patches to naturally occurring weathering for a period in excess of 15 months the optimised formulation has continued to perform, showing few signs of deterioration in stability and durability. However, ultra violet light caused some fading of the test area colours, despite this the test areas remained aesthetically pleasing. Samples with the colour of the 10% pigment withstood natural weathering to an even greater extent and in the long term outperformed the optimised formulation (with 2% pigment). However, the initial transferring of the colour and pigment cost issues remain, although the amount of pigment transferred does diminish with time. Attempts were made to further enhance the market potential of the pathway formulation by increasing the environmentally friendliness of the product. The work undertaken involved the incorporation of other waste materials, a slag based binder and ochre and red mud as pigment substitutes. The use of 10% red mud addition was encouraging and demonstrated replacement potential of expensive pigments Although the characterisation work was conducted using samples obtained from three sites, and most of the pathway formulation work was conducted using samples obtained from the Wolverhampton Recycling Centre. The need for a low specification of the final product provides scope for any other national site to be the source for this application. Therefore, should the pathway material be developed and sold commercially it could have national 4

7 implications in reducing the amount of recycled aggregates sent to landfill, reducing both the economical and environmental burden of the material. 5.2 Steel Slag Fines used for Colliery Spoil Remediation A collier spoil site located at Carwood, in Cumbria in the UK, was identified as posing a particular problem regarding acidification of run-off water and was available for undertaking preliminary investigation. Spoil from Carwood was characterised and leachate tests were conducted using a range of proportions of steel slag. Results of the leachate test are shown in Table 6. Small test patches of spoil were then laid in an unused section of a car park at the University. These patches were grassed and the progress of grass growth was monitored. The research work conducted on the remediation of colliery spoil using steel slag fines was encouraging. In the short-term the addition of steel slag fines increases the ph of the colliery spoil, consequently lowering the amount of heavy metals leached from the spoil, reducing the potential environmental impact these metals would have and providing a ph level which would support revegetation. The addition of steel slag has shown that the permeability of the spoil is not significantly lowered, although this could prove to be problematic during revegetation due to hindrance of root penetration. Analyte Untreated Spoil Spoil plus 10% SSF Spoil plus 10% LSD EA Contamination Threshold limits COD as O 2 (mg/l) ph to 9.5 Copper as Cu (µg/l) Nickel as Ni (µg/l) 62.4 <5.00 < Zinc as Zn (µg/l) Aluminium as Al (µg/l) 792 <10.0 < Iron as Fe (µg/l) 74.4 <30.0 < Manganese as Mn (µg/l) 1600 < Sulphate as SO 4 ( µg/l Conductivity (µs/cm) Sulphide as S (mg/l) <0.015 <0.015 < Alkalinity as CaCO 3 (mg/l) < Chloride ion as Cl (mg/l) SSF = steel slag fines and LSD = limestone dust. Table 6 Results of tests on eluate and EA contamination threshold limits. One of the conventional treatments for acidic colliery spoils can be the addition of limestone and this was used as a control remediating medium. The steel slag consistently outperformed the limestone dust as a remediation agent and within the confines of laboratory scale experiments appeared to be the better material for the treatment of the pyritic colliery spoil. 5

8 Steel slag released a greater amount of lime than limestone, shortly after addition and provided a greater neutralising potential for the acid content of the spoil. Eleven weeks after starting the growing trials, the appearance of the grass in the steel slag sample had grown to a thick, healthy thatch of good colour, similar in appearance to that for topsoil. The grass patches containing untreated colliery spoil had died back and were patchy in appearance. The same was observed for the area containing the limestone dust treated sample but to a lesser extent. It was also noted that grass growth on colliery spoil treated with steel slag was weed free compared to topsoil. This was because colliery spoil was most probably sterile to begin with. It thus perhaps indicated potential for developing it for growing weed free turf. In order to quantify the results seen in the vegetation growth trials, the ph of the soil mixtures in each test area was assessed using a commercially available domestic ph soil tester. Indicator solution was added to a small quantity of soil and the corresponding colour relates to a ph given by an accompanying chart. Fourteen weeks after the areas were seeded the following ph results were obtained: Colliery Spoil - ph 5.0 Colliery Spoil plus 10% Steel Slag Fines - ph 8.0 Colliery Spoil plus 10% Limestone Dust - ph 6.5 Top Soil - ph 8.0 It should be noted that the test gives only an approximate reading and the ph range is restricted between ph 5.0 and 8.0. However, for comparison purposes, it proved that steel slag fines continued to neutralise the acidity generated in the colliery spoil to a much greater extent than the limestone dust. This is a possible explanation for the differences in growth rate seen in the respective test areas. 5.3 Field Trial Using Steel Slag to Ameliorate Acid Generation in Colliery spoil A pilot field trial was installed at the Carr Wood site to assess the long term effects of steel slag fine acidity of colliery spoil. At the time of publication the proposal was to investigate the use of steel slag fines as a near surface treatment for acidic colliery spoil. Two trial areas were constructed in September 2003; the first trial was designed to treat surface runoff water in an area of 3m x 3m and 15% by mass of steel slag fines was mixed in the top 0.75 mm layer of spoil. It was envisaged that the treatment would raise the ph of the spoil, (currently between ph 2.5 to 30) to a level, which will successfully support the revegetation of the site, i.e. ph 6 to 8. For the second area of the same size, soil was dug to a depth of 0.75 m and excavated material was re-placed in the same way as the first site. However, no slag was mixed in this area. It formed the control area. Both the areas were treated with 0.5kg of grass seed. It is proposed to monitor changes in ph and heavy metals in solution over a period of 18 months to 2 years. 6

9 6 Conclusions 6.1 Conclusions Regarding Recycled Aggregates from Highway Arisings During the early stages of the current investigation, it became clear that this material had not been investigated fully thus far. This was most probably due to its low grade and varying chemical and physical properties of the material. There was thus a distinct lack of literature in this area and it highlighted the importance of the initial characterisation stage of the study. The characterisation work confirmed the variability of material and in effect imposed further restrictions on the eventual end product. It was clear that the only suitable use for the material would be the one with minimal specification where the non-homogeneous nature of the recycled aggregates would not affect the success or the working characteristics of the final application. This greatly diminished the potential outlets for the material, and the application finally adopted does appear to be the best of a limited number of options. The investigation has shown that the recycled aggregate, mixed with cement and a pigment, produces a brightly coloured, stable, and well compacted surfacing material with a durable surface. This can be used as a pathway material and at the time of publication continues to perform well in excess of 18 months after laying. Within the set criteria constraints (minimal processing and low cost requirements) the aim was to turn the waste material into a value added product and the research proved the success of the product and highlights the potential of recycled aggregates as a pathway material. 6.2 Conclusions Regarding Steel Slag Fines Finding applications and uses for steel slag has been a much more common area of research than that of recycled aggregates and consequently the available literature was more extensive. The chemical and physical properties of the material were well documented. The same constraints were imposed on the recycling of the steel slag fines as those for the recycled aggregates. Although a number of applications exist, novel work on remediation of acidity was trialled. Laboratory testing showed that addition of steel slag fines out performed the more traditional and commonly used method of neutralisation using limestone. Although probably too simplistic, experiments to assess the growth potential tests, where grass seed sown in a test patch containing a 10% steel slag fines addition performed comparably with seeds sown into ordinary topsoil. In addition, because colliery spoil is sterile, the grass seeds that were planted were not in competition with other plants and vegetation classified as weeds and developed into weed free turf. 7 Acknowledgements The authors are very grateful to Tarmac for funding this research and to Drs H Robinson and N Ghazireh for their contributions. The authors also wish to acknowledge the support both Mr Kevin Onions and Adam Jarvis or IMC. References J Jessic (2004) The Utilisation of Reclaimed Asphalt and Steel Slag Fines, PhD thesis, The University of Birmingham, Birmingham. 7

10 A, Monshi and M. K. Asgarani (1999) Producing Portland cement from iron and steel slags and limestone. Cement and Concrete Research, No. 29, pp H Y Poh, G S Ghataora and N Ghazireh (2005) Soil Stabilisation using Basic Oxygen Steel Slag Fines. ASCE Journal of Materials in Civil Engineering - submitted for publication. 8