aggregates
Mistakes and accidents are generally unavoidable, and the use of waste aggregates for construction purposes is no exception. While mistakes and accidents are unfortunate and should be avoided or minimised, there are sometimes lessons to be learned when they happen. Therefore, a number of known accidents or problems related to the use of recycled waste aggregates have been reviewed. It should be noted that only a few such incidents have been reported, mainly in the Netherlands which has a long tradition for recycling of waste aggregates as well as for monitoring and control. In addition, this type of information is not always released voluntarily by the involved parties. In some cases, the application was not carried out according to the specifications, and in such situations the applicant is at fault. In other situations, the development of unacceptable conditions was not foreseen, as the material was applied fully in line with the regulations. In this case, additional rules or guidance to users are called for. Such cases can also help improving the understanding of factors that play a role in full application but often cannot be covered by laboratory leaching tests.
Application of LD steel slag as filling material
At the end of 2004, a total amount of 105416 tons of granular LD steel slag (0-40 mm) was used as a filling and hardening/paving material in an industrial park (over an area of 80,000 m2). In January 2005, increased pH levels were found in drained (discharge) water and in the nearby surface water killing fish. When the groundwater below the application was monitored in early February 2005, high pH values (up to pH 13,4) were found (van der Sloot et al., 2007). Although the material conformed the 0-40 mm gradation, the main portion of the material was closer to the lower than to the higher range of this particle size range. This resulted in a high release of alkalinity from the slag caused by the high surface area. At the same time sulphides were leached, probably in the form of polysulphides, which resulted in oxygen depletion.. Since pH and reducing properties were not regulated in the BMD
(1995) or SQD (2007), this is a typical example of learning by experience. For now there is a recommendation to the sector to be aware of these phenomena under field conditions.
Application of LD steel slag on the bottom of a pond in a residential area
In 2001, the bottom of a pond in the middle of a small residential area was reinforced and made less permeable with LD steel slag (7-32 mm). This was done to prevent upward “seepage” of groundwater (a common problem in the Netherlands). The pond was I- shaped and had an area of about 46,000 m2. An amount of about 110,000 m3 steel slag was used on the bottom of this pond, a layer with a
thickness of about 2.40 metres. All the fish in the pond died only shortly after the application of the slag in the pond. A combination of high pH and low redox potential was measured. The problem here was the small volume of water that was not refreshed regularly to allow neutralisation by the buffering in surface water, CO2 from the air, soil and organic matter. In addition, the access of O2 from the
atmosphere was insufficient to prevent the temporary development of reducing conditions (like in a canal or big lake); van der Sloot et al., (2007). In this case, the application did not conform to the specifications to only apply the material in situations with sufficient water flow.
Utilisation of blast furnace slag in direct contact with water
Application of BFS in direct contact with ground/ surface water has resulted in discolouring of water bodies due to release of sulphides and at the same time release of hydroxide, which turned the water body alkaline (van der Sloot et al, 1995, van der Sloot et al., 2007). Application of BFS as a fill material behind a steel sheet protection in a lock In Burlington (Ontario, Canada) resulted in green, yellowish colouring of the seawater caused by sulphides released from the slag. The ultimate action has been to remove the material from that application. Application of BFS slag in road stabilisation in an area with a high groundwater table in Giethoorn (The Netherlands) resulted in similar releases of yellow, greenish liquid flowing out of the application. BFS directly in contact with ground or surface water is leading to such releases. In closed confinement (limited water flow), the high pH imposed by the slag will also affect water quality. Observed release of substances may not be coming from the slag material, but a secondary effect resulting from mobilising substances associated with soil underlying the application by the high pH imposed on the soil. A high pH mobilises dissolved organic matter (DOC), mainly in the form of humic and fulvic substances, which have the tendency to bind metals and organic contaminants.
Issues with slag stability
The application of monolithic slag (armour stone) in coastal protection has been practiced in the Netherlands already for a long time (van der Sloot et al, 1995). With the application of different types of slag, it has been found that slag stability can be an issue as upon contact with water, unreacted lime inclusions can create expansive forces leading to slag deterioration and undesirable environmental impacts. To ensure that the slag is sufficiently stable, a boiling test (EN 1367-3: Test for thermal and weathering properties of aggregates – Part 3: Boiling test for “Sonnenbrand basalt”. CEN- CMC, Brussels, Belgium, 2001) needs to be applied. In a waterworks project in Biesbosch (Netherlands) the poor slag quality was caught just in time, as the delivered slag started to deteriorate in storage. In water bodies with sufficient flow of water (rivers, canals and estuaries), the initial release of hydroxyl ions can locally result in an increased pH and release of reducing species (sulphides). The effect is very temporary (months) and, provided the water circulation is adequate, does not pose a risk to the environment.
Application of shredded tires at tennis courts
The application of shredded tires as a base material for tennis courts, which are designed to drain well, has led to increased levels of Zn release from the tire component. In several studies, this phenomenon has been observed (Hofstra, 2006 and 2007; Smolders and Degryse, 2002; Westenberg and Máscik, 2001). The key question is still whether the release is environmentally acceptable.
Application of MSWI bottom ash in embankment
Municipal solid waste incinerator bottom (MSWI) ash has been used as road subbase and embankment in many instances in Europe, but always with additional requirements, such as application with measures to reduce net infiltration, with a requirement of use at least 0.5 m above the highest recorded groundwater level and removal of the material, when the service life is finished. Over
the years there have been instances where any of these specifications were not fulfilled or were fulfilled initially, but with time changes resulted in situations no longer fulfilling the criteria. In such circumstances, measures were taken to remedy the non-compliance. An example of such a situation is the faster settling of a roadbed at a location where formerly a canal cut through the planned road section. In a few cases, local failures in the top cover placed to ensure low net infiltration have been observed and mitigated.
Lessons learned
The experience gained in these cases can be used as guidance for end users. Firstly, that the specifications associated with particular uses should always be followed, and secondly that new experiences should find their way to the user community in the form of adjusted regulation and/or as guidance in practice. Clearly, pH and redox conditions resulting from use of an alternative material were not regulated and need to be considered besides substances of direct regulatory concern. This is particularly relevant in cases where the release of leachate with a high pH can mobilise substances from underlying “natural” soil layers. Based on these observations there are further lessons to be learned. Namely, that overlying a material with potentially leachable substances with a layer of “reactive” soil that can generate DOC with a humic and fulvic acid component capable of mobilising metals as DOC complex and organic contaminants as DOC associated species. We are not aware of any cases yet, but this is likely to happen at some point. An issue to consider in practice is also that testing is often carried out at relatively high L/S values, whereas in an application, especially in applications of some height, the release of substance will occur at rather low L/S values during the first years. This condition can prove critical in field applications. The low L/S in the column test can forewarn for such potential situations.