A study of physical and hydraulic properties of green roof substrates including waste materials

Green roofs are becoming increasingly popular in the urban landscape, where levels of air pollution from numerous sources of emissions have rapidly risen compared to rural areas. Green roofs are designed to retain precipitated water, while filtering out pollutants, provide insulation, reduce roof construction costs, be lightweight, contribute to the reduction of temperatures in cities and create habitats for wildlife. When designing green roofs, substrate composition is an important consideration for water retention, plant growth and water quality of runoff. To increase further the sustainability of the green roof systems in terms of material usage, waste materials could be introduced to replace natural soils in the sustrate layer. Current standards however do not include such materials, which appears to be a missed opportunity. In this context, this paper focuses on a number of typical materials used in the green roof substrates and potential suitable alternatives to these, coming from waste streams. For the mixes, commercial growing medium/substrate (control mix) and some common natural or manufactured materials were used, trying to create appropriate substrate mixes in terms of physical and hydraulic properties. These materials were then replaced by waste materials of similar characteristics (e.g., crushed clay brick, paper sludge, and waste glass cullet aggregate, to replace respectively LECA expanded clay, peat and fines such as clay size particles, and sand). For nutrient retention, some small proportion of zeolite was also used in some of the mixes. To assess the suitability of the different mixes with waste materials as extensive green roof substrates, in comparison with commonly used substrate materials, a number of physical hydraulic property tests were performed, as well as germination tests, to attest whether the mixes with the waste materials were adequate for plant growth. The results showed that the studied mixes of different components were overall suitable as green roof substrates and supported plant growth. This encourages further research on the topic (including environmental safety testing, and microbial community studies) towards potential future inclusion of such mixes in green roof design standards

Alkali-activated cements for ground improvement

The paper studies alkali-activated (AA) slag cements for soil stabilisation, as alternatives to traditional binders such as Portland cement or lime. A number of alkali activators were considered, containing also a range of salts and a material retrieved from waste (Paper Sludge Ash, PSA). The results are discussed in terms of unconfined compressive strength (UCS) at two different curing times. In general alkali-activated cements based on the use of alkali salts only had a modest performance in terms of strength improvement; a material that showed promise however as slag activator was the PSA. Further analysis is recommended in order to gain a better understanding of the complex mechanisms involved towards engineering uses for soil stabilisation

A Study of Innovative Alkali-Activated Binders for Soil Stabilisation in the Context of Engineering Sustainability and Circular Economy

In the context of sustainability in the civil engineering industry, chemical ground improvement is becoming increasingly used, as a generally more sustainable alternative to replacing and landfilling unsuitable for construction ground. However, traditional soil stabilisers such as Portland cement or lime are not environmentally impact-free; international research effort is thus focusing on the development of innovative cementing agents. This paper presents results from a feasibility study on the development of suitable alkali-activated slag cements for the stabilisation of two soils. A number of alkali-activators were considered, comprising potassium hydroxide, a range of alkali salts, as well as a material retrieved from waste (Paper Sludge Ash, PSA) which contains free lime. Indicative results of an extensive parametric study in terms of unconfined compressive strength (UCS) are shown, followed by results of ongoing oedometer tests to determine soil compressibility and some preliminary tests on selected soil/binder mixes to observe the durability to wetting-drying cycles. Overall, all alkali-activated cement mixes increased the UCS and stiffness of the soil. Carbonates and Na 2 SiO 3 used on their own gave lower strength increases. The highest strengths were achieved from AAC with KOH and Ca(OH) 2 from PSA, which showed similar strength gain. The latter material has shown consistently a lot of promise in terms of strength, stiffness and volumetric stability of the soil as well as treatment durability. Ongoing research focuses on further mix optimisation and a comprehensive mechanical and durability property testing supported by material analysis (mineralogical, chemical and microstructural) to gain a better understanding of the complex mechanisms involved

Mechanical Properties and Durability of Concrete with Water Cooled Copper Slag Aggregate

© 2017 The Author(s)Copper slag is a voluminous waste material obtained during the manufacturing of copper (matte smelting process). As its disposal becomes a concern for environmental protection agencies and governments, possible alternative outlets for this waste material are needed. The paper presents a laboratory study on CEM-II concrete mixes, containing water-cooled copper slag waste material for a partial or full replacement of fine concrete aggregate. A series of tests were performed at two different water to cement ratios, to determine the workability, cube compressive strength, indirect tensile strength, static modulus of elasticity, and a number of durability-related characteristics (water absorption, accelerated corrosion, carbonation, alkali-silica reaction). The results showed that water-cooled copper slag had variable effects on the resulting fresh or hardened concrete properties, depending on the sand replacement level and water to cement ratio. However the measured strength values were likely to be linked to the usual variability of concrete batches, rather than a significant effect of the copper slag aggregate. This hypothesis was further supported by statistical analysis. Concerning the durability related characteristics, the overall performance of the concrete containing copper-slag was in most cases similar or better than that of normal concrete with natural sand aggregate. Based on the results water-cooled copper slag can therefore be considered to be a suitable fine aggregate for concrete. This shows promise for developing an additional viable solution to tackle the issue of copper slag waste

Use of fruit and vegetable waste as growth media in bacterial biocementation for ground improvement applications

The paper investigates the use of mixed fruit and vegetable (FV) waste to extract liquid to grow bacteria. The bacteria will be used to induce biocementation of soils and two metabolic pathways are examined. These are the ureolytic pathway and the carbonic anhydrase pathway (which absorbs CO2). The growing medium produced from fruit and vegetable waste is compared with a commercial growing medium. The results show the feasibility of using FV as a growth medium to successfully biocement soil and coal ash. A typical FV medium contains 3% total sugar and 0.302 mg/100 ml of protein. The results show that vegetable stalks and fruit peel media support the growth of both ureolytic bacteria B. licheniformis and U-1, a carbonic anhydrase-producing bacteria. The use of FV waste to grow bacteria leads to a reduction in biocementation costs for ground improvement applications

Use of paper sludge ash as a calcium-based stabiliser for clay soils

Chemical ground improvement of soils of poor quality for construction has been increasingly used as a means of promoting sustainable construction practices. The production of conventional soil stabilisers such as cement or lime involves non-renewable natural resource and energy consumption and high carbon dioxide emissions; therefore, alternative stabilisers are sought. This study used waste paper sludge ash (PSA) to treat three different clays. The aim was to assess PSA effectiveness as an alternative to lime or cement for clay stabilisation based on plasticity characteristics, unconfined compressive strength (UCS), water retention and volumetric stability. PSA-treated soil specimens were shown to perform well compared to lime-treated or cement-treated ones: (a) PSA considerably lowered the plasticity indices of the two expansive clays, in a similar way as lime; (b) in most cases PSA dosages equal to or greater than the initial consumption of lime gave UCS at least twice as high compared to those obtained using commercial limes at equivalent dosages (> 1 MPa for the two expansive soils after 7 or 28 days of curing) and in the inspected cases also higher UCS than cement; and (c) consistently with the plasticity results PSA-treated specimens swelled less during wetting and had lower volumetric strains upon drying (better volumetric stability) compared to lime-treated or cement-treated soils. Overall the results give promise for a valorisation route of this waste material in the field of ground improvement

Can waste foundry sand fully replace structural concrete sand?

Foundry sand (FS) waste creates a serious solid waste management problem worldwide due to the high volumes produced, necessitating alternatives to landfilling. A possible route is its use in concrete; however, the current consensus is that FS can only be used for modest sand replacements, based mostly on evidence on concrete with clay-bound FS (greensand). Con- versely, this study assessed salient properties of structural concrete with chemically bound FS (polymeric resin binder), for which there is very little information. Concrete mixes were prepared in which FS replaced regular concrete sand partially or fully. The results showed that unlike greensand, the tested chemically bound FS could replace regular concrete sand fully, giving highly workable mixes with good mechanical properties (compressive, splitting and flexural strengths and static modu - lus of elasticity) similar to those of mixes with regular concrete sand; the effect of FS content on these properties was not statistically significant. Durability in terms of water absorption, carbonation and alkali–silica reaction tests was not adversely affected. The possibility of using high contents of this type of FS in concrete (as opposed to greensand) gives promise for an additional outlet route for large quantities of this waste material with clear economic and environmental benefits

Concurrent Carbon Capture and Biocementation through the Carbonic Anhydrase (CA) Activity of Microorganisms -a Review and Outlook, Environmental Processes

Biocementation, i.e., the production of biomimetic cement through the metabolic activity of microorganisms, offers exciting new prospects for various civil and environmental engineering applications. This paper presents a systematic literature review on a biocementation pathway, which uses the carbonic anhydrase (CA) activity of microorganisms that sequester CO2 to produce biocement. The aim is the future development of this technique for civil and (geo-)environmental engineering applications towards CO2-neutral or negative processes. After screening 248 potentially relevant peer-reviewed journal papers published between 2002 and 2023, 38 publications studying CA-biocementation were considered in the review. Some of these studies used pure CA enzyme rather than bacteria-produced CA. Of these studies, 7 used biocementation for self-healing concrete, 6 for CO2 sequestration, 10 for geotechnical applications, and 15 for (geo-)environmental applications. A total of 34 bacterial strains were studied, and optimal conditions for their growth and enzymatic activity were identified. The review concluded that the topic is little researched; more studies are required both in the laboratory and field (particularly long-term field experiments, which are totally lacking). No studies on the numerical modelling of CA-biocementation and the required kinetic parameters were found. The paper thus consulted the more widely researched field of CO2 sequestration using the CA-pathway, to identify other microorganisms recommended for further research and reaction kinetic parameters for numerical modelling. Finally, challenges to be addressed and future research needs were discussed

A study on low energy demand materials used in glasscrete to counteract alkali-silica reactions

The potential of waste glass use in concrete as an alternative outlet to landfilling is excellent; however glasscrete (i.e. concrete with glass aggregate) suffers from durability problems caused by alkali-silica reactions (ASR). The use of pozzolanic materials to counteract ASR has been increasingly studied. This paper investigates the ability of selected low-energy demand binders/pozzolans to counteract ASR in glasscrete: these include paper sludge ash (PSA), a by-product of the paper making industry, used together with a standardised pozzolanic material for concrete, i.e. Pulverised Fly Ash (PFA) an industrial byproduct of electric power stations. A number of laboratory tests were performed on the different glasscrete mixes to assess properties (workability, compressive and tensile strengths and elasticity moduli and water absorption). Mortars were also tested for alkali-silica reaction (ASR) using the accelerated mortar bar test, which showed that ASR was effectively counteracted, towards better glasscrete durability. Glasscrete mixes were identified, with similar strengths as the respective control mixes with natural aggregates. Workability was however affected in all mixes and should be addressed in further research

Comparison of the technical performance of a discretely supported slab track system and an embedded slab track system in a high-speed railway

Slab tracks are increasingly used for High-Speed Railways (HSR) as opposed to the conventional ballasted track. This is due to many factors, including increased durability and sustainability, as the slab track can sustain higher dynamic loading with less maintenance and disruption to railway services. In line with this, this paper reports on preliminary work on the development and application of a 3D structural model using the Finite Element Analysis (FEA) software ABAQUS. The research aims at evaluating comparatively the behaviour of two types of slab track systems; namely, the RHEDA Track System (RTS), which is a German system, and the Balfour Beatty (UK) Embedded Rail System (ERS). The modelled track structures consist of a rail fastened onto a slab laid on a suitable foundation. The foundation comprises a Hydraulically Bound Layer (HBL) placed on a Frost Protection Layer (FPL) overlaying the subgrade soil. The paper reports on findings of static loading on a straight railway section investigating the relationship between slab the thickness values and the corresponding displacements (deflections) and related stresses along the load path. Ongoing research is further developing the model to assess the dynamic behaviour of HSR slab track including the railway geometry-structure interaction particularly at bends