An overview of waste materials recycling in the Sultanate of Oman

Various wastes and by-product materials are generated in the Sultanate of Oman including reclaimed asphalt pavement (RAP) aggregate, demolition concrete, cement by-pass dust (CBPD), copper slag, petroleum-contaminated soils (PCS), discarded tires, incinerator ash, and others. Recycling of such materials in construction is not practiced. Research data are also minimal into the potential use of selected materials in construction applications. This paper will present the results of several laboratory studies conducted into the use of PCS in asphalt concrete mixtures; on the use of CBPD in soil stabilization and flowable fill mixtures; on the utilization of copper slag and CBPD as cementitious materials; on the use of incinerator ash in cement mortars; and on the use of RAP aggregate in road bases and sub-bases. Laboratory data generally indicate that it is feasible to partially reuse some of these materials in construction provided that economic incentives and environmental concerns are taken into consideration.     

The reuse of municipal solid waste incinerator fly ash slag as a cement substitute

The results of the treatment of fly ash from a municipal solid waste incinerator (MSWI) by melting are described, and the safety and the effectiveness of using the slag produced by this melting treatment are studied. The properties of the MSWI fly ash slag were analyzed, to evaluate the feasibility of its reuse as a substitute for part of the cement required in mortar preparation. This MSWI fly ash slag was found to be comprised mainly of SiO₂ and CaO, which can be substituted for up to 20% of the cement content in mortar, without sacrificing the quality of the resultant concrete. In fact, the concrete thus produced has greater compressive strength, 10% higher than that without the substitution. The setting time of the fresh mortar becomes lengthens as increasing amounts of cement are replaced; while the spread flow value increases with the increasing percentage of cement substitution. X-ray diffraction analysis reveals that when the W/C=0.38 and the curing AGE=28 days, the crystal patterns in the mortar samples, prepared with different amounts of cement having been replaced by MSWI fly ash slag are similar. According to the results of the toxic characteristic leaching procedure analysis, MSWI fly ash slag should be classified as general non-hazardous industrial waste, that meets the effluent standard. Therefore, the reuse of MSWI fly ash slag is feasible, and will not result in pollution due to the leaching of heavy metals.     

垃圾焚烧飞灰处置与资源化利用研究进展

随着生活垃圾焚烧处理方式的不断推广,焚烧飞灰的产生量也不断增加。按照我国固体废物分类方法,焚烧飞灰属于危险废物,必须进一步处置才能进入填埋场或资源化利用。本文分析了飞灰物理化学特性,论述了常规处置技术（水泥固化、化学药剂稳定化、酸溶剂提取和熔融固化等）存在的问题。将原始飞灰直接应用于水泥、混凝土或路基材料,飞灰中高含量的重金属和盐类会产生新的环境问题。飞灰水洗可以高效去除其中的可溶性盐类,水洗飞灰在焙烧后重金属的浸出浓度远低于原始飞灰烧结后的相应浓度,飞灰水洗-焙烧技术具有很好的应用前景。     

Life cycle climate impacts of the US concrete pavement network

Life cycle assessment (LCA) offers a comprehensive approach to evaluate and improve the environmental impacts of pavements. First, a general pavement LCA methodology is created that describes the concepts necessary to conduct a comprehensive pavement LCA. Second, the methodology is applied to the life cycle of concrete pavements to quantify current emissions across the road network. System boundaries are drawn to include all phases of the pavement life cycle – materials production, construction, use, maintenance, and end of life. Greenhouse gas emissions are quantified for twelve functional units, which evaluate average conditions for each major roadway classification in the United States. The results present the relative contribution of each component in the life cycle, the annual emissions occurring during the 40-year analysis period, and the sensitivity of these results to model parameters. It is found for all roads that the majority of emissions occur in year one – from cradle-to-gate materials production, and pavement construction – primarily due to cement production. The results are most sensitive to traffic volume, and then to parameters affecting the cement production. Based on emissions and their sensitivity, the LCA results suggest three broad reduction approaches: reducing embodied emissions, reducing use phase emissions, and reducing end-of-life emissions.     

Soft subgrades’ stabilization by using various fly ashes

This publication presents the results of research involving different types of self-cementing fly ashes (without any other activators) for the stabilization of four different types of soft subgrades from various road sites in Wisconsin, USA. The strength approaches were applied to estimate the optimum mixture design and to determine the thickness of the stabilized layer. The stabilized soil samples were prepared by mixing fly ash at different contents at varying water contents. The performance of fly ash stabilized subbase depends both on the specific source of fly ash and the engineering properties of soils. It is suggested that the performance analysis of fly ash should be based upon the laboratory tests such as index properties, compaction, unconfined compressive strength and CBR tests of a specific site. This is suggested rather than using the study of the physical properties and chemical composition of fly ash and soil. As disclosed in the literature, the strength gain due to stabilization depends mainly upon three factors; ash content, molding water content and compaction delay. The samples were subjected to unconfined compression strength and California bearing ratio (CBR) tests after 7 days curing time to develop water content–strength relationship. To evaluate the impact of compaction delay that commonly occurs in the field during construction, the sets of samples were compacted 2 h later after mixing with water. The unconfined compression strength and CBR tests were performed and used to determine the thickness of the stabilized layer in pavement design. All of these factors were taken into account throughout this research.     

Characterisation of an unprocessed landfill ash for application in concrete

An investigation was carried out to establish the physical, mechanical and durability characteristics of an unprocessed pulverised fuel ash (PFA) from a former landfill site at the Power Station Hill near Church Village, South Wales, United Kingdom. This was aimed at establishing the suitability of the ash in the construction of the Church Village Bypass (embankment and pavement) and also in concrete to be used in the construction of the proposed highway.Concrete made using binder blends using various levels of PFA as replacement to Portland cement (PC) were subjected to compressive strength tests to establish performance. The concrete was also subjected to sodium sulphate attack by soaking concrete specimens in sulphate solution to establish performance in a sulphatic environment. Strength development up to 365 days for the concrete made with PC–PFA blends as binders (PC–PFA concrete), and 180 days for the PC–PFA paste, is reported.The binary PC–PFA concrete did not show good early strength development, but tended to improve at longer curing periods. The low early strength observed means that PC–PFA concrete can be used for low to medium strength applications for example blinding, low-strength foundations, crash barriers, noise reduction barriers, cycle paths, footpaths and material for pipe bedding.     

大庆石油管理局粉煤灰综合利用现状及前景

简要介绍了目前国内外粉煤灰利用的情况,分析了大庆石油管理局粉煤灰利用范围,主要包括筑路、生产水泥、加工切块、陶粒和步踏砖等,粉煤灰的综合利用率在70%以上。通过调查粉煤灰综合利用情况,对制作水泥和制作步踏砖存在的问题提出了解决办法。针对大庆油田的实际情况,在借鉴国内外其他方法和技术的基础上,提出了综合利用粉煤灰的其他建议,例如用于固井作业的水泥浆体系,生产铝盐,用于填埋固体废物等。     

Utilization of washed MSWI fly ash as partial cement substitute with the addition of dithiocarbamic chelate

The management of the big amount of fly ash as hazardous waste from the municipal solid waste incinerator (MSWI) has encountered many problems in China. In this study, a feasibility research on MSWI fly ash utilization as partial cement substitute in cement mortars was therefore carried out. MSWI fly ash was subjected to washing process to reduce its chlorine content (from 10.16% to 1.28%). Consequently, it was used in cement mortars. Ten percent and 20% replacement of cement by washed ash showed acceptable strength properties. In TCLP and 180-day monolithic tests, the mortars with washed ash presented a little stronger heavy metal leachability, but this fell to the blank level (mortar without washed ash) with the addition of 0.25% chelate. Therefore, this method is proposed as an environment-friendly technology to achieve a satisfactory solution for MSWI fly ash management.     

The implementation of wood waste ash as a partial cement replacement material in the production of structural grade concrete and mortar: An overview

The timber manufacturing and power generation industry is gradually shifting towards the use of biomass such as timber processing waste for fuel and energy production and to help supplement the electrical energy demand of national electric gridlines. Though timber processing waste is a sustainable and renewable source of fuel for energy production, the thermal process of converting the aforementioned biomass into heat energy produces significant amounts of fine wood waste ash as a by-product material which, if not managed properly, may result in serious environmental and health problems. Several current researches had been carried out to incorporate wood waste ash as a cement replacement material in the production of greener concrete material and also as a sustainable means of disposal for wood waste ash. Results of the researches have indicated that wood waste ash can be effectively used as a cement replacement material for the production of structural grade concrete of acceptable strength and durability performances. This paper presents an overview of the work carried out by the use of wood waste ash as a partial replacement of cement in mortar and concrete mixes. Several aspects such as the physical and chemical properties of wood waste ash, properties of wood waste ash/OPC blended cement pastes, rheological, mechanical and the durability properties of wood waste ash/OPC concrete mix are detailed in this paper.     

Leachability of dissolved chromium in asphalt and concrete surfacing materials

Leachate metal pollutant concentrations produced from different asphalt and concrete pavement surfacing materials were measured under controlled laboratory conditions. The results showed that, in general, the concentrations of most metal pollutants were below the reporting limits. However, dissolved chromium was detected in leachate from concrete (but not asphalt) specimens and more strongly in the early-time leachate samples. As the leaching continued, the concentration of Cr decreased to below or close to the reporting limit. The source of the chromium in concrete pavement was found to be cement. The concentration of total Cr produced from leachate of different cement coming from different sources that was purchased from retail distributors ranged from 124 to 641 μg/L. This result indicates that the potential leachability of dissolved Cr from concrete pavement materials can be reduced through source control. The results also showed that the leachability of dissolved Cr in hardened pavement materials was substantially reduced. For example, the concentration of dissolved Cr measured in actual highway runoff was found to be much lower than the Cr concentration produced from leachate of both open and dense graded concrete pavement specimens under controlled laboratory study. It was concluded that pavement materials are not the source of pollutants of concern in roadway runoff; rather most pollutants in roadway surface runoff are generated from other road-use or land-use sources, or from (wet or dry) atmospheric deposition.     

Use of ready-mixed concrete plant sludge water in concrete containing an additive or admixture

In this study, we investigated the feasibility of using sludge water from a ready-mixed concrete plant as mixing water in concrete containing either fly ash as an additive or a superplasticizer admixture based on sulfonated naphthalene-formaldehyde condensates (SNF). The chemical and physical properties of the sludge water and the dry sludge were investigated. Cement pastes were mixed using sludge water containing various levels of total solids content (0.5, 2.5, 5, 7.5, 10, 12.5, and 15%) in order to determine the optimum content in the sludge water. Increasing the total solids content beyond 5–6% tended to reduce the compressive strength and shorten the setting time. Concrete mixes were then prepared using sludge water containing 5–6% total solids content. The concrete samples were evaluated with regard to water required, setting time, slump, compressive strength, permeability, and resistance to acid attack. The use of sludge water in the concrete mix tended to reduce the effect of both fly ash and superplasticizer. Sludge water with a total solids content of less than 6% is suitable for use in the production of concrete with acceptable strength and durability.     

油田钻井废泥浆固化处理研究

文章结合油田的现场实际情况,针对该区钻井废泥浆污染物的特性,研究了固化的最佳工艺条件,并对固化效果进行评价。通过多组固化处理定性对比实验,从多种处理剂中筛选出处理效果较好的水泥、粉煤灰、石灰和黄土作为固化处理废泥浆的固化剂原材料。利用筛选出的固化剂组成,设计正交实验,确定固化剂最佳配方为每100mL泥浆添加10g水泥、20g粉煤灰、8g石灰、15g黄土。影响因素对固化处理效果的影响实验结果表明:当废泥浆固相含量在30%～70%,固化温度在20～40℃之间,固化时间能够满足7d的条件下,固化效果最佳。     

我国南方某城市垃圾焚烧飞灰特性及固化/稳定化处置实验

我国南方某城市主要采用焚烧法处理生活垃圾,已建7个焚烧厂日产生飞灰量为101.62 t/d。文章分析了各个垃圾焚烧厂飞灰的化学组成和重金属浸出毒性,其主要成分为CaO、SiO2、MgO、Al2O3、Fe2O3、Na2O、SO3和Cl,所有焚烧飞灰都有至少一种重金属浸出浓度超标,属于危险废物。在飞灰处置出路亟待解决的背景下,实验探讨了飞灰稳定化工艺,结果表明10%的水泥添加量可以使飞灰中超标重金属(Cd、Pb和Zn)的浸出浓度满足危险废物鉴别浓度限值和安全填埋场入场浓度限值。同时,通过人体健康风险评价分析了飞灰豁免管理的可行性,并对近期和远期该城市焚烧厂飞灰的处理处置及管理提出建议。     

The economic and environmental benefits of increased use of pfa and granulated slag

It is commonly claimed that increased use of pulverised fuel ash (pfa) and ground granulated blast-furnace slag in concrete as partial replacement for Portland cement can have economic, amenity and technical advantages. Many factors need to be considered is such an analysis and this paper is offered as a contribution to the debate of this issue, and not as a definite solution. In the long-term the major benefit may be in the savings of the energy that would otherwise be required for increased production of Portland cement.     

Long-term evaluation of coal fly ash and mine tailings co-placement: A site-specific study

This study presents the results of a laboratory investigation conducted to evaluate the efficiency of coal fly ash to control the formation of acid mine drainage (AMD) from mine waste. Site-specific materials, coal fly ash from Atikokan Thermal Generating Station and mine tailings from Musselwhite mine, were mixed at different proportions for the investigation of the drainage chemistry and the optimal mix using static testing (acid–base accounting) and kinetic (column) testing. The acid–base accounting (ABA) results indicated that the fly ash possessed strong alkaline (neutralization) potential (NP) and could be used in the management of reactive mine tailings, thus ensuring prevention of AMD in the long-term. Column tests conducted in the laboratory to further investigate long-term performance of fly ash in the neutralization and prevention of acid mine drainage from tailings similarly showed that mixing fly ash with mine tailings reduces dissolution of many heavy metals from tailings by providing alkalinity to the system. It was found that a fly ash to tailings mass ratio equal to or greater than 15% can effectively prevent AMD generation from Musselwhite mine tailings in the co-placement approach.     

废旧铝、铁片粉煤灰复合混凝剂的制备及在废水处理中的应用

我国粉煤灰和铝、铁片固体废物产量日益增多,危害越来越严重。在高温加热、搅拌条件下,用强酸浸渍粉煤灰和易拉罐碎片,使粉煤灰和废旧铝、铁片资源化,制得废旧铝、铁片粉煤灰复合混凝剂,并用所制得的废旧铝、铁片粉煤灰复合混凝剂处理屠宰废水。在一定条件下,经混凝试验处理后的屠宰废水pH值在7左右,COD去除率为92．0％,SS去除率为98．7％,浊度去除率为98．4％,色度去除率为96．6％。结果表明,粉煤灰和废旧铝、铁片资源化和在废水处理中的应用具有可行性。     

Properties of several fly ash materials in relation to use as soil amendments

Fly ash samples from five power stations in Western Australia and Queensland, and two soils used for horticulture in Western Australia, were evaluated for a series of physical and chemical properties. Soils were comprised primarily of coarse sand-sized particles, whereas most of the fly ashes were primarily fine sand- and silt-sized particles. Hydraulic conductivities in the fly ashes were 105- to 248-fold slower than in the soils. The water-holding capacities of fly ashes at "field capacity" were three times higher than those of the soils. Extractable P in the fly ashes (except Tarong and Callide) were 20- to 88-fold higher than in the soils. The pH showed considerable variation among the different sources of fly ash, with samples from Muja being the most acidic (pH = 3.8; 1:5 in CaCl2 extract) and from Gladstone the most alkaline (pH = 9.9). The toxicity characteristic leaching procedure (TCLP) values indicate that the potential for release of trace elements from the fly ashes was well below regulatory levels. When applied at sufficient rates (e.g., to achieve 10% w/w in surface layers) to sandy soils, fly ash altered texture and increased water-holding capacity. Depending on the source of fly ash used, such amendments could also provide P and aid nutrient retention by increasing the phosphorus retention index (PRI) and/or cation exchange capacity (CEC). The considerable variability in physical and chemical properties among the fly ash samples evaluated in the present study supports the notion that field trials are essential to the future development of soil amendment strategies making use of any particular source of fly ash.     

Impact of Coal Ash from Electric Power Production on Changes in Water Quality1

ABSTRACT: Since 1972 the use of coal in the U.S. has increased much more rapidly than predicted, with much of this increased use by electric power generating plants. Residue after combustion consists primarily of inorganic materials, with the majority of chemical elements concentrated in fly ash as is sulfate. The concentration of sulfate in this ash fraction most affects the chemical and physical state of other elements when fly ash comes in contact with water by lowering the pH. In a properly constructed and operated ash settling basin, however, major water quality parameters of effluent may be improved as compared to source water. It is necessary, however, to consider the solubility or deposition in sediment of potentially toxic chemical elements from fly ash as these may affect reuse of this water, or as they may impact other parts of the aquatic environment.     

Leaching characteristics of solid wastes from thermal power plants of western Turkey and comparison of toxicity methodologies

Use of lignite in power generation has led to increasing environmental problems associated not only with gaseous emissions, but also with the disposal of ash residues. In particular, use of low quality coals with high ash content results in huge quantities of both fly and bottom ashes to be disposed of. A main problem related to coal ash disposal is the heavy metal content of the residue. In this regard, experimental results of numerous studies indicate that toxic trace metals may leach when fly and bottom ashes are in contact with water. In this study, fly and bottom ash samples obtained from thermal power plants, namely Yenikoy, Kemerkoy and Yatagan, located at the southwestern coast of Turkey, were subjected to toxicity tests such as the extraction (EP) and toxicity characteristic leaching (TCLP) procedures of the US Environmental Protection Agency (USEPA) and the so-called 'Method A' extraction procedure of the American Society of Testing and Material (ASTM). The geochemical composition of ash samples showed variations depending on the coal burned in the plants. Furthermore, the EP, TCLP and ASTM toxicity tests showed variations such that the ash samples were classified as 'toxic waste' based on EP and TCLP results whereas they were classified as 'non-toxic' based on ASTM results, indicating test results are pH dependent. When the extraction results were compared with the chemical composition of water samples obtained in the vicinity of the thermal power plants, it was found that the results obtained using the ASTM procedure cannot be used to predict subsurface contamination whereas the EP and TCLP procedures can be used.     

Economic comparison of recycling over-ordered fresh concrete: A case study approach

Recycling of construction material helps save the limited landfill space. Among various types of materials, concrete waste accounts of about 50% of the total waste generation. Current off-site practices for ready mixed concrete batching plant generate a significant quantity of fresh concrete waste through over-order from construction sites. The use of concrete reclaimer is one of the methods to reclaim these concrete wastes, which separates coarse aggregate, sand and cement from fresh concrete. Although there are some concrete producers in Hong Kong providing concrete reclaimers in their plants, they are only used to flush and dilute the cement slurry from the concrete, which will still be ultimately send all to dumping areas. The reluctance of most concrete producers in reclaiming aggregate from the concrete waste is due to its high cost of treatment and lack of space around the plant. Therefore, this paper puts forth a scheme of economical considerations in recycling over-ordered concrete by concrete reclaimer. A comparative study on costs and benefits between the current practices and the proposed recycling plan is examined. The study shows that the costs of the current practices in dumping over-ordered fresh concrete waste to landfill areas are double that of the proposed aggregate recycling plan. Therefore, the adoption of concrete reclaimer in recycling the over-ordered fresh concrete can provide a cost-effective method for the construction industry and help saving the environment.