Microstructural study of carbonated cement-solidified synthetic heavy metal waste

Synthetic wastes have been widely employed to help elucidate the complex interactions between real wastes and hydraulic binders during solidification. In this work, a laboratory produced metal waste mixed with Portland cement and immediately carbonated it using an accelerated method. The microstructures of carbonated and non-carbonated control samples were distinct despite both being dominated by unusually large phenograins derived from the waste. In the carbonated sample waste phenograins remained unaltered, whereas cement grains were largely decalcified. As a consequence of decalcification, observable porosity was significantly reduced by the formation of precipitated carbonates.     

Modelling the effects of waste components on cement hydration

Ordinary Portland Cement (OPC) is often used for the solidification/stabilization (S/S) of waste containing heavy metals and salts. These waste components will precipitate in the form of insoluble compounds on to unreacted cement clinker grains preventing further hydration. In this study the long term effects of the presence of contaminants in solidified waste is examined by numerically simulating cement hydration after precipitation of metal salts on the surface of cement grains. A cement hydration model was extended in order to describe pore water composition and the effects of cement grain coating. Calculations were made and the strength development predicted by the model was found to agree qualitatively with experimental results found in literature. The complete model is useful in predicting the strength and leaching resistance of solidified products and developing solidification recipes based on cement.     

Study of the treatment effectiveness of a solidification/stabilization process for waste bearing heavy metals

The sludge from a steel processing unit bearing zinc, lead, iron, and manganese was solidified with ordinary Portland cement. The waste was stabilized in the specimens with a waste/binder ratio range of 0.16–4.0. On the basis of the available leaching and unconfined compressive strength, the performance of the solidified/stabilized waste was compared for different numbers of curing days. It was found that curing up to 28 days resulted in a performance improvement, as shown by less leaching of heavy metals and the increased unconfined compressive strength of the specimen. The treatment effectiveness of the solidification/stabilization process was assessed for the metals Pb, Zn, Fe, and Mn, and was found to be 89%, 95%, 74%, and 90%, respectively, for an optimum ratio of 4.0 after 28 days of curing.     

Stabilization/solidification of MSW incineration residues from facilities with different air pollution control systems. Durability of matrices versus carbonation

This paper discusses the stabilisation/solidification process with Portland cement applied to municipal solid waste incineration residues. Two types of residues were considered: fly ash (FA) produced in an electrostatic precipitator, and air pollution control (APC) residues from a semi-dry scrubber process. Cement pastes with different percentages of FA and APC residues were characterised according to their physical properties, the effect of the hydration products and their leaching behaviour. Portland pastes prepared with APC residues showed a rapid setting velocity in comparison with setting time for those pastes substituted with FA residues. Portland cement hydration was retarded in FA pastes. Leaching test results showed that heavy metals (such as Zn, Pb and Cd) and sulphates are immobilised within the paste, whereas chlorides are only partially retained. The carbonation process increases the leachability of S04(2-) and heavy metals such as Zn and Cr.     

Evaluation of the homogeneity of stabilised/solidified wastes by a video imaging technique

A video imaging technique is described for the homogeneity assessment of wastes that have been treated by stabilisation/solidification (S/S). The method incorporates a fluorescent tracer into the S/S reagent. A test “waste” consisting of an artificial soil was stabilised/solidified with varying degrees of mixing using Portland cement as the S/S reagent. The tracer distribution was monitored with a video camera, and the cement distribution was determined by chemical analysis for calcium. Measurement of the homogeneity of the products by the video imaging technique gave results comparable to those obtained by the chemical analysis. The results warrant use of the video imaging technique in field applications since it is easier, cheaper and faster than traditional chemical methods.     

Treatment of organic-contaminated industrial wastes using cement-based stabilization/solidification—I. Microstructural analysis of cement-organic interactions

The major deficiencies in cement-based stabilization/solidification (S/S) processes are their inability to treat inorganic wastes contaminated with organic material or organic wastes. In general, organic compounds are poorly retained in a cement matrix and frequently have a detrimental, poorly understood, effect upon cement hydration and strength development. These interactions need to be understood as fully as possible, however, if S/S processes are to be developed in ways which will assure the long-term integrity of the resultant products.The work presented in this paper investigates some fundamental aspects of the interactions of two organic compounds, 3-chlorophenol and chloronaphthalene, with a cement matrix. Phenolic compounds have previously been shown to have a detrimental effect upon the macrostructural properties of ordinary Portland cement (OPC), for example, the strength, setting rate and leachability (Montgomery et al. 1988). Microstructural studies in this work have shown that 3-chlorophenol inhibits the hydration of tricalcium silicate (C₃S in cement chemists' notation), with up to 90% of the C₃S remaining after 28 days for highly dosed 3-chlorophenol/OPC samples. The formation of ettringite was found to be increased by the presence of 3-chlorophenol and its conversion to monosulphate inhibited. Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) analysis of the samples showed that 3-chlorophenol crystallized in the cement matrix to form discrete crystals containing calcium and phenol. In contrast, chloronaphthalene had no observable effect on hydration reactions. In a subsequent paper, detailed studies will be presented showing how these deleterious effects can be minimized by the use of organophilic clays as a pre-solidification adsorbent.     

An alternative method for the treatment of waste produced at a dye and a metal-plating industry using natural and/or waste materials.

The aim of this study was to develop cost-effective, appropriate solidification technologies for treating hazardous industrial wastes that are currently disposed of in ways that may threaten the quality of local groundwater. One major objective was to use materials other than cement, and preferably materials that are themselves wastes, as the solidification additives, namely using wastes to treat wastes or locally available natural material. This research examines the cement-based and lime-based stabilization/solidification (S/S) techniques applied for waste generated at a metal-plating industry and a dye industry. For the lime-based S/S process the following binder mixtures were used: cement kiln dust/ lime, bentonite/lime and gypsum/lime. For the cement-based S/S process three binder mixtures were used: cement kiln dust/cement, bentonite/cement and gypsum/cement. The leachability of the wastes was evaluated using the toxicity characteristic leaching procedure. The applicability and optimum weight ratio of the binder mixtures were estimated using the unconfined compressive strength test. The optimum ratio mixtures were mixed with waste samples in different ratios and cured for 28 days in order to find the S/S products with the highest strength and lowest leachability at the same time. The results of this work showed that the cement-and lime-based S/S process, using cement kiln dust and bentonite as additives can be effectively used in order to treat industrial waste.     

Use of glazed ceramic waste as additive in mortar and the mathematical modelling of its strength

This study investigated the reusability of waste material from the tile manufacturing industry as an alternative material to natural pozzolan trass. Yield strength values of mortar made from Portland cement (CEM 142.5), were measured by adding glazed ceramic waste and trass at various weight ratios (5 to 40%). The test results proved that the strength values at 2, 7, and 28 days gave good results for concentrations of waste materials less than 5-10% in the cement. A decrease in strength was observed at higher concentrations. Mathematical modelling results showed a logarithmic correlation between the mortar strength and weight fraction of cement.     

Cement solidification of simulated off-gas condensates from vitrification of low-level nuclear waste solutions

Solidification in a cementitious matrix is a viable alternative for low-level nuclear waste management; it is therefore important to understand the behavior and properties of such wasteforms. We have examined the cementitious solidification of simulated off-gas waste streams resulting from the vitrification of low-level nuclear waste. Different possible methods for scrubbing the off-gasses from a vitrifier give rise to three possible types of waste compositions: acidic (from aqueous dissolution of volatile NOx and POx carried over from the vitrifier), basic (from neutralizing the former with sodium hydroxide), and fully carbonated (arising from a direct-combustion vitrifier). Six binder compositions were tested in which ordinary Portland cement was replaced at different proportions by fly ash and/or ground granulated blast furnace slag. A high solution to binder ratio of 1l/1 kg was used to minimize the volume of the wasteform and 10% attapulgite clay was added to all mixes to ensure that the fresh mix did not segregate prior to setting. The 28-day compressive strengths decreased when a high proportion of cement was replaced with fly ash, but were increased significantly when the cement was replaced with slag. The heats of hydration at early age for the various solids compositions decreased when cement was replaced with either fly ash or slag; however, for the fly ash mix the low heat was also associated with a significant decrease in compressive strength. High curing temperature (60 degrees C) or the use of extra-fine slag did not significantly affect the compressive strength. Recommendations for choice of binder formulations and treatment of off-gas condensates are discussed.     

High durability cementitious material with mineral admixtures and carbonation curing

Nuclear waste repositories need highly durable cementitious materials to function for over thousands of years while resisting leaching and degradation. The durability of cementitious material can be effectively improved by reducing permeability and by changing cement hydrates to a less soluble matrix. This paper describes the properties of carbonated new cementitious materials containing belite-rich cement and gamma-2CaO.SiO2 as main components. In addition, the long-term leaching properties are investigated and compared with ordinary Portland cement by using a predictive leaching model.     

Immobilisation of heavy metal in cement-based solidification/stabilisation: a review

Heavy metal-bearing waste usually needs solidification/stabilization (s/s) prior to landfill to lower the leaching rate. Cement is the most adaptable binder currently available for the immobilisation of heavy metals. The selection of cements and operating parameters depends upon an understanding of chemistry of the system. This paper discusses interactions of heavy metals and cement phases in the solidification/stabilisation process. It provides a clarification of heavy metal effects on cement hydration. According to the decomposition rate of minerals, heavy metals accelerate the hydration of tricalcium silicate (C3S) and Portland cement, although they retard the precipitation of portlandite due to the reduction of pH resulted from hydrolyses of heavy metal ions. The chemical mechanism relevant to the accelerating effect of heavy metals is considered to be H+ attacks on cement phases and the precipitation of calcium heavy metal double hydroxides, which consumes calcium ions and then promotes the decomposition of C3S. In this work, molecular models of calcium silicate hydrate gel are presented based on the examination of 29Si solid-state magic angle spinning/nuclear magnetic resonance (MAS/NMR). This paper also reviews immobilisation mechanisms of heavy metals in hydrated cement matrices, focusing on the sorption, precipitation and chemical incorporation of cement hydration products. It is concluded that further research on the phase development during cement hydration in the presence of heavy metals and thermodynamic modelling is needed to improve effectiveness of cement-based s/s and extend this waste management technique.     

Possibility of using waste tire rubber and fly ash with Portland cement as construction materials

The growing amount of waste rubber produced from used tires has resulted in an environmental problem. Recycling waste tires has been widely studied for the last 20 years in applications such as asphalt pavement, waterproofing systems and membrane liners. The aim of this study is to evaluate the feasibility of utilizing fly ash and rubber waste with Portland cement as a composite material for masonry applications. Class C fly ash and waste automobile tires in three different sizes were used with Portland cement. Compressive and flexural strength, dry unit weight and water absorption tests were performed on the composite specimens containing waste tire rubber. The compressive strength decreased by increasing the rubber content while increased by increasing the fly ash content for all curing periods. This trend is slightly influenced by particle size. For flexural strength, the specimens with waste tire rubber showed higher values than the control mix probably due to the effect of rubber fibers. The dry unit weight of all specimens decreased with increasing rubber content, which can be explained by the low specific gravity of rubber particles. Water absorption decreased slightly with the increase in rubber particles size. These composite materials containing 10% Portland cement, 70% and 60% fly ash and 20% and 30% tire rubber particles have sufficient strength for masonry applications.     

Use of waste ash from palm oil industry in concrete

Palm oil fuel ash (POFA), a by-product from the palm oil industry, is disposed of as waste in landfills. In this study, POFA was utilized as a pozzolan in concrete. The original size POFA (termed OP) was ground until the median particle sizes were 15.9 microm (termed MP) and 7.4 microm (termed SP). Portland cement Type I was replaced by OP, MP, and SP of 10%, 20%, 30%, and 40% by weight of binder. The properties of concrete, such as setting time, compressive strength, and expansion due to magnesium sulfate attack were investigated. The results revealed that the use of POFA in concretes caused delay in both initial and final setting times, depending on the fineness and degree of replacement of POFA. The compressive strength of concrete containing OP was much lower than that of Portland cement Type I concrete. Thus, OP is not suitable to be used as a pozzolanic material in concrete. However, the replacement of Portland cement Type I by 10% of MP and 20% of SP gave the compressive strengths of concrete at 90 days higher than that of concrete made from Portland cement Type I. After being immersed in 5% of magnesium sulfate solution for 364 days, the concrete bar mixed with 30% of SP had the same expansion level as that of the concrete bar made from Portland cement Type V. The above results suggest that ground POFA is an excellent pozzolanic material and can be used as a cement replacement in concrete. It is recommended that the optimum replacement levels of Portland cement Type I by MP and SP are 20% and 30%, respectively.     

Assessment of the leaching of metallic elements in the technology of solidification in aqueous solution

Results are presented of experiments performed to optimize the solidification/stabilization system for metallic elements in aqueous solution. This system involves mixing cement and a solution of metallic elements in a conventional mixer: the paste thus obtained is transferred drop by drop into a recipient filled with an aqueous solution of NaOH at 20% by weight, in which it solidifies immediately. The separate use of chloride solutions of Li+, Cr3+, Pb2+ and Zn2+ makes it possible to obtain granules displaying various levels of compressive strength. Three different inertization matrices were used in the experiments, the first consisting solely of Portland cement, the second of Portland cement and a superplasticizer additive, and the third of Portland cement partially replaced with silica-fume and superplasticizer. The results of the tests performed showed a very low level of leaching into the alkaline solidification solution for Cr3+, the quantity leached being under 2% as against higher levels for the other metallic elements. For all the considered elements, the best results were obtained by using silica-fume in the inertization matrix.     

Heavy metal leaching from hydroxide, sulphide and silicate stabilised/solidified wastes

A synthetic, mixed-metal solution has been stabilised by treatment with sodium hydroxide, sodium sulphide, and sodium silicate, respectively. The three stabilised filter cakes have subsequently been solidified using additions of ordinary Portland cement and pulverised fuel ash (PFA) which are typically used in UK solidification operations. Both the stabilised filter cakes and the solidified wastes have been subjected to an equilibrium extraction test, a modified TCLP test, and a series of single-extraction, batch leach tests using an increasingly acidic leachant. Metal release was found to be primarily dependent on the pH of the leachate. Under mildly acidic conditions, the percentages leached from the stabilised and the stabilised/solidified wastes were comparable for most metals. A high-volume fraction of these solidified wastes is occupied by the stabilised filter cake. When they are broken up and tested in single-extraction leach tests, the primary effect of the cementitious additives is to increase the pH of the leachate so that most heavy metals remain insoluble. When tested under acidic leachate conditions, copper, lead, and mercury were found to be particularly well retained within sodium sulphide stabilised wastes. Under similar test conditions, cadmium was leached at very low levels from the sodium silicate stabilised waste.     

Reusing fly ash in glass fibre reinforced cement: a new generation of high-quality GRC composites

New composite materials based on an alkali-resistant glass-fibre reinforced cement (AR-GRC) system are being developed by using fly ash (FA) produced at coal thermoelectric power plants, and fluid catalytic cracking catalyst residue (FC3R) from the petrol industry as cement replacement materials. These wastes are reactive from the pozzolanic viewpoint, and modify the nature and the microstructure of the cement matrix when a part of the Portland cement is replaced in the formulation of GRC. Several microstructural and mechanical aspects are being studied for AR-GRC systems. The behaviour of composites exposed to ageing shows that the pozzolanic activity of the ground FA added in high amounts and its mixture with the FC3R increase the flexural strength and no evidences of strength decay are observed. Additionally, the fibres due to the high alkalinity of the cementing matrix can be deteriorated. Fibres in the control (only Portland cement) and FC3R containing composites were attacked, whereas composites with FA and their mixture with FC3R show that the fibres have not been attacked, due to the pozzolanic activity of replacing materials that reduce the calcium hydroxide content in the cementing matrix.     

Evaluation of waste slags produced by zinc industry in bituminous hot mixtures

Owing to the large amount of waste slags produced by zinc industry, it has become necessary to recycle it in some areas. Road construction has significant potential for the use of waste materials because more material is always needed. In this study, the engineering behaviour of asphalt concrete was investigated using mineral aggregates with waste slag, which is a by-product of the zinc–lead production industry. The asphalt concrete tested in this study was fabricated using 25, 50, 75 and 100 % mixing ratios instead of the conventional fine mineral aggregate (11, 22, 33 and 44 % rate of total aggregate mixture) to determine the possibility of using slags in the binder course of bituminous hot mixtures. The asphalt concretes, made of waste slags and conventional asphalt concrete, were evaluated in terms of their fundamental engineering properties such as Marshall stability, flow, Marshall quotient (MQ), bulk specific gravity, air voids and voids filled with bitumen in the total mix characteristics. The results indicate that the addition of waste slag as mineral aggregate improves the engineering characteristic performance and that it can be used in bituminous hot mixtures. In addition, principal component analyses were applied to examine the significance of each Marshall parameter, and a regression model was developed to estimate the MQ value using effective parameters.     

Use of flue gas desulphurisation (FGD) waste and rejected fly ash in waste stabilization/solidification systems

Stabilization/solidification (S/S) processes have been used as the final treatment step for hazardous wastes prior to land disposal. Fly ash is a by-product of coal-fired power generation; a significant proportion of this material is low-grade, reject material (rFA) that is unsuitable as a cement replacement due to its high carbon content and large particle size (>45 microm). Flue gas desulphurization (FGD) sludge is a by-product from the air pollution control systems used in coal-fired power plants. The objective of this work was to investigate the performance of S/S waste binder systems containing these two waste materials (rFA and FGD). Strength tests show that cement-based waste forms with rFA and FGD replacement were suitable for disposal in landfills. The addition of an appropriate quantity of Ca(OH)2 and FGD reduces the deleterious effect of heavy metals on strength development. Results of TCLP testing and the progressive TCLP test show that cement-rFA-Ca(OH)2 systems with a range of FGD additions can form an effective S/S binder. The Leachability Index indicates that cement-based waste forms with rFA replacement were effective in reducing the mobility of heavy metals.     

Manufacture of artificial aggregate using MSWI bottom ash

This paper reports the results of an investigation on material recovery by stabilization/solidification of bottom ash coming from a municipal solid waste incineration plant. Stabilization/solidification was carried out to produce artificial aggregate in a rotary plate granulator by adding hydraulic binders based on cement, lime and coal fly ash. Different mixes were tested in which the bottom ash content ranged between 60% and 90%. To avoid undesirable swelling in hardened products, the ash was previously milled and then granulated at room temperature. The granules were tested to assess their suitability to be used as artificial aggregate through the measurement of the following properties: density, water absorption capacity, compressive strength and heavy metals release upon leaching. It was demonstrated that the granules can be classified as lightweight aggregate with mechanical strength strongly dependent on the type of binder. Concrete mixes were prepared with the granulated artificial aggregate and tested for in-service performance, proving to be suitable for the manufacture of standard concrete blocks in all the cases investigated.     

Cement clinker: An environmental sink for residues from hazardous waste treatment in cement kilns

About 70% of all of the liquid and solid hazardous wastes commercially incinerated in the United States is being burned in cement kilns. The process inevitably results in residues, primarily heavy metals, entering the clinker and waste dusts (cement kiln dust, CKD) produced by these kilns. The effects of this trend on the nature and chemical composition of cement, actual and future, are discussed. The wastes burned by cement kilns are expected to increasingly have higher levels of heavy metals per Btu. In general, the effects are very simple to describe but have as yet unknown consequences. The present American Society for Testing and Materials (ASTM) standard does not effectively control hazardous waste burning residues in Portland Cement.The regulatory and economic pressures on CKD disposal suggest that much of it, and its heavy metal residues, will, in time, end up in the clinker and the resultant cement. The end point to the trend is the ability to make cement that passes the performance specifications while containing high levels of heavy metals. The only other alternative is to maximize the levels of heavy metals in the CKD, minimize the amount of CKD, and dispose of its as a hazardous waste.It is recommended that an effort to correlate heavy metal levels in clinker with adverse effects be undertaken, a new standard for cement containing hazardous and other waste residuals be developed, and labeling be required.