Ja Stegemann

Acid corrosion resistance of different cementing materials

Abstract This study has investigated the corrosion of different hardened cementing materials, such as Portland cement (PC), alkali-activated blast furnace slag cement (ASC), lime–fly ash (LFA) blend and high alumina cement with gypsum and lime (HAC), in pH 3 nitric acid, pH 3 acetic acid, and pH 5 acetic acid solutions. Experimental results indicated that PC pastes were corroded faster than ASC and LFA pastes, and pastes consisting of HAC were quickly dissolved in these acid solutions. PC pastes are more porous than ASC pastes but much less porous than LFA pastes. Thus, the corrosion of hardened cementing materials in acid solutions depends on the nature of the hydration products rather than the porosity of the hardened cementing materials: calcium silicate hydrate (C-S-H) with a low C/S ratio is the main hydration product in ASC and LFA pastes, while C-S-H with a high C/S ratio and Ca(OH)2 are the main hydration products in hardened PC pastes. Ca(OH)2 decomposes as the pH drops below 12, and C-S-H decalcifies as the pH decreases, and decomposes for pH values below 9. The mixture of high alumina cement, gypsum, and lime results in the formation of an ettringite-based matrix, which was dissolved very quickly in these acid solutions.

Effect of Supplementary Cementing Materials on the Specific Conductivity of Pore Solution and Its Implications on the Rapid Chloride Permeability Test (AASHTO T277 and ASTM C1202) Results

The American Association of States Highway and Transportation Officials (AASHTO) Test Method T277--Rapid Determination of the Chloride Permeability of Concrete and the American Society of Testing and Materials (ASTM) C1202--Electrical Indication of Concretes Ability to Resist Chloride Ion Penetration have specified a rapid test method to rank the chloride penetration resistance of various concretes by applying a potential of 60 V DC to a concrete specimen and measuring the charge passed through the specimen during 6 hours of testing. The method is essentially a measurement of electrical conductivity of concrete, which depends on both the pore structure and the chemistry of the pore solution. Analyses based on published results have indicated that the replacement of portland cement with supplementary cementing materials, such as silica fume, can reduce the electrical conductivity of concrete more than 90% because of the change in pore solution composition in the concrete. Chemical composition of pore solution has little to do with the transport of chloride ions in the concrete; thus, it is not correct to use passed charge to rank the chloride penetration resistance of concrete made with supplementary cementing materials.

A proposed protocol for evaluation of solidified wastes

Solidification technologies are potentially useful for improving the chemical and physical properties of hazardous wastes to the extent that they are suitable for less expensive disposal or even utilization. Unfortunately, in most jurisdictions worldwide, there is no mechanism for reclassifying a treated, previously hazardous waste, as non-hazardous. In response to the need for such a mechanism, the Wastewater Technology Centre has proposed a protocol of test methods for cement-based solidified wastes. The suggested test methods examine partitioning of contaminants as a result of their chemical speciation, and potential for slow release of contaminants, based on the mobility of the contaminants in the solidified waste matrix, and the durability of the matrix. Most of the suggested tests are standards from the fields of hazardous and radioactive wastes, some of which have been evaluated in a cooperative study with vendors of solidification processes, initiated by Environment Canada. Based on the performance of a solidified product in the tests, it is considered for four utilization and disposal scenarios unrestricted utilization, controlled utilization, segregated landfill and sanitary landfill. The protocol represents a first attempt to develop a management tool for solidified wastes that accounts for their physical and leaching characteristics, in the context of different disposal scenarios.

Lysimeter washing of MSW incinerator bottom ash

Stockpiled municipal waste incinerator bottom ash is frequently considered for utilization as a construction material. Two 360 kg lysimeter experiments were conducted to study percolation washing of contaminants from stockpiled MSW bottom ash. One lysimeter was leached with a concentrated sodium hydroxide solution, as a possible pre-treatment for improvement of the bottom ash characteristics prior to utilization, while the other was leached using distilled water. The lysimeter leachate was analysed, and at the end of the 2-year leaching period, the bottom ash from each lysimeter was subjected to several laboratory tests to assess the effect of the treatments. The laboratory tests showed that distilled water leachability of both treated ashes was an order of magnitude lower than that of fresh ash, but long-term contaminant leachability under acidic conditions had not changed. Although alkaline washing clearly resulted in greater contaminant removal than did distilled water washing, the chemical properties of the alkaline-leached bottom ash were not significantly different from those of the water-leached ash.

Screening tests for assessing treatability of inorganic industrial wastes by stabilisation/solidification with cement

Stabilisation/solidification with cementitious or pozzolanic binders (S/S) is an option for reducing leachability of contaminants from residual, predominantly inorganic, industrial wastes and contaminated soils before disposal or reuse. Treatment by S/S is complicated by the fact that the presence of impurities, such as the contaminants and bulk matrix components present in industrial wastes, can have deleterious effects on cements. Therefore, careful laboratory development and testing of S/S formulations are required prior to full-scale application, to avoid technology failures, including problems with handling and contaminant retention. An understanding of cement chemistry and contaminant immobilisation mechanisms has been used to propose a series of test methods and performance thresholds for use in efficient evaluation of the treatability of industrial wastes by S/S, and optimising S/S formulations: measurement of stabilised/solidified product workability, bleeding and setting time (for flowable mixtures) or Proctor compaction (for compactable mixtures), together with unconfined compressive strength, leachability in a batch extraction with distilled water, and hydraulic conductivity.

Stabilization/solidification of petroleum drill cuttings: Leaching studies

This work explores the effectiveness of Portland cement (CEM I), with the addition of high carbon fly ash (HCFA), as a novel binder, for the improvement of leachability-related properties of stabilized/solidified (s/s) petroleum drill cuttings. A factorial design experiment was adopted to investigate the effects of waste-to-binder ratio, HCFA addition, and curing time on leachate pH, acid neutralization capacity (ANC), and metal, chloride and hydrocarbon leaching. The leachate pH and ANC of all products suggested successful formation of a calcium-silicate-hydrate-based matrix with good resistance to acid attack, and little detrimental effect from drill cuttings addition. Leaching of amphoteric metals was significantly affected by pH, which was a function of other studied factors. All studied factors also affected leaching of chloride and hydrocarbons. CEM I, without HCFA addition, was more effective in immobilizing chlorides, but the overall chloride immobilization was poor in all runs. HCFA addition significantly reduced the leaching of hydrocarbons. Comparison of milligram of contaminant leached per kilogram of drill cuttings from the s/s products and untreated drill cuttings provided clear evidence of hydrocarbon and chloride immobilization. This work shows that HCFA improved the immobilization of organic contaminants and may represent an inexpensive binder for stabilization/solidification of organic wastes.

Prediction of leachate pH for cement paste containing pure metal compounds

Neural network models, to predict the leachate pH for single batch extraction leaching tests conducted on Portland cement pastes containing pure compounds, were constructed using existing data from the literature. The models were able to represent the known non-linear dependency of pH on acid addition, and were used to show that Cu increases, and Zn and NO3- decrease, the leachate pH for addition of 8 meq acid/g dry cement (to achieve a mid-alkaline pH). Ba, Cd, Cr(III), Ni, Pb, Cl- and OH- had no detectable effect on the acid neutralisation capacity (ANC) of the cement pastes in the concentration ranges investigated. The laboratory where testing was conducted was found to be an important predictive variable, which acted as a surrogate variable for laboratory specific variables that were not adequately reported in the literature, such as cement characteristics, sample preparation details, and leaching test and pH measurement details. This work has shown that development of good empirical predictive models for solidified product leachate pH is feasible, and is limited only by the availability of data.

Stabilization/solidification of petroleum drill cuttings.

A systematic treatability study was conducted for the treatment of drill cuttings, a waste generated during petroleum exploration and production, by stabilization/solidification with Portland cement (CEM I), with the addition of high carbon power plant fly ash (HCFA), an industrial by-product, as a novel sorbent for organic contaminants. A factorial design experiment was adopted to investigate the effects of waste-to-binder ratio, binder formulation, and curing time on response variables including unconfined compressive strength (UCS), hydraulic conductivity, porosity, leachate pH, and acid neutralization capacity (ANC) of the s/s products. Results show that all factors had significant effects on the properties of the s/s products. Drill cuttings and HCFA addition both reduced UCS, but HCFA improved hydraulic conductivity, relative to CEM I only s/s products. Drill cuttings addition had little effect on the ANC of products prepared with CEM I only, and improved that of products containing HCFA. Management options assessment based on performance criteria adapted from regulatory and other guidance suggests that the s/s products could find application as controlled low-strength materials, landfill liner, and landfill daily cover. This work demonstrates how a systematic treatability study can be used to develop a s/s operating window for the management of a particular waste type.

Summary of an investigation of test methods for solidified waste evaluation

Abstract A study was undertaken by Environment Canada (EC), in conjuction with the United States Environmental Protection Agency (USEPA), Alberta Environment, and 15 industrial participants involved in developing or marketing solidification technology, to develop and validate 16 laboratory test methods for evaluating the physical and chemical properties of solidified wastes. Environment Canada and USEPA provided the 15 industrial participants with 5 untreated hazardous wastes. The industrial participants applied their proprietary processes to the wastes and returned the solidified products to four laboratories in Canada and the United States for testing. Seven physical tests, five leaching tests, and four micromorphological characterization methods were applied to the solidified products. Although the reproducibilities of the different methods were found to range from excellent to only fair, this study showed that all of the methods were sufficiently reproducible to be valuable tools for evaluating the properties of a wide variety of solidified wastes. The results of this study show that the majority of solidified products are strong, durable materials of intermediate permeability that immobilize heavy metal contaminants effectively, while providing little containment of organic compounds.

Prediction of unconfined compressive strength of cement paste with pure metal compound additions

Abstract Neural network analysis was used to construct models of unconfined compressive strength (UCS) as a function of mix composition using existing data from literature studies of pure compound additions to Portland cement paste. The models were able to represent the known nonlinear dependency of UCS on age and water content, and generalised from the literature data to find relationships between UCS and contaminant concentrations, resulting in the following ranking of the UCS values predicted for addition of the contaminants, on an equimolar basis: at 7 days, Cl≈Cr(III)>NO3−≈Cd>control>Zn≥Ni>Pb>Cu≫Ba; at 28 days, Cl>Cr(III)>NO3−≈control≥Zn≥Cd>Ni>Pb>Cu≫Ba. Application of the best neural network to other data suggested that Cs is a retarder and Cr(VI) has no effect. No trends could be discerned for Hg, K, Mn, Na and SO42−. The root-mean-square error for the best neural network seems to be an estimate of the interlaboratory error for UCS.