Fly ash concrete subjected to thermal cyclic loads

The present study describes the behaviour of concrete as well as fly ash concrete when subjected to varying number of high temperature heating cycles. A Concrete mix (1:2.37:2.98) with 340 kg/m³ cement and w/cm ratio 0.45 was prepared. Cement was replaced by varying percentages (0%, 20%, 40%, 50% and 60%) of fly ash by weight of cement. The concrete was subjected to a constant temperature of 200°C for 7, 14, 21 and 28 heating cycles. One heating cycle corresponds to 8 h heating and subsequent cooling in 24 h. Subsequently the effect of temperature on the properties of the concrete was investigated and compared with that of the properties of unheated concrete. The compressive strength of plain as well as fly ash concrete increased when it was subjected to thermal cyclic loads. Moreover, the compressive strength increased with an increase in number of heating cycles. Thermal conductivity of concrete was found to decrease with an increase in the fly ash content.     

Fly ash reactivity: Extension and application of a shrinking core model and thermodynamic approach

In the present paper a theoretical study is presented on the dissolution (reaction) of pulverised powder coal fly ash. A shrinking core model is derived for hollow spheres that contain two regions (outer hull and inner region). The resulting analytical equations are applied to the dissolution experiments by Pietersen (Mat.Res.Soc.Symp.Proc., Vol. 178, Materials Research Society, 1990, p. 139; Ph.D. Thesis, Delft University of Technology, The Netherlands, 1993), yielding reaction rates at various temperatures and pH for two class F fly ashes. It is revealed that the available amount of reactive fly ash is proportional to the glass content of the fly ash, and that the reaction rate is proportional to this glass content as well. Moreover, it is concluded that the outer region is less reactive than the inner region, and that these reactivities are proportional to a power of the hydroxyl concentration. Subsequently, experimental data and model are used to assess the magnitude of inner and outer region. It seems that the outer hull of solid spheres and cenospheres are having the same thickness, about 2 m. Based on the observed trends a reaction mechanism is proposed which accounts for the glass content and composition of the fly ash (and that is applicable to slags as well). Finally, using the reaction product, thermodynamic properties of the studied fly ashes are derived: the free energy, enthalpy and entropy of reaction.     

Fluid transport in high volume fly ash mixtures with and without internal curing

The transport of fluid and ions in concrete mixtures is central to many aspects of concrete deterioration. As a result, transport properties are frequently measured as an indication of the durability that a concrete mixture may be expected to have. This paper is the second in a series investigating the performance of high volume fly ash (HVFA) mixtures with low water-to-cementitious ratios (w/cm) that are internally cured. While the first paper focused on strength and shrinkage, this paper presents the evaluation of the transport properties of these mixtures. Specifically, the paper presents results from: rapid chloride migration (RCM), rapid chloride penetration test (RCPT), apparent chloride diffusion coefficient, surface electrical resistivity, and water absorption. The test matrix consisted of mortar samples with two levels of class C fly ash replacement (40% and 60% by volume) with and without internal curing provided with pre-wetted lightweight fine aggregates (LWA). These mixtures are compared to plain ordinary portland cement (OPC) mortars. The results indicate that HVFA mixtures with and without internal curing provide benefits in terms of reduced transport coefficients compared to the OPC mixtures.     

Fly ash cenospheres as multifunctional supports of g-C3N4/N-TiO2 with enhanced visible-light photocatalytic activity and adsorption

The ternary composites of g-C₃N₄/N-TiO₂/FACs (FAC: Fly Ash Cenospheres) were synthesized by an in-situ hydrolysis method to improve the photocatalytic activity and their stability. When TiO₂ was anchored on FAC, it was easily to be separated from the aqueous solution and could be repeatedly utilized. In the present experiments, the degradation rate remained for more than 68% even after the composite reused for seven times. The band gap of g-C₃N₄/N-TiO₂/FAC was 2.75?eV, which might be owing to the synergistic effect between N-TiO₂ and g-C₃N₄. The composite of g-C₃N₄/N-TiO₂/FAC had an ideal activity of 72.2% under visible light illumination for 180?min. It was about 1.3 times of N-TiO₂/FAC and 3.5 times of g-C₃N₄. The synergistic effect of SiO₂, Fe₂O₃ and TiO₂ components resulted to the improvement of photocatalytic performance.     

Synthesis of zeolites NaA and analcime using rice husk ash as silica source without using organic template

Silica was extracted from Fajr rice husk ash (RHA) with approximately 97% purity, converted to sodium and potassium silicate solutions and used as silica source for the synthesis of NaA and analcime zeolites. NaOH and KOH without organic templates were used in the synthesis of NaA and analcime zeolite, respectively. Conventional heating and microwave-assisted hydrothermal were used in zeolite synthesis. Zeolites were characterized with XRD, SEM and IR spectroscopy. Heating time and Si/Al ratio were optimized for the synthesis of zeolites.     

MSW fly ash stabilized with coal ash for geotechnical application

The solidification and stabilization of municipal solid waste (MSW) fly ash for the purpose of minimizing the geo-environmental impact caused by toxic heavy metals as well as ensuring engineering safety (strength and soaking durability) are experimentally evaluated. The mixtures of MSW fly ash stabilized with cement and fluidized bed combustion coal fly ash (FCA) were used for unconfined compressive strength tests, leachate tests, and soaking tests. The behavior of soluble salts contained in the MSW fly ash significantly affects strength development, soaking durability, and the hardening reaction of the stabilized MSW fly ash mixtures. The cement stabilization of the MSW fly ash does not have enough effect on strength development and soaking durability. The addition of cement only contributes to the containment of heavy metals due to the high level of alkalinity. When using FCA as a stabilizing agent for MSW fly ash, the mixture exhibits high strength and durability. However, the Cd leachate cannot be prevented in the early stages of curing. Using a combination of cement and FCA as a MSW fly ash stabilizer can attain high strength, high soaking durability, and the containment of heavy metals. The stabilized MSW fly ash with cement and FCA can be practically applied to embankments.     

A warning against misuse of quarter-point elements

Quarter-point elements are used very frequently for fracture mechanics computations, because the quarter-point technique yields the required singular interpolation without any modification to existing software. This advantage is particularly significant for three-dimensional stress intensity factor computations because of the difficulty of implementing other techniques. However, in practical 3-D applications, the crack front is usually curved, and this note proves that a crack front distortion leads to a negative Jacobian in the region surrounding the crack front. The numerical difficulties to be expected depend on the aspect ratio of the elements.     

Laboratory compaction of fly ash and fly ash with cement additions

The use of power-industry wastes as a material for earthen structures depends on its compactibility. It has been confirmed that a fly ash/bottom ash mix compacted several times in Proctor's moulds are not representative. The relationship between dry density of solid particles and water content for re-used waste samples was determined. The re-compaction effect on grain-size distribution, density of solid particles, specific surface and sand equivalent of wastes was investigated. Tests were conducted on fly ash samples compacted by the Standard and Modified Proctor methods. Another aim of the paper was to investigate the influence of cement additions on the compactibility of a fly ash/bottom ash mix. Waste samples in the natural state and with different percentages of cement additions (2, 5 and 10%) were compacted by both impact compaction methods to obtain compactibility curves rhod(w). It was found that cement addition resulted in an increased rhod max value, while wopt decreased. Linear regression relationships for changes in compaction parameters after cement stabilisation are also given.     

Properties of geopolymer with seeded fly ash and rice husk bark ash

In the present work, compressive strength of inorganic polymers (geopolymers) produced of seeded fly ash and rice husk bark ash has been investigated. Different specimens made from a mixture of fly ash and rice husk bark ash in fine and coarse form were subjected to compressive strength tests at 7 and 28 days of curing. The curing regime was different: one set of the specimens were cured at room temperature until reaching to 7 and 28 days and the other sets were oven cured for 36 h at the range of 40-90 °C and then cured at room temperature until 7 and 28 days. The results indicate that in both 7 and 28 days regimes, the highest strengths are related to the specimens by SiO₂/Al₂O₃ ratio equals 2.99 cured at 80 °C. For these specimens, those contained finer fly ash particles show more compressive strength. Thermogravimetric analysis and Fourier transform infrared spectroscopy both also are in agreement with the obtained results from compressive strength tests. In addition, SEM micrographs of the specimens show that the finer the particle size of the utilized ashes, the denser the microstructure which confirms the results obtained by the strength tests.     

Child restraint system use and misuse in six states

This project addressed use and misuse of child restraint systems (CRS) in the nation. CRS use and critical misuse observations were collected in the Fall of 2002 for 5527 children less than 36 kg (80 pounds) in 4126 vehicles in six states: Arizona, Florida, Mississippi, Missouri, Pennsylvania, and Washington. Results showed that 62.3% of these children were restrained in a CRS; 25.9% were restrained in a safety belt (SB); and 11.8% were unrestrained. By weight class, CRS use was 97.1% for children less than 9 kg (20 pounds); 86.4% for children between 9 and 18 kg (20 and 40 pounds); 41.7% for children between 18 and 27 kg (40 and 60 pounds); and 10.9% for children between 27 and 36 kg (60 and 80 pounds). Overall critical CRS misuse was 72.6%. Most common critical misuses were loose harness straps and loose vehicle SB attachment to the CRS. Other types of CRS misuses were also observed and recorded in the study. Recommendations are provided for field observation techniques, periodic monitoring, and research for education and enforcement strategies.     

Dielectric properties of fly ash

This paper reports the dielectric properties of fly ash. The dielectric measurements were performed as a function of frequency and temperature. The sample of fly ash shows almost similar behaviour in the frequency and temperature range studied. The large value of dielectric constant in the typical frequency range is because of orientation polarization and tight binding force between the ions or atoms in the fly ash. The sample of fly ash is of great scientific and technological interest because of its high value of dielectric constant (10⁴).     

Glass-ceramic from sewage sludge ash

Glass-ceramic was produced by adding limestone to sewage sludge incinerated ash. Black glass was produced by melting a blended ash batch at 1450°C. For nucleation, this glass was reheated at 800°C for 1 h, and reheated at 1100°C for 2 h to form glass-ceramic. The main components of sewage sludge incinerated ash are SiO2 and Al2O3. Because small amounts of Fe2O3, sulfur and carbon are included, the addition of limestone alone can generate the crystal nucleant, FeS, to form anorthite (CaO·Al2O3·2SiO2) crystal. The glass-ceramic showed superior characteristics of high strength and acid resistance for use in construction materials. This revised version was published online in November 2006 with corrections to the Cover Date.     

Cement-stabilized fly ash base courses

Various demonstration projects have been carried out in The Netherlands with cement-stabilized fly ash as a base course. Usually these courses were made of 100 parts by mass of fly ash; 10 parts by mass of cement; 20 to 30 parts by mass of water. However, the projects were not quite successful since delamination was observed, and long-term strength, after a period of six years of performance, appears to be much smaller than expected on the basis of preliminary laboratory research. A model for pozzolanic reaction of fly ash recently developed by Fraay and Bijen pointed out that the reactivity of fly ash is influenced greatly by the pH value of the pore water. A pH of at least 13 is required to initiate fly ash pozzolanic reaction in a Portland cement environment. Pore water extraction measurements showed that the pH of cement-stabilized fly ash often has a substantially lower value. In this high-volume fly ash application the effect of the acidity of fly ash is much greater than in ordinary concrete with cement replaced by fly ash up to 30%. By addition of NaOH and/or sodium silicate to the mixing water, the pH value can be increased above the ‘threshold’ value.

Tests were carried out with different types of class-F fly ashes and with different NaOH concentrations in the mixing water. The results show an increase in compressive strength of up to 300% depending on the type of fly ash, and a substantial decrease in the rate of water absorption.     

Kinetics of fly ash geopolymerization

Based on the wet chemical analysis, we measured and modeled the kinetics of reactions between fly ash and KOH at various temperatures and water-to-solid mass ratios (W/S). We find that three consecutive rate-limiting processes control reaction progress: (1) dissolution or alteration of the glass phase in the fly ash, (2) classical Fick diffusion through a surface layer, and (3) diffusive transport through a more complex gel structure (interstitial gel). This sequence of processes is independent of W/S (0.35–40), temperature (22–75 °C), and KOH concentration (5–10 M). The relative contribution of each process to the overall reaction progress changes with experimental conditions. Only if and when the third process is rate limiting, a fly ash geopolymer forms and develops mechanical strength (sufficiently low W/S ratio provided). The rate of reaction progress decreases significantly, due to slow transport of reacting species to the surface of the glass particles.     

Biotoxicity evaluation of fly ash and bottom ash from different municipal solid waste incinerators

Different types of municipal solid waste incinerator (MSWI) fly and bottom ash were extracted by TCLP and PBET procedures. The biotoxicity of the leachate of fly ash and bottom ash was evaluated by Vibrio fischeri light inhibition test. The results indicate the following: (1) The optimal solid/liquid ratio was 1:100 for PBET extraction because it had the highest Pb and Cu extractable mass from MSWI fly ash. (2) The extractable metal mass from both fly ash and bottom ash by PBET procedure was significantly higher than that by TCLP procedure. (3) The metal concentrations of fly ash leachate from a fluidized bed incinerator was lower than that from mass-burning and mass-burning combined with rotary kiln incinerator. (4) The TCLP and PBET leachate from all MSWI fly ash samples showed biotoxicity. Even though bottom ash is regarded as a non-hazardous material, its TCLP and PBET leachate also showed biotoxicity. The pH significantly influenced the biotoxicity of leachate.     

粉煤灰吸附剂的研究进展

对国内外粉煤灰作吸附剂的情况进行了综述,阐述了粉煤灰吸附剂在处理废水和废气中对不同污染物的吸附情况,归纳总结了粉煤灰吸附剂的发展历史,对粉煤灰吸附剂中存在的问题给予了解析,并为粉煤灰吸附剂提出了建设性意见.为粉煤灰吸附剂发展方向和开发粉煤灰的资源化利用提供了理论依据.     

粉煤灰抑制ASR的机制

本文研究粉煤灰对ASR的抑制作用.测定了不同粉煤灰掺量砂浆棒膨胀率、水泥石中Ca(OH)2含量以及主要水化产物C-S-H凝胶的微观结构和化学组成,并对粉煤灰抑制ASR的机制进行探讨.结果表明:随粉煤灰掺量增加,其对ASR具有显著的抑制效果.水泥石中Ca(OH)2含量不断减少,二次反应生成大量低n(Ca)/n(Si)的C-S-H凝胶.随n(Ca)/n(Si)降低,C-S-H凝胶中固溶了大量的Al,凝胶的固碱能力增强,从而使混凝土孔溶液中的有效碱大大减少,减轻了ASR程度.凝胶的固碱能力增强除了因n(Ca)/n(Si)降低,使C-S-H凝胶的固碱能力增强之外.还可能因为Al3 代替Si4 形成[AlO4]5-四面体,由于Al3 比Si4 少一价,引起电价不平衡,系统带负电,因此碱离子更容易进入C-S-H凝胶中,减少了碱与活性集料反应的机会.     

Seat belt misuse among children transported in belt-positioning booster seats

Objective

Observe and report seat belt use among children transported in belt-positioning booster seats.

Design

We conducted a cross-sectional, observational survey of children transported in motor vehicles between 2006 and 2007. While drivers completed a survey reporting the child's age, weight and gender, and the driver's age, gender, race, income, education, and relationship to the child; a child passenger safety technician recorded vehicle seating location, restraint type, and use of the car safety seat harness or seat belt as appropriate for the child.

Setting

Twenty-five fast food restaurants and discount department stores throughout Indiana.

Participants

A convenience sample of drivers transporting children younger than 16 years.

Main exposure

Seat belt use among children transported in belt-positioning booster seats.

Outcome measure

Seat belt misuse.

Results

Overall, 1446 drivers participated, 2287 children were observed with 564 children in belt-positioning booster seats. At least one seat belt misuse was observed for 64.8% of the children transported. Common misuses were the shoulder belt being placed over the booster seat armrest (35.8%); shoulder belt not at mid-shoulder position (28.5%), seat belt was too loose (24.5%), and the shoulder belt was either behind the child's back (9.1%) or under their arm (10.0%).

Conclusion

There is a high frequency of seat belt misuses among children transported in booster seats. Advice to parents on appropriate car seat selection, and encouragement to parents to supervise seat belt use may decrease misuse.