Showing papers on "Fly ash published in 2007"

Workability and strength of coarse high calcium fly ash geopolymer

Abstract: In this paper, the basic properties viz., workability and strength of geopolymer mortar made from coarse lignite high calcium fly ash were investigated. The geopolymer was activated with sodium hydroxide (NaOH), sodium silicate and heat. The results revealed that the workable flow of geopolymer mortar was in the range of 110 ± 5%–135 ± 5% and was dependent on the ratio by mass of sodium silicate to NaOH and the concentration of NaOH. The obtained compressive strength was in the range of 10–65 MPa. The optimum sodium silicate to NaOH ratio to produce high strength geopolymer was 0.67–1.0. The concentration variation of NaOH between 10 M and 20 M was found to have a small effect on the strength. The geopolymer samples with high strength were obtained with the following practices: the delay time after moulding and before subjecting the sample to heat was 1 h and the optimum curing temperature in the oven was 75 °C with the curing duration of not less than two days.

Evaluation of bagasse ash as supplementary cementitious material

Abstract: The utilization of waste materials in concrete manufacture provides a satisfactory solution to some of the environmental concerns and problems associated with waste management. Agro wastes such as rice husk ash, wheat straw ash, hazel nutshell and sugarcane bagasse ash are used as pozzolanic materials for the development of blended cements. Few studies have been reported on the use of bagasse ash (BA) as partial cement replacement material in respect of cement mortars. In this study, the effects of BA content as partial replacement of cement on physical and mechanical properties of hardened concrete are reported. The properties of concrete investigated include compressive strength, splitting tensile strength, water absorption, permeability characteristics, chloride diffusion and resistance to chloride ion penetration. The test results indicate that BA is an effective mineral admixture, with 20% as optimal replacement ratio of cement.

Durability of alkali-activated fly ash cementitious materials

Abstract: The study described in the present paper addresses the durability of alkali-activated fly ash (AAFA) cement under different conditions: specifically, cement performance is measured in a number of aggressive environments (deionized water, ASTM seawater, sodium sulphate and acidic solutions) and with respect to alkali–silica reaction-induced expansion. The chief parameters studied are: weight loss, compressive strength, variations in volume, presence of the products of degradation and microstructural changes. The results show that AAFA pastes perform satisfactorily in aggressive environments and that degradation in these materials is distinctly different from such processes in OPC paste. These mortars are also compliant with the 16-day expansion limit stipulated in ASTM standard C1260-94 on potential alkali–silica reactivity.

Use of High Volumes of Fly Ash to Improve ECC Mechanical Properties and Material Greenness

Abstract: Environmental sustainability considerations are taken into account in this report on high-performance fiber-reinforced cementitious composite (HPFRCC) development. Featuring high tensile ductility with high volumes of fly ash (HVFA) replacement of cement (up to 85% by weight), unique HPFRCC members are engineered cementitious composites (ECC). While the material design process sees application of micromechanics in many of its aspects, this study emphasizes how fly ash content alters material microstructure and mechanical properties. While they incorporate high recycled fly ash volumes, HVFA ECCs are shown in experimental results to be able to retain an approximately 2 to 3% long-term tensile ductility. Significantly, there is reduction in both free drying shrinkage and crack width with a fly ash amount increase, though which HVFA ECC structures' long term durability may benefit. That HVFA ECCs' fiber/matrix interface frictional bond increase is responsible for tight crack width is indicated through micromechanics analysis. Use of industrial waste stream material instead of cement reduces environmental impact and achieves more saturated multiple cracking, meaning a robustness improvement is shown in HVFA ECCs.

Engineered Cementitious Composites with High-Volume Fly Ash

Abstract: Engineered cementitious composites (ECCs) are a breed of high-performance fiber-reinforced cementitious composites (HPFRCC) with significant strain-hardening behavior under tension. As ECCs impart ductility and durability to the structure, the high cement usage in the mixture causes environmental and economical impacts. In this paper, the mechanical performance of ECCs incorporating high volume fly ash and bottom ash is reported. Emphasis is placed on the influence of fly ash content on the key micromechanics properties relevant to composite ductility. It is revealed that a high volume faction of fly ash tends to reduce the polyvinyl alcohol (PVA)fiber/matrix interface bond and matrix toughness in favor of attaining high tensile strain capacity. The limit of cement substitution with ash is constrained by compressive strength development.

Influence of Fly Ash as Cement Replacement on the Properties of Recycled Aggregate Concrete

Abstract: The use of high percentages of recycled aggregates in concrete would usually worsen the concrete properties. This paper tries to address the deficiency of the use of recycled aggregates by systematically presenting results on the influence of incorporating Class F fly ash on concrete properties. In this study, two series of concrete mixtures were prepared with water-to-binder (W/B) ratios of 0.45 and 0.55. The recycled aggregate was used as 0, 20, 50, and 100% by weight replacements of natural aggregate. In addition, fly ash was used as 0, 25, and 35% by weight replacements of cement. The results showed that the compressive strengths, tensile strengths, and static modulus of elasticity values of the concrete at all ages decreased as the recycled aggregate and the fly ash contents increased. Further, an increase in the recycled aggregate content decreased the resistance to chloride ion penetration and increased the drying shrinkage and creep of concrete. Nevertheless, the use of fly ash as a substitute for...

Attenuated Total Reflectance Fourier Transform Infrared Analysis of Fly Ash Geopolymer Gel Aging

Abstract: Structural changes in fly ash geopolymers activated with different sodium hydroxide and silicate concentrations are investigated using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy over a period of 200 days. A strong correlation is found between the concentration of silicate monomer in the activating solution and the position of the main Si−O−T stretching band in the FTIR spectrum, which gives an indication of the relative changes in the gel Si/Al ratio. The FTIR spectra of geopolymer samples with activating solution concentrations of up to 1.2 M SiO2 indicate that an Al-rich gel forms before the final gel composition is reached. The time required for the system to reach a steady gel composition depends on the silicate activating solution concentration and speciation. Geopolymers activated with solutions containing predominantly high-order silicate species rapidly reach a steady gel composition without first forming an Al-rich gel. A minimum silicate monomer concentration...

In Situ ATR-FTIR Study of the Early Stages of Fly Ash Geopolymer Gel Formation

Abstract: The kinetics of geopolymer formation are monitored using a novel in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopic technique. Reaction rates are determined from the intensity variation of the bands related to the geopolymer gel network and the unreacted fly ash particles. Comparison with deuterated geopolymer samples provides critical information regarding peak assignments. An initial induction (lag) period is observed to occur for hydroxide-activated geopolymers, followed by gel evolution according to an approximately linear reaction profile. The length of the lag period is reduced by increasing the concentration of NaOH. An increase in the rate of network formation also occurs with increasing NaOH concentration up to a maximum point, beyond which an increased NaOH concentration leads to a reduced rate of network formation. This trend is attributed to the competing effects of increased alkalinity and stronger ion pairing with an increase in NaOH concentration. In situ analysis also shows that the rate of fly ash dissolution is similar for all moderate- to high-alkali geopolymer slurries, which is attributed to the very highly water-deficient nature of these systems and is contrary to predictions from classical glass dissolution chemistry. This provides for the first time detailed kinetic information describing fly ash geopolymer formation kinetics.

Influence of Fly Ash, Lime, and Polyester Fibers on Compaction and Strength Properties of Expansive Soil

Abstract: An experimental program was undertaken to study the effects of polyester fiber inclusions and lime stabilization on the geotechnical characteristics of fly ash-soil mixtures. An Indian fly ash was mixed with expansive soil in different proportions. The geotechnical characteristics of fly ash-soil specimens, lime-soil specimens and lime-fly ash-soil specimens mixed with different proportions of randomly oriented fibers were investigated. Lime and fly ash were added to an expansive soil at ranges of 1–10% and 1–20%, respectively. Test specimens were subjected to compaction tests, unconfined compression tests and split tensile strength tests. Specimens were cured for 7, 14, and 28 days after which they were tested for unconfined compression tests and split tensile tests. Based on optimum values obtained for lime and fly ash, tests were conducted on test specimens prepared from fly ash-expansive soil- lime-fiber mixture after 28 days of curing. Samples were tested with 0, 0.5, 1.0, 1.5, and 2% plain and crimped polyester fibers by dry weight. Based on the favorable results obtained, it can be concluded that the expansive soil can be successfully stabilized by the combined action of fibers, lime, and fly ash.

The influence of calcium content on the structure and thermal performance of fly ash based geopolymers

Abstract: The influence of calcium and its content on the structure formation, hardening, and performance of fly ash based geopolymeric binder was the objective of our investigation. Calcium hydroxide was added to fly ash in different amounts. Since it is known that the formed structure determines certain properties of the material, the coherence between different types and various ratios of the reaction products on thermal properties such as strength after thermal treatment up to 1,100 °C, thermal resistance under load, creep in compression, and axial dilation were investigated. The results were compared to the type and composition of the reaction product, which was detected, for example, by 29Si NMR spectroscopy and X-ray diffraction. Along with calcium containing zeolitic phases, the calcium built C–S–H-phases using the silicon from the fly ash, both of which crystallize or convert into new phases at elevated temperatures.

Opc-fly ash cementitious systems: study of gel binders produced during alkaline hydration

Abstract: In the present paper, experiments were performed on blended cements containing 30% Portland cement clinker and 70% fly ash. The powdery material was mixed with deionised water for “normal” hydration, and with two different alkaline solutions for “normal” alkaline activation. The mechanical strength developed by this highly blended cement differed significantly depending on the hydrating solution used. XRD, FTIR and 29Si MAS-NMR characterisation studies were conducted to obtain information on the complex structural nature of the hardened matrices, which in all cases consisted of a mixture of amorphous gels (C-S-H + N-A-S-H gel). These highly blended cements are able to comply with the specifications defined in the European Standard EN-197-1:2000.