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Research on Surface Modification of Fly Ash and Its Application in Prepartion of Blended Cement

Author: ZhangPingPing
Tutor: YuQiJun
School: South China University of Technology
Course: Materials Science
Keywords: Fly ash De-hydrated phase Bonding Blended cement Durability
CLC: X773
Type: Master's thesis
Year: 2011
Downloads: 201
Quote: 0
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Abstract


Fly ash is a by-product of coal fire power plant, which is regarded as a man-made pozzolanic material. Utilization of fly ash as a supplementary cementitious material in blended cement manufacture is one of the most effective reusing ways. Fairly poor bonding between fly ash particles and hydration products is observed due to low pozzolanic activity and the pozzolanic activity mainly takes place in late ages. As a result, the properties of blended cements with fly ash are much poorer than Portland cement, and the addition of fly ash is also limited to avoid un-acceptable properties of blended cement. It is reported that the pozzolanic activity of fly ash is largely depended on its particle size and the condition where it placed. Effects of particle size and pore solution on the activity of fly ash were closely investigated. Surface cementitious properties of coarse fly ash were improved by rehydration of dehydration products on fly ash particles, and high performance blended cements with treated fly ash were also prepared. An effective way was provided by which the coarse fly ash with high carbon content can be used in blended cements manufacture. The research will be very useful to utilization of fly ash and energy and resources conservation, emission reducing, and sustainable of cement industry.Hydration process of fly ash in different solution was determined. The results indicated that fly ash-water and fly ash- saturated Ca（OH）2 solution remain un-hydrated. SiO2 and Al ion concentrations in pore solution of Fly ash-CaO-water paste are very low, as a result, a very slight hydration of fly ash was observed and the hydration degree of fly ash-CaO-water paste is 8.3% after 28 days curing. The hydration process of fly ash-5%CaO-alkali solutions increases significantly, and hydration mainly occurs in 14 days. Fly ash-5%CaO-0.2 mol/L NaOH solution paste has a hydration degree of 19.2% at 28 days.Effect of size fraction on composition and pozzolanic activity of high calcium fly ash was investigated. As the particle size gets finer, the SiO2 and Fe2O3 content of fly ash fractions decreases remarkably, while CaO, SO3 and alkali content increases significantly. Although the carbon content of fine fly ash fractions is much higher than coarse fractions, the pozzolanic activity and activity index of fine fly ash fractions is high than coarse fly ash fractions. For instance, both 3 and 28 days activity indexes of coarse fly ash fractions are lower than 70%, while the fly ash fraction D50=1.45μm shows a 3 days activity index of 85% and a 28 days activity index higher than 100%. Thus fly ash fractions with mean size under 8μm are recommended as a supplementary cementitious material for Portland cement-based materials. Effects of calcination temperature and hydration degree of fly ash were also considered in this study. The results show that during the calcination at 750°C, C-S-H gel is mainly transformed into a new nesosilicate, which is similar to a less crystallineβ-C2S. It is expected that surface cementitious properties of coarse fly ash were improved significantly due to rehydration of generated C2S. The formation of melilite from hydration products is also noticed at 900°C, however, this will not contribute to rehydration of calcined fly ash. The calcined fly ash presents highest activity index when the hydration degree of fly ash is about 5%.Gap-graded blended cements were prepared by mixing fine blast furnace slag fraction （<8μm）, middle clinker fraction （832μm） and coarse fly ash fraction （>32μm） homogeneously. Properties of blended cements with calcined or original coarse fly ash were compared. The results shown that blended cement pastes present high packing density and low water requirement for normal consistency due to a proper particle size distribution. The hydration processes of clinker and blast furnace slag were accelerated after addition of calcined fly ash. Rehydration of new generatedβ-C2S on the surface of coarse fly ash leads to a better bonding between coarse fly ash particles and hydration products. As a result, both early and late mechanical properties of gap-graded blended cements containing 25% cement clinker and 39% calcined coarse fly ash are higher than those of 100% Portland cements. In addition, blended cements with calcined fly ash also have superior durability than blended cements with original fly ash.

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CLC: > Environmental science, safety science > Processing and comprehensive utilization of waste > Power of industrial waste treatment and comprehensive utilization > Electric Power Industry
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