Some studies on the reaction between fly ash and lime

The reaction between fly ash (FA) and lime is extensively exploited for the manufacture of building bricks, blocks and aggregates. To get a better idea of this reaction, FA from different sources were mixed in different ratios with lime and compacted. The compacts were treated both by ordinary water and hydrothermal curing to promote lime bearing hydrate bond formation e.g. CaO- SiO₂-H₂O (C-S-H), CaO-Al₂O₃-H₂O (C-A-H) etc. The decrease in free lime content in these compacts was measured as a function of curing time and curing process. This drop in this content was correlated to the chemical composition of the fly ashes. The mathematical relationships between free lime remaining in the compacts after its maximum decrease in concentration and lime binding modulus (a ratio between the amount of added lime and the total amount of lime binding constituents present in FA) for both types of curing were developed. Further, the rate of decrease in free CaO content under both types of curing conditions was compared from kinetic study. From this study the orders of the reactions and rate constants were found out.     

Effect of heat treatment on properties of steam cured fly ash-lime compacts

Four different varieties of class F fly ashes, collected from different sources from the state of West Bengal (India) were mixed with lime in 9 : 1 wt ratio, followed by compaction of the mixes. The compacts were subjected to steam curing to develop an optimum strength by the reaction between fly ash and lime. The steam cured compacts were heated at different elevated temperatures and free lime content, compressive strength, bulk density and water absorption tendency of these compacts were measured and FTIR spectral changes were studied as a function of the heating temperatures. Kinetics of thermal dehydration of the compacts was also studied from thermogravimetric measurements under non-isothermal condition to ascertain the order of dehydration process and the associated activation energy.     

Strength of lime–fly ash compacts using different curing techniques and gypsum additive

Lime–fly ash mixtures are exploited for the manufacture of fly ash bricks finding applications in load bearing masonry. Lime–pozzolana reactions take place at a slow pace under ambient temperature conditions and hence very long curing durations are required to achieve meaningful strength values. The present investigation examines the improvements in strength development in lime–fly ash compacts through low temperature steam curing and use of additives like gypsum. Results of density–strength–moulding water content relationships, influence of lime–fly ash ratio, steam curing and role of gypsum on strength development, and characteristics of compacted lime–fly ash–gypsum bricks have been discussed. The test results reveal that (a) strength increases with increase in density irrespective of lime content, type of curing and moulding water content, (b) optimum lime–fly ash ratio yielding maximum strength is about 0.75 in the normal curing conditions, (c) 24 h of steam curing (at 80°C) is sufficient to achieve nearly possible maximum strength, (d) optimum gypsum content yielding maximum compressive strength is at 2%, (e) with gypsum additive it is possible to obtain lime–fly ash bricks or blocks having sufficient strength (>10 MPa) at 28 days of normal wet burlap curing.     

Properties of lime stabilised steam-cured blocks for masonry

The paper addresses certain issues pertaining to the technology of lime-stabilised steam-cured blocks used for masonry construction. Properties of lime-stabilised steam-cured blocks using expansive soils and tank bed soils have been examined. Influence of parameters like steam curing period, lime content and fly ash content on wet strength of blocks is studied. Steam curing of lime stabilised blocks at 80°C for about 20 hours at atmospheric pressure leads to considerably higher strengths when compared with curing under wet cloth at ambient temperatures. Clay-fly ash fractions of the mix control the optimum lime content yielding maximum strength. Long-term strength behaviour of steam-cured blocks has been monitored. The results indicate a favourable lime-clay ratio for stable long-term strength. A small-scale steam cured block production system has been designed and implemented to construct a load bearing masonry structure, thus demonstrating the potential of steam-cured block as a material for masonry construction.     

Mechanical characteristics of clay structural ceramics containing coal fly ash

In this work, the mechanical characterization of ceramic products processed from red clay with different amounts of added coal fly ash was investigated. Coal fly ash produced by power plants is a waste material that constitutes an alternative source of minerals for the production of traditional building ceramics, as it is a complex mixture of several oxides such as SiO₂, Al₂O₃, CaO, Fe₂O₃, Na₂O, TiO₂, which are usually present in the composition of such ceramics. A powder technology and firing route was followed for the processing of the clay and coal fly ash based ceramics. Different proportions of waste (10, 25 and 50%, by weight) were added to red clay, and then the mixed powders were pressed to form compacts that were fired at a selected temperature in the range 850–1,150°C. The effects of waste content and of heating conditions on the microstructure and mechanical characteristics of the obtained materials were investigated. The density, porosity, water absorption, flexural strength, hardness and fracture toughness of the produced materials were evaluated. A comparison was made between the properties of the produced ceramics with those of traditional ceramic materials used in construction, e.g. floor or wall tiles, and it was observed that the clay based products with coal fly ash additions may be used in similar applications.     

Microconcrete with partial replacement of Portland cement by fly ash and hydrated lime addition

The reduction in Portland cement consumption means lower CO₂ emissions. Partial replacement of Portland cement by pozzolans such as fly ash has its limitations due to the quantity of calcium hydroxide generated in the mix. In this work we have studied the contribution of the addition of hydrated lime to Portland cement + fly ash systems. We have also studied several levels of cement replacement, ranging from 15% to 75%.The best mechanical results were obtained replacing 50% of Portland cement by the same amount of fly ash plus the addition of hydrated lime (20% respect to the amount of fly ash). In these systems, an acid-base self-neutralization of the matrix has occurred through a pozzolanic reaction of fly ash with portlandite liberated in the hydration of Portland cement and the added hydrated lime. It has been identified for these mixtures a significant amount of hydrated gehlenite, typical reaction product from rich-alumina pozzolans.     

Mechanical performance of low cement reactive powder concrete (LCRPC)

The possibility of producing a reactive powder concrete (RPC) with low cement content was aimed in the scope of this study. Cement was replaced with class-C fly ash (FA) up to 60% for this purpose. Three different curing conditions (standard water curing, autoclave curing and steam curing) were applied to specimens. Two series of RPC composites were prepared with bauxite and granite aggregates. Mechanical properties such as compressive strength, splitting tensile strength, flexural strength and fracture energy of composites were investigated. Test results showed that, compressive strength of 200 MPa can be reached with low cement by using high-volume fly ash. Thermally treated specimens showed compressive strength beyond 250 MPa and high volume fly ash RPC have superior performance. Furthermore, compressive strength values reached up to 400 MPa with external pressure application during setting and hardening stages.     

Mechanical performance of low cement reactive powder concrete (LCRPC)

The possibility of producing a reactive powder concrete (RPC) with low cement content was aimed in the scope of this study. Cement was replaced with class-C fly ash (FA) up to 60% for this purpose. Three different curing conditions (standard water curing, autoclave curing and steam curing) were applied to specimens. Two series of RPC composites were prepared with bauxite and granite aggregates. Mechanical properties such as compressive strength, splitting tensile strength, flexural strength and fracture energy of composites were investigated. Test results showed that, compressive strength of 200 MPa can be reached with low cement by using high-volume fly ash. Thermally treated specimens showed compressive strength beyond 250 MPa and high volume fly ash RPC have superior performance. Furthermore, compressive strength values reached up to 400 MPa with external pressure application during setting and hardening stages.     

Hydration of high-volume fly ash cement pastes

The hydration processes of high-volume fly ash cement paste were investigated by examining the non-evaporable water content, the CH content, the pH of pore solution and the fraction of reacted fly ash, curing at either 20°C or elevated temperatures after an initial curing at 20°C. The replacement percentage levels of fly ash were 40%, 50% and 60% by weight, respectively. The results revealed that the non-evaporable water content in high-volume fly ash cement pastes does not develop as plain cement pastes does, so it may be improper to apply the non-evaporable water content to evaluate the hydration process in high-volume fly ash cement matrix. The reduction in CH content increases with the progressing of hydration process and varies linearly with the logarithm of curing age. The addition of 3.0% of Na₂SO₄ could accelerate the pozzolanic reaction of fly ash at early ages. At 20°C, the pH of pore solution of high-volume fly ash cement paste was reduced to a great extent at early ages and it continued to decline at later ages due to the inclusion of large amount of fly ashes. At elevated temperatures, however, this trend was not found. The fraction of reacted fly ash directly reflects the pozzolanic reactivity of fly ash both at normal and elevated temperatures. There is some inherent correlation between the reduction in CH content, the pH of pore solution and the fraction of reacted fly ash. For specified matrix, the consumption of CH and the pH of pore solutions change linearly with the increase of the fraction of reacted fly ash.     

制革污泥及其固化块的微观结构研究

采用扫描电镜-能谱分析(SEM-EDS)对制革污泥及其固化块的微观结构进行研究。结果表明每千克污泥掺加石灰、粉煤灰和煤渣分别为0.12、0.02和0.08kg,经过6、8、10、15、20d自然条件养护后,污泥固化块污泥致密性及抗压强度均随养护时间的增加而增强。污泥固化块经过20d养护后通过5000倍扫描电镜观察,发现污泥固化块内部出现明显的针状结构。污泥固化块的抗压强度主要来源于石灰和活性硅酸盐料(粉煤灰、煤渣)与水反应生成的钙矾石,养护时间的延长也是制革污泥固化块抗压强度增大的主要原因。     

Prefabricated building elements based on FGD gypsum and ashes from coal-fired electric generating plants

Mixtures of fly ash, bottom ash and Flue Gas Desulphurized (FGD) gypsum, all solid wastes from coalfired electric generating plants, can be combined with lime and 10% of water to produce a damp powder which can be moulded at a pressure of 20–40 MPa and then steam-cured in less than 1 day at 35–80°C. The resulting building materials-in the form of bricks, blocks or slabs-produced by this Pressure Forming (PF) process, are stronger and sounder than the corresponding materials produced by a slip casting (SC) process. The physical and mechanical properties of the materials manufactured through the PF process are based on the reaction of amorphous silica and alumina of the ash with lime or lime and gypsum respectively, so that calcium silicate hydrate and ettringite are produced. When the temperature of the steam curing is as low as 35°C, the hardened material is sound in the air, but it swells and is quickly destroyed by the action of water. This effect can be ascribed to the formation of ill-crystallized ettringite. On the other hand, with thermal treatment at higher temperatures (60–80°C), the material is stronger and sound even in the presence of water in service. The well-crystallized ettringite fibers, favoured by the higher temperature of the steam curing treatment, are considered to be responsible for the better mechanical performances and the lower change in length. In general, the physical and mechanical properties of the ash-gypsym-lime cementitious system are better than those of the traditional clay-based ceramic products manufactured at temperatures as high as 1000°C. Therefore, this process based on steam curing at 60–80°C appears to be very useful for both the re-utilization of solid wastes and the saving of energy in the production of building materials.     

Suppression of solidification of calcium-rich incinerator fly ash during thermal treatment for decomposition/detoxification of dioxins

Dioxins and dioxin-like compounds released from municipal and industrial solid waste incinerators have been a serious problem from the viewpoint of environmental pollution control. Since these compounds are concentrated especially on fly ash, supplemental treatment systems to decompose/detoxify them are required after collecting the fly ash either by a bag filter or an electrostatic precipitator. The present work is aimed at developing a heat treatment technique for fly ash, which contains a large amount of calcium (Ca) derived from hydrated lime, at a temperature higher than 500°C by adding chemical additives to prevent the solidification of Ca-rich fly ash. As calcium hydroxychloride (CaClOH) in the Ca-rich fly ash was found to cause solidification of fly ash at high temperatures, sodium hydroxide, mullite and coal fly ash were added as additives prior to the heat treatment. As a result, the additives studied in the present work are effective for decomposing CaClOH and therefore suppressing the solidification of fly ash, and yet they promoted the decomposition/detoxification of dioxins.     

Influence of flue gas SO2 on the toxicity of heavy metals in municipal solid waste incinerator fly ash after accelerated carbonation stabilization

The influence of CO₂ content and SO₂ presence on the leaching toxicity of heavy metals in municipal solid waste incinerator (MSWI) fly ash was studied by examining the carbonation reaction of MSWI fly ash with different combinations of simulated incineration flue gases. Compared with raw ash, the leaching solution pH of carbonated ash decreased by almost 1 unit and the leaching concentrations of heavy metals were generally lower, with that of Pb decreasing from 19.45 mg/L (raw ash) to 4.08 mg/L (1# carbonated ash). The presence of SO₂ in the incineration flue gas increased the leaching concentrations of heavy metals from the fly ash to different extents after the carbonation stabilization reaction. The pH of the leaching solution was the main factor influencing the leaching concentrations of heavy metals. The increase in buffer capacity with the pH of carbonated ash caused an increase in heavy metal stability after the carbonation reaction. Accelerated carbonation stabilization of MSWI fly ash could reduce its long-term leaching concentrations (toxicity) of Cu, Pb, Se, and Zn. The leaching concentrations of heavy metals from carbonated ash also likely had better long-term stability than those from raw ash. The presence of SO₂ in the incineration flue gas increased the proportion of exchangeable state species of heavy metals; slightly increased the long-term leaching toxicity of Cu, Pb, Se, and Zn; and reduced the long-term stability of these metals in the fly ash after the carbonation reaction.     

Preparation of aluminium-fly ash particulate composite by powder metallurgy technique

Aluminium-fly ash mixtures containing different weight percentages of fly ash were prepared and compacted at pressures from 138–414 MPa. The compacts prepared at 414 MPa were sintered in nitrogen atmosphere at 600, 625 and 645°C, respectively. The time of sintering ranged from 0.5–6 h. The densification parameter and the green densities of the compacts were determined as a function of compacting pressure and fly ash weight per cent. Density, hardness and strength of the sintered compacts were determined as a function of weight per cent of fly ash particles. Volume changes during sintering of green compacts were also evaluated as a function of increasing fly ash weight per cent. Microscopic studies of green and sintered compacts were done to study the effectiveness of sintering. Green and sintered density of the compacts were found to decrease with increasing weight per cents of fly ash. Sintering results in slight decrease in density and increase in volume of green compacts within the range investigated. Strength of the sintered compacts decreased with increasing weight per cent of fly ash under the present experimental conditions; however, the hardness was found to increase slightly up to 10 wt% fly ash, beyond which it decreased. This revised version was published online in November 2006 with corrections to the Cover Date.     

基于饱和面干骨料的粉煤灰混凝土配合比设计及碳化性能研究

本文以设计试验研究基于饱和面干骨料的粉煤灰混凝土为主,并通过试验数据分析得出结论：相同水胶比的情况下,随着粉煤灰掺量的增加,混凝土7d、28d强度逐渐下降,拌和物坍落度也平稳下降;自然养护条件和标准养护条件对碳化深度有一定的影响;饱和面干骨料粉煤灰混凝土的强度测定期应当延迟。     

改性磷石膏的强度与孔结构的研究

本工作研究了矿物掺合料(矿渣、粉煤灰)和激发剂(熟石灰和水泥)对磷石膏强度的影响,并且探索了水泥对磷石膏耐水系数的影响。此外对磷石膏改性处理后的微观形貌和孔结构进行了分析。研究结果表明:矿渣和粉煤灰均能提高磷石膏的强度,且矿渣对磷石膏强度的增强作用更明显;但两者对磷石膏耐水性的增强作用并不明显,矿渣掺量过多时会由于延迟钙矾石的形成而导致石膏开裂。水泥和熟石灰作为激发剂时可以增强磷石膏的强度,熟石灰的增强作用更明显。水泥对磷石膏的耐水性能有一定的增强作用。磷石膏的水胶比、养护龄期和矿物掺合料可以改变其孔隙率,但不会改变其孔径分布;粉煤灰可以提高石膏的孔隙率,并且改变其孔径分布;水泥会降低石膏的孔隙率并改变其孔结构。     

Effects of different curing regimes on the compressive strength properties of self compacting concrete incorporating fly ash and silica fume

This paper presents the effect of air curing, water curing and steam curing on the compressive strength of Self Compacting Concrete (SCC). For experimental study, SCC is produced with using silica fume (SF) instead of cement by weight, by the ratios of 5%, 10% and 15%, and fly ash (FA) with the ratios of 25%, 40% and 55%. It is observed that mineral admixtures have positive effects on the self settlement properties. The highest compressive strength was observed in the concrete specimens with using 15% SF and for 28 days water curing. Air curing caused compressive strength losses in all groups. Relative strengths of concretes with mineral admixtures were determined higher than concretes without admixtures at steam curing conditions.     

Influence of 15 and 80 nano-SiO2 particles addition on mechanical and physical properties of ternary blended concrete incorporating rice husk ash

This study demonstrates the effects of SiO₂ nanoparticles as additives with two different sizes of 15 and 80?nm on compressive strength and porosity of rice husk ash (RHA) blended concrete. Up to 20% of ordinary Portland cement (OPC) was replaced by RHA with average particle size of 5 micron. Also, SiO₂ nanoparticles were added to the above mixture at four different weight percentages of 0.5, 1.0, 1.5 and 2.0 and cured in lime solution. The results indicated that compressive strength of Portland cement–nano SiO₂–rice husk ash (PC–NS–RHA) ternary blended concrete was considerably increased. Moreover, the total amount of porosity decreased to a minimum with respect to the control concrete. This improvement was observed at all the curing ages and replacement levels, but there was a gain in the optimal point with 20% of RHA plus 2% of 80?nm SiO₂ particles at 90 days of curing.     

Alkali-activation potential of biomass-coal co-fired fly ash

Co-fired fly ash, derived from the co-combustion of coal and biomass, is examined as a potential precursor for geopolymers. Compared to a coal fly ash, two co-fired fly ashes have a lower vitreous content and higher carbon content, primarily due to differing combustion processing variables. As a result, binders produced with these co-fired fly ashes have reduced reaction potential. Nevertheless, compressive strengths are generally highest for all ashes activated with solutions with a molar ratio of SiO₂/(Na₂O + K₂O) = 1, and these mixes reach the highest extent of reaction among those studied. Activation with sodium hydroxide solution forms zeolitic phases for all ashes. The thermal and dilatometric behavior of the coal and co-fired fly ash geopolymers is similar between equivalent mix designs. These results indicate that co-fired fly ashes can be viably used to form alkali-activated geopolymers, which is a new beneficial end-use for these emerging waste materials.     

富氧型高活性吸收剂制备及同时脱硫脱硝特性研究

以粉煤灰、工业石灰、氧化性添加剂为原料,制备了具有良好同时脱硫脱硝性能的富氧型高活性吸收剂.研究了添加剂加入量、Ca/(S+N)比等重要因素对烟气脱硫脱硝效果的影响;通过SEM分析和BET数据分析,探讨了吸收剂的物理特性与活性关联性.该高活性吸收剂具有99%的脱硫和86%的脱硝特性.