Role of microwave radiation in curing the fly ash geopolymer

Fly ash geopolymer requires rather long heat curing to obtain reasonable strength development at an early age. However, the long heat curing period limits the application of the fly ash geopolymer. High strength development and a reduction in heat curing duration have been considered for energy saving. Therefore, this research proposed a process using 90-W microwave radiation for 5 min followed by conventional heat curing for high-calcium fly ash geopolymer. Results showed that the compressive strengths of geopolymer with microwave radiation followed by conventional heat curing were comparable to those of the control cured at 65 °C for 24 h. Microwave radiation gave the enhanced densification. In addition, SEM images showed that the gels formed on the fly ash particles owing to the promoted dissolution of amorphous phases from fly ash. This method accelerated the geopolymerization and gave the high compressive strength comparable to the conventional curing.     

Synthesis of polypropylene fiber/high-calcium fly ash geopolymer with outdoor heat exposure

Solar energy is an important source of renewable and sustainable energy. Thailand is near the equator and thus experiences hot weather throughout the year. The average maximum temperature is 35 °C and can reach 40 °C in the summer time. This outdoor heat exposure (OHE) was, therefore, used for the curing of a polypropylene (PP) fiber fiber-reinforced high-calcium fly ash geopolymer composite, in order to reduce energy consumption. Fly ash is an abundant solid waste generated from the coal-power generation process. In this research, a high-calcium fly ash was used as a source material for the geopolymer synthesis. PP fiber was also incorporated in the composites to improve tensile characteristics and control crack development. The results show that the incorporation of PP fiber in composites led to improved tensile strength, crack control, and resistance to acid solution. OHE could thus be used as an energy source for the heat curing of high-calcium fly ash PP-fiber geopolymers, resulting in a strong matrix.     

Fire-resistant geopolymer bricks synthesized from high-calcium fly ash with outdoor heat exposure

This research proposed an alternative utilization of high-calcium fly ash to produce geopolymer bricks for fire-resistant applications. Outdoor heat exposure (OHE) was applied to cure geopolymer mortar. The temperature was up to 40 °C. Geopolymer brick was created with a 30-day compressive strength of 47 MPa via OHE curing for 3 days. The brick experienced a low weight loss after the firing test, which indicated its fire-resistant property. For the flame test, the maximum temperature on the opposite side of the brick from the flame was lower than 380 °C, with no observable cracks, complying with the fire-test requirement. Therefore, high-calcium fly ash geopolymer cured with OHE is suitable for use as a fire-resistant material. In addition, outdoor heat exposure is a promising renewable means to cure geopolymer.     

Reduction of metal leaching in brown coal fly ash using geopolymers

Current regulations classify fly ash as a prescribed waste and prohibit its disposal in regular landfill. Treatment of the fly ash can reduce the leach rate of metals, and allow it to be disposed in less prescribed landfill. A geopolymer matrix was investigated as a potential stabilisation method for brown coal fly ash. Precipitator fly ash was obtained from electrostatic precipitators and leached fly ash was collected from ash disposal ponds, and leaching tests were conducted on both types of geopolymer stabilised fly ashes. The ratio of fly ash to geopolymer was varied to determine the effects of different compositions on leaching rates. Fourteen metals and heavy metals were targeted during the leaching tests and the results indicate that a geopolymer is effective at reducing the leach rates of many metals from the fly ash, such as calcium, arsenic, selenium, strontium and barium. The major element leachate concentrations obtained from leached fly ash were in general lower than that of precipitator fly ash. Conversely, heavy metal leachate concentrations were lower in precipitator fly ash than leached pond fly ash. The maximum addition of fly ash to this geopolymer was found to be 60wt% for fly ash obtained from the electrostatic precipitators and 70wt% for fly ash obtained from ash disposal ponds. The formation of geopolymer in the presence of fly ash was studied using 29Si MAS-NMR and showed that a geopolymer matrix was formed. X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) imaging showed the interaction of the fly ash with the geopolymer, which was related to the leachate data and also the maximum percentage fly ash addition.     

An examination of the reactivity of fly ash in cementitious pore solutions

Fly ash is frequently used to replace cement in concrete, but it is difficult to predict performance based only on the oxide composition, which is typically the only compositional information available. In order to better utilize fly ash in concrete, it is important to develop more meaningful characterization methods and correlate these with performance. The research presented here uses a combination of analytical methods, including X-ray powder diffraction, scanning electron microscopy coupled with multispectral image analysis, and solution analysis to determine the compositions of the glassy phases in a specific fly ash and to examine the fly ash’s reactivity in late- and early-age cement pore solutions, ultrapure water, and sodium hydroxide. The dissolution of individual glassy phases in the fly ash was tracked over time and the precipitation of reaction products monitored. A high-calcium aluminosilicate glass was the most reactive, a low-calcium aluminosilicate glass was of intermediate reactivity and a medium-calcium aluminosilicate glass had the lowest reactivity in the solutions tested for a specific fly ash. This result suggests the glass composition has a strong effect on reactivity, but that that there is not a strict correlation between calcium content and glass reactivity.     

Modification of phase evolution in alkali-activated blast furnace slag by the incorporation of fly ash

The microstructural evolution of alkali-activated binders based on blast furnace slag, fly ash and their blends during the first six months of sealed curing is assessed. The nature of the main binding gels in these blends shows distinct characteristics with respect to binder composition. It is evident that the incorporation of fly ash as an additional source of alumina and silica, but not calcium, in activated slag binders affects the mechanism and rate of formation of the main binding gels. The rate of formation of the main binding gel phases depends strongly on fly ash content. Pastes based solely on silicate-activated slag show a structure dominated by a C–A–S–H type gel, while silicate-activated fly ash are dominated by N–A–S–H ‘geopolymer’ gel. Blended slag-fly ash binders can demonstrate the formation of co-existing C–A–S–H and geopolymer gels, which are clearly distinguishable at earlier age when the binder contains no more than 75 wt.% fly ash. The separation in chemistry between different regions of the gel becomes less distinct at longer age. With a slower overall reaction rate, a 1:1 slag:fly ash system shares more microstructural features with a slag-based binder than a fly ash-based binder, indicating the strong influence of calcium on the gel chemistry, particularly with regard to the bound water environments within the gel. However, in systems with similar or lower slag content, a hybrid type gel described as N–(C)–A–S–H is also identified, as part of the Ca released by slag dissolution is incorporated into the N–A–S–H type gel resulting from fly ash activation. Fly ash-based binders exhibit a slower reaction compared to activated-slag pastes, but extended times of curing promote the formation of more cross-linked binding products and a denser microstructure. This mechanism is slower for samples with lower slag content, emphasizing the correct selection of binder proportions in promoting a well-densified, durable solid microstructure.     

粉煤灰地聚物反应体系下的反应影响因素分析

以粉煤灰基地质聚合物为研究对象,研究了粉煤灰在地聚物反应体系下的反应影响因素。主要研究内容有:反应时间、反应温度和反应碱浓度对反应过程及宏观强度的影响及相关反应机理。结果表明:在反应中后期,粉煤灰反应速率明显下降而此时地聚物的宏观性能反而呈现最大值。以75℃为例,反应时间从24h延长到672h的过程中,反应程度仅从20.8%增加到了32.4%,平均反应速率仅为0.0179%/h,而此时样品的抗压强度则从1.31MPa增加到了7.98 MPa;在地聚物反应体系下,反应产物为无定形的硅铝胶凝体,该物质的致密程度与地聚物宏观性能直接相关;温度的升高可有效提高粉煤灰的反应速率及地聚物的宏观性能,促进无定形胶凝体的形成和硬化。反应24h,75℃下的反应程度和抗压强度可以达到20.8%和1.31 MPa,而同期35℃下的反应程度只能达到7.8%且尚未形成宏观强度;碱浓度的变化不仅可影响反应速率和宏观性能,还可改变粉煤灰在地聚物反应体系下的最终反应程度,在反应温度为75℃和50℃时,10 mol/L体系下672h的反应程度比5 mol/L体系分别高了90%和28.6%。     

Characterization of the high-calcium fly ash geopolymer mortar with hot-weather curing systems for sustainable application

Fly ash geopolymers are an alumino-silicate material and thus enable the utilization of waste containing alumino-silicate effectively. Geopolymeric reaction occurs as a result of the activation of fly ash with alkali solutions. In Thailand, a large amount of high-calcium fly ash is available due to the use of low-grade lignite coal feedstock for pulverized coal combustion process and the calcium content becomes very high. In this study, heat curing at 35 °C as a representative of a high ambient temperature (hot weather) and low cost was investigated. Curing at temperature of 65 °C and room temperature of 25 °C were also conducted to compare the results. Geopolymeric products were tested for compressive strength and characterized by XRD, IR, SEM and TGA techniques. The results showed that heat curing enhanced the geopolymerization resulting in the formation of network product. Heat curings at 35 °C and 65 °C led to the formation of calcium silicate hydrate (C-S-H) and alumino-silicate (geopolymer bonding). Without heat curing, the product was predominantly C-S-H compound and the matrix was as strong as the heat-cured product. The immersion of samples in 3% sulfuric acid solution revealed that the performance of the heat-cured samples were better than those cured at room temperature. In addition, application of research results was to produce the geopolymer brick with outdoor heat exposure of 35 °C. Pedestrian pathway was demonstrated.     

Influence of curing conditions on properties of high calcium fly ash geopolymer containing Portland cement as additive

This paper investigated the mechanical properties and microstructure of high calcium fly ash geopolymer containing ordinary Portland cement (OPC) as additive with different curing conditions. Fly ash (FA) was replaced with OPC at dosages of 0%, 5%, 10%, and 15% by weight of binders. Setting time and microstructure of geopolymer pastes, and flow, compressive strength, porosity and water absorption of geopolymer mortars were studied. Three curing methods viz., vapour-proof membrane curing, wet curing and temperature curing were used. The results showed that the use of OPC as additive improved the properties of high calcium fly ash geopolymer. The strength increased due to the formation of additional C–S–H and C–A–S–H gel. Curing methods also significantly affected the properties of geopolymers with OPC. Vapour-proof membrane curing and water curing resulted in additional OPC hydration and led to higher compressive strength. The temperature curing resulted in a high early compressive strength development.     

Resistance to acid and sulfate solutions of microwave-assisted high calcium fly ash geopolymer

In this paper, 90-W microwave radiation for 5 min plus a shortened heat curing period was applied to cure the fresh geopolymer paste. Results showed that microwave radiation contributed to the dissolution of fly ash in the alkaline solution. Numerous gel formations were observed in microscopic scale. This resulted in a dense composite and strong bonding between the fly ash and the geopolymer matrix leading to high strength gain compared to those of the control pastes cured at 65 °C for 24 h. In addition, resistances to the sulfuric acid and sulfate attacks of the microwave geopolymer were superior to that of the control as indicated by the relatively low strength loss. The microwave radiation also helped the geopolymer attaining thermal stability as the dense matrices were obtained.     

How does aluminium foaming agent impact the geopolymer formation mechanism?

Aluminium is widely used as a foaming agent in lightweight geopolymer concretes. The impact of aluminium reaction on the phase evolution of fly-ash based geopolymers is investigated. In-situ Fourier transform infrared spectroscopy (FTIR) is used to track the changes in the nanostructure of geopolymers. By combining the FTIR functional group analysis with Atomic-force microscopy (AFM) results, it is possible to evaluate the phase development during the early hours of reaction. At the initial stages of geopolymerization, aluminium reaction induces the formation of aluminium hydroxide gel which precipitates on fly ash and conceals the reactive surface of ash particles. Therefore, the dissolution rate of fly ash declines and the strength development is delayed. The high release rate of alumina into the solution, as a result of the aluminium metal reaction, leads to a better connectivity of unreacted particles and better microstructural development as shown in SEM images. Also, by consuming part of the sodium hydroxide catalyst at the beginning of the reaction, aluminium powder protects some of the alkali content of geopolymer matrix from carbonation.     

The utilization of recycled concrete aggregate to produce controlled low-strength materials without using Portland cement

This paper reports the results of an experimental study that investigated the feasibility of using fine and coarse recycled concrete aggregate (RCA) with slag or fly ash to produce Controlled Low-Strength Materials (CLSM). The main objective was to produce CLSM using only recycled and by-product materials without the need to add Portland cement. In addition to the hydraulic activity of slag and high-calcium fly ash (HCFA), their pozzolanic reaction was activated by the alkalis and calcium hydroxide present in the residual paste of the RCA. Preliminary tests showed mixtures with slag to have 7-day compressive strengths 70% higher than mixtures with fly ash.Two types of CLSM with slag were investigated in further detail: one with fine and the other with fine/coarse RCA. The results showed that the developed CLSMs are suitable for a wide range of applications particularly those requiring structural support and fast hardening.     

Controlling ettringite formation in FBC fly ash geopolymer concrete

Fluidized bed coal combstion (FBC) is extensively used in small self-generation power plants. The fly ash obtained from this FBC process contains high quantity of calcium and sulfate compounds which hinders its use in the construction industry. In addition, its reactivity is low and additional source material or additive is, therefore, needed to increase the reaction. This research studied the use of Al(OH)₃ and high concentrations of NaOH to control ettringite formation in the FBC fly ash geopolymer. Two replacement levels of 2.5 wt.% and 5.0 wt.% of Al(OH)₃ and three NaOH concentrations of 10, 12 and 15 M were used in the study. Results indicated that the NaOH concentration affected the ettringite formation and strength of the FBC geopolymer. No ettringite was formed at high NaOH concentration of 15 M which helped the dissolution of calcium sulfate and formed the additional calcium hydroxide. The subsequent pozzolanic reaction led to strength gain of the geopolymer. For 15 M NaOH, the addition of 2.5 wt.% Al(OH)₃ promoted the reaction and formed a dense matrix of alumino silicate compound. Relatively high 7-day compressive strength of 30 MPa was obtained.     

污泥-高钙煤系废物制备地聚合物的技术与性能

鉴于地聚合物是一种低碳排放且能源、资源消耗较少的新型胶凝材料,其制备技术倍受关注。采用热活化污泥和高钙煤系废物制备地聚合物,并与采用高钙粉煤灰和矸石制备地聚合物的反应机制与性能进行了对比分析,确定了制备污泥-高钙煤系废物地聚合物的最佳工艺参数。采用XRD、SEM、TG-DTA及FTIR等对制备的地聚合物的原料组成和性能进行了深入分析。研究表明: 经900℃焙烧45 min掺量为40%(＜50 μm)热活化污泥-煤系废物制备的地聚合物具有较好的抗压强度。无定形地聚合物胶凝包裹在球状粉煤灰颗粒周围,有类沸石矿物生成,Al—O/Si—O对称伸缩峰及Si—O—Si/Si—O—Al弯曲振动峰明显。     

Evaluation of sodium content and sodium hydroxide molarity on compressive strength of alkali activated low-calcium fly ash

Activation of low calcium fly ash is investigated using activating solutions of sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃). The oxide ratio of Na₂O relative to the total reactive silica in the activated mix provides consistent results in achieving the highest ultimate strength. The total reactive silica used for calculating the ratio consists of the reactive silica contributed by fly ash and the silica from the activating solution. There is an increase in the ultimate compressive strength on increasing the total sodium content relative to the total reactive silica content in the activated system. Increasing the sodium content beyond a certain limit does not provide additional gain in the ultimate compressive strength. The ratio of total reactive SiO₂ to Na₂O in the activated system equal to 4.72 is shown to provide the highest compressive strength and there is no further increase in the ultimate strength on increasing the sodium content. A N-A-S-H type gel with reaction products containing Si, Al and Na, is formed in the activated system. The ultimate strength achieved is directly related to the reaction product content in the system and is dependent on the extent of glassy phase dissolution from fly ash. The extent of glassy phase dissolution and the quantity of reaction product formed in the system increases with an increase in the molarity of NaOH, which also contributes to an increase in the sodium content in the activating solution. The decrease in the unreacted glassy phase content of fly ash is sensitive to temperature at a lower molarity of NaOH. The Al/Na ratio in the reaction product approaches a value of 0.9 on increasing the sodium content in the activated system. The Si/Al ratio in the reaction product varies within a range of 2.3–2.8.     

The capacity of ternary blends containing slag and high-calcium fly ash to mitigate alkali silica reaction

The efficiency of ternary blends containing high-calcium fly ash and slag in mitigating alkali-silica reaction (ASR) was evaluated. The concrete prism expansions showed that the ternary blends did not offer significant advantage over binary blends of portland cement and either of the individual material at the same total SCM content. The ability of a particular blend to mitigate ASR was related to its capacity to retain alkalis in its hydration products, as evaluated by an alkali leaching test. For the slag and fly ash used in this study, the capacity to retain alkalis increased with the ability of the blend to consume Ca(OH)₂ during its pozzolanic reaction. For the blends investigated here, the alkali leaching test was more realistic than the accelerated mortar bar test in predicting the 2-year expansion of concrete prisms. The adopted alkali leaching test is proposed to be used as a tool to compare the efficacy of different cementing blends to mitigate ASR.     

Use of OPC to improve setting and early strength properties of low calcium fly ash geopolymer concrete cured at room temperature

Most previous works on fly ash based geopolymer concrete focused on concretes subjected to heat curing. Development of geopolymer concrete that can set and harden at normal temperature will widen its application beyond precast concrete. This paper has focused on a study of fly ash based geopolymer concrete suitable for ambient curing condition. A small proportion of ordinary Portland cement (OPC) was added with low calcium fly ash to accelerate the curing of geopolymer concrete instead of using elevated heat. Samples were cured in room environment (about 23 °C and RH 65 ± 10%) until tested. Inclusion of OPC as little as 5% of total binder reduced the setting time to acceptable ranges and caused slight decrease of workability. The early-age compressive strength improved significantly with higher strength at the age of 28 days. Geopolymer microstructure showed considerable portion of calcium-rich aluminosilicate gel resulting from the addition of OPC.     

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.     

水热法合成NaP1型粉煤灰沸石的性能表征

通过碱性介质中的水热反应,由粉煤灰合成了单一沸石矿物种的NaP1沸石,并对合成产品进行了表征.经粉晶X射线衍射鉴定,合成产物中主要矿物成分为NaP1沸石,另有少量尚未反应的石英和莫来石.在电子显微镜下,粉煤灰颗粒呈球形且表面光滑,而合成产物颗粒表面粗糙.粉煤灰合成沸石含有大量的交换性Ca2 ,且与粉煤灰原料相比,SiO2含量明显减少,Al2O3稍有增加,SiO2/Al2O3比值由3.3降至1.8.红外光谱分析和差热分析证实了合成的粉煤灰沸石中沸石水的存在.NaP1型粉煤灰沸石的阳离子交换容量(CEC)达213 cmol/kg,比表面积达29 m2/g,分别比粉煤灰高约100倍和26倍.     

Comparative study of adsorption properties of Turkish fly ashes. II. The case of chromium (VI) and cadmium (II)

The purpose of the study described in this paper was to compare the removal of Cr(VI) and Cd(II) from an aqueous solution using two different Turkish fly ashes; Afsin-Elbistan and Seyitomer as adsorbents. The influence of four parameters (contact time, solution pH, initial metal concentration in solution and ash quality) on the removal at 20+/-2 degrees C was studied. Fly ashes were found to have a higher adsorption capacity for the adsorption of Cd(II) as compared to Cr(VI) and both Cr(VI) and Cd(II) required an equilibrium time of 2h. The adsorption of Cr(VI) was higher at pH 4.0 for Afsin-Elbistan fly ash (25.46%) and pH 3.0 for Seyitomer fly ash (30.91%) while Cd(II) was adsorbed to a greater extent (98.43% for Afsin-Elbistan fly ash and 65.24% for Seyitomer fly ash) at pH 7.0. The adsorption of Cd(II) increased with an increase in the concentrations of these metals in solution while Cr(VI) adsorption decreased by both fly ashes. The lime (crystalline CaO) content in fly ash seemed to be a significant factor in influencing Cr(VI) and Cd(II) ions removal. The linear forms of the Langmuir and Freundlich equations were utilised for experiments with metal concentrations of 55+/-2mg/l for Cr(VI) and 6+/-0.2mg/l for Cd(II) as functions of solution pH (3.0-8.0). The adsorption of Cr(VI) on both fly ashes was not described by both the Langmuir and Freundlich isotherms while Cd(II) adsorption on both fly ashes satisfied only the Langmuir isotherm model. The adsorption capacities of both fly ashes were nearly three times less than that of activated carbon for the removal of Cr(VI) while Afsin-Elbistan fly ash with high-calcium content was as effective as activated carbon for the removal of Cd(II). Therefore, there are possibilities for use the adsorption of Cd(II) ions onto fly ash with high-calcium content in practical applications in Turkey.