Reaction Activity Evaluation And Gelling Mechanism Of Alkali Activated Coal Fly Ash And Gasification Residue

In recent years,coal still occupies the main position in China’s energy consumption.Coal-fired thermal power generation is the main way of coal consumption in China at this stage,and a large amount of fly ash is emitted during the coal combustion process.In addition,coal gasification technology is widely used to improve the energy utilization efficiency of coal.The coal gasification process generates a large amount of coal gasification slag.The massive stockpiling of fly ash and coal gasification slag（coal-based ash and slag）is harmful to the ecological environment while occupying land resources.At present,the production of construction materials is the main way to utilize a large amount of coal-based ash.However,traditional cement-based building materials have a high carbon footprint.Geopolymer is one of the most promising new materials to replace cement-based building materials,with the advantages of low production energy consumption,low carbon emissions and a wide range of raw material sources.The high silica-alumina content of coal-based ash and slag allows the preparation of geopolymers by means of alkali activation.Due to the different formation conditions,the physicochemical properties of coal-based ash vary greatly,which poses a great challenge to the evaluation of the reactivity of raw materials and the study of the reaction mechanism in the alkali activation process.Based on this,circulating fluidized bed fly ash（referred to as CFB ash）,coal pulverized furnace fly ash（referred to as PC ash）and Shell gasification slag（referred to as CGS）discharged from industrial boilers,as well as simulated slags prepared at different temperatures in the laboratory,were selected as objects of study in this paper based on the differences in ash formation temperatures.The dissolution behavior of silicon and aluminum from these raw materials in NaOH solution was investigated using the experimental method of static dissolution,and the effects of key factors such as NaOH concentration,curing temperature and liquid-solid ratio on the dissolution behavior of silicon and aluminum were examined.The above raw materials were activated using NaOH solution and NaOH-Na₂SiO₃ mixture solution as alkali activator,respectively,to prepare geopolymer test blocks.The effects of key factors such as NaOH concentration,activator modulus,alkali admixture,water-cement ratio and curing temperature on the strength of the ground polymers were investigated.The following main conclusions were drawn:（1）By comparing the dissolution behavior of CFB ash,PC ash and CGS under static dissolution conditions,it was found that the silica-alumina dissolution ratio of all three raw materials increased with the increase of dissolution time until the dissolution equilibrium was reached.The dissolution rate of Si and Al in CFB ash is fast,and the dissolution equilibrium can be reached in 28 days.In contrast,the dissolution rates of Si and Al in PC ash and CGS are relatively slow,and the dissolution equilibrium time is longer.The dissolution equilibrium has not been reached by 90 days.The early dissolution ratio of Si and Al in CFB fly ash is higher than the other two.When the dissolution equilibrium is reached,the magnitude of Si dissolution ratio is:CGS>PCFA>CFBFA,and the magnitude of Al dissolution ratio is:CGS>CFBFA>PCFA.（2）The geopolymer specimens were prepared by alkali activation of the three raw materials using NaOH solution and NaOH-Na₂SiO₃ mixture solution,respectively.Under optimal conditions,the magnitude of 90d compressive strength of alkali activated specimens is:CGS>PCFA>CFBFA.CFBFA based geopolymers have higher early strength,while PCFA and CGS based geopolymers have lower early strength but higher later strength.The strengths of CFBFA based geopolymer specimens were basically the same when a single NaOH solution and a NaOH-Na₂SiO₃ mixture solution were used as the activator.And the other two had higher strength of the specimen blocks when NaOH-Na₂SiO₃ mixed solution was used as activator.CFB fly ash is suitable for activation with low concentration NaOH solution,CGS can be activated with high concentration NaOH solution,and PC fly ash is suitable for activation with NaOH-Na₂SiO₃ composite activator.（3）From the results of the dissolution experiments,the reactivity of Si and Al in CFB fly ash in NaOH solution is not low.However,the compressive strength of alkali-activated test blocks is the lowest and difficult to improve.The main reasons include two aspects.On the one hand,the particles are loose and have a high calcium content,which leads to the poor flowability of the slurry during the preparation of the geopolymer and the high water-cement ratio required（0.7～0.8）.On the other hand,during the maintenance process,the gel is easily transformed into crystals such as zeolite-like leading to an undense matrix structure.（4）Under the same chemical composition,the size of dissolution activity of four temperature simulated coal ash is:1500℃FA>600℃≈815℃FA>1300℃FA.The magnitude of compressive strength of alkali activated test blocks is:1500℃FA>600℃≈815℃FA>1300℃FA.It indicates that the reactivity of low-temperature amorphous silica-aluminates is lower than that of high-temperature vitreous,which is consistent with the conclusions obtained from industrial boiler ash.The lower dissolution activity and alkali-activated gelling activity of 1300℃ simulated ash residue are due to its high crystalline phase content,which can participate in the reaction with less active silica-aluminate.