Alkaline Activation Reaction Mechanism Of Fly Ash And High Preformance Study For Its Foamed Cementious Materials

In the 21 st century,the explosion of energy,resources,and environment crises has brought into focus the energy saving,environmental protection and waste recycling.As one of the largest industrial solid wastes around the world,(coal)fly ash is in urgent need to be well disposed due to its degraded environmental impacts: occupying large-area soil land,causing air dust pollution once fly ash blowing into air with wind,and further threatening local people's health and ecological environment.Therefore,unutilized fly ash is considered to be one of the adverse factors that caused the unsustainable development of the society.On the other hand,the construction energy saving efficiency level in our nation is far below that of developed western countries.The utilization of high-performance thermal insulation materials in buildings is one of the most effective methods to improve construction energy saving efficiency.However,the manufacturing energy consumption of traditional thermal insulation materials is high and poor properties hindered their application.New style organic types materials perform excellent in thermal insulation,but they are not safe,as their poor heatresistance.This paper conducted a system study on the preparation of high-performance and lightweight foam materials using chemical alkaline activation reaction and chemical foaming methods,and fly ash as raw materials,aimed to apply the materials in thermal insulation field of buildings.The purpose of this project exactly meet with the two problems above,and propose a solution that meets the best of both worlds.This project firstly prepare low-carbon and environmental materials using industrial solid waste as raw materials,and secondly apply the materials for construction energy saving fields,no doubt,that will be of great significance to science and reality.This research project employed circulating fluidized bed combustion fly ash as the only raw binding materials,sodium hydroxide and water glass as alkaline activators,and used chemical foaming methods to prepare the fly ash based foam geopolymers.This paper focused the study mainly on two parts: the alkaline activation reaction mechanism of fly ash and preparation and high-performance study of the foam geopolymers.The main details and results are as follows:(1)Study on alkaline activation reaction mechanism of fly ashThe content of this part mainly included in the 3rd section of the paper,is based on the available research works,and is the supplement and innovation of the alkaline activation reaction(AAR)mechanism of fly ash.For more convenient analysis and identification of the changes of fly ash during the alkaline activation reaction processes,this paper firstly conducted a comprehensive study on fly ash particles.For the material,we focused on its particulate study in terms of the elementary existence phase form and their morphologies.The results showed that:(1)the CFA particles in this study are predominantly irregular in shape,with amorphous phase as its main phase composition;(2)Si and Al were the two richest elements existing in CFA,and the two elements are usually accompanied as amorphous aluminosilicate phase;(3)The calcium constituent in CFA was noted to be existed in the form of anhydrite(CaSO4),lime(f-CaO),and amorphous calcium content;(4)Element of Fe was recognized to exist in the form of crystal hematite and oxygen-contained ferrite,in spherical morphology(hematite)or in irregular shape that included in CFA particles.Then,the calcium constituent phase evolution at early alkaline activation reaction age was studied.This study firstly investigated the AAR isothermal heat flow curves of CFA at 60 oC for the initial 24 hrs.Based on the AAR heat flow curves,the AAR was divided into 4 stages,and then the four stages were respectively characterized and analyzed to deduce the AAR calcium evolution processes.The results showed that,the calcium constituent would rapidly dissolved in the alkaline solutions,and then developed into mainly two directions: 1)under the force of chemical kinetics,Ca2+ would rapidly combine with OH-and form the layer-stacked Ca(OH)2;However,Ca(OH)2 was not stable,as they will be carbonated by CO2 in air;2)another fraction of Ca2+ would combine with OH-and [OSi(O H)3]-to form the immature amorphous gels.In addition,the produced Ca(OH)2 and CaCO3 are thermodynamics unstable,and they will also convert into amorphous gels.The leached Al played a key role in bridging different immature gels to form three dimensional C,N-A-S-H gels networks,which were identified as the mature gels.With AAR went on,amorphous gels would convert into crystal zeolites.Moreover,this paper innovatively studied and characterized the C,N-A-S-H gels nano-scale networks and its formation processes during AAR,and furthermore,an illustration of fly ash based C,N-A-S-H gels formation processes was proposed for the study.Concerning the C,N-A-S-H gels formation mechanism,under the chemical attack of strong alkaline solutions,active Al,Si,Ca etc.elements will leach out from CFA particles,and then combined the ionic WG to form monomers.Quickly,once the monomers reach its super-saturation concentration,larger dimensional intermediates would be formed.What's more,with the help of monomers bonds,neighbor intermediates would also be cross-linked with each other to further form mature gel particles.Consequently,countless gels particles stacked together to compose the three dimensional C,N-A-S-H gels networks.(2)Preparation and high-performance study of fly ash based foam geopolymersThis part mainly includes chapter 4 and 5 of this paper.Based on the study of the fundamental preparation factor of alkaline activated fly ash foam materials,we further conducted a study on the adjustment and optimization of the foam structures,aiming to achieve the high performance of the materials.On the fundamental preparation technical factors,this paper mainly focused on the study of concentration of NaOH,SiO2/Al2O3 molar ratio,curing temperature and curing time,dosage of foaming agent,and reinforced fibers on the properties of materials.The results showed that,the optimism NaOH concentration and SiO2/Al2O3 molar ratio are about 4 M and 3.4,respectively.The role of NaOH concentration is to control the leaching rate of the elements from fly a sh,and the SiO2/Al2O3 molar ratio of alkaline activation system play a role in controlling the chemical composition and polymerization level of the produced gels.Curing time and and curing temperature also have significant effect on the properties of foa m geopolymers.Curing time plays the role in maintaining the phase composition of the products,short curing time lead to the low yield of C,N-A-S-H gels,and further result in low strength of materials;While,curing temperature mainly influenced the foam microstructures,and influenced the performance of materials.Too high temperature would damage the pore structures and further have negative effect on materials performance.Foaming agent is the key component for the alkaline activated materials system to form porous microstructures,and its dosage play the key role in determining the porosity and bulk density of materials,and further step influencing properties of materials.Organic fibers have been identified an effective method to overcome the toughness failure of alkaline activated materials.It was shown in this paper that,the improvement tendency on geopolymers strength and flexural toughness is same,and ranked as: PAN > PET > PP;The optimism fiber dosage and fiber length is 0.1 wt.% and 12 mm respectively,correspond to the maximum flexural elastic modulus of 0.074 GPa,which improves about 172 % comparing with the blank control.Based on above fundamental study,this paper further studied the pore structures control and adjustment with different foam stabilizer: SDBS based and calcium stearate based ones.In addition,HGB incorporation could lowed the density with little strength decreased.The results showed that:(1)with the increase of SDBS based foam stabilizer,pore diameter decreased and hole rate increased,and the optimism dosage is 2 %.However,SDBS foam stabilizer had little effect on the improvement of the pore sealing and strength,as material pores possess high hole-rate;(2)compared with SDBS based foam stabilizer,calcium stearate performs much better in controlling pore structures,as it can(a)make pores diameter distribution more uniform;(b)obviously lower the hole rate,improve mechanical strength,and decrease thermal conductivity coefficient.With the increase of calcium stearate,the pore diameter gradually decreased,but hole rate increased.In summarization,considering the comprehensive performance in terms of mechanical strength,bulk density,and thermal conductivity,calcium stearate was demonstrated to behave much better in controlling pore structures of alkaline activated foam materials;(3)By both HGB incorporation and chemical foaming methods,this paper successfully prepared ultra-lightweight porous alkaline activated materials with excellent strength and ultra-low thermal conductivity coefficient,and the optimism properties in terms of bulk density vs.compressive strength vs.thermal conductivity coefficient were 200 kg/m3 vs.1.4~1.6 MPa vs.0.0522 W/(m�K).