Study On The Reaction Process Of Alkali-activated Materials Based On Digital Holographic Microscopy

Alkali-Activated Materials（AAM）have the advantages of high early strength,good permeability resistance,strong acid and alkali corrosion resistance,and low energy consumption,so they have broad application prospects.However,due to the great difference between the reaction process and the traditional Portland cement hydration process,AAM are difficult to accurately and directly characterize the reaction process,which leads to the inability to clarify the effect of the reaction process on the material properties,and is limited in practical engineering applications.In this paper,an off-axis transmission Digital Holographic Microscopy（DHM）system was designed and constructed to directly observe the real-time reaction process of alkali-activated materials（slag and fly ash particles in sodium silicate solution）and to characterize the volume change and reaction rate.At the same time,the reaction process and reaction mechanism of alkali-activated materials were comprehensively studied by combining the ion dissolution and heat evolution analysis of slag and fly ash particles in sodium silicate solution.In addition,by studying the reaction products,microstructure and strength of the hardened pastes of alkali-activated slag and alkali-activated fly ash,the influence of the alkali-activated materials reaction process on the properties of the hardened pastes was discussed.First,this paper analyzes the basic principle and implementation methods of DHM,and finally selects the Angle spectrum method as the reconstruction method of DHM system in this topic.An off-axis transmission digital holographic microscopy system suitable for observing the reaction process of cementitious materials was established by optimizing the design of optical path system,building of physical device and software design.The observation results of standard samples show that the DHM system is fast,efficient and reliable,and the degree of confidence in horizontal and vertical observations are above 98%and 87%,respectively.So it can be used to study the reaction process of cementitious materials.Second,the DHM observation slag,fly ash particles in the dissolution of sodium silicate solution-reaction process,get dissolution rate curve,the results show that the dissolution-reaction process of slag can be divided into 5 periods:initial fast dissolution period,low-speed dissolution period equal to the induction period,acceleration dissolution period,slow dissolution period and steady period;The dissolution-reaction process of fly ash can be divided into 3 periods:initial fast dissolution period,induction period and subsequent dissolution-reaction period.Under the condition of a large liquid-solid ratio（1:200）,the initial dissolution rate of slag and fly ash particles is large,and the volume of slag particles can be reduced by more than 50%after 2 h.For particles with equivalent particle size of 5-10μm,the average dissolution rate is about 30μm³/h.It disappeared completely 24 h ago.At 2 h,the volume of fly ash particles decreases to less than 40%,and at 12 h,the residual volume of fly ash particles is about 4%.For particles with an equivalent particle size of 3-4μm,the average dissolution rate is about 1.68μm³/h.At 24 h,the residual volume was less than 1.5%,and the dissolution was relatively slow.The dissolution rate of slag particles at 12 h is about 20 times that of fly ash particles,so the early compressive strength of alkali-activated slag is much bigger than that of alkali-activated fly ash.The influence of sodium silicate modulus on the dissolution-reaction process of slag and fly ash is different.When the sodium silicate modulus is in the range of 0.8-1.6,the smaller the modulus is,the faster the dissolution-reaction rate of slag particles is.The larger the modulus,the faster the dissolution-reaction rate of fly ash particles.Then,the ion dissolution and heat exothermic reaction of slag and fly ash particles in sodium silicate solution are studied.The results show that:the Ca²⁺concentration in sodium silicate and slag system has experienced a rapid growth,slow growth,falling and stabilizing process.The Al³⁺concentration in sodium silicate and fly ash system has experienced rapid growth,fast falling,small amplitude fluctuation and very slow falling process.They have good correlation with DHM results of slag and fly ash.The heat evolution curve of the sodium silicate and slag system is consistent with the DHM dissolution rate curve,which can be divided into 5periods.In the fast dissolution period,there is a main initial peak and an additional initial peak.The main initial peak is from the fast dissolution of slag particles heat evolution,and the additional initial exothermic peak is from the reaction of Ca²⁺dissolved from slag particles and Si O₄^4-in sodium silicate solution;After the induction period,there is a second accelerated exothermic peak,which is mainly the reaction between Ca²⁺dissolved from slag particles and Si O₄^4-dissolved from slag particles themselves.The heat evolution curve of the sodium silicate and fly ash system is consistent with the dissolution rate curve of DHM,which can be divided into 3 periods.There is an exothermic peak in the initial rapid dissolution period,which is mainly due to the exothermic peak in the dissolution of fly ash,and there is no obvious exothermic peak thereafter.After the induction period,the cumulative exothermic heat increases steadily and slowly and lasts for a long time,which is mainly due to the exothermic reaction in the formation of silica-aluminum gel and N-A-S-H gel products.So alkali-activated fly ash materials have the low strength and slow strength growth in the early stage.Finally,the composition structure and strength of early hardened pastes in sodium silicate and slag system and sodium silicate and fly ash system were studied.The results show that:At0.5 h,a small amount of product was generated in both sodium silicate and slag systems.With the extension of age to 2 h and 4 h,the amount of product increased significantly,and Ca/Si（molar ratio）of gel products gradually increased,and the degree of reaction increased.The reaction degree of sodium silicate and fly ash system increases slowly,but the reaction product Al/Si increases gradually,indicating that the product is constantly polymerizing.The smaller the sodium silicate modulus,the higher the reaction degree of slag hardened pastes and the larger Ca/Si of gel products.The larger the sodium silicate modulus is,the larger and more homogenous the Al/Si of the fly ash hardened pastes is,indicating that the reaction degree is relatively higher and the chemical composition of the product is more uniform.The 1 d compressive strength of the hardened pastes of slag and fly ash prepared with different modulus of sodium silicate is in good agreement with the early solution-reaction process of slag and fly ash particles in sodium silicate solution observed by DHM.However,the development law of the strength of hardened pastes with early solubilization rate is different:The higher the early dissolution rate,the greater the increase of 28 d compressive strength of fly ash hardened pastes.The increase of 28 d compressive strength of slag hardened pastes is smaller.This paper is the first to try to design and build a complete off-axis transmission digital holographic microscopy system and reconstruction methods in China,and the digital holographic microscopy technology is used to study alkali-activated materials,which is of great significance to solve the problem of alkali-activated materials reaction mechanism research and expand its research methods.At the same time,digital holographic microscopy is used to characterize the reaction process of alkali-activated materials at multiple latitudes,which can reveal the reaction mechanism more clearly,accurately and completely,and provide a theoretical basis for its popularization and application,as well as provide ideas for the research of other cementitious materials.