Effect Of Nano Zinc Oxide On Shrinkage Properties Of Alkali Activated Fly Ash/Slag

Alkali activated material is the most promising green cementing material,with high early strength,high temperature resistant,resistant to chemical corrosion,low hydration heat,low carbon dioxide emissions,impermeability,frost resistance is good,and many other advantages,but alkali activated material autogenous shrinkage and drying shrinkage is larger,larger shrinkage leads to the more easy to produce the problem such as crack and structure defects,Further,the durability of alkali activated materials is sharply reduced.At present,materials such as shrinkage reducer,expansion agent,fiber and lightweight aggregate have been widely studied to improve the early shrinkage of alkali-excited materials.Compared with the above materials,nanomaterials have the advantages of large specific surface area,good adsorption performance,strong coordination bonds,easy to disperse in alkaline solution,etc.The effects of nano-silica,nano-magnesia,nano-clays,nano-tio₂ and carbon nanotubes on alkali-activated materials have been studied.However,the effects of zno nanoparticles with high thermal capacity,low thermal expansion rate and high melting temperature on the shrinkage properties,early reaction process,mechanical properties and internal structure of alkali-excited materials are not clear.In this paper,fly ash and slag are used as the main raw materials to prepare alkali activated fly ash/slag net slurry,and nano zinc oxide is used to study the change rules of self-shrinkage and drying shrinkage of alkali activated fly ash/slag slurry.The influence of nano-zno on the early reaction process of alkali activated fly ash/slag slurry was studied by early reaction heat release,internal relative humidity and mass loss during drying.The reaction products were characterized by X-ray diffractometer,infrared spectrometer,thermogravimetric analyzer and scanning electron microscope,and the mechanism of action of nano-zno on alkali excited fly ash/slag system was studied.Finally,the effects of nano-zno on compressive strength,flexural strength,setting time and pore structure of alkali excited materials were studied.The results show that nano-zno can delay the reaction rate of alkali activated fly ash/slag composite system,and hinder the development of chemical shrinkage and the refinement of pore structure.The addition of nano-zinc oxide can effectively inhibit the self-shrinkage of alkali-excited fly ash/slag composite slurry.When the content of nano-zinc oxide increases from 0%to 3%,the self-shrinkage of alkali-excited fly ash/slag composite slurry gradually decreases.Smaller particle size of nanometer zinc oxide particles can better dispersion and fill in the smaller hole,make the gel more compact,thus reducing the shrinkage rate,in addition of nanometer zinc oxide content increased from 0%to 3%when the internal relative humidity increases,the addition of nanometer zinc oxide can reduce system caused by the dry larger pore pressure,thereby reducing the generation of autogenous shrinkage.The size of drying shrinkage is positively correlated with the mass loss of samples during drying.The higher the content of nano-zno is,the smaller the drying shrinkage and mass loss of alkali-excited materials are,indicating that the addition of nano-zno can reduce the free water evaporation and the drying shrinkage of alkali-excited slurry.5 h after the reaction,it was observed that zinc ions reacted with dissolved calcium ions in the slag to form meso-phase CZ（Ca（Zn（OH）₃）₂·2H₂O）.As calcium ions continued to react preferentially to form CZ phase,The calcium concentration in the pore solution did not reach the critical limit of nucleation and growth of calcium silicate aluminate hydrate（C-A-S-H）gel,which delayed the formation of C-A-S-H gel.Therefore,the addition of nano-zno hinders the development of chemical shrinkage of alkali activated slag/fly ash slurry and the refinement of pore structure,and the self-drying process and pore pressure increase of slurry are alleviated,thus reducing the self-shrinkage rate.Compared with the control group without nano-zno,the nitrogen adsorption amount of alkali-activated slurry with 1%and 2%nano-zno decreases,indicating that the addition of nano-zno reduces the porosity of alkali-activated slag/fly ash slurry and the pore structure is more compact.However,the alkali activated slurry with 3%nano-zinc oxide has the largest nitrogen adsorption capacity and the pore structure is not dense,which is consistent with the results that adding 3%nano-zinc oxide can inhibit the strength of the activated fly ash and slag composite slurry.When the content of fly ash is constant and the content of sodium oxide increases from 4%to 6%,the compressive strength of alkali activated slurry increases gradually.The flexural strength of alkali activated slurry shows a similar trend to the compressive strength.With the increase of nano-zno content,the setting time of alkali-activated gelling system increases gradually.