| The addition of steel slag as aggregate into concrete can not only realize the resource utilization of steel slag,but also alleviate the excessive dependence on natural sand and gravel.However,the poor volume stability of steel slag aggregate leads to the serious expansion and damage of specimens in concrete,which greatly limits its application.In this paper,the idea of researching the stability of steel slag aggregate in acid pickling is put forward.The steel slag aggregate pretreated with acid activator is proposed to replace sand aggregate to prepare steel slag mortar with good volume stability and excellent mechanical properties.Based on the theory of molecular dynamics and cementitious material science,the hydration characteristics of acid-activated steel slag sand were studied from the Angle of volume stability and mechanical properties.Combined with scanning electron microscope,X-ray diffraction analysis and other microscopic detection methods,the internal mechanism of steel slag aggregate pickling stability was revealed,and the internal mechanism of steel slag aggregate surface activation on strength development was analyzed.Exploratory and innovative work has been done in the following aspects:(1)Study on Stability Mechanism of Volume Stability of Acid-activation Steel Slag AggregateThe volumetric expansion of steel slag sand specimen was studied under different types,dosage and excitation duration.Combined with the micro-analysis method,the occurrence state of f-CaO in steel slag was revealed,and the internal mechanism of stability in pickling steel slag aggregate was analyzed.The results show that the stability effect of different acid activators on steel slag and mortar is different.Oxalic acid and glacial acetic acid have poor stability effect on steel slag and mortar,while sulfuric acid has good stability effect on steel slag and mortar,but the compressive strength of the specimen is low.Hydrochloric acid has the best stability effect and the mechanical properties of the specimens are good.The volume stability of steel slag mortar is good when the dosage of activator and excitation duration are appropriate.The f-CaO in steel slag is lumped and wrapped by solid solution.After pickling,H~+chemisorption and erosion take place on the surface of steel slag aggregate,and the f-CaO is dissolved out,thus eliminating the block f-CaO which causes the poor stability of steel slag volume.(2)Study on Mechanism of Surface Activation of Steel Slag Aggregate on Strength DevelopmentThe effects of different kinds of activators,dosage and excitation duration,and different steel slag aggregate dosage,particle size and pretreatment methods on the compressive strength of alkali activated steel slag mortar were studied,and the best kinds of activators,dosage and excitation duration were determined.The characteristics of transition zone at the interface between surface activated steel slag aggregate and cement stone were studied by X-ray diffraction(XRD)and scanning electron microscopy(SEM)analysis,and the internal mechanism of surface activation of steel slag aggregate on strength development was revealed.The results show that the improvement effect of different activators on the mechanical properties of steel slag mortar is different.Appropriate amount of activator and excitation time can make the best mechanical properties of steel slag mortar.With the increase of steel slag aggregate content,the mechanical properties of steel slag mortar show a trend of first increase and then decrease.Through microscopic analysis shows that natural aggregate-sand interface is water slurry of cement and sand mechanical interlocking of interface,and steel slag aggregates-cement interface transition zone for the activation of steel slag aggregates and hydration products of cement paste by chemical action after the formation of the interface,the interface formed stronger than mechanical interlocking of chemical bonding,so as to improve the mechanical properties of the concrete.(3)Study on Mechanism of alkali activation of Steel Slag Samples on Strength DevelopmentBy changing the activator modulus and the content of steel slag aggregate,the surface expansion damage degree of steel slag sand specimen before and after autoclaving was observed,and the compressive strength of alkali excited steel slag sand for 3d,7d and 28d was measured.The influence law of these factors on the volume stability of steel slag mortar was analyzed,and the strength development model was established to deeply analyze the internal stability mechanism of alkali activated steel slag aggregate.The results show that the compressive strength of steel slag mortar increases first and then decreases with the increase of steel slag aggregate content.The compressive strength of steel slag mortar is the best when the alkali activator modulus is about 1.0.The surface f-CaO of steel slag aggregate is dissolved in a high alkalinity environment and spontaneously polymerizes with silicate ions in the activator,so that the f-CaO in steel slag aggregate,which causes poor volume stability,can be transformed into C-S-H gel with cementitious activity,achieving the dual effects of stability and activation activity.There are 39 figures,14 tables and 94 references in the paper. |
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