Study On Evolutionary Regularity And Micro Mechanisms Of Macro Properties Of Composite Limestone Powder-fly Ash-slag Concrete

The increase in infrastructure investment has led to an increase in demand for cement in China,resulting in great environmental pollution and resource consumption.By using mineral admixtures such as limestone powder instead of cement as supplement cementitious materials?SCMs?to make composite concrete,it can effectively achieve recycle solid waste and further energy conservation and emission reduction.This paper systematically studies the evolutionary regularity and micro mechanism of macro properties of composite limestone powder-fly ash-slag concrete.Basic properties of composite mortar and concrete mix design method are studied,and then the hydration heat performance of SCMs,autogenous shrinkage performance,basic mechanical properties,uniaxial compression performance of limestone powder concrete and the bond behavior between it and steel bar are studied.The durability of the composite concrete in general atmospheric environment is explored.The micro mechanism of composite limestone powder concrete is revealed.The utilization efficiency of comprehensive utilization of green limestone powder is improved.The achievements can provide theoretical guidance and technical support for the transformation and upgrading of the concrete industry.The main conclusions and innovations are as follows:?1?The basic properties variation regularity of composite limestone powder mortar was revealed,and the optimization design method of composite concrete mixing ratio was proposed.Limestone powder,fly ash and slag have different improvement degrees on the fluidity of the mortar.The order from high to low was:fly ash>limestone powder>slag.When the dosage was less than 20%,the limestone powder can improve the early strength of mortar.The compressive strength development regularity of mortar was similar to the flexural strength.The multi-blending SCMs can improve the mechanical properties of mortar better than the single-blending,but the late strength will decreased with the increasing of the limestone powder amount.An optimized design method of composite limestone powder concrete mix proportion was proposed based on controlled slurry-fixed basic parameter method.?2?The hydration heat performance evolutionary regularity of the SCMs was obtained,and the acceleration mechanism of limestone powder in the early stage was revealed.In the initial stage,the hydration heat release of the SCMs continued to increase and released 50%to75%of heat in the first 3 days,and gradually decreased in the later stage.The early heat release and total hydration heat release of cement were significantly reduced as mineral admixtures blended into,and the limestone powder had the largest reduction.At the 0.40water-binder ratio,limestone powder-fly ash double blended mortar had the lowest hydration heat release is decreased 47.6%of the cement mortar.The hydration heat release and early heat release rate of the SCMs increased with the increase of the specific surface area of limestone powder,and the total amount of hydration heat and hydration heat released increased with the increase of water-binder ratio.The microcrystalline nucleation of limestone powder can accelerate the early hydration of C₃S,and improve the second exothermic peak of mortar with limestone powder appeared in advance,but the peak of exothermic rate was reduced.?3?The autogenous shrinkage performance evolutionary regularity of composite limestone powder concrete was revealed,and the prediction model was established.The autogenous shrinkage rate of composite limestone powder concrete increased rapidly with age first and then gradually flattened,and the rate of first 20hours reached more than 60%of72hours.With the same water-binder ratio,the rate of concrete increased with the increase of the specific surface area of limestone powder.The rate of 450m²/kg and 650m²/kg concrete increased by 5%²5%compared to the 350m²/kg concrete.With the same specific area of limestone powder,the rate decreased with the increase of water-binder ratio,0.40 and0.35water-binder ratio concrete were 5%²0%and 20%³0%bigger than the 0.49water-binder ratio concrete respectively.A three dimensional model on composite limestone powder autogenous shrinkage was established,which can accurately reflect the relationship between the autogenous shrinkage rate of composite limestone powder concrete and the age and limestone powder specific surface area.?4?The mechanical properties variation regularity of composite limestone powder concrete was given,and the strength prediction models were established.The compressed concrete was failed in tensile cracking,while the splitting tensile and uniaxial compressed concrete were failed in splitting section of coarse aggregate and cement mortar in the early stage,and then in coarse aggregate broken in the later stage.The mechanical properties of the concrete improved slightly as the fineness of the limestone powder increased.The initial strength was still lower than that of ordinary concrete.The 28-day strength was difficult to reach the 95%of design value until the 60days.The compressive strength of limestone powder-fly ash concrete was generally lower than that of limestone powder-slag and limestone powder-fly ash-slag concrete.Based on the simplex-centroid experimental design,the strength prediction models of composite limestone powder-fly ash-slag concrete were established considering the water-binder ratio,specific surface area of limestone powder and blending ratio of SCMs.The calculated results were in good agreement with the test results.?5?The uniaxial compression performance evolutionary regularity of composite limestone powder concrete was given and the constitutive model between stress and strain was established.Under the ultimate load,the prism specimens were mostly shear failure along the diagonal.90%of the cylinders were mainly fractured along the vertical,and 10%was shear failure along the surface.The relationship between the cylinder and the prism was achieved with the test results regression.Relationship between the test results of prism and cylinder specimen is f_c=0.90665f^',reflecting the shape and size effect.The elastic modulus of composite limestone powder concrete increased with the increase of peak stress,which exceeded 11%of the calculated elastic modulus of the standard.The formula of the elastic modulus was obtained by modifying the standard,and the constitutive model of composite limestone concrete under uniaxial compression was established.The calculated results were in good agreement with the test results.?6?The evolutionary regularity of the bond performance between steel bar and composite limestone powder concrete was given,and the bond anchorage feature values were obtained.The specimens were cracked and the steel bar pulled out under the ultimate load.Plastic failure occurred as the horizontal stirrup was put.The interface between the concrete and steel bars was extruded or even flattened after the failure.The curve can be divided into five stages as the common concrete:slight slip,slip,split,drop and residual.The ultimate bond stress was positively correlated with the compressive strength of the concrete.The parameter?had a quadratic parabolic relationship with the specific surface area of limestone powder,and approximately linear with the water-binder ratio.The bond anchorage features of the composite limestone powder concrete were obtained,which confirmed that the bond performance was improved as limestone powder,fly ash and slag were mixed.?7?The freeze-thaw resistance degradation regularities and carbonization depth growth of limestone powder concrete were revealed.With the increase of freeze-thaw cycles,the apparent morphology of the composite limestone powder concrete specimens continued to deteriorate,the mass loss rate continued to increase,and the relative dynamic elastic modulus continued to decrease.As the water-binder ratio increased,the mass loss increased.As the water-binder ratio increased,the carbonation depth of concrete increases.As the water-binder ratio and the specific surface area of limestone powder were same,the limestone powder-fly ash double-concrete concrete had twice the carbonization depth of limestone powder-slag double concrete or limestone powder-fly ash-slag three concrete.The carbonization depth growth curve had good relevance to_td?28?a t.?8?The microstructure of SCMs was verified,and the micro mechanism of the macro properties of the composite limestone powder concrete was revealed.The content of C-S-H increased with the hydration time.The secondary hydration of fly ash and slag led to the decrease of Ca?OH?₂ content in the later stage.The limestone powder effectively inhibited the conversion of early AFt to AFm.The hydration heat release micro mechanism of SCMs was given as follows:the nucleation matrix of C-S-H accelerated the hydration of cement.The amount of hydration products were reduce as the proportion of cement clinker decreased,while the secondary hydration of slag and fly ash led to the increase of the diffraction peak of C-S-H.The autogenous shrinkage micro mechanism of concrete was that the volume increased due to the reaction between CaCO₃ in limestone powder and C3A,compensating for the shrinkage.As the specific surface area of limestone powder increased,capillary negative pressure was promoted by clinker hydration,causing the autogenous shrinkage of concrete.Secondary hydration of fly ash reduced the shrinkage of concrete.The mechanical properties evolutionary micro mechanism of concrete was that the effective slurry volume and early strength of concrete in the early stage were reduced because of the low activity of SCMs.Hydrated calcium silicate and other gels produced during the secondary hydration of fly ash,enhancing the overall strength of the mortar.The admixture improved the gradation of concrete particles,resulting in a later strength increase.The concrete durability degradation micro mechanism was interpreted that large water-binder ratio or low specific area of limestone powder both increased the porosity of concrete,thus the compactness decreased and the durability deteriorated.Especially when the limestone powder-fly ash were double blended with large amount,the dispersion and dilution effects were particularly significant,therefore it was necessary to properly control the blending ratio of SCMs and the preferred materials.