Microscopic Characterization And Early Age Performance Of MgO And Nano-MgO High Performance Concrete

In order to improve the bearing capacity and service life of the structure,the construction engineering materials are gradually developed towards the direction of high-strength and high-performance concrete,and the structural strength and durability are greatly improved.When high performance concrete is applied in bridge engineering,the cement content is relatively high,the hydration heat release of the cementitious material is concentrated,and the structural system produces temperature shrinkage deformation.When the shrinkage deformation of the structure is too large,the shrinkage stress will easily generated in the high performance concrete,which increases the possibility of brittle cracking in the interface between the aggregate and the cement slurry,affects the durability and load-bearing capacity of the system,and shortens the normal service life of the structure.MgO,CaO or sulphoaluminate expansion agent are added into cement-based materials to hydrate and form Mg(OH)2,Ca(OH)2 or AFt to provide external volume expansion and effectively compensate for shrinkage and deformation of cement.The micro-expansion effect produced in the crystallization process of hydration products can generate precompression stress in the structure system,which can resist the early shrinkage stress and reduce the shrinkage cracking of concrete in the plastic stage and condensation hardening stage.By studying the microstructure and early age properties of MgO and nano-MgO high performance concrete,this paper provides effective data support and theoretical reference for the engineering application of nano-expansion materials and the hydration inhibition mechanism of mineral powder on magnesium expansive agent.Based on the application of expansive agent in hydraulic structure and ordinary concrete,this paper took the hydration expansion of MgO and nano-MgO in cement environment as the research point,and the microscopic characterization and early age performance and modification test of MgO and nano-MgO concrete structures were studied.The main research work and conclusions of this paper were as follows:(1)Study on the basic physicochemical characteristics of MgO and nano-MgO and their effects on the basic working performance of cement.The basic physical and chemical properties of MgO and nano-MgO,such as purity,particle size,crystal activity and hydration rate,were characterized by XRD,SEM and thermal analysis.Based on the material characteristics of MgO and nano-MgO,the mechanism of MgO and nano-MgO on the basic performance of cement such as standard consistency water consumption,setting time and water reduction rate of mortar were studied.The experimental results showed that the nano-MgO crystal particles were smaller,the lattice growth integrity was lower,and the ability of the molecules to participate in the reaction was stronger.MgO significantly increased the water consumption and setting time of the standard consistency of the cementing system.When the MgO content reached 10%,the setting time of the cementing system exceeded the requirements of the specification,which seriously affected the quality and performance of the cementing material.Under the action of the expansion agent with the same mixing ratio,the standard consistency water consumption of the cement-nano-MgO cementing system was smaller,and nano-MgO can effectively reduce the porosity inside the structure.When the nanometer MgO content ratio is greater than 4%,the hydration resistance of the cementitious system was effectively reduced,and the hydration of unhydrated cement cementitious particles was promoted,and the setting time of the cementitious material system was significantly reduced.(2)Preparation and mechanical and microscopic properties of MgO and nano-MgO high performance concrete.The effects of MgO and nano-MgO expansion agents on the strength,toughness and expansion properties of the structure during concrete plasticity stage,condensation hardening stage and later strength stability stage were studied.The hydration and expansion effects of MgO and nano-MgO in the cement environment were analyzed by XRD and SEM microscopic methods,and the mapping mechanism of micro performance characterization and macro performance of high performance micro-expansive concrete was studied by combining the microstructure of concrete and crystal distribution of expansion products.The experimental data of MgO and nano-MgO micro-expansion concrete were compared and analyzed,and the results showed that the effect of nano-MgO was better than that of MgO expansive agent.In the cement environment with low water-cement ratio,nano-MgO hydrated to form a large number of Mg(OH)2 rod-shaped crystals,which were cross-grown to form a compact structure,improve the mechanical properties and wthe toughness of the structure,and the crystal growth produce expansion effect,realizing the coordinated development of the mechanical properties and expansion properties of high performance concrete.(3)Experimental study on modification of MgO and nano-MgO high performance concrete with active mineral admixture.The active mineral admixture was added to the low water-to-binder ratio concrete to study the evolution of strength,toughness and expansion properties of MgO and nano-MgO micro-expansion concrete in the ore-pollen environment.Through SEM and TG-DSC microscopic tests,the mechanism of action of mineral powder active admixtures in high performance concrete on hydration degree and expansion performance of MgO and nano-MgO was analyzed,and the evolution law of macroscopic properties and microstructure of high performance concrete under the combined action of mineral powder,MgO and nano-MgO was explored.The experimental results showed that the strength and toughness of MgO and nano-MgO concrete were effectively improved by the mineral powder active admixture,but it was not conducive to the development of hydration expansion effect of MgO and nano-MgO.Combined with microscopic data analysis,the OH-concentration of the cementitious system decreased when the mineral powder mixing ratio reached 40%,which adversely affected the hydration degree and effective expansion efficiency of MgO and nano-MgO,and weakened the volume expansion deformation of high performance concrete structures;nano-MgO was more sensitive to the change of alkalinity of cement slurry,and the decrease of alkalinity seriously hindered the hydration process of nano-MgO.