| The structural mechanical properties of Reactive Powder Concrete(RPC)were enhanced by optimizing the fractal gradation of aggregate and the amount of steel fiber.In this paper,the flexural and compressive strength,dynamic elastic modulus,hydrate phase,and micromorphology of RPC with different minerals(fly ash,quartz powder,and slag)are studied.By using industrial waste slag such as fly ash and slag,a new green high-performance concrete method is proposed in this paper.The main research results are as follows:The single-size quartz sand aggregate cannot form a dense spatial structure in the concrete.Therefore,using the fractal principle,in the experimental scheme with different fractal dimension values F,the mechanical properties of the RPC block are better than that of the single-size aggregate.At F = 2.34,the reinforced effect of aggregate-grade RPC concrete is the most obvious.Its compressive strength reaches 226.3MPa,which is 50.5MPa higher than that of single fine quartz sand aggregate RPC,an increase of 28.7%;the flexural strength reaches 30 MPa,which is 1.7MPa higher than that of single coarse quartz sand aggregate RPC,an increase of 7.1%,and its dynamic elastic modulus is 58.7GPa,which is 8.1% higher than the dynamic elastic modulus of single coarse aggregate RPC.Fly ash and slag have pozzolan activity and can be used as a mineral admixture to partially replace quartz powder in RPC.When 50% fly ash is used instead of quartz powder,the sample shows the best mechanical properties.The compressive strength reaches198.4MPa,which is an increase of 12.9% compared to the quartz powder sample,and the flexural strength is 33.0MPa,compared to the quartz powder.,the sample increased by 5.1%.Under this proportioning scheme,when 7 vol% long steel fibers were added,a 401 MPa RPC sample was prepared.The analysis of the phase and microstructure of this type of RPC shows that the hydration of tricalcium silicate and dicalcium silicate in the RPC matrix does not change much with the content of fly ash and slag replacing quartz powder.In SEM scanning electron microscopy,the RPC matrix showed a dense microstructure and morphology.In the internal water-filled space,there were calcium hydroxide and needle-shaped tobemullite.Under autoclaved curing conditions,fly ash and slag have similar hydration characteristics to quartz powder in the RPC matrix.The test results of adjusting the calcium-silicon ratio of the RPC system by replacing some cement with fly ash show that when the replacement amount of fly ash is less than 50% and the calcium-silicon ratio is greater than 0.30,the fly ash has a good reinforcing effect.After increasing the amount of fly ash instead of cement,the strength of the sample decreased due to the Ca content in the system being much lower than the Si content.When 0-80wt%cement is replaced by slag,the calcium-silicon ratio in the RPC system changes from 0.79 to 0.44.At this time,RPC still has excellent mechanical properties.The KC30 sample has the highest compressive strength,reaching 395.8MPa,and the KC80 sample also has excellent The mechanical properties and compressive strength reach 310 MPa.In terms of adjusting the calcium-silicon ratio of the RPC system,the slag enhancement effect is better than that of fly ash.Using the excellent performance of fly ash and slag in RPC,a series of high-performance concrete above 300 MPa can be prepared by compounding the two materials.Steel fiber-reinforced RPC has significant anisotropy.Studies have shown that anisotropy is significantly related to the RPC molding method,fiber length,fiber content,and load direction.With the increase of steel fiber content in the sample from 3vol% to 5vol%,the anisotropy index of RPC anisotropy of 12-14 mm steel fiber increased from 0.231 to 0.245;while the change of anisotropy index of RPC of 5mm short steel fiber increased from 0.203 to 0.249.The larger the aspect ratio,the more anisotropic the mechanical properties are.Therefore,in the process of stressing anisotropic RPC components,it should be noted whether the force surface at this time is a fiber-reinforced surface.If the conventional test results are still used to indicate the strength of the steel fiber-toughened RPC sample,and use it In engineering practice,there may be some hidden risks. |
