| With the rapid development of infrastructure construction in China,many important construction projects or special engineering components require high-performance cement-based materials,for instance the large-span beams and plates,bridges and plates of highways and railways,military protection engineering with high-strength and special components or products used in civil construction engineering.However,the porous structure and high porosity of cement-based materials seriously restrict the development of high performance of it.Reducing the initial water-cement?w/c?ratio of cement-based materials and applying pressure molding technology are effective ways to increase the density and realize its high performance.However,reducing the w/c ratio will result in the uneven dispersion of water in cement,making it easy to form a?liquid bridge?between some water-rich particles,resulting in heterogeneous agglomeration of cement particles,as well as heterogeneous hydration of some water-poor particles,resulting in uneven composition and microstructure of cement-based materials and great fluctuations in performance.Therefore,solving the problem of uniform dispersion of water under pressure and low w/c ratio is the key to the high performance of cement-based materials with low w/c ratio.It is difficult to totally eliminate the?liquid bridge?once it is formed.Therefore,from the point of view of?no initial liquid bridge?,this paper proposes the?micro-structure uniform control technology?of?solid mixing water?to prepare cement based materials with super low w/c ratio.The main study content of the paper including the uniform control of microstructure and multi-scale characterization of the material,the hydration characteristics of cement at super low w/c ratio with homogeneous microstructure,the failure mechanism of homogeneous microstructural hardened cement paste with super low w/c ratio,the effects of micro powders on properties of the homogeneous microstructural materials,and the modifying of homogeneous microstructure cement-based materials by carbon fiber.In addition,the three dimensional X-ray microscope?X-CT?,mercury injection apparatus?MIP?,X-ray diffraction apparatus?XRD?,scanning electron microscope?SEM?,thermogravimetric analyzer?TG?and hydration heat analyzer et al are employed to study the mechanism of action.The main findings of the study are as follows:1)High performance cement-based materials with homogeneous components and microstructure were successfully prepared by using the technology of?solid mixing water?at super low w/c ratio.The fluctuation range of compressive strength of hardened prepared by?solid mixing water?technology was less than 4.4%of its own strength,which was reduced by 66.3%.The amplitude of ultrasonic transmission increased by 94.4%,and the integral value of the Fourier transform curve of ultrasonic transmission decreased by 51.9%.The pore size distribution range of the hardened paste was 0.1?7?m,which was obviously lower than that of the reference from 0.01?200?m.The results of X-CT test showed that,there was not any pore with diameter more than 9?m of the hardened paste prepared by?solid mixing water?,which was unlike the reference.Compared with the reference,the size fluctuation range of unhydrated cement particles with the same particle size distribution in the hardened paste prepared by?solid mixing water?technology was reduced by 62.1%.2)The maximum hydration reaction rate of cement changes under super low w/c ratio.In general,the maximum hydration reaction rate will increase with the increase of w/c ratio.However,when the w/c ratio was 0.16,the maximum hydration reaction rate was34.51×10-4mW/mg,which was slightly higher than 34.12×10-4mW/mg when the w/c ratio was 0.30.This is mainly because when the w/c ratio is 0.16,the alkalinity of the pore solution with larger concentration of Na+and K+ions is higher,accelerating the hydration of cement.3)The hydration mechanism of cement-based materials with homogeneous micro-structure cement changes under super low w/c ratio.When the w/c ratio was lower than 0.16,the hydration mechanism of Portland cement changed from the crystallization nucleation and crystal growth?NG?-phase boundary reaction?I?-diffusion?D?mechanism when the water-cement ratio was 0.30 to the crystallization nucleation and crystal growth?NG?-diffusion?D?mechanism.4)The change regularity of porosity and compressive strength of hardened cement pastes with homogeneous microstructure changed at super low w/c ratio.Generally,the porosity of hardened cement paste increased and the compressive strength decreased with the increase of w/c ratio.However,the porosity decreased and the compressive strength increased with the increase of w/c ratio at super low w/c ratio.This is because the initial porosity was determined by the molding pressure,and the amount of hydration products and the filling rate of hydration products to the initial void at the same curing age increase with the increase of the w/c ratio.5)The study on the failure mechanism of homogeneous microstructural hardened cement paste with super low w/c ratio suggested that the relationship between the compressive strength and total porosity is a function of y=366.2exp?-7.382x?,which proves that the total porosity is an important factor in determining the mechanical properties of homogeneous microstructural cement-based materials with super low w/c ratio.6)There was an important effect of compact pressure on the porosity and mechanical properties of cement-based materials with super low w/c ratio.The initial porosity decreased with the increase of compact pressure,and the relationship between the two parameters was the exponential function of y=21.68exp?-0.0062x?+16.78.The porosity of hardened cement paste decreased with the increase of compact pressure.The 28-days porosity decreased to 9.7%and the compressive strength increased to 215.4 MPa and the flexural strength increased to27.5 MPa at the compact pressure of 300 MPa.7)The microstructure and mechanical properties of cement-based materials with super low w/c ratio could be effectively improved by slag,and its optimal content is 10%.At this dosage,the 28-days porosity decreased to 4.8%and the compressive strength reached 247.5MPa and the flexural strength reached 35.4 MPa.In addition,There was important influence of slag on the hydration mechanism of cement at super low w/c ratio.The hydration mechanism of cement changed from the crystallization nucleation and crystal growth?NG?-diffusion?D?mechanism to the crystallization nucleation and crystal growth?NG?-phase boundary reaction?I?-diffusion?D?mechanism when the content of slag powder reached up to 20%.8)The microstructure and mechanical properties could be effectively improved by silica fume,and its optimal content is 15%.At this dosage,the 28-days porosity is only 2.8%,and the compressive strength reached up to 296.3 MPa,and the flexural strength reached up to43.8 MPa.9)The shrinkage and porosity of hardened cement paste could be reduced by quartz sand,and the mechanical properties could be enhanced by it.The optimal content of quartz sand was 20%in this paper.At this dosage,the chemical shrinkage was reduced to 0.1129 ml/g and 150-days dry shrinkage was reduced to 147.4×10-6.Compared with the reference,the chemical shrinkage and the dry shrinkage decreased by 37.9%and 38.1%.The 28-days porosity was reduced to 2.2%,which was 21.4%lower than that of the reference.The28-days compressive strength of the hardened cement paste reached up to 311.5 MPa and the flexural strength reached up to 47.4 MPa,an increase of 5.1%and 8.2%respectively compared with the reference.10)The flexural strength,the ratio of flexural strength-to-bulk density?FS/BD?and the frost resistance could be effectively enhanced by carbon fiber,and the dry shrinkage could be reduced by it.The optimal dosage of carbon fiber was 0.3%in this paper.At this content,28-days flexural strength reached up to 56.1 MPa,and the ratio of FS/BD reached up to0.0212,and the loss of mass and strength decreased to 2.23%and 12.0%after 200freeze-thaw cycles,and 150-days dry shrinkage decreased to 117.0×10-6.The flexural strength increased by 18.4%,and the ratio of FS/BD increased by 19.1%,and the loss of mass and strength decreased by 58.6%and 35.8%,and the dry shrinkage decreased by 20.3%compared with the reference. |
PDF Full Text Request