Study On FA Critical Dosage Of Hydraulic Concrete Based On The Microstructure Characteristic And Deterioration Mechanism

Fly ash (FA) has become one of important components in the hydraulic concrete. While improving resource utilization of solid waste, due to replacing cement by FA, hydration heat of cement is reduced, mechanical properties and durability of concrete is improved, which take on the technical and economic double benefits. However, due to the limited measuring methods previously, it is devoid of the systematic study about C-S-H structure, composition, surface morphology and micro-mechanics properties due to the mixing fly ash, FA critical dosage during the microstructure formation and degradation process microstructure in the cement-fly-ash materials system and the relationship between macro-properties and microstructure. Through the thorough analysis of these problems, there is the important practical significance on materials research and engineering application and using fly ash with high efficient.Financial supported by National Basic Research Program of China (973) (No. 2009CB623200), National Natural Science Foundation Project (No.50972109) and Science and Technology Project of China Hydropower Engineering Consulting Group Company (CHECC). Aiming at the above-mentioned problems, the systematic study in this thesis focus on the correlation between microstructure and macroscopic mechanical properties, including microstructure formation and deterioration process of the cement-based materials, which is the low water to binder ratio and the different dosages fly ash. The main research results are listed as follows.1. The limit dosage of fly ash in the process microstructure formation is put forward based on the correlation between microstructure and macroscopic mechanical properties in the cement pastes incorporating with the different dosages fly ash.(1) When FA dosage is 40%, there is the best activity effect and pore structure at the early age. However, with increase of curing age, the lapse of FA dosage is 50%. There is the better relationship between the parameters of pore structure(α和λ) and strength of the cement pastes with the different FA dosages at the different curing ages.(2) Decrease of strength in the cement pastes is not obvious with increasing FA dosages compared to reference before FA dosages less than 50%. Pore structure is also improved with increasing FA dosages until more than 50% dosages, that is, the optimum parameters value of pore structure could achieve when FA dosage increasing to 50%, but after increased continuously to exceed 50%, strength and pore structure of cement pasts begin to be deteriorated.(3) FA dosage 50% is critical in that crystallinity is declined with the highest extent and structure and composition of C-S-H are varied significantly. When FA dosages less than 50%, the silicate polymerization and mean silicate chain length and aluminous of C-S-H are increased. The center of overlaping peak of Si-O stretching vibration is migrated to the high wave number. But after FA dosages more than 50%, chains begin to reduce. There are two center of absorption band of Si-O stretching vibration.(4) With the increase of fly ash dosages, silicon-oxide bodies with end groups are reduced in the cement pastes. An important observation in this thesis is that silicon-oxide bodies with end groups C-S-H in the cement pastes was more advantaged to the development of strength when comparing to mean silicate chain length C-S-H by XRD/NMR/FTIR testing.2. A kind of method is put forward for measuring the surface morphology and C-S-H cluster quantitative cohesion in the cementitious pastes.(1) Through the analyses and comparison of the advantages and disadvantages of four kinds sample preparation methods(natural section, polishing, replica and spinning) respectively, it is provided that the polishing method is the most suitable method for measuring the nano-scale surface morphology of C-S-H clusters in the cementitious pastes by AFM.(2) Characteristics of surface morphology and particle packing structures of C-S-H clusters by AFM are studied by adopting the single relatively component of the C3S pastes, which is synthesized by sol-gel method. C-S-H clusters are packed with the round and round amorphous nano-scale particles, whose size ranges are from twenty to thirty nanometers.(3) Comparison with the curing 3d, the packing density of C-S-H clusters is increased by the surface morphology research after 90d. Moreover, packing density of the cement pastes improved obviously than reference cement. In addition, C-S-H clusters in the pastes with incorporated 30% FA become more irregular and flat when contrasting with reference sample.(4) Interactions between the nanoscale C-S-H clusters in the cementitious pastes and the AFM Si3N4 tip are described by measuring force-distance curve in the real atmospheric conditions. Comparison with reference sample, force values of interactions in the cement pastes incorporated 30% FA are decreased significantly at the early age. But just the opposite at the later age, force values of interactions in the cement pastes incorporated 30% FA are almost equal with ones of the reference sample, which is coincident with their test results of the macroscopic mechanical properties.3. The optimal dosage of fly ash in the degradation process is put forward based on the correlation between microstructure and macroscopic mechanical properties in the cement pastes incorporating with the different dosages fly ash.(1) According to cumulative CaO dissolution, decreasing rate of compressive strength and pore structure in the cementitious pastes, mixing FA is advantage to dissolution resistance of the cement pastes. With increasing FA dosages, cumulative CaO dissolution was declined, but decreasing rate of compressive strength and specific surface area and porosity took on the first reduce after increased, the highest dissolution resistance is achieved at the 50% FA. (2) Due to condensation polymerization of dissolution, silicon-oxide bodies with end groups are inverted into mean silicate chain length C-S-H. Q'/Q2 of C-S-H after dissolution of the reference sample is declined more obviously than others, which is responding exactly to its macroscopic mechanical properties after dissolution decreased maximum. However, Q1/Q2 of C-S-H after dissolution of the sample with 50% FA dosages is increased slightly, which is responding exactly to its macroscopic mechanical properties after dissolution decreased minimum, that is, the highest dissolution resistance is achieved when FA dosage is 50%.(3) Due to the dissolution, the relative intensity of Ca-OH stretching vibration in the cement pastes is declined remarkable when comparing to their control sample, which indicate that Ca(OH)2 is also declined with the different range. An important observation in this thesis is that silicate polymerization and mean silicate chain length of C-S-H are increased in all cementitious pastes, which means that the dissolution of Ca(OH)2 as well as decalcification of C-S-H gel is happened in all cementitious pastes meanwhile the dissolution of unhydrated cement clinker.(4) Compared with mortar with 50% FA, after dissolution, fibrous C-S-H is significantly increased in the reference sample, its surface morphology becomes more loose and porous, which indicates also that silicon-oxide bodies with end groups are inverted into mean silicate chain length C-S-H with the high specific surface area. Mortar mixing 50% FA after dissolution, C-S-H is inverted from fibrous into thin sheets, which shows that the low ratio of calcium and silicon C-S-H produced by the pozzolanic reaction of FA has the better stability for dissolution resistance.