Research On The Influence Of Micro-and Nano-powder On Cement Physico-mechanical Performance And Its Mechanism

The various techniques which can be used to improve cement performance have been or will be the focus of investigation into cement-based materials in 21^st century, and one of the mostly used methods is composite technique. Especially with extension of application area of ultra-fine mineral powder, the research on the application of ultra-fine mineral powder in cement-based materials just starts to expand in many countries in the world. The techniques which use ultra-fine power to advance cement have not only its necessity which develop high performance cement-based materials, but also great significations which push forward the development of high performance cement and concrete and reform traditional cement and concrete industry by high-tech.In this thesis, the influence of micro- and nano-powder, such as nano-silica powder, silica fume and ultra-fine calcium carbonate, on physico-mechanical performance and its mechanism of Portland cement and sulphoaluminate cement are studied by modern testing methods, such as SEM, XRD, TG-DTA, IR, BET, laser granulometer and microcalorimeter, etc.. Based on the studies, the influence of ultra-fine slag powder and ultra-fine fly ash on cement physico-mechanical performance is discussed. The suitable formulation and production technology by which the green high performance composite ultra-fine mineral powder are made using slag, fly ash and limestone, etc, are also investigated. The results obtainedare as follows:It shows that nano-silica powder and silica fume can both improve strength of Portland cement, and the strength of the sample containing nano-silica powder obviously is higher than that of the one containing silica fume after curing three days. The reason is that the pozzolanic property of nano-silica powder is higher than that of silica fume, which is proved by that the grain size of nano-silica powder is smaller than that of silica fume, the surface energy of nano-silica powder is higher than that of silica fume, and the polymerization degree of [SiO₄]^4- of nano-silica powder is lower than that of silica fume. The hydration extent of the sample containing nano-silica powder is higher, the amount of CSH gel grows more and the content of the crystalline Ca(OH)₂ is lower. So the inner specific area and the total pore volume of harden pastes of the sample containing nano-silica powder are smaller, the microstructure is more homogeneous and dense, so that the strength of Portland cement is higher. The reasons that the strength was improved by use of nano-silica powder are the high pozzolanic property and the perfect filling effect. The high pozzolanic property of nano-silica powder comes into play in early, middle and late period, but the perfect filling effect of that comes into play in middle and late period.After mixing nano-silica powder into sulphoaluminate cement, the inner specific area, the total pore volume and mean pore diameter of harden pastes is lower than that of the reference, and microstructure becomes more dense and homogeneous. The late period strength of the sample containing nano-silica powder will not fall down, but stable increase.Carbide ash was successfully used to prepare ultra-fine calcium carbonate in the laboratory. The influence of ultra-fine calcium carbonate on physico-mechanical performance and its mechanism of Portland cement are studied. The results show that 1-day-strength of the sample containing ultra-fine calcium carbonate can be increased by 50 percent to 80 percent than that of the reference, and is higher by 30 percent to 50 percent than that of the one containing grounded limestone powder, but 28-day-strength of the samples with or without ultra-fine calcium carbonate are almost same.It shows that the drawback of the downfallen bending strength of sulphoaluminate cement can be overcame by addition of ultra-fine calciumcarbonate into sulphoaluminate cement, and the strength of sulphoaluminate cement in early and late period can also be enhanced. The reasons that ultra-fine calcium carbonate improve the strength are the perfect filling effect, the dilution effect and the chemical effect on the sulphoaluminate cement system. The chemical effect shows off by that ultra-fine calcium carbonate can react with C4AF to produceC3ACaCO3llH2O and have the reaction(3C4A3S+3CSH2+2CC+3CH+92H=2C6A53H32+2C4ACHi,+5AH3) to produce AFt, C3ACaCO3llH2O and A1(OH)3to accelerate clinker hydration and make microstructure more homogeneous and close-grained, and that calcium carbonate has the role of calcium sulfate which can prevent AFt transform into AFm, which make the late strength not draw back.Based on the results above, green high performance composite ultra-fine mineral powder by mixing slag, fly ash and limestone has stepwise hydration effect on Portland cement, i.g. early strength of Portland cement is obtained by clinker and limestone, middle strength of that is obtained by slag, and late strength of that is obtained by slag and fly ash. The addition of composite ultra-fine mineral powder successfully resolves the problem which the strength of the sample containing slag and fly ash fall down. The production line which manufactures composite ultra-fine mineral powder was established and can use a great deal of fly ash and limestone. The composite ultra-fine mineral powder can be used to manufacture green high performance cement and concrete by its micro-and nano-effect. 3-day-strength and 28-day-strength of the sample containing 55 percent composite ultra-fine mineral powder are 1.3 to 1.9 times and 1.6 to 2.2 times, respectively, as that of the reference.lt is obvious that the application of composite ultra-fine mineral powder to the production of green high performance cement and concrete in industrial scale can not only cut down the cost, but also have largely pronounced society benefit.