Abstract: As the major structure of silica fume, the change of silica tetrahedron in the pozzolanic reaction during the hydration has not been revealed clearly in previous studies. In this study, 29Si solid-state MAS NMR was used to characterize the silica tetrahedron change of the silica fume in saturated alkali solution with 0.9, 1.2, 1.5 and 1.8 four different calcium/silica ratios. The amorphous Q4 silica tetrahedron structure in silica fume changed into Q1 silica tetrahedron at 1 day. Q2 silica tetrahedron formed from Q1 silica tetrahedron within 3 days. Q1 and Q2 silica tetrahedron reached a balance until silica fume completed pozzolanic reaction and the Q4 silica tetrahedron exhausted. The coexistence of Q1 and Q2 silica tetrahedron benefited the physical properties increase of cementitious system. 29Si solid-state MAS NMR results proved that the chain length of silica tetrahedron in C-S-H shortened in the silica fume hydration while the C/S ratio increased.
Abstract: Early cement paste (0h-6h) made with cement of different Blain specific surface area was investigated with 23MHz 1H low-field NMR. T2 distribution, T2 against age and T2 distributions against age were obtain. Results showed: 1) T2 distribution of fresh cement paste made with cement of common Blain specific surface area has two peaks and corresponded to water within flocculation and among flocculation; 2)T2 against age could reflect a stage-like behaviour of cement hydration; 3) T2 distributions against age showed evolution of microstructure in early cement paste.
Abstract: In this paper, the freezing-melting hysteresis was first obtained in two mesoporous molecular sieves with similar pore size to analyze the reason lead to the different results in nuclear magnetic resonance cryoporometry (NMR-C) experiment between them. Then the hardened white cement paste was explored by the same method. The primary results shows that the freezing branch can offer more details to the pore size distribution (PSD) obtained from the melting branch, which will probably improve the accuracy of PSD.
Abstract: The water distribution in hardened cement paste with different ages, water to cement ratio (w/c) and different cured methods were investigated by low field NMR. The transverse relaxation time, T2, was used as a parameter to describe the water phase constrained in pores. The results show that the T2 distributions of pastes in the early age are bimodal distribution. As the curing time increase, the T2 distribution peaks shift gradually to the short T2 values reflecting the decrease of mean pore dimension as well as the increase of specific surface area resulted from the gel products. In addition, the influences of initial water to cement ratio and cure methods on water distribution are occurred mainly during the early age. When cured to 28 days, the differences of water distribution in various samples are unconspicuous.
Abstract: The quantitative characterization of hydration of cement pastes has always been one of focuses of researchers’ attention. Rietveld phase analysis (RPA), a combination of quantitative X-ray diffraction (QXRD) and the Rietveld method, supplies a tool of an enormous potential for that. Although a few of related researches were conducted by RPA, the reported attention was not paid to the neat cement paste with a low w/c ratio. Therefore, this work aimed at the quantitative study on hydration of such a cement paste chiefly by this method, meanwhile, cooperated with the hyphenated technique of thermogravimetry with differential scanning calorimetry (TG-DSC), as a spot check. Results indicated that RPA was a reliable method in quantitatively characterizing hydration of cement pastes, and gave a clear decription of evolution of all main crystal phases in cement pastes; and that the evolution of monosulphate(Afm_12) was also able to be tracked quantitatively. This will help to understand better the hydration mechanism of cement pastes, as well as to investigate quantitatively effects of mineral and chemical admixtures on hydration of composite cementitious systems.
Abstract: The variations of dielectric constant and the amplitude of reflected EM wave of concrete during the first 3 days are measured with Ground Penetrating Radar (GPR) at 20 oC. The amplitude decreases sharply after mixing with water, and then increases till a stabilized stage, followed by a gradual decline. The relative dielectric constant decreases with increasing hydrating time. The results show that the dielectric properties of concrete can be used as an effective way of studying the kinetics of concrete setting and hardening process at early ages.
Abstract: A kinetic model originally developed by Tomosawa is analyzed for the hydration of Portland cement. The complicated kinetic equation of Tomosawa model is transformed into three simple equations of simplified model which represent mass transfer, phase-boundary reaction and diffusion process at different stage, respectively. Based on the isothermal calorimetry and the simplified model, the relationship between the degree of reaction and reaction rate is discussed. The derived result could reflect the development of degree of reaction and reaction rate during the cement hydration. The calculation results of simplified model are compared well with the experimental results. Studying the connection among the three processes is helpful to understanding the complicated hydration mechanism of Portland cement.
Abstract: The influence of water to binder (W/B), types and dosage of supplementary cementitious materials (SCM) on the internal relative humidity (IRH) and autogenous shrinkage (AS) of cement pastes caused by self-desiccation were investigated, and their relationship was discussed. The results show that, W/B is a main factor that affects IRH change and AS of cement pastes with SCM. With the decrease of W/B, IRH of cement pastes decreases, but AS of cement pastes increases. Different types and dosages of SCM affect the IRH differently; fly ash (FA) reduces AS, silica fume (SF) increases AS, and the effect of GBFS on AS is between FA and GBFS. The linear correlation between the change of IRH and AS of cement pastes with SCM is established.
Abstract: Abstract. With the more application of pervious concrete in China, its ecological benefits was paid more attention, especially improve "heat island effect"of the urban; In this paper, according to the mechanism of pervious concrete how to improve atmospheric humidity, the testing device for humidity-control performance of pervious concrete for laboratory testing was designed, humidity sensor is used in combination with automatic acquisition program, the equipment can test the humidity-control of pervious concrete under different conditions.