Self-desiccation is one common phenomenon of high-performance cementitious materials characterized by low water to cementitious material ratio (w/c). Autogenous shrinkage is closely related to the internal relative humidity (RH) drop and capillary pressure induced by self-desiccation in the cement pastes. However, there is debate about the determination of time-zero, the time at which autogenous shrinkage begins to develop. The objective of this study is to provide an accurate determination of time-zero based on the relationship between the internal RH and autogenous shrinkage of low w/c ratio cement pastes. And according to the time-zero, cement pastes blended with zeolite were prepared to investigate the potential of zeolite as internal curing agent. The autogenous shrinkage was conducted according to the standard method ASTM C1698. Internal RH was performed on the sealed cement pastes at very early age by conventional method of hygrometer. Setting time was determined by the Vicat needle apparatus according to the standard method ASTM C191. Experimental results revealed that no internal RH drop was observed around the final setting time determined by the Vicat method. Besides, a knee point was observed in the shrinkage curve at the time when the internal RH began to decrease. This is the so-called time-zero. And zeolite was found to be a potential internal curing agent according to the autogenous shrinkage tests measured from the new time-zero.
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Supplementary cementitious materials (SCMs) like fly ash (FA) and blast furnace slag (BFS) are normally used to replace parts of Ordinary Portland cement (OPC) to reduce the cost and CO2 emission. Some consequences are the reduction of portlandite (CH) content and the formation of C-S-H with low Ca/Si ratio, due to pozzolanic reactions. It is known that carbonation of portlandite leads to a reduction in the porosity which is ascribed to the positive difference of molar volumes between CH and CaCO3. However, the influence on the porosity caused by the carbonation of C-S-H is still controversial. The molar volume change due to the carbonation of C-S-H depends on the properties of C-S-H (like Ca/Si ratio, water content) and the water remained in silica gel. Moreover, the decalcification of C-S-H with the Ca/Si ratio lower than 1.2 can cause more structure changes and shrinkage of C-S-H. During the carbonation of cement paste blended with SCMs, less portlandite but a relatively high amount of C-S-H with low Ca/Si ratio will be carbonated. The pore structure will evolve in a different way, comparing with Portland cement paste. Therefore, it’s very important to figure out the pore structure development of cement paste blended with SCMs under carbonation. In this paper, the binary cement pastes (B70, blended with blast furnace slag, and F30, blended with fly ash) and ternary cement pastes (F10B54 and F30B30, blended with blast furnace slag and fly ash) are studied and compared with Portland cement paste. Mercury Intrusion Porosimetry (MIP) and nitrogen adsorption isotherm are used to determine the pore volume and size distribution of capillary pores and gel pores (2-37 nm), respectively. Thermogravimetric analysis (TGA) is used to determine the amounts of portlandite and CaCO3. The results show that the amount profiles of portlandite and CaCO3 can be used as a more accurate method to study the carbonation in blended cement paste, comparing with the phenolphthalein test. Carbonation of most of the species of C-S-H results in the increase of the porosity of cement paste. CaCO3 contributed by the carbonation of low Ca C-S-H is dominant in blended cement paste B70, F10B54 and F30B30. Both the total and effective capillary porosity increases in the above-mentioned paste after the carbonation. Moreover, total porosities of B70 and F10B54 increase with the increasing amount of C-S-H involving in carbonation. However, the increment of the total porosity of F30B30 decreases with the increasing amount of C-S-H being carbonated. Carbonation of C-S-H increases the volume and size of the small gel pore, and creates more capillary pores. This peculiar phenomenon is more evident for the mixture with a higher proportion of SCMs, like B70 and F30B30. The results reveal that the carbonation of C-S-H formed in pozzolanic reactions cause the increase of the total and capillary porosity in cement paste blended with SCMs, which will bring adverse effects on the durability of blended cement concrete exposed to the carbonation.
The lattice Boltzmann (LB) method is an efficient numerical approach which is widely utilized in the modeling of mass transport in porous media within a few decades because of the easy implementation of complex boundary conditions, inherent parallelism and high numerical stability of this approach. This paper presents the simulation of the effective ionic diffusivity of cement paste using the LB method. The entire simulation consists of three main steps. Firstly, the three-dimensional (3D) microstructure of hydrating cement paste is developed by the cement hydration model HYMOSTRUC3D. Cement paste is modeled as a composite material consisting of capillary pores, un-hydrated cement particles and hydration products (inner-, outer-, contact-). Secondly, the obtained 3D microstructure is digitized into a 3D array of voxels at a certain identical resolution (voxel size). Each voxel is assigned to be pore or solid depending on the position of its center. Finally, the diffusion process driven by the concentration difference between inlet and outlet is simulated using LB model. The operations such as collision and streaming are running until the steady-state diffusion condition is achieved. On the basis of the calculated steady-state molar flux across the whole domain, the effective ionic diffusivity can be estimated. Taking the diffusion of chloride ions for example, the simulated effective chloride diffusivities of cement paste are compared with the experimental results presented in literature. The simulation shows a good agreement with the experimental results. In addition, the influence of resolution on the simulated effective chloride diffusivity is studied.
The authors introduced and described the main contents of new standard EN 868 Packaging materials for terminally sterilized medical devices in details,summarized the development situation of medical device packaging at home and abroad,and put forward suggestion and proposal for development of medical device packaging in China.
Alternating current (AC) impedance spectroscopy has been applied in characterizing microstructural evolution and electrochemical properties of cement-based systems. In the present paper, an equivalent circuit model was proposed to study the influences of chloride binding on microstructure and solid-liquid interfacial properties of cement paste. Chloride concentration index of pore solution was measured to correlate to the parameters in equivalent circuit model corresponding to electrical double layer at solid-liquid interface. The results showed that the parameters of equivalent circuit model can properly indicate the microstructure and interfacial properties of cement paste. Resistance of continuous pores was gradually decreased with chloride concentration in soaking solution due to the higher conductivity of pore solution. The capacitance of electrical double layer was increased with chloride concentration in pore solution due to more content of chloride ions in electrical double layer. While the thickness of electrical double layer was decreased as chloride concentration increased, which is in agreement with mathematical calculation.
In the present paper pollen grains of 8 species in the genus Dracaena (Liliaceae) from China were examined by scanning electron microscope, are reported here for the first time. As a result, four types of the pollen exine sculpture are distinguished: (1) foveolate, D. cochinchinensis and D. menglaensis; (2) regulate, D. cambodiana and D. terniflora; (3) reticulate, D. gracilis, D. angustifolia and D. ensifolia; (4) tuberculate-baculate, D. hokouensis. The differences in pollen morphology of Chinese Dracaena can serve as characters for delimitating species.
