1,721,094 research outputs found
Influences of chloride immersion on zeta potential and chloride in concentration of cement-based materials
Full text linkIn this paper, the zeta potential of freshly mixed cement paste and hardened cement pastes, as well as the concentration index, was measured. The influences of chloride concentration in mixing water and slag content on zeta potential of freshly mixed pastes were studied. A proposed model was expressed to explain the relationship of zeta potential and concentration index of hardened cement pastes immersed in chloride solution. The results showed that the increase of chloride concentration in mixing water and slag replacement improved the zeta potential of freshly mixed cement, the hydration rate and concentration of ions in mixed water affects the zeta potential. With the increase of chloride concentration in soaking solution, the chloride concentration index and zeta potential of hardened cement paste all gradually decreased. The addition of slag gave some changes on chloride in concentration and zeta potential. The relationship among chloride concentration index, chloride concentration in soaking solution and slag replacement revealed by Gouy-Chapman model was in good agreement with the measured results
Effect of post-fire lime-saturated water and water-CO2 cyclic curing on strength recovery of thermally damaged high-performance concrete with different silica contents
No full textThis study investigates the effects of lime-saturated water and water–CO2 cyclic recuring on strength recovery of thermally damaged high-performance concretes (HPC). The HPC samples were subjected to elevated temperatures up to 1000 °C in 200 °C increments and underwent recuring. Phase assemblage and distribution, microstructure evolution, and pore structure of the HPC samples were identified. According to the results, recovered compressive strength of the HPC samples with low silica content can surpass their original strength after 600 and 800 °C exposure and recuring. In contrast, HPC with high silica content is unfavorable for strength recovery at temperatures above 800 °C because the low-calcium phases formed have low reactivity. After 1000 °C exposure, only water–CO2 cyclic recuring coalesces the disintegrated microstructure and recovers the compressive strength. Strength recovery primarily depends on healing the microcracks and large pores rather than the coarsened cement paste
Optimizing paste proportions to enhance early age strength of high volume fly ash (HVFA) concrete.
No full textUse of higher proportions of fly ash as a cement replacement in concrete has obvious environmental benefits such as reducing cement consumption and increased use of fly ash (alleviating issues with fly ash disposal). HVFA concrete benefits from enhanced workability and cohesion, lower heat of hydration and increased long term strength and durability. However, high volumes of fly ash are not commonly used due to perceived lower early age strengths. In this investigation, addition of cement kiln dust (CKD) and gypsum to activate the fly ash was studied and the proportions used in the paste mixes were designed to optimize the mixture ingredients to achieve the highest early age compressive strength. It was found that CKD was much more effective in activating the fly ash than gypsum and appreciable early age compressive strengths were achieved for fly ash contents up to 60% of the binder material
Autogenous shrinkage of zeolite cement pastes with low water-binder ratio
Full text linkSelf-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
On the Optimisation of Cement Microstructure for Nuclear Waste Encapsulation - Probing the Effect of Hydration on Migration Pathways
No full textThe hydration process of Ordinary Portland Cement (OPC) has been investigated using 2D Digital Image Correlation (DIC). OPC blended with SrCl2 solution was investigated, with a focus to understand the effect of hydration on microstructure development and associated leaching behaviour. The aim of the study was to establish a link between chemical shrinkage during hydration and OPC microstructure evolution using in-situ imaging techniques. The development of heterogeneous local surface displacements was observed, resulting in a granular displacement pattern at the sample surface. The observed displacement pattern are currently correlated with preferred migration pathways in cement microstructure, by using a combination of in-situ leaching tests with 2D optical and 3D X-ray Computed Tomography (X-ray CT) techniques. X-ray Diffraction (XRD) of cement microstructure after acid leaching has also been explored to optimize in-situ leaching experiments. A general overview of the experimental methodologies is provided in this paper
Experimental investigation of hydration of ternary blended cement paste
Full text linkIn this paper, ternary and binary blended cement pastes as well as pure Portland cement paste were prepared by Portland cement, ground granulated blast-furnace slag (GGBFS), limestone powder and water, which were then hydrated from 1 to 91 days at 20 °C in a sealed environment. At each curing age, the hydration kinetics of cement and slag was determined by XRD/Rietveld and selective dissolution method (EDTA), respectively. The content of CaCO3 (limestone) was quantified by Thermogravimetric analysis (TGA) technique.
The degree of hydration of cement clinker was distinctly accelerated by the single addition of slag or limestone within 91 days of hydration. The coexistence of slag and limestone in ternary blended cement accelerated the hydration of cement clinker within the first 14 days of hydration, but lowered the degree of hydration of cement clinker after 91 days of hydration compared with other pastes. The degree of reaction of slag in blended cement pastes was about 8% and 35% after 1 and 91 days of hydration, respectively, which was almost not influenced by the addition of limestone powder. A small amount of limestone, i.e. around 2% of the total solid raw materials, was reacted in pastes, and mainly occurred at the early age.
Based on the experimental investigation, the results show that the hydration of calcium silicate phases of cement in pastes was enhanced by the presence of limestone, but hampered by slag. The hydration of calcium alumina phases of cement was greatly accelerated by the addition of slag, and also enhanced by the presence of limestone powder in binary blended limestone cement paste at early age. However, the coexistence of limestone with slag in ternary blended cement paste restrained the hydration of calcium alumina phases of cement
Mechanism of microstructural modification of the interfacial transition zone by using blended materials
Full text linkApplying blended materials with finer particle size or high reactivity could be an effective and economic way for improving the microsturcture of interfacial transition zone (ITZ). In this study, the porosity characteristics of ITZ in concrete made with OPC and blended binders were determined quantitatively by using backscattered electron microscopy (BSE) image analysis and mercury intrusion porosimetry (MIP) measurements. This paper especially focused on the effects of slag and limestone filler on the thickness and pore structure of the ITZ. Results indicated that the porosity at each distance reduces with increasing limestone filler from 0 to 5%, and a significant increase is observed in the sample with 10% of limestone filler. The addition of 5% of limestone filler is able to densify the pore structure of both ITZ and bulk matrix. The reduction in pore volume in the range coarser than 100 nm contributed to the largest decrease in the total pores. Increasing the incorporation level of limestone filler to 10% resulted in an increase in the total porosity. The influences of slag on the porosity characteristics were highly dependent on the replacement level and the determined pore size regions. The addition of 35% of slag reduces the porosity at all distances and produces a denser microstructure both in the ITZ and bulk cement matrix. However, this improvement disappears when the substitution amount reaches to 70%. The incorporation of slag as a partial substitute for Portland cement tends to refine the pore structure
Development of porosity of cement paste blended with supplementary cementitious materials after carbonation
Full text linkSupplementary 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
Bacteria based repair and self-healing of concrete
Full text linkDurability problems in concrete can often be linked to a high permeability, which is either caused by a high matrix permeability or the presence of cracks. Therefore, treatments that reduce the permeability of the matrix, or that close the crack from ingress of aggressive agents carried by water or air, would substantially enhance the service life of a concrete structure. Several chemical products are currently in use for consolidation and crack repair, but a new technique that has been the focus of much research efforts over the last decade is the bacteria-based calcium carbonate precipitation. This technique is now slowly making its way towards practical applications. The principles of the technique, the important influential parameters and the recent advances related to its use for consolidation, surface protection, external crack repair, and self-healing of cracks in concrete are discussed in this article. Also the wider applicability for mechanical strengthening and consolidation of natural stone and soils is shortly treated
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