CEMENT最新文献

This study presents an overview of numerical models simulating frost action in cement-based materials. Most of the frost action models are grouped in one of three main groups named poroelastic models, lattice models, and rigid body spring models formed according to the followed mechanical principles providing stress estimation and volume change. Other models are further grouped based on underlying physical or empirical principles and potential applications. It is the intention that the overview of numerical models highlights aspects of frost action that are known to be important in experimental research but considered very sporadically in numerical modeling. This study can help new model builders to choose a modeling approach, and important factors need to be considered for their own work.

This paper examines the acid resistance of cement pastes where a portion of the cement clinker is replaced with limestone (LS or calcium carbonate, CaCO₃) or ground silica (GS). Specifically, the work is intended to better understand the acid resistance of ASTM C595 IL cement as compared with ASTM C150 cement. The performance of OPC, OPC + GS, and OPC + LS systems were tested in sulfuric acid baths where the pH was held constant at 2.0 and 3.0 using an automated setup that uses titration to add acid. The degradation of the cement paste was measured as a function of time. Thermogravimetric analysis (TGA) was used to quantify changes in the calcium hydroxide (Ca(OH)₂) and calcium carbonate (CaCO₃) contents of the paste. In addition, the flexural strength of the cement paste specimens was measured. Results indicate that the dissolved sulfate and calcium concentrations due to acidification were not noticeably different for the OPC + GS and OPC + LS mixtures exposed to the same pH. However, as expected, differences were observed between the samples immersed in the solution of pH∼2 and pH∼3 sulfuric acid with the lower pH corresponding to more severe deterioration. TGA results showed that Ca(OH)₂ is more susceptible to acid attack than limestone as evidenced by the larger Ca(OH)₂ and sulfuric acid consumption in samples immersed at pH∼2. The additional acid consumption that is beyond the consumption of Ca(OH)₂ can be explained by the acid attack of other hydration products such as CSH and unreacted cement phases. This results in a significant B3B flexural strength loss for the samples immersed in a pH∼2 as compared to those in the pH∼3 solution. The results demonstrated that the performance of ASTM C595 IL cements was promising and comparable with ASTM C150 cements.

Ettringite formation is an expansive reaction that causes cracking in the hydrated cementitious materials. This research has investigated the mechanisms of ettringite formation by examining the chemical and physical structure of the reactants and products involved in the process of late age (over 15 years) ettringite formation, and subsequent expansion and cracking. For this, seven different types of commercially available cement with their unique composition, and an elevated heat curing temperature of up to 100 °C were applied. The physical expansion of mortar bars due to delayed ettringite formation was monitored by the length change comparator. Environmental scanning electron microscopy (ESEM) was used to qualify and quantify changes in the microstructure and chemical composition of the cementitious matrix. Results revealed that the high-temperature heat curing accelerated the onset of expansion but limited the over-magnitude of the expansion. Results also revealed that the expansion may take years to initiate, likely due to a critical pore size threshold necessary to induce stresses. If expansion is delayed, the expansion magnitude is greater than those that expanded immediately.

The production of AYF (alite-ye'elimite-ferrite) clinker was tested at laboratory and semi-industrial scale using by-products from the metallurgical industry: AOD slag; ladle slag; and fayalitic slag. Alite could be produced with ye'elimite using fluorine originating from AOD (argon oxygen decarburisation) slag as a mineraliser. After a successful laboratory demonstration, the clinker production was scaled to a semi-industrial trial. It was discovered that the reason for the absence of alite formation in a semi-industrial demonstration was that the AOD slag from the specific batch did not perform the designed mineralisation effect for alite formation. This study demonstrates that alite-ye'elimite can be produced at 1260 °C at laboratory scale by using fluorine mineralisation originating from an industrial by-product – in this case, AOD slag. However, the utilisation of by-products for delicate reactions requires detailed determination of the properties of the slag, as the variability from the same source yields different clinker chemistries and mineral phases.

Reactive magnesia cement (RMC) is an emerging class of green cement that hardens by sequestering CO₂. However, CO₂ diffusion into RMC is restricted to a few millimeters by the carbonation-induced dense microstructure on the outer layer, which severely slows down the strength growth and CO₂ sequestration. To address this issue, this work employed hollow natural fibers (HNFs) to facilitate CO₂ diffusion into the deep regions of RMC. The effects of HNFs contents on the mechanical strength development, holistic porosity, CO₂ sequestration, CO₂ diffusivity, and microstructure of RMC were investigated through different techniques. The findings revealed that the compressive strength could be more than doubled with the addition of adequate sisal fiber. Moreover, the CO₂ sequestration and diffusivity could be continuously enhanced with the increasing HNFs content. However, overdosage of HNFs could induce a higher porosity and additional defects, which slightly compromises the mechanical strength. Finally, the durability of HNFs in simulated RMC and Portland cement (PC) environment was compared by accelerated aging test, showing that the alkaline-induced deterioration of HNFs could be almost eliminated in RMC. Therefore, this preliminary study reinforces the function of RMC as a carbon reservoir and lays the foundation for the large-scale utilization of HNFs in RMC.

In this study, concentrated CO₂ gas was used to create foamed cement paste, and concentrated CO₂ gas was intermixed in fresh cement paste to produce regular cement paste materials that were not foamed. The potential of both methods to form CaCO₃ from calcium ions that would form Ca(OH)₂ was investigated. After curing the materials for different ages, the Ca(OH)₂ and CaCO₃ contents were measured using thermogravimetric analysis; the large void size distributions, dynamic modulus, and compressive strength were tested and compared against Control (no gas added), and N₂ foamed and N₂ intermixed specimens. A 10% increase in dynamic modulus and compressive strength was measured in CO2 foamed specimens compared to N2 foamed specimens. The increase in mechanical properties was the result of both a narrow void diameter distribution and CaCO3 formation in place of Ca(OH)₂. The CO₂ foamed cement generation method shows the potential to sequester 0.06 ton of CO₂ for every ton of cement which is a CO₂ emission reduction of 7.0% of the CO₂ production associated with cement production. For the CO₂ intermixing method, the void content, compressive strength, and dynamic modulus results were consistent between the Control and CO₂ intermixed specimens while the N₂ intermixed specimens showed a decrease in compressive strength and dynamic modulus. The CO₂ intermixing method showed potential to sequester 0.04 ton of CO₂ for every ton of cement or a 4.7% CO₂ emission reduction of the total CO₂ production associated with cement manufacturing. The reported CO₂ emissions are not based on life cycle assessment and do not account for emissions associated with CO₂ collection, transportation, and intermixing. The present paper does not investigate the mechanisms of hydration under CO₂ intermixing.

The attractive properties of ye'elimite (C₄A₃$) are receiving particular attention in cement and repairing materials. The present paper aims to further explore the difference in hydration performance of different C₄A₃$ phases (o-C₄A₃$ and c-C₄A₃$) using the Ga-doping method. On this basis, C₄A_3-xG_x$ specimens (noted as C₄A₃$, C₄A_2.3G_0.7$, and C₄A_1.6G_1.4$) were first synthesized and their hydration characteristics were systematically studied. Results showed that Ga-doping enhanced the hydration activity of ye'elimite. With increasing Ga³⁺ ions addition, the intensity of the initial peak gradually increased, but the duration of the induction period and the hydration heat development rate were gradually reduced. The conductivity and pore solution analysis also demonstrated a higher ion concentration precipitation at the early stage for cubic C₄A_1.6G_1.4$ compared to pure C₄A₃$. The main hydration products of these specimens were AFt, AFm, and AH₃ (gel). Moreover, the addition of Ga³⁺ ions improved the crystallinity of AFt and AFm with recorded a relatively higher decomposition temperature. Overall, this study demonstrated that the addition of Ga³⁺ ions can regulate the hydration characteristics of ye'elimite.

Cementing around the casing in oil and gas wellbores provides multiple benefits such as proper zonal isolation, casing support, and prevention of fluid migration. Wellbore cement is an important part of the completion and abandonment process. However, wellbore cement has some drawbacks such as micro-annuli formation or loss of zonal isolation. Nanoparticles (NPs) have been shown to improve the characteristics of wellbore drilling fluids but have not been used extensively in cement. The objective of this paper is to show the effect of NPs’ concentration on wellbore cement characteristics such as thickening time, viscosity, and fluid loss properties. Nanoparticle barite and magnetite were added to heavy cement and bentonite was added to light cement in intervals of 1, 3, and 5 % by weight of cement to test the resulting cement characteristics. The results showed that the thickening time increased for all concentrations of nanoparticles, except for the 5 % magnetite. The resulting yield stress of both cement mixtures increased for all concentrations of nanoparticles. The viscosity for all concentrations of nanoparticles in the heavy cement was greater than the control case, while no change in viscosity was seen with the light cement. Fluid loss generally decreased by increasing nanoparticle concentrations for both heavy and light cement. The results of this work in combination with results from the literature show that the addition of barite, magnetite, or bentonite nanoparticles can enhance wellbore cement without diminishing the pumpability and curing time.

Carbonation development and reinforcement corrosion were investigated on concretes exposed for a five-year period at 90% RH, 20 ℃, and 5% CO₂, and for a six-year period at natural carbonation. Portland cement-based binders with 0%, 18%, and 30% fly ash were investigated. The fly ash blends showed lower carbonation resistance compared to PC both at laboratory and field exposure, a large difference in carbonation performance was observed between the laboratory exposed specimens. The carbonation rate was fastest on the laboratory specimens and showed square-root time dependency the first 2.5 years, but reduced rate at later age. Deeper carbonation depths were in general observed in the vicinity of the reinforcement compared to the unreinforced laboratory exposed specimens. Not all specimens were fully carbonated at the steel-concrete interface. The correlation between degree of carbonation of the steel-mortar interface, the open circuit potential, and the observed corrosion of the steel bars varied between binders and bar position (top or bottom). The measured corrosion rate in the laboratory exposed (90% RH, 20 ℃, and 5% CO₂) carbonated concrete was on average 0.2 μA/cm², with an upper value of 0.6 μA/cm². The highest corrosion rate was measured in the fly ash concrete. No corrosion rate data are yet available for the field exposed concretes.

In this study, an inorganic retarder and a synthesized dispersant (based on PCE) were used to improve the retention and workability of alkali-activated ground granulated blast furnace slag (GGBFS), with NaOH as the sole activator. The objective of the study was to formulate pumpable concrete mixtures with workable time of more than 90 min. The prolonged retention in slump was attained by the addition of the retarder. The effect of the dispersants, synthesized with different monomer to macromonomer ratios, on the workability of the paste was investigated by analyzing the fundamental rheological parameters. The addition of dispersant reduced the initial storage modulus and improved the workability of the alkali-activated paste mixtures. The interaction between the dispersant and NaOH-activated GGBFS systems was investigated by means of adsorption studies and zeta potential measurements. The dispersing ability and the amount adsorbed on GGBFS increased with an increase in the anionic charge of the dispersant. Zeta potential measurements suggested that the dispersion mechanism is primarily due to steric hindrance. Concrete mixtures of compressive strength in the range of ordinary concrete with pumpable workability for 90 and 120 min were achieved with the addition of both retarder and dispersant. The study concludes that a retarder is necessary to prolong the workable times, whereas a dispersant with a higher anionic charge is required to improve the workability of sodium hydroxide-activated GGBFS mixtures.