CEMENT最新文献

Ye′elimite (C₄A₃$\overline{S}$), a main compound in calcium sulfoaluminate (CSA) clinker, is an important ingredient as expansive additive in shrinkage compensating cement. This study proposes to modify the expansive additive by encapsulating it with polyethylene glycol (PEG). The polymer provides a matrix structure, in which the ye′elimite particles are embedded. When the modified expansive additive come into contact with water, the polymer matrix acts as a water barrier, but can dissolve away. This slowly exposed C₄A₃$\overline{S}$ to hydration, resulting in gradual early-stage ettringite formation; hence control early expansion in expansive cement. The study compared the ettringite formation between the unmodified and the modified expansive additive using thermogravimetric analysis (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) from 1 hour to 3 days. The results show that the unmodified expansive additive generated more ettringite than the modified ones at the same hydration time. The study subsequently investigated the mortar properties with the unmodified and modified expansive additives admixtures. The results showed that the modified expansive cement showed superior flowability and drying shrinkage behaviours, while the compressive strength of the finished products underperformed that of the untreated expansive additives.

The mechanical properties of Ultra High Performance Fibre Reinforced Cementitious Composite (UHPFRCC) is basically influenced by the type of fibres and reactive binders used. Fibres primarily influence the ductility whereas reactive binders influence the compressive strength of UHPFRCC. Among the commonly used reactive binders, Ground Granulated Blast Furnace Slag (SL) with its vitreous nature has the ability of influencing both the compressive strength and ductility of UHPFRCC. This study discussed the microstructure and mechanical properties of six different mixtures made up of 0%, 20%, 40%, 60% 75% and 90% cement replacement of SL. The XRD results indicated that, increased levels of C-S-H and ettringite retard the hydration process leading to lower compressive strength and vice versa. The SL-cementitious composite can achieve a compressive strength of up to 108.1MPa and ductility of up to 1.67% without the use of fibres. The maximum compressive strength and ductility were achieved with 40% SL replacement of cement whereas the minimum compressive strength and ductility were achieved with 60% and 20% SL contents, respectively. Moreover, the optimum mechanical properties (i.e. compressive strength, tensile strength, flexural strength, and tensile strain) were achieved with a 40% SL replacement of cement in the cementitious composite.

Alternative cementitious materials (ACMs) may exhibit superior mechanical properties and durability to certain environments, and that also may be produced with relatively less environmental impact compared to traditional portland cement. Differences in ACM composition, reaction products, and microstructure produces variations in their performance, including their resistance to fluid and ion and to corrosion of embedded steel. Understanding relationships between composition, structure, and corrosion performance in ACM systems is essential for designing durable reinforced concrete from these materials. Here, five commercially available ACMs are evaluated and compared against ordinary portland cement (OPC). The five ACMs include one calcium aluminate cement (CAC); one ternary blend of calcium aluminate, portland cement, and calcium sulfate (CACT); one calcium sulfoaluminate cement (CSA) as well as the same CSA cement with polymer-modification (CSAP); and one activated aluminosilicate binder system (AA). Water sorption, chloride ion ponding, bulk conductivity, formation factor measurements, and accelerated corrosion tests were performed to evaluate the porosity, mass transport, chloride ion binding capacity, and resistance to corrosion of embedded reinforcement. The results demonstrate that mixtures with high pore structure interconnectivity and low binding capacity (such as CSA and CAC investigated in this paper) or mixtures with significantly low binding capacity (such as AA investigated in this paper) should be avoided to minimize damage due to chloride-induced corrosion. Polymer addition could be an important strategy to improve the corrosion resistance of mixtures that have high interconnectivity. Overall, one ACM, CACT, evaluated in this study showed the best corrosion resistance among the materials considered – including OPC.

This paper details a study on the efficacy of portland-limestone cements (PLCs) in combination with supplementary cementitious materials (SCMs) to prevent expansion due to alkali-silica reaction (ASR). The PLCs studied include both interground (10–15% limestone by mass) and interblended (10% limestone by mass) systems. In this study, ASTM Type II/V cements, five different SCMs, two very-highly reactive fine aggregates, and six SCM combinations were investigated. A total of 100 mixtures were assessed using three different accelerated laboratory test methods to investigate if the SCM combinations that are used with OPCs can be utilized as-is, increased, or decreased when used instead with PLCs. The test methods used to evaluate ASR included the Pyrex mortar bar test (PMBT, ASTM C441), the accelerated mortar bar test (AMBT, ASTM C1567), and the miniature concrete prism test (MCPT, AASHTO T 380). The difference in performance between PLCs with SCMs and parent OPCs with SCMs in the MCPT conditions was further evaluated using pore solution alkalinity and electrical resistivity analysis. The efficacy of the SCM combinations to prevent ASR was also evaluated with a pozzolanic reactivity test. The expansion results from the accelerated laboratory test methods revealed that the mixtures with PLCs and SCMs had similar or better overall performance when compared to the mixtures with the parent OPCs and SCMs. It was observed that the particle size of the added limestone in interblended PLC with SCM mixtures could have a significant influence on the ASR expansion that may alter the output of the test (pass/fail). Consequently, the SCM combinations that are used with OPCs can likely be utilized as-is when used with interground PLCs with up to 15% limestone to prevent ASR. The pore solution and bulk electrical resistivity analysis showed that the lower pore solution alkalinity and higher resistance to mass transport are the main contributing factors towards PLCs’ overall improved performance for ASR mitigation in the presence of SCMs.

The complex composite material cement paste (CP) is under high pressures in underwater applications and when impact loading occurs. The mechanical behavior of cement paste to hydrostatic compression results from mechanical deformations of each phase, including unhydrated and hydrated minerals. Molecular Dynamics was used to study the atomistic deformation of individual unhydrated cement phases with increasing hydrostatic pressures. The pressure-specific volume Birch-Murnaghan equation of state (EoS) and the bulk modulus at zero pressure were determined for each phase. Results show that the bulk modulus and compressibility are pressure dependent. For tricalcium silicate (C₃S), dicalcium silicate (C₂S), and tricalcium aluminate (C₃A), the bulk modulus increases, while the volume compression decreases with increasing pressure. The C₃S and C₃A phases are stable during hydrostatic compression and exhibit isotropic behavior. The C₂S phase is not stable and shows anisotropic behavior. These results explain the effect of unreacted cement clinkers on cement paste mechanical behavior under high pressure based on the response of individual phases.

This study presents the influence of different curing temperatures and the availability of unbound H₂O on the phase changes during the drying process of a simplified calcium aluminate cement bond castable. A mixture of CAC and alumina was hydrated for 48 h at 5, 23 and 40 °C, which represents different working conditions during casting. After the curing process, these samples were heated up to 180 °C, and in some of them, the remaining unbound H₂O had been removed by vacuum drying beforehand. The quantitative phase composition was determined by QXRD. Thermogravimetric analysis and gravimetric measurements were also used to characterize the differently cured samples. While the mineral phases in the samples cured at 40 °C were barely affected by the heating process in the investigated temperature range, the initial conditions before the drying of the samples cured at 5 and 23 °C strongly affected the final phase composition.

Nanoconfinement is known to affect the property of fluids. The changes in some thermo-mechanical properties of water confined in C-S-H are still to be quantified. Here, we perform molecular simulations to obtain the adsorption isotherms in C-S-H as a function of the pore size (spanning interlayer up to large gel pores). Then, fluctuations formula in the grand canonical ensemble are used to compute the isothermal compressibility (and its reciprocal, the bulk modulus), the heat capacity, the coefficient of thermal expansion and thermal pressure, and the isosteric heat of adsorption of confined water as a function of the (nano)pore size. All these properties exhibit a pore size dependence, retrieving the bulk values for basal spacing above 2 nm. To understand why property changes with confinement, we compute structural descriptors including the radial distribution function, apparent density, hydrogen bonds counting, and excess pair entropy of water as a function of the confinement. These descriptors reveal significant structural changes in confined water. The heat capacity shows a good linear correlation with the apparent density, entropy, and hydrogen bond number. The values of water property as a function of the basal spacing are a valuable input for multiscale modeling of cement-based materials.

Permeability-reducing admixtures (PRAs) are marketed as an option to improve the concrete durability and reduce water ingress in structures. Two categories of PRAs that have become more prominent recently are hydrophobic pore blockers and crystalline waterproofers. A literature review was performed to determine the composition, mechanism of action, test methods to indicate durability, and performance of PRAs in concrete, with focus on their use in infrastructure. The test methods for evaluating the performance of PRAs and their effects have varying degrees of frequency and standardization and there is a lack of consistency in the experimental methods used to evaluate PRAs based on the studies found in the literature; the dosage, water to cementitious ratio (w/cm), testing age, and mixture designs were variable. There remains a need for studies with both field and lab data to establish relationships between lab results and field performance to determine laboratory test method validity for PRAs.

Cement is the most widely used building material worldwide. Its production demands a large amount of natural resources, in addition to high energy consumption. Thus, the cement industry has been looking for solutions that effectively reduce the use of these resources as well as greenhouse gas emissions. This research aims to show the technical feasibility of incorporating civil construction waste (CCW) in the production of Portland cement. To this purpose, the physicochemical characterization of the raw materials used was carried out, dosage and manufacture of the raw mix, which were calcined at 1450 °C. Clinkers were characterized mineralogically, by means of XRD, to verify the formation of the crystalline phases. Subsequently, the clinkers were ground, resulting in Portland cements, and their physical-mechanical properties were evaluated. The results showed the potential of using CCW as an alternative raw material, since experimental cements presented performance similar to industrial cements used as a reference.

In continuation of earlier work on early hydration, this study evaluates the late hydration of CAC and CaCO₃ using QXRD and thermodynamic modelling at different temperatures. Experiments were performed at 5, 23, 40 and 60 °C for up to one year. As stated in the preceding study, C₂AH_X might act as a precursor for monocarbonate in early hydration, and thus no or only little monocarbonate should form at temperatures below 20 °C. At 5 °C, monocarbonate starts precipitating after 7 d and remains alongside CAH₁₀. At all other investigated temperatures, monocarbonate is the dominant hydrate phase. Primarily formed CAH₁₀ at 23 °C is visible up to 14 d but then becomes unstable with respect to monocarbonate. At 23 °C and 40 °C the thermodynamically stable phase assemblage is reached within one year. However, the precipitation of C₃AH₆ is detected in all samples at 60 °C, which results from an insufficient w/CAC ratio for carbonate-AFm in the paste due to the inevitable evaporation of mixing water for this condition. However, C₃AH₆ can partly be “re-converted” at 60 °C when the sample is subsequently stored under water and monocarbonate is stable again.