CEMENT最新文献

This paper examines the sustainability of cementitious materials and concrete. Although the environmental impact of these materials is often evaluated based on their CO₂ emissions per ton of cement or m³ of concrete, incorporating performance parameters into sustainability indices is crucial for a more comprehensive assessment. This study evaluates the sustainability of concretes with and without supplementary cementitious materials (SCM), considering compressive strength and durability performance as performance parameters. Results show that the most sustainable concretes have the highest compressive strength and best durability performance. Furthermore, the importance of using locally available materials is highlighted, as transporting SCM over long distances can outweigh the benefits of using them as a replacement for Portland cement.

This paper presents a study investigating for the first time the effects of the inclusion of graphene nanoplatelets (GNPs) on the mechanical and microstructural properties of alkali activated binders produced with fly ash and slag and using superplasticizer for the GNPs dispersion in the mixing water. Compressive strength and water absorption of mortar cubes, flexural strength of mortar beams, X-ray computer tomography scanning and mercury intrusion porosimetry, Fourier transform infrared spectroscopy, scanning electron microscopy with Energy Dispersive X-Ray Spectroscopy, and X-ray diffraction, were carried out on samples cured either at room temperature or at 40 °C. Results showed that the inclusion of GNPs at a dosage of 0.05% provided a considerable increase in compressive strength at both curing conditions. Microstructural observations suggested that the presence of GNPs improved the formation of hydrated gel, and the research demonstrated through porosity measurements the shift from the capillary to the gel pore region due to the inclusion of GNPs. This study represents a step forward in understanding the effects of GNPs inclusion on alkali activated binder microstructure.

Purpose

Companies that manufacture poles generate several negative environmental impacts, whose extent needs to be assessed to find ways to mitigate them.

Methods

In this research, Life Cycle Assessment (LCA) was used as a methodology to measure the potential environmental impacts throughout the poles' life cycle. Primary data (amount of cement, gravel, sand, steel rebars, energy, water) were collected from industries located in Teresina, Piauí, Brazil, and information from the Ecoinvent 3.7.1 database (transport, solid waste, liquid effluents, particulate matter) was used.

Results and discussion

The literature addresses pole production from a different perspective, making this study relevant to disseminate the life cycle thinking in concrete pole production. However, the literature points to a correlation trend for ecotoxicity and human toxicity indicators, as well as the results found in this research. Waste disposal stands out as an important source of impact for these industries, confirming the necessity of efficient management of these materials at the end of their lifespan and during the production process. The scenario analysis showed that is possible to reduce the potential impacts of these industries.

Conclusion

The reuse of waste within the industry itself is feasible (using a shredder for this purpose) and can contribute to decreasing the extraction of natural deposits in various production processes related to the poles' life cycle and reducing their accumulation in the environment. The use of inputs from closer suppliers is a strategy that contributes to mitigating the potential impact of gaseous emissions, reducing the impact that generates global warming and climate change. In addition, other papers show viable alternatives in different scenarios, based on complex laboratory studies. Nevertheless, his approach shows how impacts can be mitigated with the adoption of simple actions such as the reuse of effluents and residues from these industries. It is possible to redefine the production process through a scenario close to the ideal, bringing environmental sustainability to the sector.

Spent fluid cracking catalyst (FCC) waste is produced to convert petroleum crude oil into gasoline, and its main component is a reactive zeolite known as faujasite. This paper studies low-energy treatments to enhance reactivity. When untreated, the spent FCC has outstanding activity, and a fast set which delivered significant strength (6–10 MPa) and a high mechanical index (MI=14). Calcination (up to 800 °C) is not enough to amorphize the faujasite and increase reactivity. However, NaOH-fusion is highly efficient. Even at low temperature (450 °C), NaOH-fusion breaks down the zeolite structure, dissolving Si⁴⁺ that forms cementing hydrates with high Ca/Si and Si/Al ratios which delivered high strengths. NaOH-fusion at 450 °C totally amorphized the zeolite resulting in high strength (9–13 MPa) and outstanding MI>22; superior to pozzolans, and closer to cementitious materials. Fusion at 600 °C reorganises some of the amorphous phase into a silicate whose hydrates provided the greatest strengths (over 16 MPa) and an outstanding MI of 24.

Na₂CO₃-fusion at 600 °C did not alter the spent FCC but provided CO₃²⁻ which formed calcite cements. These initially densified the matrix providing strength but lowered long-term strength and workability.

Acid-etching partially dissolved spent FCC particles which improved early activity but caused a loss of soluble Si⁴⁺ and Al³⁺ that reduced the ultimate strength. Due to the low organic matter in the spent FCC, oxidation did not increase reactivity.

The spent FCC is highly pozzolanic, it can safely reduce the embodied carbon of cements: concentrations of heavy metals are either traces or insignificant. Therefore, they can easily immobilise in a stable matrix.

The aim of this research was to investigate the relation between leachate data and expansion due to delayed ettringite formation (DEF). These correlations have the potential for identifying the probability of delayed ettringite formation in a shorter time than the traditional method of monitoring expansion over time. Ions leached from the samples after heat curing of the cement paste were measured and the resultant data were used to calculate relative quasi-crystallization pressures within the cement microstructure. Potential relationships exist between these pressures, time to expansion initiation, and overall percent expansion of samples. The results indicate that sulfates and alkalis affect the onset and overall percent expansion observed. The calculated quasi-crystallization pressures strongly correlate with the observed overall percent expansion of mortar bars. Time to expansion initiation was negatively correlated with alkali content, indicating that cement with higher amounts of alkalis tends to expand at earlier ages. Overall, the leachate test corroborates earlier findings in the determination of DEF potential in cementitious materials and allows for the possible prediction of expansive behavior in 28 days or less using experimental results as opposed to cement composition.

This research investigates the chemical reactivity of waterglass, a sodium silicate (Na₂O·nSiO₂·yH₂O). This research establishes a framework for thermodynamic modeling of waterglass systems that contain calcium hydroxide and potassium hydroxide. Conventional pozzolanic reactivity tests used for supplementary cementitious materials (e.g., fly ash) that rely on heat release and calcium hydroxide consumption cannot adequately capture the waterglass reactivity, primarily due to the high reaction rates. Capturing the heat released using isothermal calorimetry requires procedural changes in the testing protocol. Specifically, the test is modified by lowering the temperature of the test to slow the reaction rate and using internal mixing to capture the initial reaction. The heat release and calcium hydroxide consumption are used to quantify the reactivity. The theoretical relationship between heat, reactivity, and calcium hydroxide consumption is related to the molar ratio of SiO₂ to Na₂O, also known as the waterglass modulus (n). Thermodynamic modeling and X-ray powder diffraction results demonstrate that the mixtures react to produce Tobermorite-like calcium-silicate-hydrate (C–S–H, C/S = 1.42), which increases in amount with waterglass modulus. Finally, the developed approach demonstrates how the quantified reactivity is used in thermodynamic calculations to predict the reaction products and paste properties.

Natural clinoptilolite zeolite has been a popular supplementary cementitious material (SCM) due to its acceptable pozzolanic performance and the overall lower environmental footprint. Previous research established that milling is an effective pretreatment technique to further increase the pozzolanic reactivity of zeolitic tuffs leading to an increased specific surface area and amorphous contents. Therefore, the present study characterized the zeolite particles after ball milling for 1 and 3 h using phase analysis by X-ray diffraction (XRD), particle size distribution by laser diffraction, microstructural analysis by scanning electron microscopy (SEM), moisture absorption rate, and relative chemical dissolution. The performance of milled clinoptilolite zeolite as a SCM with the replacement of up to 20% portland cement was evaluated through hydration kinetics (heat of hydration, setting time, chemical shrinkage, degree of hydration), workability, compressive strength, autogenous shrinkage, drying shrinkage, and alkali-silica reaction (ASR). Results revealed that 1 and 3 h of milling led to an increased specific surface area, moisture absorption capacity, and relative dissolution of particles, but had no visible effects on the crystalline structure of zeolite particles compared to the unmilled zeolite particles. For the hydrated system, both 1 and 3-h milled zeolite increased the overall heat of hydration leading to an increased silicate and aluminate reaction along with the acceleration effects in the setting time. The compressive strength of up to 20% milled (1 and 3 h) zeolite samples was increased by about 20 to 25% compared to the unmilled zeolite samples at an early age which suggested an increasing pozzolanic response of milled zeolite particles in the system due to an increased volume of hydrated phases and degree of hydration. Milling slightly decreased the workability by demanding a higher content of fresh water which was released at a later age leading to a higher drying and autogenous shrinkage. In addition, milling reduced the internal curing capacity leading to damage to the porous structure of zeolite particles. The use of up to 20% 3-h milled zeolite reduced the deleterious expansion by about 80% due to ASR compared to the control sample and the overall performance of milled clinoptilolite zeolite as the SCM was satisfactory in the hydrated system.

Oxidation of aggregates containing iron sulfide minerals has recently been linked to severe degradation in housing foundations in the northeast United States and the Trois-Rivières area of Quebec, Canada. Existing performance-based approaches mainly rely on the use of oxidizing solutions, which may create harsh environments and lead to unexpected results. This work evaluated the effectiveness of a mortar test by using atmospheric oxygen (a more realistic exposure condition) as an oxidizing agent and employed a design of experiments approach to investigate the effects of relative humidity (50% and 95%), oxygen content (20.9% and 35%), temperature (5°C and 60°C), and water-to-cement ratio (0.45 and 0.65) on the oxidation potential of iron sulfide-bearing aggregates. Results show that length changes of the mortar samples are mainly attributed to drying shrinkage within the experimental duration (more than 400 days), which is highly dependent on the relative humidity levels, whereas minimal to no expansion was observed under laboratory conditions. Recent efforts to simulate iron sulfide deterioration in laboratories by performance-based tests are then reviewed. Their advances and challenges as well as comparison with the proposed test are summarized, leading to a call for further development of experimental methods.

Studies on the properties of pure C₃A phases are often limited to methods requiring small sample amounts due to the lack of a convenient laboratory synthesis yielding sample amounts exceeding 100 g. Here, we report a simple and large scale lab method for the synthesis of C₃A polymorphs with yields of up to 500 g per batch. Commercial calcium aluminate cement (CAC) was used to prepare cylindrical green bodies of CaCO₃ and Al₂O₃ (and NaNO₃ for orthorhombic and monoclinic polymorphs). The green bodies were sintered at 1300 °C and 1400 °C respectively. The chemical and mineralogical compositions of the obtained C₃A polymorphs were analyzed by X-ray powder diffraction and X-ray fluorescence spectroscopy. The reactivities of these C₃A polymorphs were compared to conventionally synthesized C₃A (using mechanical powder compaction prior to sintering) via in-situ isothermal heat flow calorimetry. Additionally, we demonstrate that synthetic C₃A retains its reactivity over one year if stored appropriately. As the new synthesis protocol yields hundreds of grams of C₃A, it enables experimental methods such as slump flow testing with pure phases, which is also reported for all polymorphs.

Partial replacement of ordinary portland cement (OPC) with supplementary cementitious materials (SCMs) is a ubiquitous and effective approach to design concrete mixtures with lower embodied carbon and improved durability compared to plain OPC concrete mixtures. However, the global supply of common industrial SCMs, like fly ash (a byproduct of coal combustion) and blast-furnace slag (a byproduct of steelmaking), is dwindling due to global decarbonization efforts and sustained demand from the concrete industry. The newly standardized ASTM C1897 rapid, relevant, and reliable (R3) test is an effective screening method to measure the chemical reactivity of potential SCMs. However, the sample quantity requirements impede the rapid-throughput screening of new SCM sources that may currently be available only in small quantities. The objective of the current study is to design and validate a small-scale modified R3 test to enable standardized characterization and rapid-throughput screening of novel SCMs. The results substantiate that the ASTM C1897 R3 bound water method can be performed with sufficient accuracy at a much smaller scale (i.e., 0.01 g of SCM per test) using the thermogravimetric method developed and validated herein.