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Analysts at JPMorgan recommended selling or "underweight" emerging market local currency sovereign debt because of the global impact of the Ukraine-Russia crisis.
JPMorgan estimated that fixed-income assets in emerging markets have lost 6-9 percent of their value since Russia invaded Ukraine a month ago, with nervousness about the war and its impact on global energy and food prices adding to existing pressures.
Some leading emerging market central banks are signaling that interest rates now need to rise faster than previously expected, which fuels fears of "stagflation". Stagflation means when high inflation and higher interest rates undermine economic growth.
With both the U.S. Federal Reserve and emerging market central banks raising interest rates, JPMorgan also said it made sense to "underweight" emerging market assets by taking advantage of the recent pullback in local currency bond yields relative to TREASURIES.
JPMorgan said major metals exporters such as South Africa, Chile, and Peru could still do well, but warned that emerging market fixed income assets now faced a more "stagflationary" trajectory.
The markets and prices of many commodities, metals, chemicals like the polycarboxylate superplasticizer still face uncertainty.
Concrete is an indispensable raw material in the construction of construction projects. In the process of preparing concrete, in order to improve the performance of concrete, it is necessary to add water reducing agent after adding water, stone, sand and cement. Then sufficient mixing can greatly improve the fluidity and strength of concrete. The more common superplasticizers in daily work are mainly polycarboxylate superplasticizers. This water reducing agent is widely used due to its advantages of high water reduction rate, high slump retention performance, and good economy.
1 Analysis of the mechanism of action of polycarboxylate superplasticizers
Polycarboxylate superplasticizer is mainly composed of carboxylic acid grafted polymer, light brown, is a transparent liquid. Its main mechanism of action includes two kinds of maintaining dispersion mechanism and dispersion mechanism. Polycarboxylate superplasticizers are currently widely used in the field of construction engineering because polycarboxylate superplasticizers have very stable chemical properties compared to ordinary superplasticizers; they are transparent liquids and are easy to transport. In this way, a lot of transportation costs are saved, and the waste of resources is also reduced.
At present, the most used polycarboxylate water reducer is mainly acrylic acid as the main chain, but it is the hydroxyl and carboxyl molecules that play a role. These two molecules can combine with water molecules in concrete, thereby promoting The flocculation structure is destroyed, so that the microstructure of the concrete changes, which makes the concrete set and harden faster. The relevant chemical molecules in the polycarboxylate water reducer will be combined with the cement particles, which will speed up the fluidity of the concrete, and then obtain the desired ideal state, which can further highlight the characteristics of the concrete. However, in the process of actually using polycarboxylate water reducer, this water reducer has higher requirements on the outside temperature range during the use of concrete. Whether the construction temperature is too high or too low, it will affect the characteristics of the polycarboxylate water reducer, so that the ideal effect cannot be achieved.
2 Water reduction rates of different dosages
In this paper, ten kinds of water reducing agents (PCE-1~PCE-10) are selected for analysis, and the specific water reducing rate is shown in Figure 1.
When the content of polycarboxylate superplasticizer is 0.15%, the water reduction rates of PCE-1, PCE-2, PCE-4, PCE-6 and PCE-7 are all higher than 20%, showing a higher Water reducing rate; once the content of polycarboxylate water reducing agent is higher, that is, when it reaches 0.2%, these water reducing rates will be higher (25%-30%); When the dosage is not higher than 0.3% and not lower than 0.25%, the water reduction rates of PCE-2, PCE-4, PCE-6, PCE-7 and PCE8 all reach (30%) ~35%).
3 Analysis of the influence of concrete raw materials on polycarboxylate water reducer
3.1 Influence of cement on polycarboxylate superplasticizer
There are essential differences between ordinary lignosulfonate-based superplasticizers, naphthalene-based superplasticizers and polycarboxylic acid-based superplasticizers. Polycarboxylate superplasticizers have the advantages of being environmentally friendly, harmless, strong in slump retention and high in water reduction rate, so they are widely used. However, this water reducing agent also has certain defects, that is, the problem of poor adaptability, but this is also a common defect of ordinary water reducing agents. For example, when a certain cement is applied, there may be characteristics such as poor fluidity of the cement slurry and poor water reduction effect, and the main reason for this phenomenon is due to the molecular structure of the substance itself. composition and surface properties.
3.2 Influence of fine aggregate on polycarboxylate superplasticizer
Machine-made sand and natural sand are the more commonly used fine aggregates in concrete, and are one of the raw materials for concrete preparation. Natural sand is divided into three types: river sand, mountain sand and sea sand. However, it should be noted that in the process of preparing concrete, do not use a large amount of polycarboxylate water-reducing agent. Once excessive, it will inevitably react with fine aggregates, which will not only affect the slump of concrete, but also affect the concrete. strength is adversely affected. In addition, another important factor that affects the adaptability of concrete and polycarboxylate superplasticizers is the content of stone powder in the sand. If sand with high stone powder content is used in the process of preparing concrete, it is very likely to have a series of effects on the fluidity of concrete. In addition, other characteristics of fine aggregates will also affect the adaptability of concrete and polycarboxylate superplasticizers, such as the thickness of sand. Generally speaking, if the fineness of the sand is less than the standard value, it will cause rapid loss of fluidity of the concrete. At the same time, the sand ratio and the residual sulfate and chloride ions in the sea sand will reduce the water of the polycarboxylate system. The adaptability of the agent has a certain degree of influence. In addition to the above factors, the content of mud in the sand will also affect the adaptability and performance of the polycarboxylate superplasticizer. The main reason is that the soil has a strong adsorption force, and this strong adsorption force will affect the water reducing rate of the water reducing agent. The mud content of different substances will also be different. Generally speaking, the content of soil in the sandstone will have a greater effect on the polycarboxylate superplasticizer. When the mud content in the sandstone exceeds 3%, the performance of the water reducer will be significantly reduced. , even adding an appropriate amount of mixing will not promote the fluidity of concrete.
3.3 Influence of coarse aggregate on polycarboxylate superplasticizer
Another important factor affecting polycarboxylate superplasticizer is coarse aggregate. This performance is mainly reflected in the gradation of the stone and the content of needle flakes. Even for stones with the same gradation, if the acicular content of the stones increases, the flow rate of the concrete will inevitably slow down, and the decrease in expansion may lead to bleeding or segregation. In addition, the water absorption of coarse aggregates with different thicknesses will also be different. Generally speaking, the water absorption of fine aggregate is higher than that of coarse aggregate. Even changing the air content or dosage of the water reducing agent cannot achieve the expected purpose. Only by adjusting the mix ratio of concrete can it be changed. Therefore, in the actual construction process, it is necessary to continuously adjust the stone gradation and the mud content of the sand according to the actual situation of the raw materials, and seek the best solution.
In summary, admixtures play a vital role in the preparation of high-performance concrete, and are therefore indispensable raw materials in the preparation of concrete. At present, polycarboxylate superplasticizers are widely used due to their many advantages. In order to better promote the further development of this superplasticizer, the mechanism and influence of concrete raw materials on polycarboxylate superplasticizers must be investigated. Carry out in-depth research, and at the same time focus on mastering its laws, so that high-performance polycarboxylate water reducers can be prepared in the later stage.
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