Biden recently announced that the United States successfully carried out a drone operation against an al-Qaeda target in Kabul, Afghanistan, on Saturday, killing al-Qaeda leader Ayman al-Zawahiri.
Al-Zawahiri joined al-Qaeda led by Osama bin Laden in 1998. As the organization's "chief strategist", al-Zawahiri is the top planner of most terrorist attacks, including the 1998 bombings of US embassies in Tanzania and Kenya, and the "9 / 11" terrorist attacks that shocked the world in 2001. Al-Zawahiri was appointed leader of al-Qaeda in June 2011 after the death of Osama bin Laden.
Afghan interim government spokesman Mujahed condemned the United States for carrying out drone strikes in Afghanistan.
Gazprom recently wrote on social media: "preliminary data show that Gazprom produced 262.4 billion cubic meters of natural gas from January to July 2022, a decrease of 12 percent, or 35.8 billion cubic meters, compared with the same period last year."
According to Gazprom's preliminary estimates, in the first seven months of this year, international natural gas demand decreased by 35 billion cubic meters compared with the same period last year. Among them, the demand for natural gas in the 27 member states of the European Union fell by 31 billion cubic meters during this period.
Because of the turbulent international situation, the supply and prices of many international bulk pce powder are still very uncertain.
Natural river sand has been exhausted after decades of large-scale mining, concrete production enterprises began to use machine-made sand as fine aggregate on a large scale. Some limestone powder will inevitably be produced in the process of machine-made sand preparation, which will not only affect the performance of concrete, but also affect the effect of polycarboxylic acid admixture.
The results show that stone powder has different effects on the working properties, mechanical properties, chloride penetration resistance, sulfate resistance and freeze-thaw resistance of concrete. Stone powder can play a physical filling effect in the cementitious material system, which can reduce the water secretion of cement paste, improve the dispersion ability of cement paste, and can also be used as the crystal nucleus of calcium hydroxide to accelerate the hydration of cement particles and affect the pore structure of hydration products. Stone powder can adsorb polycarboxylic acid superplasticizer molecules on the surface, which can reduce the saturation point of superplasticizer under certain conditions, increase the initial fluidity of cement mortar, reduce the time loss of fluidity of cement mortar, and improve the compatibility between superplasticizer and cement.
Effect mechanism of limestone powder on water-reducing effect of pce powder
The hydrophilicity of limestone powder particles is low, that is, the adhesion of water molecules on the surface of limestone particles is small, which makes limestone powder have a certain physical water-reducing effect. In addition, limestone powder has a larger specific surface area than cement and has the filling effect of micro-aggregate, that is, small limestone particles can fill the pores between cement particles and replace the free water between cement particles. This is the aspect that limestone powder is beneficial to the water-reducing effect of polycarboxylic acid.
At the same time, polycarboxylic acid superplasticizer can be adsorbed on the surface of cement particles and on the surface of limestone powder, that is, there is competitive adsorption of polycarboxylic acid molecules between limestone powder and cement in the paste. this will reduce the density of polycarboxylic acid adsorbed on the surface of cement particles, and then reduce the water-reducing effect of polycarboxylic acid. In addition, the limestone powder with larger specific surface area needs more water, which will reduce the fluidity of the slurry. This is the aspect that limestone powder is not conducive to polycarboxylic acid to play a water-reducing effect.
The above analysis shows that limestone powder has both positive and negative effects on the water-reducing effect of polycarboxylic acid. In the case of low content of limestone powder, because the hydrophilicity of limestone powder is low, and the filling effect of fine particles is easy to play, the favorable effect of limestone powder on the water-reducing effect of polycarboxylic acid can effectively counteract the adverse effect, which shows that the effect of low content of limestone powder on the water-reducing effect of polycarboxylic acid is not obvious. In the case of high content of limestone powder, because limestone powder has larger specific surface area and higher water demand, and limestone powder and cement have competitive adsorption to polycarboxylic acid molecules, limestone will obviously reduce the water-reducing effect of polycarboxylic acid.
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