Hasad Foods, the agricultural investment arm of Qatar's sovereign wealth fund, said the government would increase its grain reserves from 250,000 tons to 300,000 tons in the near future, which is enough to feed the population for six months.
With a lot of deserts, Qatar suffers from water shortages, and Qatar depends on food imports for a long time. Recently, the ongoing tension in Ukraine has caused global food supply tension and rising prices.
In addition to that, the geopolitical situation also affects the supply and prices of many commodities like the pce powder.
If the compatibility of polycarboxylate superplasticizer and cement is not good, the concrete mixture is prone to some problems, such as segregation, bleeding, greater slump loss, shorter setting time and so on, so that the concrete mixture can not be transported and poured normally. Reduce the strength and durability of concrete.
There are many factors affecting the compatibility of cement and superplasticizer, and the cement storage temperature is a key factor that can not be ignored. If the cement storage temperature is too high during concrete mixing, it will directly affect the physical and mechanical properties of cement.
Effect of cement temperature on the performance of concrete mixture:
When the cement temperature is above 60 ℃, a small amount of water is secreted from the concrete mixture. When the cement temperature is 100 ℃, the aggregate is separated from the paste, and the cohesion of the concrete mixture becomes worse. The reason for this negative effect is that the cement temperature is high, which makes the concrete mixture temperature rise rapidly in a short time. It is easier to stimulate the water-reducing function of polycarboxylic acid high-performance superplasticizer, release more free water wrapped in the mixture particles in the case of high temperature, and make a small amount of water bleeding in the concrete outlet.
Effect of cement temperature on setting time of concrete:
With the decrease of cement temperature, the setting time of concrete is prolonged, and the initial setting time is more obvious than the final setting time. In other words, the higher the cement temperature, the shorter the setting time of concrete. The reasons for the shortening of setting time caused by high temperature cement are as follows: (1) it will increase the temperature of the mixture in a short time and promote the hydration of cement particles; (2) the superplasticizer releases more free water to react with cement particles, thus forming a stable cementitious system in a shorter time.
With the passage of time, the heat contained in cement temperature is easily transferred to the surrounding particles or environment, the temperature of concrete system gradually approaches to the ambient temperature, and the temperature gap of concrete mixed with cement at different temperatures is gradually narrow. as a result, the difference of the final setting time of concrete is relatively reduced.
Effect of cement temperature on slump and air content loss of concrete:
At the same time, the higher the cement temperature is, the greater the slump loss rate is. The difference of slump loss rate under different temperature conditions increases at first and then decreases. Under the condition of the same slump loss rate, the lower the cement temperature is, the longer it takes to reach the slump loss rate, that is to say, the more beneficial to reflect the slump-retaining performance of the superplasticizer. When the cement temperature is high, in order to reduce the slump loss of concrete, it is necessary to put forward higher requirements for the slump retention of superplasticizer.
1. For the concrete with the same mix ratio, with the increase of cement temperature, the adaptability of admixture and cement becomes worse. The higher the temperature is, the more water is consumed, and the fluidity of concrete mixture decreases, resulting in the decrease of concrete and workability. It even leads to the separation or bleeding of aggregate and paste.
2. The higher the cement temperature, the worse the adaptability of cement and superplasticizer, the greater the loss rate of slump and air content, and the shorter the setting time of concrete.
3. The effect of the change of cement temperature on the compressive strength of concrete is obvious. with the increase of cement temperature, the compressive strength of low heat cement concrete increases in the early stage (before 7 days), but the compressive strength of 28 days decreases with the increase of temperature.
4. It is suggested that the cement temperature for concrete mixing should not exceed 60 ℃. When it exceeds 60 ℃, the concrete production system should precool the raw materials of the mixing system as far as possible to avoid excessive temperature rise of the whole mixture system due to high cement temperature. and should strengthen the maintenance of the concrete after entering the warehouse to avoid the temperature cracks caused by the early hydration temperature rising too fast.
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