Influence of cement dosage
The solid phase volume of ordinary portland cement increases during hydration and hardening, while the cement + water system shrinks. Secondly, the cement hydration process is accompanied by thermal effects, which cause the initial volume to expand and shrink when cooling, resulting in an increase in apparent shrinkage. Besides, there is also a phenomenon of self-shrinkage caused by self-absorption during cement hydration. Therefore, in general, if other conditions are the same, the amount of cement will increase, and the shrinkage of foam concrete will increase accordingly. At the same time, cement is one of the important factors to ensure strength, so there is a suitable range of cement dosage.
The impact of cement types
Not all the volume of cement before and after hardening shrinks. The volume of expanded cement not only shrinks but also swells before and after hardening. Therefore, if an appropriate amount of expansion cement is used, the overall shrinkage of the foamed concrete can be compensated or reduced to a certain extent. However, the expansion cement not only affects the volume change but also affects a series of other properties. If it is too much, it will cause the structure of the hardened foam concrete to be damaged. Therefore, the type and dosage of the expansion cement must be determined through experiments.
The impact of aggregate
Experimental and engineering statistics show that the shrinkage of ordinary cement concrete is the smallest, the shrinkage of cement paste is large, and the shrinkage of foam concrete is the largest. This is because ordinary concrete is mixed with a large amount of coarse aggregate with constant volume, and the total volume of cement paste without aggregate is reduced before and after hardening. The largest shrinkage of foam concrete is, on the one hand, because there is no coarse aggregate therein, and on the other hand, because it contains a large number of pores, most of the gaps are filled with water. During use, as the water in the pores evaporates, the appearance shows a volume shrinkage. It can be seen that the addition of aggregate is undoubtedly one of the measures to reduce the shrinkage of foamed concrete, and foamed concrete can only be blended with a part of fine aggregate. At the same time, because the aggregate is chemically inert, excessive addition will result in a significant decrease in the strength of the foamed concrete, and thus its amount will be limited. After the foam concrete's density, water-cement ratio, and other process parameters are determined, the fine aggregate will increase, and the amount of cement will decrease. Therefore, there is also a suitable choice of whether the aggregate is mixed or not.
Impact of water-cement ratio and conservation system
The moisture loss and drying shrinkage test results of foam concrete at 60℃show that there is a clear and close synchronization between water escape and shrinkage change of hardened foam concrete. This shows that the escape of moisture directly causes the shrinkage of the foam concrete, and when the moisture stops escaping, the foam concrete stops shrinking. According to classic cement chemistry theory, the amount of water required for complete cement hydration, that is, the theoretical water-cement ratio should be 0.38. In contrast, the foamed concrete's molding water-cement ratio is often as high as 0.7 or even 0.8. Excess water will remain in the pores of the hardened foam concrete, and this part of the water will account for about 1/2 of the amount of molding water. Once the surrounding relative humidity is low or the ambient temperature is high, the moisture evaporates and then escapes. Especially in the early stage of hardening, the structure of foamed concrete is still relatively weak. If it is not well maintained, moisture is easily lost, leading to large shrinkage and surface cracking, weakening the internal structure of the hardened body, and triggering high water absorption of the hardened foamed concrete. According to this, the initial water-cement ratio of foamed concrete becomes a prerequisite factor affecting the shrinkage of hardened foamed concrete. One of the key technologies for preparing low-shrink foam concrete is to control the low water-cement ratio. After pouring the foam concrete sample with a density of 1100 kg/m3 for 24 hours, it was divided into two batches of samples with a surface nylon film sealed and no surface treated. The samples were cured in the same environment, like temperature and humidity. Dry shrinkage. Through comparison, it is known that the shrinkage of the sample after the surface sealing treatment is much smaller than that of the sample with an open surface, and the shrinkage tends to be stable at the age of 5 days. The results once again proved the close relationship between water escape and shrinkage and the importance of early water retention to control foam concrete shrinkage.
Technical measures to reduce shrinkage and cracking of foamed concrete
According to the above analysis, the technical measures to reduce the shrinkage of foamed concrete and prevent cracking and water absorption mainly include the following aspects:
(1) The appropriate amount of cement;
(2) Adding an appropriate amount of expansion cement;
(3) Low molding water-cement ratio;
(4) Optimize the conservation system and strengthen early water retention;
(5) the use of waterproofing agents (mixed or surface coating);
(6) Add barbed wire (crack prevention net) to the insulation layer to prevent the cracking of the foam concrete.
By optimizing the concrete mix ratio and controlling construction measures, early cracking of the concrete can be reduced.
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