The shrinkage, cracking and water absorption of foamed concrete are three closely related problems.Generally speaking, foamed concrete due to poor early maintenance, water conservation measures are not enough or in the use process conditions are more stringent, will cause its internal moisture evaporation, resulting in volume shrinkage, cracking or significant water absorption.And foam concrete absorbs water too much to be able to reduce heat preservation heat insulation effect again.From the preparation process of foamed concrete and the observation and study on the section of hardened body, it is found that most of the holes in the foamed concrete are relatively independent closed holes. Therefore, the well-preserved foamed concrete is soaked in water, and its water absorption is mainly concentrated in the surface layer, but does not have great water absorption.The main factors affecting the shrinkage, cracking and water absorption of foamed concrete are as follows:
1. Influence of cement dosage
In the process of hydration and hardening, the solid volume of ordinary Portland cement increases, while the cement + water system shrinks.Secondly, the hydration process of cement is also accompanied by thermal effect, causing initial volume expansion and cooling and shrinkage, resulting in an increase in apparent shrinkage.In addition, there is a phenomenon of self - shrinkage caused by water absorption in the process of cement hydration.Therefore, in general, if other conditions are basically the same, the amount of cement increases, the shrinkage of foam concrete will increase accordingly.Cement is also one of the important factors to ensure the strength, so there is a suitable range of cement dosage.
2. The influence of cement types
Not all cement before and after the hardening of the volume is shrinkage, expansion of cement before and after the hardening of the volume is not shrinkage but expansion.Therefore, if appropriate expansion cement is adopted, the overall shrinkage of foamed concrete can be compensated or reduced to a certain extent.However, expansion cement not only affects the volume change, but also affects a series of other properties, too much introduction will cause damage to the structure of hardened foam concrete, so the variety and dosage of expansion cement must be determined through the test.
3. Influence of aggregate
Foamed concrete can only be mixed with a part of fine aggregate, because of the chemical inertia of aggregate, excessive mixing will lead to a significant reduction in the strength of foamed concrete, so its content is limited to a certain extent.When the density, water-cement ratio and other technological parameters of foamed concrete are basically determined, the amount of cement will decrease as the fine aggregate increases. Therefore, there is an appropriate choice between whether to mix the aggregate or not and how much.
4. Influence of water-cement ratio and conservation system
The molding water-cement ratio of foam concrete is often as high as 0.7 or even 0.8, and the excess water will remain in the pores of hardened foam concrete, which accounts for about 50% of the molding water.When ambient relative humidity is low or ambient temperature is high, water evaporates and escapes.Especially in the early stage of hardening, the structure of foamed concrete is still relatively weak, if the maintenance is not good, water loss is extremely easy, leading to greater shrinkage and surface cracking, weaken the internal structure of the hardening body, leading to high water absorption of hardened foam concrete.Therefore, the initial water-cement ratio of foamed concrete becomes a precondition for the shrinkage of hardened foamed concrete.One of the key techniques to prepare low shrinkage foamed concrete is to control low water cement ratio.
5. Technical approach to reduce shrinkage and cracking of foamed concrete
According to the above analysis, the technical measures to reduce the shrinkage, prevent cracking and water absorption of foam concrete mainly include the following aspects:
(1) appropriate cement dosage;
(2) add appropriate expansion cement;
(3) low molding water-cement ratio;
(4) optimize the maintenance system and strengthen early water conservation;
(5) use waterproof agent (mixing or surface coating).
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