When the foam concrete is not properly maintained and the water conservation measures are not enough, it will cause volume shrinkage due to the excessive evaporation of internal water, resulting in surface cracking. The strength and thermal insulation properties of foam concrete after water absorption are significantly reduced.
1. The influence of cement dosage and type. The solid volume of ordinary portland cement increases in the process of hydration and hardening, while the water adding system of cement shrinks. Secondly, the hydration process of cement is accompanied by exothermic, which causes the initial volume expansion and shrinkage when cooling, resulting in the increase of apparent shrinkage. In addition, there is self shrinkage in the process of cement hydration. Therefore, under normal circumstances, if other conditions are basically the same and the amount of cement increases, the shrinkage of foam concrete will also increase correspondingly. Not all cements are shrinkage when they are hydrated. For example, the volume of expansive cement after hydration not only does not shrink, but also increases, which can make up for the volume shrinkage of the system to a certain extent. But expansive cement also affects other properties of foam concrete.
2. Influence of aggregate. Adding volume invariable aggregate can effectively reduce the shrinkage of concrete, but in the foam concrete, only fine aggregate can be added. However, because the aggregate is inert material, too much admixture will lead to the reduction of concrete strength.
3. Influence of water cement ratio and curing conditions. As mentioned above, the evaporation of water has a direct effect on the cracking of foam concrete. The water cement ratio of foam concrete is often as high as 0.7 or even 0.8. When excess water is exposed to high temperature or dry environment, it will evaporate away, thus making the foam concrete shrink and crack. Especially in the early stage of molding, when there is a lack of effective maintenance, this situation is particularly serious. It can be seen that low water cement ratio is a key factor to ensure the non cracking of foamed concrete.