For building materials the difference between specific gravity and relative density is usually negligible and the terms are often used interchangeably. If a material does have some open, interconnected pores, liquid such as water can penetrate it. If the liquid is able to travel right through a material it is described as permeable. When materials absorb liquid, usually water, they tend to expand. When they dry they contract.
Materials also expand and contract due to changes in temperature.
All materials , porous and non-porous, metals, ceramics and organics, are dimensionally unstable under normal conditions. This situation is complicated by the fact that different materials expand and contract at different rates. The simplest and most practical way of dealing with this in your building is to carefully consider the joints between materials and to make provision for the movement. The whole building is composed of parts. Joints connect the parts.