A void is an empty space in lime mortar. The type,
size, shape, arrangement and abundance of voids and pores are factors
controlling many important properties. The volume air voids is depend on
mix design, preparation and application methodologies of lime mortars. To study
the texture of lime plaster, it is essential to know the profusion of various
types voids. Air voids present are formed during the mixing lime
mortar. Diffusion of vapourous lime mortar into air voids may cause free growth
of needle-like crystals from inner walls of such voids as in the
crystallization in miarolytic cavities 31 and 32.  Particle size and particle size distibution
in the lime mortar  and the planar
surface on which the lime mortar applied are 
also caused heterogenetity of lime mortar. They may be studied under
scanning electron microscope (SEM).  The
size and shape of the voids can be used as indicators of type and origin of
voids. The luster and texture of the interior of the voids may be used in the
recognition of voids caused by accumulation of water and passageways for water.
The properties on which distinctions may be made between the various types of
voids.  Because these distinctions are
made on the appearance of a void on the surface of a slice, many large voids
will be classified as entrained voids when they are really entrapped voids
(>1mm bubble like (FHWA-RD, 1997). A cross section that is larger than the
defined maximum for entrained voids (100µm – 1mm) must be a section of an entrapped
air void or a water-formed void. A significant number of large cross sections indicate a
great number of large voids. SEM study is a valuable technique to analyze
quantitative determination of the abundance of various types of voids. Air
voids are classified as entrained or entrapped. During recrystallization of
fine-grained minerals into coarse crystalline ones, the whole material tries to
loose the excess of internal energy generated during  the deformation when  the crystal lattice defects occurs. During
such process the shape and size of grains change and the texture of the
material changes. The intergranular boundaries of mineral grains are irregular.
During recrystallization of gypsum and other sulphate minerals, intergranular
boundaries are migrated or rotated (Nimmo, 2004). Adjacent  grains differs in density.The defect poor
gypsum grain bulges into the deffect rich adjacent grain, and shape of the  gypsum grain distorted. It results
spontaneous growth of new grains.