Formation of hydrated minerals from calcium
carbonate saturated solutions is rather complex, but they form at natural
condition at low temperature condition and at relatively high-pressure
conditions. Such an environment is possible to occur in pore spaces occurring
in building structures in tropical and temperate regions. The volume changes
during hydration and dehydration of these hydrates induce hairline cracks
during the course of time by repeated influxes and evaporation of capillary seepage
pore fluids.

3.2
Chloride Phases

3.2.1 Physical and chemical
characterization of pore fluids

The lime mortar is relatively soft and porous
than Ordinary Portland Cement (OPC) mortars 19 and it is compatible to repair
and renovate heritage structures. The sizes of the pores present in lime mortar
materials used in the current study are widely vary from >10?m to < 1?m. The shape of the pores, orientation and interconnections with adjoining pores also widely varied. The most pores are isolated. Some are interconnected and therefore, the permeability of the lime mortar plaster is widely varied. The sizes of larger pores vary between 200×50?m and 250×100?m with length and breadth ratio varying between 4 and 2.5. The linear pores are found along inter-granular boundaries with length and breadth ratio varies between 5 and 4. The smaller pores less than 10?m are well rounded. The well-developed globular grains amidst the matrix of saccharoidal carbonate materials are seen in the crystal form of druses with large volumes of cavities around these crystals. The matrix is enriched with needles of fibrous carbonate materials (20×0.5?m); are seen as bundles of radiating aggregates (10?m from the centre to 20?m in peripheral portions) in some samples. The nature and form of these minerals indicate that they are composed with significant amount of water, hydroxides or other anionic components. The chloride crystals are rapidly developed by the process of evaporation of droplets from their peripheral portions by leaving hallow spaces at their centre. These hopper or skeletal crystals of sodium chloride enriched halite were started crystallization initially at the peripheral portions; indicating that they were rapidly crystallized by evaporation process leaving a hollow space at the centre. The sizes of hollow spaces present at the centre of these crystals vary from 0.1×0.1?m to 2×1?m. The peripheral rims of halite vary between 0.2 to 0.5?m. The sizes of halite crystals vary between 1.5×1.0?m and 0.5?m. A linear chain of such halite crystals indicate that they were crystallized along a linear crack of 5×2?m; tapering at both ends and appear to be a flake like fracture at interstitial spaces of the matrix of carbonate minerals of host; by capillary influx of pore fluid and subsequent evaporation. However, the matrix and co-existing minerals are characteristically of non-chlorides and are carbonates. A larger cavity of 4×3?m size is completely filled with numerous skeletal crystals of halite with cavities at their centre as aggregates. The successive individual layers of halite are also seen in this cavity. Some globular beads of halite are also seen within the cavity. The size of halite also varies from 0.2?m cubes to prisms of 4?m ×0.75?m with length and breadth ratio exceeding over 5 indicating its rapid free growth in a cavernous environment.  Generally capillary forces are effective between the pore sizes of 1 and 10?m 19. The EDS analyses showed that the matrix was essentially composed of carbonate materials. Table 3 presents a number of saline minerals are composed of hydroxides, sulphates, chlorides, bicarbonates, carbonates and water of hydration. The table 3 is compiled from the data acquired from standard mineralogy text books and other references (5, 20, 21 and 22). These saline minerals occurring in pores or interstitial spaces of mineral grains or in the matrix; are often hydrates subjected to dehydration during summer and hydration owing suction of groundwater by capillary action through the pore sizes ranging between 10?m and 1?m. A free flow by gravitation forces may be initiated through the pore sizes of sizes >10?m. On the contrary, if the
pore sizes reduced to <1?m by adsorption process; the groundwater may be penetrated into the mortar structure 19. The table 4 shows that during hydration, the volume of mineral component increases (during monsoon period) and dehydration (dry season) reduces the volume of the mineral. The pore spaces are completely or partially filled with ionic components of water, water vapor and air. During differential expansion and contraction cracks are induced along the inter-granular spaces or along the direction of orientation of pores in the lime-plaster with its brick structure. The presence of HCO3 in natural groundwater promotes formation of hydrated minerals in pore spaces. Natural hydration of these minerals increases volume and dehydration reduces the volume of minerals; respectively decreasing and increasing their specific gravities (Table 4). The volumes of water vapor (humidity) and air interlocked in the pore spaces are subjected to volume increase due to rising of temperature from monsoon to peak summer 23. 

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