In the past half a century, cement has come to predominate all the architectural surface finishes. Cements are Portland and to a limited extent Alumina cements, with many additives and fillers. Some additives and fillers contribute positively to improve the qualitative aspects such as strength, water proofing, homogeneity, shrinkage, rapid or low setting. While others are used to increase the bulk, to reduce cost with or without affecting the quality. Sand, lime, plaster of Paris, stone aggregates, cinders, expanded aggregates like vermiculite, fly ash, pozzolana, surkhi, gravel, metal turnings, asbestos, fibreglass etc. and chemicals like metal stearates, silicates, polymers aluminium powder pastes etc. are used as additives and fillers.
Cement + water pastes have a tendency, for flocculation and form minute air bubbles an emulsion like mass, and tendency to set falsely. The paste as result has very poor workability and is good for very thin float type applications.
Cement + sand mixtures overcome the flocculation, emulsion like mass forming and false setting problems, and yet provide a reasonable workability. Addition of lime or proprietary chemicals improves the workability, but slows down the setting of cement. Plaster of Paris due to its affinity to water helps in through wetting, improve rendering of cement, but with increased time for setting.
Asbestos, glass fibres etc. reinforce the inter-particle bondage, i.e., homogenize the cement finish. Chemical like aluminium powder or flakes react with silicates of cement to form a foamy air-entrained paste, which on setting and drying retains sufficient spongy mass to insulate the base. Sodium silicate, sodium carbonate and certain sulphate form the basis of proprietary water proofing compounds.
PROBLEMS WITH CEMENT FINISHES
Cement finishes whether smooth or textured, new or old, pose some of these problems:
● fine hair cracks
● honey comb voids
● unbounded loose particles
● foreign particles stuck on the surface
● foreign particles deposited on the surface.
● washable salts leached out from the surface
● salts and compounds formed over the surface by the constituents of the environment
● mould and fungi type bacterial growth
● disengagement from the substrate -peel off.
Cracks appear in both thick and thin masses of cement finishes. Very quick drying and sucking up of moisture by the thirsty substrate, the water needed for complete reaction, cause fine cracks. Saturated substrates often release excess moisture due to temperature change, and this disturbs the bonding to substrates. Unevenly bonded mass soon splits up. Unsound impurities like magnesium salts cause expansion and crack the surface. Where thick finishes are required multi coat application system, allow each coat to readjust the shrinkage stresses and thus reduce cracking. Where thin coats are unavoidable, the surface should be textured so that hair cracks are not easily visible.
Honey comb voids usually occur due to improper mixing and over stirring of ingredients. Method of application (pouring, placement, vibration, compaction) can overcome the air entrained voids, Trowel or float pressure on the surface can squeeze out excess water and consolidate the mass. Pressure spraying and guniting give a well-compacted mass. The voids, not only increase the total area of atmospheric exposure but also the moisture retention capacity of the surface. Consequently leaching of salts, decomposition of surface ingredients, bacterial growth increases.
Ingredients of cement surface finish remain loose or unbounded due to, lack of sufficient binder, poor wettability of fillers, flocculation due to very fine size of particles, oversized particles in terms of surface finish thickness and due to inadequate trowelling pressure. Loose particles get off the surface when fricative forces become operative. This may not damage the surface, but does contaminate the surroundings (e.g. drug or micro electronic plants). Beside the obvious remedies such as use of leaner mix (high wet plasticity), use of wetting agents, proper mixing, judicious selection of particle sizes and proper trowelling, air blasting and pressure cleaning can remove the unbounded particles from the cement surface.
If proper protection is not provided during hardening and setting periods, foreign or redundant particles such as, aggregates, sand, wood chips, saw dust, metal turnings get stuck up on the cement surface. Many of these materials, though, are inert and removable, but do dent the surface.
Moisture reacts with or dissolves the soluble ingredients, the compounds formed after setting and other deposited impurities. Resultant softened, dissolved and loosened substances get carried over to the micro cavities of the surface, where they dry out and remain till mechanically removed, blown off by air or washed away by water shower. However, in the intervening period some of these substances react with atmospheric carbon dioxide, sulphur dioxide, air borne bacteria and fungi, sea salts etc. to form newer substances. Which if insoluble and harmful may have to be removed by rubbing, sanding or treating with suitable chemicals. Cement finish compounds are usually alkaline, so can be neutralized with mild acids, and then dislodged by plain water washing.
Voids, dents, downs of a rough surface are full of moisture and provide an excellent culture for growth or mould, fungi etc. The bacteria are capable of laying latent for months and then revive with the monsoon. The only possible way to eliminate permanently this menace is to make the surface smooth and impervious. Where this is not achievable, periodic treatment of mouldicide and fungicide can help.