Post – by Gautam Shah
Cement mortars are rendered on masonry, concrete and other surfaces. The colour of cement, sand, amount of water, mixing, trowel pressure and rendering technique determine the colour of surfaces.
Cement mortars are mixed at two distinctive stages.
1. In dry state mixing of cement is mixed with other ingredients such as lime, chalk, sand, clay, various sizes and shapes of aggregates. Dry mixing requires continuous agitation till water is added, otherwise heavier ingredients settle down. Dry mixing is much easier, requires less energy then any wetted mass. Dry mixing helps penetration of water and thereby reduces the flocculation (wetting of cement particles due to enveloping air). In fast setting or hardening varieties of cements or due to presence of such additives, mixing of wet-mass becomes very difficult due to the resistance.
2. Dry state mixed mass is combined with water. Very intensely mixed mortars are air entrained and allow calcination of calcium hydroxide (instead of allowing it to combine with silica alumina compounds for cementing action). Air entrained mass also have lighter colour for a very temporary period, due to formation of bubbles.
Colour of the wet mixed cement-based mass and of the cured-dried mass are nearly uncertain conditions. There are great many operative variants. Cement mixed mortars are affected (tinged) by the colour of the aggregates. So it is very difficult to produce a perfectly white marble mosaic tile or washed chips plaster, unless only pure white aggregates are used. Cement and aggregate flooring such as IPS – Indian Patent Stone, Red Madras floor and Ironite (cast iron milling waste)are all strongly affected by the colour of the constituents.
Coloured mortars have pigments of iron oxides (black, red and yellow). Green, blue and other colours (though not sun-fast or long lasting) are achieved by use of chrome pigments. For vertical and ceiling surfaces ziki plaster formed with marble dust containing substantial amounts of fine mica and talc. Similarly pearl glow and smooth surface can be achieved by including sea shell dust. Slow setting and engravable cement mortars require high workability, and are achieved by addition of fully calcined gypsum or lime containing such compounds.
Use of fine sands increases the air entraining effect and reduces the workability. Angular or flaky sands are difficult to use in sand face plasters.
Finishes for Concrete Surfaces
A cement agglomerate like concrete can have a finish depending on several factors, such as:
1 Form work surface, joints and continuity, use of a release agent, absorbency of the form work surface and setting or hardening enhancer and retarders used.
2 Concrete mix proportions, ingredients’ colour, size, and texture (lighter toned aggregates and sands produce a light-coloured concrete (Colour of cement is variable not only from plant to plant but often batch to batch). Degree of mixing and air entrainment that occur in the mass. Free limes in water creates a soapy foam which also affects the colour.
3 Insufficient or uneven curing affects the hydration and eventually the colour of the concrete.
4 Inadequate vibration causes minor pockets of air bubbles, which affects the texture.
.Joints and Lining works
Cements Joints or lining is rendered over stone, brick and other masonry blocks. Cement is used as a joint material for masonry work and component assemblies, coating material for masonry and other surfaces, casting materials as conglomerated compounds and bonding material for free particulate.
Cement joints, from a surface finish point of view are very important elements in exposed masonry faces. The joint is usually of a contrasting colour and of smoother texture, then the main material surface. A raked joint or protruding joint looks much darker than a flushed joint or flat joint.
Cement composites can be rendered to many types of finishes, patterns and textures. Type of ingredients, particle size, grain distribution, colour, structure, shape; techniques of application (trowel float, sprayed gunited, brush coated, sponge sucked, machine vibrated etc.) and post finish cares (premature wash etching, sand blasting, chipping, etc.) all form the quality of surface finish of cement composites.
Cement is used in types of composites. One of the cheapest combinations is soil-cement, a combination to produce masonry blocks and temporary (light traffic) pathways. Cement and dry agricultural waste or wood chips are used in production of low-cost insulation boards. Cement is also used in binding wood charcoal, lignite, mineral coke and other combustible waste to produce briquettes or fuel pallets.