Post 124 -by Gautam Shah
- Finishes generated over the cast cement products,
- Rendered finishes of cements,
- Cement coatings’ finishes.
The major problem with cement finishes is the consistency of
colour over the surface. Other problems happen due to
differences in application (casting, rendering or coating), mixing
water content and quality and curing processes. The colour
differences arise due to many reasons: ingredients used in
cement manufacturing, particulate size and its distribution, size
and colour quality, proportionate quantity of other ingredients,
quality-conditioning additives and mixing procedures.
● Cement cast products are plain or steel reinforced structures,
pre cast blocks, units, etc. A cement finish of a cast product
emerges due to the form-work, compaction, level of vibration
(causing aeration), proportion of water and technique of casting
(towards gravity, centrifugal or centripetal, short or long depth
fall).
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 light-coloured concrete (colour of cement is variable not only from
plant to plant but often batch to batch). Degree of mixing and air entrapment affect the colour. Free lime 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.
● Rendered finishes of cement include plasters, sprays,
guniting, masonry pointing, screeds and daubing. These are
comparatively of thinner mass. The compaction, if any is part of
the application or rendering-levelling by trowels, plasters boards,
etc. For rendered finish the surface quality is fairly consistent for
small-extent surfaces. For very large surfaces, such as multi-storeyed buildings, the surface differences are noticeable.
These anomalies can be reduced by dividing the surface with
wide intervening elements of different colour, texture,
projections or depressions. Same technique is used for pointing
to the masonry faces of stones or bricks. Pointing is designed to
enhance the joints’ pattern, as strongly horizontal, vertical, or
both. 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 is used for creating in-situ and precast floor blocks. Use
of fine sands increases the air entraining effect and reduces the
work-ability. Angular or flaky sands are difficult to use in sand
face plasters.
Cement plasters and concrete were new finishes in later part of 19th C and so, architects preferred to experiment its colour and texture. For this old forms were not suitable, and new forms, volumes were tried out. The new material was placed inside and outside, where the illumination, shadows and textures were the key expressions. Entirely a new vocabulary came into being. Brutalism was just one such expression.
The mixed mortars are affected 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 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.
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| Brick masonry pointing with cement |
For vertical and ceiling surfaces ziki plaster formed with marble
dust containing substantial amounts of fine mica and talc. Similarly pearl glow and a smooth surface can be achieved by
including sea shell dust. Slow setting and engravable cement
mortars require high workability, are achieved by addition of fully
calcined gypsum or lime containing such compounds.
● Cement coatings’ finishes are in the form of cement paints
and high viscosity or bodied rendering formulations. Cement
paints have limited life of 3-5 monsoons and re-applicability of
5-7 coatings. Cement paints are alkaline materials and applied
on similar substrates, and as a result the colour range is mostly
of Oxides colours. Some blues and greens of darker range are
available. The chief problem is the process of adding the water,
which creates flocculation and aerated mass. Next problem is
application on a dried out substrates with chances of poor
adhesion and difficult brushing. Rendering formulations have
high viscosity, and the success usage relies more on the craft
of application.
Cement Surface Finishes have few basic 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
- disengagements from the substrate -peel off.
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