CEMENT SURFACE FINISHES

Post 124 -by Gautam Shah

 Cement surface finishes occur in three different conditions:

• 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). 

Cement Concrete Surfaces can have finishes 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 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.

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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