Concrete for pools
Normal Portland cement is manufactured in a plant and comprises the following raw materials:
- Lime (CaO) from alkali waste, calcite, cement rock, chalk, clay, and seashells;
- Iron (Fe) from blast furnace flue dust, iron ore, mill scale, and ore washings;
- Silica (SiO2) from fly ash, sand, shale, marl, quartzite, and rice hull ash;
- Alumina (Al2O3) from aluminum ore waste, bauxite, and copper slag;
- Gypsum (CaSO4·2H2O) from anhydrite, calcium sulphate, and gypsum; and
- Magnesia (MgO) from cement rock, limestone, and slag.
These raw materials are properly proportioned (with a maximum of five per cent limestone when creating normal Portland cement) and processed in a modern-day manufacturing plant to produce cement powder.
It is interesting to note, approximately 50 per cent of all industrial byproducts (from other types of manufacturing) have potential as raw materials for manufacturing cement powder. As such, concrete is an extremely ‘green’ building material with important environmental benefits.
The manufacturing process
The first step is the quarrying and blending of raw materials using drilling, blasting, and crushing operations. Once obtained, the materials are stored in separate silos and fed into a roller mill in the proper proportions where they are combined with hot gasses; dry mixed, and blended in additional silos. The mixed-and-blended material is then stored until being fed into a kiln system where it is preheated, burned, cooled, and stored as a ‘clinker’ (i.e. dark grey lumps or nodules), which are typically 3 to 25 mm (0.11 to 1 in.) in diameter.
At this stage, the ‘cement’ can be stored for long periods of time (i.e. months) with the right humidity conditions. It is often transported in this stage to other worldwide cement plants that do not have their own raw material processing operations.
In the final stages, five per cent gypsum is added to control the concrete’s setting properties and the ‘clinker’ is pulverized into the more familiar cement powder and kept in bulk storage silos until the material is ready to be shipped via tanker truck or rail, or packaged into 10- or 20-kg (22- or 44-lb) bags for distribution.
The role of aggregates
The aggregates used to manufacture concrete comprise a mix of sand and stone. By volume, aggregates makeup approximately 60 to 75 per cent of the concrete. Rocks compose most of the coarse aggregates, while minerals compose most of the fine aggregates. In comparison to cement powder, aggregates are inexpensive as there is little involved in the manufacturing process.
Essentially, aggregates are extracted from the earth via mechanical means (blasting, excavation, etc.), processed (crushed, washed, and sifted), and used in the concrete mix as per the proper proportions described by the particular mix design, depending on the strength required and application type.
In general, the aggregate should be good quality and must not contain any deleterious properties, which could weaken the concrete’s overall strength.
Aggregates are selected based on the right properties for producing a good quality concrete mix. They include:
- Abrasion resistance: contributes to the surface’s long-term wear and durability;
- Resistance to freeze/thaw: controls the concrete’s volume expansion in the winter;
- Particle shape and surface texture: contributes to the concrete’s shear strength;
- Grading: helps build a uniform heterogeneous mixture with consistent properties;
- Absorption and surface moisture: allows the mix water proportions to be controlled; and
- Alkali reactivity: aggregates used in concrete cannot react with alkali to cause abnormal expansion and map cracking.
| ADVANTAGES OF USING CONCRETE AS A STRUCTURAL BUILDING MATERIAL |
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| • Cost: In comparison to many other structural materials, concrete is an economical building material, both by weight and volume; • Availability: Concrete is readily available. There are dozens of concrete plants in most major metropolitan cities, which in most cases are minutes away from the jobsite; • Ease of use: In general, concrete is easy to work with and can be used to fill any form or space as required; • Strength: Concrete is extremely strong in compression and shear and is well suited for columns, walls, foundations, and slabs; • Permeability: Concrete is relatively impermeable to the flow of fluids. Although it is a porous structure, the pores are not continuous. This also makes it resistant to freeze/thaw damage; • Portability: Concrete does not require a ‘factory’ setting to work with. It is easily and effectively used outside, making it a versatile, economical material; • Durability: Properly mixed, placed, and cured concrete will last many years without much more than minor abrasion and cracking; • Time: Concrete construction is usually faster than building with other structural materials; • Weather resistance: Structures built from concrete can withstand tornadoes, hurricanes, and other extreme weather conditions; • Fire: Concrete is fairly resistant to the damaging effects of fire; and • Insects: Concrete structures are not subject to damage from insects as is the case with other structural materials (e.g. timber). |
Turning cement into concrete
When cement, water, sand, and stone are mixed together in the proper proportions—usually according to a specific mix design (recipe)—the four materials combine to form concrete. Each material performs a specific function and plays an important role in the end product.
The cement powder is obviously the most important, since without it, the mixture would not harden into concrete. Once water is introduced to the cement, it reacts to form a cement paste, which is basically the glue that holds everything together. Mixing the two together (hydration) creates an exothermic reaction (i.e. a chemical reaction that produces heat). During this process, tiny microscopic hair-like structures develop and create a complex mesh of fibres that bind the aggregate matrix together. This calcium silicate hydrate is by far the most important cementing compound of concrete. Hydration will continue indefinitely; however, most will occur during the first few days and directly attributes to the concrete’s gain in strength.
As only a certain amount of water is required for the hydration process (according to the mix design and the volume of cement powder and aggregates), it is important not to add any excess water. Should excess water be added, it will weaken the concrete mix as the water will not be absorbed into the crystalline structure of the cement paste matrix, which binds the aggregates together.
This is the reason why adding water to the concrete mix on the job site should be avoided. Instead, chemical admixtures should be used to improve the workability of the concrete mix.
Finally, aggregate is added as filler. Essentially, it is used as an inexpensive way to create volume in the concrete. The cement paste and gel matrix basically coat the aggregate and adhere to its surface. As the gel hardens, it binds the aggregates together to produce hardened concrete; hence the importance of quality aggregate material. Aggregate must be clean, angular, hard, and somewhat porous, so the cement paste can adhere to the surface and form a strong mechanical bond. The overall strength of the concrete is directly related to the aggregate’s strength and physical properties.
