WHAT SIZE CEMENT BAGS DOES BUILDERS STOCK?
Cement traditionally comes in 50 kg bags, which is what Builders stocks.
WHAT IS THE DIFFERENCE BETWEEN CEMENT AND CONCRETE?
Although people tend to interchange the terms cement and concrete, cement (a mixture of a very fine powder made from limestone, sand, clay, and iron ore) is in fact an ingredient of concrete. When mixed with gravel and sand, and water, the finished product is concrete.
WHAT’S THE DIFFERENCE BETWEEN MORTAR AND PLASTER?
Mortar is a mix of cement, water and fine aggregate sand, which glues together bricks or blocks, and gives strength and stability to a wall. Plaster comprises even finer sand than mortar and is used to render outside and inside walls.
WHAT ARE THE DIFFERENT TYPES OF CEMENT?
Most common types of cement are referred to as Portland cement. Cement is classified CEM I to V. CEM I is basic Portland cement, while the other four categories are factory blends of CEM I with other types of materials to give them different properties.
White Portland cement contains little or no iron or manganese, which give cement its grey colour. It’s more expensive to produce than the conventional product, so is more suited to decorative applications.
WHAT ARE THE DIFFERENT STRENGTH CLASSES OF CEMENT?
There are three main strength classes for cement: 35,5, 42,5 and 52,5. The strength class of most cements is measured in a standard test at ages two or seven days, and at 28 days. You’ll also find the letters N or R on cement bags. N refers to a class of cement with normal strength development and R (rapid) denotes cement that achieves better early strength.
HOW DO I KNOW WHICH TYPE OF CEMENT TO CHOOSE?
The type of cement you use is entirely dependent what you’re planning to build and the strength that is required for that structure. However, as a rule of thumb, 32,5N classified cement is a good general purpose choice. When in doubt, ask one of the experts at Builders.
HOW DO I WORK OUT THE MIX RATIO FOR CONCRETE?
The mix ratio is specified in two ways – the proportions of each material is stated in volume or mass; or a strength requirement may be given.
Volume is expressed in ratios, for example 1:4:5, refers to cement, sand and stone respectively. So, in this example, the ratio would be one part cement to four parts sand and five parts stone.
A similar example of ratio in mass, would be 50 kg of cement to 200 kg of each sand and stone.
If concrete is specified by strength, this is given in megapascals (MPa), or the pressure that concrete can withstand in a crushing test.
HOW DO I CALCULATE THE QUANTITY OF CEMENT FOR DIFFERENT APPLICATIONS (IE SCREEDING)?
According to the National Home Builders Registration Council, you need:
Mortar: Two bags of cement and eight wheelbarrows of sand will lay 770 bricks in a hollow-cavity or single leaf wall; 640 bricks in a double-leaf wall; 770 (390 x 190 x 90), 510 (390 x 190 x 140) and 360 (390 x 190 x 190) blocks.
Plaster: Two bags of cement and six wheelbarrows of sand will cover 20 m2 at 15 mm thickness.
Screeding: 9.1 bags of cement, two wheelbarrows of sand and two wheelbarrows of stone (6.75 or 9 mm) will make 1 m3 of screed.
HOW DO I MIX CEMENT?
If you’re not using a ready mixed cement, the most important thing is to get the ratios right to contain your costs. For small projects, mixing can be done in a wheel barrow. For larger jobs, measure out the sand and place in a long, thin heap on a flat, clean, hard surface, pour the cement on top of the sand and add clean water to a runny consistency. Once it’s thoroughly mixed, add the stone, which will stiffen up the mixture.
HOW DO I PAINT CEMENT?
Make sure the surface is thoroughly clean and dry before applying paint.
Acrylic paint: apply bonding agent to the surface, and leave to cure for at least two hours. Apply two coats (allowing the first to dry for at least two hours before applying the second) with a wool roller.
Epoxy: Mix the two components together, add 15% lacquer thinners. Use a brush or mohair roller to apply. Leave to cure overnight then reapply without diluting the paint.
Enamel: Apply plaster primer with a mohair roller or brush. Leave to cure overnight, then apply stoep enamel with the same sort of brush.
HOW DO I WATERPROOF CONCRETE?
By its nature, concrete is not waterproof. There are admixtures available on the market that are added to the cement mix stage. Or there are coatings, sealers and membranes that can used on the concrete once it’s set. It’s a good idea to waterproof concrete in areas exposed to water, such as the kitchen or bathroom indoors, and concrete ponds and water features outside.
CAN I USE CEMENT AS GROUT?
Grout is basically a runny cement mix, so technically, yes you can. But unless you know what you’re doing, things could go horribly wrong. Builder’s has a whole range of grouting products, so rather use the right product for the job.
CAN I ADD A PRODUCT TO CEMENT TO MAKE IT DRY QUICKER?
Calcium-based accelerators can be used as admixtures to the cement mixtures to speed up the rate of hardening and early gain in strength of concrete. Bags with an R on them also indicate a rapid-hardening cement.
WHAT’S THE SHELF LIFE OF CEMENT?
Stored in a dry place, the shelf life of cement is six months inland and three months at the coast.
CAN I USE CEMENT WITHOUT MIXING WITH SAND?
You can, but you’re going to end up with a product that will eventually crumble. Sand fills the voids between the cement and the aggregate to make it stronger.
IS CEMENT HARMFUL TO MY HEALTH?
Cement has an alkali base, so it can cause burns when it comes into contact with your skin. The fine powder can also irritate your eyes and lungs. But with the right safety precautions, wearing protective gear, injuries can be avoided.
HOW DO I REMOVE CEMENT FROM CLOTHES WHEN IT’S ALREADY DRY?
If you’ve got dried cement on your clothes, scrape any lumps off the fabric with a blunt knife. A good home-grown remedy is one tablespoon of salt and one cup of vinegar added to cold water, with the clothes soaked in the mixture.
HOW DO I PROTECT MY CONCRETE AND KEEP IT LOOKING GOOD?
You can apply a sealer to concrete to protect it from spills and stains. There are different types depending on the location of the surface.
- Penetrating sealers (silanes, siloxanes, silicates, and siliconates) are best for outdoors, where your concrete is exposed to temperature variations.
- Acrylics (solvent- or water-based) can be used both indoors and outdoors, but solvent-based acrylic sealer stand up better to outdoor applications.
- Polyurethanes (solvent- or water-based) are also good for interior and exterior use, such as floors in high-traffic areas, and on coloured, stamped or exposed aggregate concrete and concrete countertops.
- Epoxies can yellow with age, so are better for interiors.
HOW SHOULD I STORE CEMENT?
Cement is activated by water, so storage spaces need to be as leakproof and moisture-free as possible. Cement bags need to be stacked on wooden planks, 15 to 20 cm off the off the floor, and away from walls. Always store the cement so that it can be used in a first in, first out basis. If there are lumps in the cement that can’t be crumbled by hand, the cement isn’t fresh and should be discarded.
HOW DO I REMOVE STAINS FROM CONCRETE?
Stains can be removed from concrete using dry or wet methods.
Dry methods include sandblasting, flame cleaning and shot-blasting, grinding, scabbing and scouring. Be cautious of using steel-wire brushes – they can leave metal particles behind that may rust and stain.
Wet methods involve water or chemicals depending on the nature of the stain. Chemicals either dissolve the stain so it can be blotted up from the surface of the concrete or bleach it so it will not show.
HOW DO I POLISH CONCRETE?
You polish concrete like you sand wood. Use a heavy-duty polishing machine with progressively finer grits of diamond-impregnated segments or disks, starting with the most coarse. This could take three or four applications, depending on the condition of the concrete.
Next, use the same technique with diamond abrasives embedded in a plastic or resin matrix, until the surface has the desired sheen.
Lastly, a commercial polishing compound can be applied.
HOW LONG DOES CEMENT TAKE TO DRY?
Generally, concrete sets in 24 to 48 hours and partially cured within a week (keep the cement free and clear of heavy equipment during this period) at which you can continue building/construction. Most mixes fully cure at 28 days.
HOW MANY BAGS OF CEMENT WOULD I NEED FOR 1 M3?
The number of bags depends on the strength of concrete you require.
- Low-strength concrete (10 - 15 MPa) 4,6 bags
- Medium-strength concrete (20 - 29 MPa) 6,1 bags
- High-strength concrete (30 - 35 MPa) seven bags
WHAT ARE AGGREGATES?
Aggregates are the granular materials used in making concrete. Coarse aggregate is the stone component of concrete that adds bulk and lessens the cost. The larger the aggregate the less water and cement is needed for the mix. Fine aggregate comprises sand and is a void filler, closing the gap between the stone and cement/water mix. The sand needs to be builder’s quality and free of twigs, leaves and other organic matter.
HOW DO I COLOUR CONCRETE?
There are various ways of colouring or staining concrete, depending on which stage of the process you add colour. The professionals at Builders will point you in the right direction.
WHAT WILL HAPPEN IF I ADD TOO MUCH CEMENT/SAND TO MY MIX?
Increasing the cement content increases the strength, but there is a tipping point where the reverse starts happening. The result is a finished product that is brittle.
A very high amount of sand will make your mix very brittle and weak.
WHICH TOOLS ARE NEEDED WHEN WORKING WITH CONCRETE?
- Timber (formwork and pegs)
- Builder’s wheelbarrows
- Square-mouthed shovels
- Screed board (smooth, heavy board about 900 mm wider than the formwork)
- Wooden float (flat piece of wood with a handle)
- Steel trowel
- Edging tool to make cross joints and edging
- 15 mm grooving tool
- Garden rake
- Mallet or heavy hammer
- Plastic sheeting to cover the areas to be concreted
WHY DO CONCRETE SURFACES FLAKE AND SPALL?
To prevent flaking and spalling, make sure there is no water sheen or excess bleed water on the surface before finishing the concrete, otherwise the water content in the surface concrete will have too high, making it weak and less durable. Similarly, too much water in the mix will contribute to early flaking and spalling of the surface.
WHY DOES CONCRETE CRACK?
There are two common types of cracks (the result of shrinkage as concrete dries). Both are related to bleeding (when the mass of the paste sinks and water rises to the top) of the plastic concrete, but in different ways.
- Shrinkage: These are the random, non-continuous cracks that you’ll often find on large slabs. It usually happens shortly after the concrete has been finished, and is a result of the evaporation rate being faster than the bleeding rate. The best way to avoid these cracks is to lay the concrete during the coolest part of the day and avoid extreme weather conditions.
- Settlement: These cracks happen after the cement is compacted, and the solid particles settle and the water rises to the surface. The settlement process continues until the concrete stiffens.
WILL CONCRETE HARDEN UNDER WATER?
Because it sets and hardens due to a chemical reaction with water, cement will harden under water.