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Choosing the right cement for the right application
To build structures that will last, cement consumers need the right type of cement. We take a look at the different types of cement and its applications.
The three main strength classes of cement All cement is mainly classified into three strength classes:
• 32,5Mpa
• 42,5Mpa
• 52,5Mpa
This figure is typically accompanied by either an ‘N’ or ‘R’, which denotes the time it takes for the cement to reach the suitable strength.
• The N implies that it will have normal strength gain
• The R implies early/rapid strength gain.
This is important to know, as a structure that requires early stripping of formwork could benefit in being poured with a concrete containing an ‘R’ type of cement over an ‘N’ of the same class. The ‘R’ cement will achieve a higher early age strength.
CEMENT STANDARDS
Questionable materials will produce questionable results. For the South African built environment, it is recommended to use cement that complies with the requirements of South African National Standards (SANS) 50197-1. All cement products sold in South Africa should fully conform to the requirements of the strength classes as specified by SANS 50197-1, as well as the National Regulatory Of Compulsory Specifications (NRCS) requirements.
General purpose cement that gains strength at an early stage
The 32,5N is a general-purpose blended cement that reaches strength at a normal age and that is suitable for general use in:
• Concrete for home or building structures
• Mortar, plaster and screed applications.
General purpose cement that has normal strength gain
The 42,5N general purpose blended cement is a highly versatile cement that is suitable for a comprehensive repertoire of concrete, mortar and screed application. It can be blended to achieve all the required strengths of concrete for the same applications as the 32,5N mentioned above. This cement type is more forgiving where only poor quality local sand is available.
High performance cement that requires early strength
42,5R and 52,5N cement are high-performance cements that are suitable for civil engineering and structural concrete applications that require early strength and cold-weather concreting. This includes:
• High-strength concrete, pre-cast elements and ready-mix
• Water-tight, structural and pre-stressed concretes
• Precision grouts, repairs and formulated cement products.
It is also ideal for use with cement extenders that enhance the concrete properties such as silica fume and fly ash or Slagment.
CONCRETE MIXING PROPORTIONS
Concrete is made up of four elements: cement, sand, stone (aggregate) and water. The cement: sand: stone: water proportions are critical as it determines the compressive strength of the concrete, which in turn establishes the capacity of the structure you build to withstand loads.
Choose the right cement for the right application
Point of departure would be to select the right cement for the right application and to check that it complies with the relevant specifications to ensure a lasting end product.
Input equals output
Apart from choosing the right type of cement for the right application, it is also critical to take an uncompromising approach to the quality of the cement you choose, to ensure structures that will last.
Consult manufacturers’ guidelines for ideal mixing proportions
As soon as you’ve selected the cement that is most suited to your application, you will need to consult the manufacturers’ guidelines to determine the ideal concrete mixing proportions. The mixing proportions are also subject to the strength of concrete you need, which can generally be classified in the following five groups:
• High strength (25 – 30 MPa)
• Medium strength (20 – 25 MPa)
• Low strength (10 – 15 MPa)
• Mortar (5 MPa)
• Dry, lean mixes (7 – 14 MPa).
Examples of concrete mixing ratios
For the purposes of this illustration, we will take a look at two examples.
The Sephaku 32, a 32,5R general purpose blended cement that reaches compressive strength at a normal rate and that is suitable for an extensive range of building applications, including general domestic or building concrete, mortar, plaster and screed. The guidelines allow for minimal wastage of between 5-10% and are based on a 65-litre builder’s wheelbarrow. Please note that one 65-litre wheelbarrow load of cement is equal to two 50kg bags in volume.
• Dry, lean mixes for brick and block manufacturing: One wheelbarrow of cement (= 2 x 50kg pockets) and eight wheelbarrows of sand
• General purpose mortar for bricklaying and plastering: One wheelbarrow of cement and six wheelbarrows of sand
• Low strength concrete for foundations, domestic flooring, steps, paths and fencing: One wheelbarrow of cement, four wheelbarrows of sand and four wheelbarrows of stone
• Medium strength concrete for paths, driveways and light-duty floors: One wheelbarrow of cement, three wheelbarrows of sand and three wheelbarrows of stone
• High strength concrete for columns, slabs and heavy-duty industrial flooring: One wheelbarrow of cement, two wheelbarrows of sand and two wheelbarrows of stone.
AfriSam All Purpose Cement, a top performer in the typical 42,5N cement strength class. It is a specially blended high quality cement for use in all structural, building and masonry applications.
USE CEMENT AND CONCRETE SAFELY
Concrete is the world’s most widely used building material, and, as with most materials, there are potential risks involved in handling or working with it. Portland cement is a complex combination of compounds that includes minute quantities of trace elements. Although South African cements typically contain less than two parts per million of Hexavalent Chrome (widely regarded as a safe level), it may serve as an aggravating factor in cases of exposure to alkaline fluids. There have been some reports of allergic dermatitis after exposure to these fluids.
RECOMMENDED PRECAUTIONARY MEASURES
Physical hazards Bagged cement is usually packaged in 50kg units. Handling and moving cement bags should be undertaken with due regard for the possibility of back and other physical strains. Similar caution should be exercised when mixing, transporting, placing and finishing concrete, as many of these operations may involve unaccustomed physical effort or working in awkward or uncomfortable positions for long periods. Exposure to cement and other dust Cement: This is an abrasive fine powder and, when handled, some dust may become suspended in the air in the working area. Users should avoid inhaling cement dust as this may cause irritation of the nose and throat. Cement dust may also cause irritation of the eyes. This will occur as a result of the chemical reaction of the suspended dust with the moist mucous membranes. Every attempt should be made to keep airborne cement dust to a minimum to avoid these problems. Should this be impractical, then the use of goggles and dust masks is strongly recommended.
Silica dust: Many of the aggregates used in concrete have high silica contents. The fine silica dusts created when crushing or handling these aggregates can cause lung problems, and precautions should be observed to avoid breathing in such dusts.
First aid and remedial treatment
• Carefully remove wet concrete-soiled clothing. Although some first aid books state that clothing should not be removed from a burn wound, concrete burns are chemical alkaline – not heat burns – and clothing will not normally adhere to the wound
• Rinse the affected skin as soon as possible with cool, clean water. If the skin is treated soon enough (before ulceration), vinegar can be added to the rinse water to neutralise the alkalis (half a bottle of vinegar to a bucket of water).
Milk (which is a good natural buffer) canthereafter be applied to the skin with a pad toneutralise any further traces of alkalis withoutrisk of acid burning the skin. The skin shouldthen be dried by gently dabbing with a towel,and lanolin may be applied to replace lost fatsand oils and to restore suppleness
• If ulceration has already set in, do not attemptto treat the wound with anything other thanclean water without first seeking expert medicaladvice. The risk of spreading an infection to thewound at this stage is severe
• A sterile burn dressing can be applied overthe wound and bandaged. An ice pack canbe applied afterwards above the dressing butnot directly to the skin
• Do not delay getting medical treatment as thealkalis will continue to destroy tissue.
IMPORTANT!
It is advisable to notify any medical authorities that the victim should be treated for ‘alkali burns’ as many medical practitioners may be unaware that concrete is a highly alkaline material that can cause third degree burns.
