Chemistry of cement

The main components of standard Portland cement are:

• Lime (calcium oxide: 66%) in the form of limestone;

• Silica (silicum dioxide: 22%), a component in most quartz, which forms the particles of clays;

• Aluminum oxide (4%), found in large quantities in many clays. The proportion of aluminium oxide in the clay can be adjusted by the addition of bauxite, which is mainly water-soluble aluminium oxide;

• Iron oxide (3%), found in iron ore and in clay;

• Magnesium oxide (2%);

• sulphur dioxide (2%);

• Miscellaneous components (1%).

The manufacturing process aims to produce a material with a high content of tricalcium silicate, usually 55–62 percent of the crystals in the clinker. Other crystals formed are: about 15 percent dicalcium silicate (the same component as the hydraulic binder in hydraulic lime), 8–10 percent tricalcium aluminate and 9 percent tetracalcium aluminate ferrite. As cement sinters during burning, it is very important for no calcium oxide (quicklime) to remain in the finished product. The quicklime will remain embedded in the clinker, even after very fine grinding, and will not be available for slaking until the hardening process of the cement is quite far advanced. When the quicklime particles are finally slaked, they expand and break the structure already developed. The proportion of limestone in the initial mix must therefore be no more than 0.1 percent.

When cement is mixed with water, it initiates the chemical reactions that are so important for hardening. The most important of these is the formation of tricalcium disilicate hydrate, ‘mineral glue’, from hydrated calcium oxide and silica.

2(3CaO SiO2) + 6H2O = 3CaO 2SiO2 3H2O + 3Ca (OH)2

                                        And

2(2CaO SiO2) + 3H2O = 3CaO 2SiO2 3H2O + Ca (OH)2

The reaction between dicalcium silicate and water is slow and does not contribute to the strength of the concrete until a considerable time has elapsed. Aluminate would interfere with these processes, hence the addition of gypsum at the end of the manufacturing process. The gypsum forms an insoluble compound with the aluminate.

During the hydration process, the cement chemically binds water corresponding to about one-quarter of its weight. Additional water evaporates, leaving voids, which reduce the density, and therefore the strength and durability, of the end products.

Suggested use for various concrete grades and nominal mixes

Grade	Nominal mix	Use
C7 C10	1:3:8 1:4:6 1:3:6 1:4:5 1:3:5	Strip footings; trench fill foundations; stanchion bases; non-reinforced foundations; over site concrete and bindings under slabs; floors with very light traffic; mass concrete, etc.
C15 C20	1:3:5 1:3:4 1:2:4 1:3:3	Foundation walls; basement walls; structural concrete; walls; reinforced floor slabs; floors for dairy and beef cattle, pigs and poultry; floors in grain and potato stores, hay barns, and machinery stores; septic tanks and water storage tanks; slabs for farmyard manure; roads, driveways, paving and walks; stairways.
C25 C30 C35	1:2:4 1:2:3 1:1.5:3 1:1:2	All concrete in milking parlours, dairies, silage silos, and feed and drinking troughs; floors subject to severe wear and weather conditions, or weak acid and alkali solutions; roads and paving in frequent use by heavy machinery and lorries; small bridges; retaining walls and dams; suspended floors, beams and lintels; floors used by heavy, small-wheeled equipment, such as lift trucks; fencing posts and precast concrete components.
C40 C50 C60		Concrete in very severe exposure; prefabricated structural elements; prestressed concrete

Batching measuring is done by weight or by volume. Batching by weight is more exact but is only used at large construction sites. Batching by volume is used when constructing farm buildings. Accurate batching is more important for higher grades of concrete. Batching by weight is recommended for concrete of grade C30 and higher. Checking the bulk density of the aggregate will result in greater accuracy when grade C20 or higher is batched by volume. A 50 kg bag of cement can be split into halves by cutting across the middle of the top side of a bag lying flat on the floor. The bag is then grabbed at the middle and lifted so that the bag splits into two halves.

A bucket or box can be used as a measuring unit. The materials should be placed loosely in the measuring unit and not compacted. It is convenient to construct a cubic box with 335 mm sides, as it will contain 37 liters, which is the volume of 1 bag of cement. If the box is made without a bottom and placed on the mixing platform while being filled, it is easy to empty by simply lifting it. The ingredients should never be measured with a shovel or spade.