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In the most general sense of the word, cement is a binder that sets and hardens independently and can bind other materials together. Cement used in construction is characterised as hydraulic or non-hydraulic.
Hydraulic cements (e.g., Portland cement) harden due to hydration, a chemical reaction that occurs independently of the mixture’s water content. They can harden even under water or when constantly exposed to wet weather. The chemical reaction that results when the anhydrous cement powder is mixed with water produces hydrates that are not water soluble.
Non-hydraulic cements (e.g., lime and gypsum plaster) must be kept dry in order to retain their strength.
Chemically, cement is a mixture of calcium silicates and calcium aluminates with a small amount of gypsum. The essential raw materials used in the manufacture of cement are clay, limestone and gypsum. Gypsum (calcium sulphate, CaSO4) is used to regulate the setting rate.

Manufacture of Cement

In the wet process, the raw materials, properly proportioned, are ground with water, thoroughly mixed and fed into the kiln in the form of a ‘slurry’ (containing enough water to make it a fluid). The slurry is fed into the kiln from the higher end. At the same time, hot air is blown from the lower end. This process drives out the unwanted elements present in the slurry (in the form of gases). The remaining elements combine to form a new substance with new physical and chemical characteristics. The new substance, called clinker, is formed in pieces of about the size of marbles (see Figure 10.2).
The collected clinkers are cooled, and then finely powdered by grinding and mixed with 3% gypsum.


Setting of Cement

When mixed with water, cement sets to a hard mass. It first forms a plastic mass which hardens after some time due to three-dimensional cross-links between –Si-O-Si– and –Si-O-Al– chains. The first setting occurs within 24 hours, whereas the subsequent hardening requires a fortnight, when it is covered by a layer of water. This transition from plastic to solid state is called setting.
Reactions involved in setting of cement
  • On hydration, silicates and aluminates of calcium get converted into their respective hydrated colloidal gels.
  • At the same time, hydrolysis precipitates calcium hydroxide and aluminium hydroxide.
    This calcium hydroxide binds calcium silicate particles together. On the other hand, aluminium hydroxide fills the interstices (an intervening space) rendering the mass impervious (not affording passage to a fluid).
  • Role of gypsum: Gypsum reacts with tricalcium aluminate.

The fast setting tricalcium aluminate is removed to slow down the setting process. A quick setting will give rise to a crystalline-hydrated calcium aluminate. A slower setting yields the colloidal gel that imparts greater strength to the set mass. Thus, gypsum helps in regulating the setting time of cement.

Curing of Cement

Care needs to be taken to properly cure the concrete and to achieve the best strength and hardness. Cement requires a moisture-controlled environment to gain strength and harden completely. The cement paste hardens over time, i.e. initially sets and becomes rigid though very weak, and gains in strength in the following weeks. In around 3 weeks, over 90% of the final strength is typically reached, though it may continue to strengthen for decades.
Hydration and hardening of concrete during the first three days is critical. Abnormally fast drying and shrinkage due to factors such as evaporation of water due to wind during placement may lead to increased tensile stress during the initial stage when it has not yet gained significant strength. This results in greater shrinkage cracking. The early strength of the concrete can be increased by keeping it damp for a longer period during the curing process. Minimising stress before curing minimises cracking. High early-strength concrete is designed to hydrate faster, often by increased use of cement that increases shrinkage and cracking. The strength of concrete changes (increases) up to three years. It depends on cross-sectional dimension of elements and conditions of structure exploitation.
During this period, concrete needs to be in a controlled temperature and humid atmospheric conditions. In practice, this is achieved by spraying or ponding the concrete surface with water, thereby protecting concrete mass from ill-effects of ambient conditions. For example, two methods to achieve this are by ponding, i.e. submerging setting concrete in water, and by wrapping in plastic to retain the water content in the mix.
Properly curing the concrete leads to increased strength and lower permeability and avoids cracking where the surface dries out prematurely. Care must also be taken to avoid freezing, or overheating due to the exothermic setting of cement. Improper curing can cause scaling, reduced strength, poor abrasion resistance and cracking.

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