Frequently Asked Questions

You might have some questions in mind about the cement industry. Below are the most commonly asked questions. Feel free to browse and read the contents.

What is clinker?

Clinker is a coarse agglomerate of synthetic minerals that is produced by burning a raw meal, consisting of a selected mixture of raw materials, at a very high temperature in a specialized kiln system. The clinker mostly appears as a dusty granular mixture of dark grey/ black particles up to 40mm in size.

What are the unique chemical features of a clinker attributable to quality of cement?

Major chemical compounds in a clinker and their effect

  • Tricalcium Silicate (C3S) hardens rapidly and is largely responsible for initial set and early strength.
  • Dicalcium Silicate (C2S) hardens slowly and contributes largely to strength increases at ages beyond 7 days.
  • Tricalcium Aluminate (C3A) liberates a large amount of heat during the first few days of hardening and, together with C3S and C2S may somewhat increase the early strength of the hardening cement (this effect being due to the considerable heat of hydration that this compound evolves). It does affect set times.
  • Tetracalcium Aluminoferrite (C4AF) contributes very slightly to strength gain. However, acts as a flux during manufacturing. It contributes to the color effects that makes cement gray.
What is cement

Cement is a hydraulic binder, i.e. a finely ground inorganic material which, when mixed with water, forms a paste which sets and hardens by means of hydration reactions and processes and which, after hardening, retains its strength and stability even under water. Cements when appropriately batched and mixed with aggregate and water shall be capable of producing concrete or mortar which retains its workability for a sufficient time and shall after defined periods attain specified strength levels and also possess long-term volume stability.

How many types of cement are there?

There are 27 products in the family of cement grouped into five main cement types as

  • CEM I Portland cement;
  • CEM II Portland-composite cement
  • CEM III Blast furnace cement
  • CEM IV Pozzolanic cement
  • CEM V Composite cement.
Requirements of cement

a. Chemical requirement Chemical requirements of cement are those which are attributed to the chemical content of the cement. It includes SiO2, Al2O3, Fe2O3, CaO , MgO , SO3 content that should be present in cement and their respective tests including Loss On Ignition (LOI), Insoluble residue (IR),Alkali Equivalent etc.

b. Physical requirement Are those requirement cement should possess as per the standard and have to be confirmed through the following tests; expansion test (volume stability), fineness, Specific surface area (Blain Air permeability), setting time

c. Mechanical requirements Those properties which can be tested mechanically like comprehensive strength test, flexural test etc so as to qualify the cement as per the standard.

What is the difference between cement and concrete?

Most people use cement and concrete interchangeably. Cement is actually an ingredient of concrete. Concrete is basically a mixture of aggregates, water, and cement. Aggregates can be sand, gravel, or crushed stone. The cement acts as a binder, a substance that sets and hardens as it dries. Cement reacts with carbon dioxide in the air dependently and binds the aggregates together resulting to a rock-like mass.

What is the difference between OPC and PPC?

Ordinary Portland Cement (OPC) and Portland Pozzolana Cement (PPC) differ primarily in their composition, which ultimately affects its applicability.

PPC, typically, has pozzolan material added to the cement mixture (either before clinkering or added to ground clinker).

Pozzolan has an effect in reacting with calcium hydroxide formed after hydration of alite and belite phases in cement. Their composition is primarily silica and alumina, though they are essential for formation of alite and belite in cement clinker, this external addition is an attempt to control the free calcium hydroxide presence in the paste.

In terms of utility, long term strength of such cements are strength gaining with the hydration period extending quite longer than OPC. On the downside, the early strength properties are going to be affected. The fresh properties are going to be slightly better because of action of fly ash as filler.

What are the important guideline for storing cement?
  • Do not store bags in a building in which the walls, roof and floor are not completely weatherproof.
  • Do not store bags in a new warehouse until the interior has thoroughly dried out.
  • Do not store in rooms with badly fitting windows and doors.
  • Do not stack bags against the wall. Similarly, don’t pile them on the floor unless it is a dry concrete floor. If not, bags should be stacked on wooden planks or sleepers.
  • Pile the bags close together.
  • Do not pile more than 15 bags high and arrange the bags in a header-and-stretcher fashion.
  • Do not disturb the stored cement until it is to be taken out for use.
  • Do not take out bags from one tier only. Step back two or three tiers.
  • The principle of first-in first-out should be followed in removing bags.
What is the shelf life of cement?

Cement is a hygroscopic material, meaning that it absorbs moisture in presence of moisture it undergoes chemical reaction termed as hydration. Therefore, cement remains in good condition as long as it does not come in contact with moisture. If cement is more than three months old then it should be tested for its strength before being taken into use.

What is initial and final setting time of cement?

Initial set is when the cement paste loses its plasticity and stiffens considerably. Final set is the point when the paste hardens and can sustain some minor load.

How do you control the strength of a concrete?

The easiest way to add strength is to add cement. The factor that most predominantly influences concrete strength is the ratio of water to cement in the cement paste that binds the aggregates together. The higher this ratio is, the weaker the concrete will be and vice versa. Every desirable physical property that you can measure will be adversely affected by adding more water.

What is "curing" and why is it so important?

The term "curing" is used to include maintenance of a favorable environment for the continuation of chemical reactions, i.e. retention of moisture within, or supplying moisture to the concrete from an external and protection against extremes of temperature.

How do you remove stains from concrete?

Stains can be removed from concrete with dry or mechanical methods, or by wet methods using chemical or water. Common dry methods include sandblasting, flame cleaning and shot-blasting, grinding, scabbing, planing and scouring. Steel-wire brushes should be used with care because they can leave metal particles on the surface that later may rust and stain the concrete. Wet methods involve the application of water or specific chemicals according to the nature of the stain. The chemical treatment either dissolves the staining substance so it can be blotted up from the surface of the concrete or bleaches the staining substance so it will not show.

What are recommended mix proportions for good concrete?

Good concrete can be obtained by using a wide variety of mix proportions if proper mix design procedures are used. A good general rule to use is the rule of 6's:

  • A minimum cement content of six bags per cubic yard of concrete
  • A maximum water content of 6 gallons per bag of cement
  • A curing period (keeping concrete moist) a minimum of six days
  • An air content of 6 percent (if concrete will be subject to freezing and thawing)
© Copyright National Cement Share Company 2022.
Designed by ICT Department, National Cement Share Co., Dire Dawa, Federal Democratic Republic of Ethiopia.