Frequently Asked Questions

FAQ's - Cement

 

(Click on the questions below as this will take you directly to the answer, or scroll down to see all questions and answers)

  1. What is cement?

  2. What is the difference between cement and concrete?

  3. How is portland cement made?

  4. Are there different types of portland cement?

  5. Is there a universal international specification for portland cement?

  6. How has the cement specification evolved? Can I still order OPC or PC15 cement?

  7. Does the 'R' in 32.5R, 42.5R and 52.5R cements refer to a concrete that will set quicker than a concrete made with a cement in a 'N' strength class?

  8. There are 3 main strength classes of cement 32.5 – 42.5 and 52.5. Does that mean that I need to use 32,5 strength class cement to produce a 30MPa concrete and a 52.5 strength class cement to produce a 50-60MPa concrete?

  9. With so many different cement products on the market, what would a cement with the following specification CEM II / B-M (V-S)32,5N actually mean?

  10. What is the difference between a CEM I cement and CEM II – CEM V cements?

  11. The CEM II B-L 32,5N has historically been specified for soil stabilization. What does the 'L' stand for? Lime?

  12. What is the recommended cement content that can be used in 1.0 m3 of concrete?

  13. When mixing a 1:6 ratio do I use 1 bag of cement and 6 wheelbarrows of sand?

  14. How should cement be stored?

  15. What is the storage (shelf) life of cement?

  16. What are masonry cements?

 


What is cement?

Portland cement is an extremely fine grey powder manufactured from some of the earth's most common minerals.

  • Limestone -CaCO3
  • Alumina source -Al2O3
  • Iron ore -Fe2O3
  • Silica source -SiO2

Mixed together in the correct proportions and heated up to ± 1400°C, it creates a binder (clinker) with hydraulic properties. This is the ‘glue’ that binds sand and gravel together into the rock-like mass we know as concrete.

(back to top)


What is the difference between cement and concrete? 

Although the terms cement and concrete often are used interchangeably, cement is actually an ingredient of concrete. Concrete is basically a mixture of aggregates and paste. The aggregates are sand and gravel or crushed stone; the paste is water and portland cement. Concrete gets stronger as it gets older. Portland cement is not a brand name, but the generic term for the type of cement used in virtually all concrete. Cement comprises 10 to 15% of the concrete mix, by volume. Through a process called hydration, the cement and water harden and bind the aggregates into a rocklike mass.

So, there is no such thing as a cement floor, or a cement mixer; the proper terms are concrete floor and concrete mixer.

(back to top)


How is portland cement made?  

Materials that contain appropriate amounts of oxides of lime, silica, alumina and iron are crushed and screened and placed in a rotating cement kiln. Ingredients used in this process are typically materials such as limestone, marl, shale, iron ore, clay, and fly ash.

The kiln resembles a large horizontal pipe with a diameter of 3 to 4 metres and a length of 90 metres or more. One end is raised slightly. The raw mix is placed in the high end and as the kiln rotates the materials move slowly toward the lower end. Flame jets are at the lower end and all the materials in the kiln are heated to high temperatures of about 1400 and 1450 degrees Celsius. This high heat drives off, or calcines, the chemically combined water and carbon dioxide from the raw materials and forms new compounds (tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite). For each ton of material that goes into the feed end of the kiln, two thirds of a ton comes out the discharge end, called clinker. This clinker is in the form of marble sized pellets. The clinker is very finely ground to produce portland cement. A small amount of gypsum is added during the milling process in order to to control the cement's set or rate of hardening.

(back to top)


Are there different types of portland cement?  

Though all portland cement is basically the same, five types of cement are manufactured to meet different physical and chemical requirements for specific applications:

  • CEM I  portland cement with a maximum of 5% minor additional constituents
  • CEM II portland cement containing varying additions of secondary materials, i.e. fly ash, pozzolana, slag, silica fume, or limestone.
  • CEM III blast furnace cement
  • CEM IV pozzolanic cement
  • CEM V composite cement

With the wide range of secondary product addition amounts allowable under the South African standard for common cements (SANS 50197-1), there are now potentially 27 products in the family of “common cements”. The Concrete Institute in South Africa supplies a downloadable leaflet on the full composition details of these different cements. Alternatively, a full copy of the specification can be obtained from the South African Bureau of Standards.

White portland cement is made from raw materials containing little or no iron or manganese, the substances that give conventional cement its grey colour. White cement is used primarily for decorative purposes.

(back to top)


Is there a universal international specification for portland cement?  

Each country has its own standard for portland cement, so there is no universal international standard. South Africa uses the specifications prepared by the South African Bureau of Standards:

SANS 50197-1 Cement: Part 1 Composition, specifications and conformity criteria for Common Cements and this is supported by SANS 50197-2 2000 Cement:

Part 2:Conformity evaluation

SANS 50413-1: 2004 Masonry cement. Part 1: Composition, specifications and conformity criteria. SNAS 50413-2 covers the test methods which apply to masonry cements only.
 
There are a few other countries that also have adopted their own standard. Unfortunately, many do not use the same criteria for measuring properties and defining physical characteristics so they are virtually "non-translatable." The European Cement Association located in Brussels, Belgium, publishes a book titled "Cement Standards of the World."

(back to top)


How has the cement specification evolved? Can I still order OPC or PC15 cement?  

1971
* SABS 471:1971 Portland cement (ordinary, rapid-hardening and sulphate-resisting)
* SABS 831:1971 Portland cement 15 (ordinary and rapid-hardening)
* SABS 626:1971 Portland blastfurnace cement

1988
* SABS 1466:1988 Portland fly ash cement

1996
* SABS ENV 197-1:1992* (Based on ENV 197-1:1992)

2002
* SABS EN 197-1:2002 (Based on EN 197-1:2000)

2004
* EN 197-1:2000/A1 – low heat cements

2007
* EN 197-1:2000/A3 – fly ash LOI labeling

2013
* SANS 50197-1:2013 (based on EN 197-1:2011)  

In 1996 South Africa adopted the “EN” European specification for cements and with that the classification changed to “CEM I” for the old OPC and “CEM II – CEM V” for the blended cements.

So, one cannot order an OPC but one can order a CEM I.

(back to top)


Does the 'R' in 32.5R, 42.5R and 52.5R cements refer to a concrete that will set quicker than a concrete made with a cement in a 'N' strength class?

No, the ‘N’ or ‘R’ refers to the early age strength gain at 2 or 7 days and not the setting time. Both the ‘N’ and ‘R’ cement in a specific class needs to achieve the same minimum setting time.

The early age (2 day) strength gain of a class of cement is equal to the class just higher. Also note that the strength requirement is measured (as per the specification) using a standard sand and a fixed C/W ratio meaning that the cement performance is the only variable and this achieved strength is not directly relatable to cement performance in concrete.

Example: a 42,5R cement needs to achieve 20MPa in 2 days which is the same requirement as for a52,5N cement.

Strength Class

Compressive Strength , MPa

Early Strength

Standard Strength

2 days

7 days

28 days

32,5 L

-

> 12,0

> 32,5

< 52,5

32,5 N

-

> 16,0

32,5 R

> 10,0

-

42,5L

-

>16,0

> 42,5

< 62,5

42,5N

> 10,0

-

42,5R

> 20,0

-

52,5 N

> 20,0

-

>52,5

-

(back to top)


There are 3 main strength classes of cement 32.5 – 42.5 and 52.5. Does that mean that I need to use 32,5 strength class cement to produce a 30MPa concrete and a 52.5 strength class cement to produce a 50-60MPa concrete?

No, the strength classes as set out, are for control testing with standard sand to assess the cement performance. When using the cements in concrete you can achieve high and low strength concrete with each/any type of cement. But with a lower strength class cement there is an economical limit on achieving high to very high concrete strength.
Example: Commercial Ready Mix concrete producers normally uses the 42,5R or 52,5N strength class cements for concrete production. With this cements they are able to produce high strength concretes of +50-70MPa. The 42,5R cement can produce the same high strength concrete as the 52.5N strength class cement, due to it having similar 2day and 28day strength requirement as 52,5N cements.

(back to top)


With so many different cement products on the market, what would a cement with the following specification CEM II / B-M (V-S)32,5N actually mean?

pic-faq-cements

The ‘M’ denotes that there are multiple constituents in the cement. In this case it means Fly Ash (V – siliceous Fly Ash) and Slag (S), with more Fly Ash than Slag due to the V being mentioned first.

(back to top)


What is the difference between a CEM I cement and CEM II – CEM V cements?

All cements, including the CEM I cements, are allowed 5% minor constituents which is usually limestone. Thus, a CEM I consists of between 95-100% clinker where the CEM II – V cements can range 20-94% clinker.

Blended cement in South Africa is obtained by mixing CEM I with mineral admixtures or additives like fly ash, slag, silica fume and limestone. Blended cements are now being used more extensively, as they exhibit good technical and environmental advantages:

a. Technical Advantages
• It reduces water demand and therefore the water-cement ratio can be reduced.
• It improves workability for the same water content.
• The blended cements have a better particle size distribution (grading) as compared to CEM I, therefore the permeability of concrete is less which results in improved densities.
• Fly Ash, Slag and Silica fume have hydraulic properties creating secondary crystal growth which makes for more dense concrete and better durability

b. Environmental Advantages
• Energy saving: Blended cements are obtained by adding mineral admixtures with clinker. The energy, which would have otherwise been utilized for production of CEM I, is thus saved.
• Conservation of natural resources: The used mineral admixtures are the by-products of power generation and steel plants. By using these products, we are conserving the natural resources like limestone, clay and silica, etc.
• Pollution control: By reducing the production of clinker, pollution is also controlled as cement is an energy-intensive product

The CEM II B-L 32,5N has historically been specified for soil stabilization. What does the 'L' stand for? Lime? 

The 'L' stands for Limestone and not Lime. Limestone is added as inert filler into the cement and thus does not react with the clay in soil to reduce the PI (Plasticity Index) the same way that Lime would.

(back to top)


What is the recommended cement content that can be used in 1.0 m3 of concrete?

The cement content of a concrete mix will differ according to the strength class, material used and workability required. It is advisable to contact your cement supplier for the recommended cement content. As the main factor that determines the strength of concrete is the water to cement ratio, it is also important to consider the amount of water to be added to the mix.

(back to top)


When mixing a 1:6 ratio do I use 1 bag of cement and 6 wheelbarrows of sand? 

No. You need to mix 1 wheelbarrow of cement with 6 wheelbarrows of sand. The volume of one builder's wheelbarrow is 66L and the volume of 1 bag of cement is 33L thus you need 2 bags of cement per wheelbarrow.

(back to top)


How should cement be stored?  

Cement should be stored in a building which is dry, leak-proof and as moisture-proof as possible. There should be as few windows as practical in the storage building. Stack the cement bags off the floor on wooden planks in such a way, that they are about 150 mm to 200 mm above the floor and away from the walls. The floor may comprise lean cement concrete or two layers of dry bricks, laid on well consolidated earth. Always store the cement in such a way that it can be used in a first in, first out basis.

(back to top)


What is the storage (shelf) life of cement?

The shelf life of cement is estimated to be 6 months inland and 3 months in coastal areas. But this would still depend on the condition of the storage and how well moisture ingress is prevented.

(back to top)


What are masonry cements?

Masonry cements are governed by:
SANS 50413-1:2014 Masonry Cement - Part 1: Composition, specifications and conformity criteria.

There are three different classes of masonry cement:
MC 5; 12,5; 22,5

And two different types: (Example)
MC 22,5 and MC 22,5 X
The normal masonry cement MC 22,5 needs to entrain 8-22% air where the MC 22,5X needs to entrain less than 6% air.
Air entrainment in mortar and plaster helps with cohesion in the mix making it nice and “fluffy”.

(back to top)


 Also see:  Concrete - Fresh FAQ's   |   Concrete - Hardend FAQ's

 

 

Members Login

Please login to activate the Members Section menu bar