Types Of Cement And Its Influence On Concrete
To better understand concrete groupings according to CIRIA R108 (1-7) it is important to recognise how different kinds of cement and admixtures influence this classification. There are five kinds of cement (CEM I, II, III, IV and V) according to standard strength which are grouped into three classes: class 32.5, class 42.5 and class 52.5 (compressive resistance in MPa). There are also three classes of early strength: N – class with ordinary early strength, R – class with high early strength and L – class with low early strength. The latter can be applied only to CEM III cements. See below table for further clarification:
|CEM I||Portland Cement||Manufactured to conform to British Standard BS EN 197-1. It is the cement that has been most commonly used throughout the world in civil engineering and building works. However, CEM I is the least sustainable type and use of alternatives is in the ascendancy. For concrete class C40/50 and higher.|
|CEM II||Portland – Composite Cement (PCC)||Produced by grinding clinker and a certain amount of gypsum, fly ash, slag and limestone. It reaches high strength early and its lighter colour in comparison with CEM I makes it easier to dye. For concrete class C8/10 – C35/45, mostly pre-cast elements.|
|CEM III||Blast Furnace Cement||A type of cement made from a blend of ordinary Portland cement and crushed slag from a blast furnace. It has lower setting properties than ordinary Portland cement but it’s more resistant to sulfates. For concrete class C8/10 – C35/45.|
|CEM IV||Pozzolanic Cement||Mixtures of Portland cement and a pozzolanic material that may be either natural or artificial. Typical pozzolans include: metakaolin, silica fume, fly ash, slag, VCAS (vitrified calcium alumino-silicate). Its properties are similar to blast-furnace cement.|
|CEM V||Composite Cement||Mixtures of Portland cement, slag and pozzolanic materials. Its characteristics are low early strength but very high strength in longer perspective, very good workability of concrete mix based on CEM V and high resistance to chemical aggression.|
Letters A, B, C (e.g. CEM IIIA etc.) define slag content (A – lowest, C – highest). Cement, depending on its kind, class and presence of admixtures will also have a different heat of hydration reactions as well as resistance to sulfates or alkalinity. This means that cement options are vast, and when deciding what kind and class of cement should be implemented in a concrete mix production, the following characteristics have to be taken into consideration:
- What class (strength) of concrete is required?
- What are the environmental conditions (exposure class)?
- How long will it take to transport the concrete mix to site?
- What will be the method of concrete compaction?
- What will be curing time and conditions?
Concrete class is defined by a structural engineer, whilst concrete mix characteristics depend on the type of construction, placing, compaction technique and transport time. When designing the composition of a concrete mix, the above must be taken into consideration so that the final result is optimal from both economical and technological point of views.
Concrete Type Depending On Application
BS 8500-1:2006, a complementary British Standard to BS EN 206-1, defines specification, production and conformity of fresh concrete. In this document one can learn about parameters which concrete should have at different exposure classes (Table A.8) or what designated concrete (its min. strength class, max w/c ratio, cement and combination types) is required for different applications (Table A.31).
Concrete Group Criteria
When looking at the table of guidance on concrete groupings for CIRIA R108 formwork pressure calculations, we can see that concrete group depends on the type of cement implemented and the presence of retarding admixtures. The latter prolongs the process of hydration reaction in its initial period and slows down the rate of heat release. Retarding admixtures extend the transition time of concrete mix from plastic to rigid state. When using retarding admixtures initial concrete strength may be lower when compared to concrete without admixtures, whereas the final strength is normally higher. Retardants are usually added during the process of preparing concrete mix, normally with mixing water. In some cases, however, it may be necessary to add a retarding admixture to a ready concrete mix e.g. in case of prolonged transport caused by failure or a traffic jam. The use of retarding admixtures is necessary when a need of long distance transport occurs and for works carried out in conditions of high ambient temperatures.
Concrete mix design depends on many variables. To design it efficiently one must know the requirements for concrete strength, durability, application, means of transport, compacting methods on site and details of RC elements. Based on this information the cement type, w/c ratio, thermal requirements and necessity of introducing admixtures etc. can be determined. There is no direct correlation between concrete class (strength) and concrete group. It is impossible to derive what group concrete belongs to based just on its strength. To be able to classify concrete mix to a certain group, one must seek additional information, particularly the exact cement type and presence of any retarding admixtures.