Q.
b) State four (4) types of Portland a Cement and explain the main properties of each type of these cement. (18 marks)
(Part b) of, 2011 Q5)
A.
b)
Portland cement comes in a variety of different types. In the United States, these types are classified as Type I, II, III, IV and V. Only Types I and III are necessary for consideration by concrete countertop fabricators; the benefits of Type II cement are generally irrelevant to the concrete countertop industry.
- Type I is ordinary Portland cement, and it is available in white or gray.
- Type II is a moderate sulfate resistant cement, important when concrete is cast against soil that has moderate sulfate levels.
- Type III is a high early strength cement. It is ground finer and reacts faster than Type I, so the early strength gains are greater. However the ultimate strength is not higher than Type I. Concrete made with Type III will have slightly higher 28 day strengths than concrete made with Type I, all else being equal. Type III is available in white or gray, but white Type III is difficult to find in small (less than pallet) quantities; it often has to be special ordered.
- Type IV and V are often used in special construction applications where high sulfate resistance is required or a low heat of hydration is important. Neither of these types are practical choices for countertops.
http://www.concretecountertopinstitute.com/library.item.4/portland-cement-type-i-ii-iii-what-039-s-the-difference.html
Further details below.
3.8 Types of Portland cement
Classification |
Characteristics |
Applications |
|
Type I |
General purpose | Fairly high C3S content for good early strength development | General construction (most buildings, bridges, pavements, precast units, etc) |
Type II |
Moderate sulfate resistance | Low C3A content (<8%) | Structures exposed to soil or water containing sulfate ions |
Type III |
High early strength | Ground more finely, may have slightly more C3S | Rapid construction, cold weather concreting |
Type IV |
Low heat of hydration (slow reacting) | Low content of C3S (<50%) and C3A | Massive structures such as dams. Now rare. |
Type V |
High sulfate resistance | Very low C3A content (<5%) | Structures exposed to high levels of sulfate ions |
White |
White color | No C4AF, low MgO | Decorative (otherwise has properties similar to Type I) |
The differences between these cement types are rather subtle. All five types contain about 75 wt% calcium silicate minerals, and the properties of mature concretes made with all five are quite similar. Thus these five types are often described by the term “ordinary portland cement”, or OPC.
Types II and V OPC are designed to be resistant to sulfate attack. Sulfate attack is an important phenomenon that can cause severe damage to concrete structures. It is a chemical reaction between the hydration products of C3A and sulfate ions that enter the concrete from the outside environment. The products generated by this reaction have a larger volume than the reactants, and this creates stresses which force the concrete to expand and crack. Although hydration products of C4AF are similar to those of C3A, they are less vulnerable to expansion, so the designations for Type II and Type V cement focus on keeping the C3A content low. There is actually little difference between a Type I and Type II cement, and it is common to see cements meeting both designations labeled as “Type I/II”. The phenomenon of sulfate attack will be discussed in much more detail in Sections 5.3 and 12.3, but it should be noted here that the most effective way to prevent sulfate attack is to keep the sulfate ions from entering the concrete in the first place. This can be done by using mix designs that give a low permeability (mainly by keeping the w/c ratio low) and, if practical, by putting physical barriers such as sheets of plastic between the concrete and the soil.
Type III cement is designed to develop early strength more quickly than a Type I cement. This is useful for maintaining a rapid pace of construction, since it allows cast-in-place concrete to bear loads sooner and it reduces the time that precast concrete elements must remain in their forms. These advantages are particularly important in cold weather, which significantly reduces the rate of hydration (and thus strength gain) of all portland cements. The downsides of rapid-reacting cements are a shorter period of workability, greater heat of hydration, and a slightly lower ultimate strength.
Type IV cement is designed to release heat more slowly than a Type I cement, meaning of course that it also gains strength more slowly. A slower rate of heat release limits the increase in the core temperature of a concrete element. The maximum temperature scales with the size of the structure, and Type III concrete was developed because of the problem of excessive temperature rise in the interior of very large concrete structures such as dams. Type IV cement is rarely used today, because similar properties can be obtained by using a blended cement.
White portland cement (WPC) is made with raw ingredients that are low in iron and magnesium, the elements that give cement its grey color. These elements contribute essentially nothing to the properties of cement paste, so white portland cement actually has quite good properties. It tends to be significantly more expensive than OPC, however, so it is typically confined to architectural applications. WPC is sometimes used for basic cements research because the lack of iron improves the resolution of nuclear magnetic resonance (NMR) measurements.
Ref:
http://iti.northwestern.edu/cement/monograph/Monograph3_8.html