3.8     Types of Portland cement

     The ASTM has designated five types of portland cement, designated Types I-V.  Physically and chemically, these cement types differ primarily in their content of C3A and in their fineness.  In terms of performance, they differ primarily in the rate of early hydration and in their ability to resist sulfate attack.  The general characteristics of these types are listed in Table 3.7.  The oxide and mineral compositions of a typical Type I portland cement were given in Tables 3.4 and 3.6.

Table 3.7.  General features of the main 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.

<-previous    next->

Home Pictures Site Map About the Authors Links