Editor’s Note: This is the second article in a year-long series explaining common raw materials used in precast.
By Kayla Hanson, P.E.
Evidence of cementitious material use dates back to the beginning of recorded history. Egyptians used a blend of cementitious materials as a mortar to secure each 2.5-ton quarried stone block of the Great Pyramid more than 4,500 years ago. Romans employed a pozzolanic cementitious blend to construct aqueducts and other engineering marvels including the Pantheon, whose roof is still the largest unreinforced concrete dome in the world. Europeans in the Middle Ages used hydraulic cement to construct canals and fortresses, some of which still stand today.
Today, we primarily use portland cement in our concrete. Ingredients in modern portland cements are carefully selected, manufactured, tested, and regulated for quality and consistency. Portland cement is available in numerous varieties, each comprised of precise quantities of various materials that are designed for specific concreting applications.
Portland cement specifications
ASTM C150, “Standard Specification for Portland Cement,” outlines 10 cement types, five of which are generally regarded as the primary types of cement used in precast plants:
Type I – Normal/General Purpose
Type II – Moderate Sulfate Resistance
Type III – High Early Strength
Type IV – Low Heat of Hydration
Type V – High Sulfate Resistance
Type I cement is considered a general, all-purpose cement and is used when the special properties of the other cement types are not required.
Type II cement is specified in scenarios where the concrete product is required to exhibit increased resistance to sulfates. Concrete made with Type II cement can be useful for underground structures in areas where soil and groundwater contain moderate levels of sulfates, as well as in roadways, transportation products, and more.
Type III cement offers expedited early-age strength development. Because colder ambient temperatures can cause cement to hydrate slower, Type III cement is often used in cold weather concreting applications to expedite strength development in the early stages of cement hydration. Type III cement is also beneficial when precasters cast the same form twice in one day.
Type IV cement generates less heat during hydration and curing than ordinary Type I portland cement. When conducting mass pours or casting large-volume concrete products, Type IV cement is often used to lessen the amount of heat generated and reduce the risk of flash setting or thermal shock. Type IV cement’s ability to generate less heat during hydration is also beneficial in hot weather concreting applications where fresh concrete may cure at an expedited rate due to high ambient temperatures.
Type V cement is used in concrete products where extreme sulfate resistance is necessary. Coastal structures, piers, underwater tunnels, submerged structures, foundations, roadways and transportation products are all common applications for Type V cement.
Portland cement is manufactured first by producing clinker in a massive kiln. Production of portland cement clinker relies primarily on limestone, clay, sand, iron ore and gypsum. These source materials are excellent providers of calcium, iron, silica and alumina among other elements. The prevalence of these elements in portland cement is determined by the proportion of each source material used during the production of clinker. The amount of each element present in cement will affect the cement’s physical characteristics and behavior.
Four predominant phases or compounds make up each type of portland cement: C3S, C2S, C3A and C4AF.1 Each phase plays a unique role in cement’s performance. The proportion of each phase found in portland cement clinker is attributed to the amount of the source material used.
- C3S (tricalcium silicate) comprises 50% to 70% of portland cement clinker. C3S hydrates and hardens rapidly, and, as a result, is largely responsible for early-age strength gain and initial set. As portland cement’s C3S content increases, so does its ability to contribute to early-age concrete strength development.
- C2S (dicalcium silicate) comprises 10% to 25% of portland cement clinker. C2S hydrates and hardens slowly, and, as a result, contributes primarily to concrete strength development beyond one week.
- C3A (tricalcium aluminate) comprises up to 10% of portland cement clinker. Although it only contributes slightly to early-age strength development, C3A is the most reactive of the four main phases and readily generates heat during the first few days of hydration. Cements with lower percentages of C3A are more resistant to soils and water containing sulfates.
- C4AF (tetracalcium aluminoferrite) comprises up to 15% of portland cement clinker. Its contribution to concrete strength development is minimal. Portland cement’s typical gray color is largely attributed to C4AF.2
Figure 1, below, shows C3S and C2S at about 400X magnification.
Impact of phase composition
The chemical makeup of each ASTM C150-compliant cement type must meet a required limit or fall within a specified range set in the standard. Certain composition requirements apply to all cement types. For example, each type of ASTM C150-compliant cement is allowed a maximum magnesium oxide content of 6%. Magnesium oxide causes slight expansion during cement hydration, so the amount of this material must be limited.
Composition requirements for Type II through Type V are tailored to help the cements perform in accordance with their intended purpose.
Refer to Figure 2 to correlate the relative reactivity of each phase to the following cement attributes.
Lower C3A contents in cement correspond to increased sulfate resistance. Therefore, Type II cement, which is intended for moderate sulfate resistance, is allowed a maximum C3A content of 8%. Similarly, Type V cement, which is intended for high sulfate resistance, is allowed a C3A content no greater than 5%.
Early-age strength development and increased heat of hydration
C3A is also a major contributor to portland cement’s heat of hydration. Type III cement, which is specified in scenarios where high early strength or increased heat of hydration is desirable, allows a relatively high C3A content of up to 15%.
Lower heat of hydration
Conversely, Type IV cement, which is specified when low heat of hydration is necessary, allows a maximum C3A content of 7%. Additionally, Type IV cement requires a minimum C2S content of 40% because C2S hydrates and hardens slowly and contributes to strength gain beyond one week. This helps ensure slower strength development and less heat generation at early ages.
C3S hydrates rapidly and is a significant contributor to early-age strength development and initial set. Therefore, Type IV cements allow a maximum C3S content of 35%, which regulates early-age strength gain and heat generation.
Impact of physical characteristics
Blaine fineness is a measure of the fineness of the cement particles, determined in accordance with ASTM C204, “Standard Test Methods for Fineness of Hydraulic Cement by Air-Permeability Apparatus.”
The total surface area of particles filling a given volume increases as the particle size decreases. Therefore, smaller particle sizes provides more contact area for mix water. Increased cement surface area and greater contact area for mix water allows finer cements to react more readily with water, which can expedite hydration, early-age strength gain and setting time. Some of the primary cement types have particle size requirements in the form of Blaine fineness limits to help the cements perform as designated by their type.
For example, Type III cement will have a higher proportion of smaller particle sizes to help achieve greater early-age strength development, while Type IV cement is likely to have a greater proportion of larger particle sizes to help regulate set time and provide lower heat of hydration.
ASTM C150 also outlines minimum compressive strength results for pastes made with each of the primary cement types. It is important to note these are minimum values and they are not representative of the concrete’s compressive strength at these ages. Figure 3 shows average set times for certain portland cement samples.
Paste made with Type I cement is required to achieve a minimum compressive strength of 1,740 psi at 3 days and 2,760 psi at 7 days. Paste made with Type II cement is required to achieve a compressive strength of 1,450 psi at 3 days and 2,470 psi at 7 days. Paste made with Type V cement is required to exhibit a minimum compressive strength of 1,160 psi at 3 days, 2,180 psi at 7 days and 3,050 psi at 28 days.
Because Type II and Type V cements have lower C3A contents to achieve greater sulfate resistance, it is reasonable to expect slightly lower compressive strength results at early ages. Paste made with Type III cement for use when higher early-age strength is desired is required to exhibit a minimum compressive strength of 1,740 psi at 1 day and 3,480 psi at 3 days. No further strength requirements are outlined because early age generally applies to the first few days of hydration.
Paste made with Type IV cement is required to achieve a minimum compressive strength of 1,020 psi at 7 days and 2,470 psi at 28 days. Type IV cement’s low C3S content reduces heat of hydration by slowing the rate at which cement reacts, which in turn reduces early-age strength gain. Therefore, compressive strength requirements for paste made with Type IV cement are lower than the requirements for the other cement types.
Cement for every application
Each cement type has a different range of chemical and physical requirements that promote preferential behaviors in a concrete mix to optimize it for most any application. As cement’s characteristics are continually tailored, precasters can achieve enhanced concrete performance in more demanding conditions.
Consider reviewing your cement mill certifications for information about each lot’s composition. Because a distinct range of values is allowed for many components in a cement type, it could be beneficial to use the details outlined in the mill certificate to anticipate fresh or hardened concrete characteristics or to troubleshoot minor inconsistencies. Consult with your cement supplier to learn more about your cement and how it interacts with other materials in your mix design to achieve your best results.
Kayla Hanson, P.E., is NPCA’s director of technical services.
1. These are shorthand designations for the chemical compounds. Per ASTM C150, when expressing phases, C = CaO, S = SiO2, A = Al2O3, F = Fe2O3.
2. PCA Design and Control of Concrete Mixtures, 15th Edition
3. Kosmatka, Steven H. and Wilson, Michelle L., Design and Control of Concrete Mixtures, EB001, 16th edition, Portland Cement Association, Skokie, Illinois, USA, 2016, 632 pages.