4  Portland Cement

Portland cement is the chief ingredient in cement paste - the binding agent in portland cement concrete (PCC).  It is a hydraulic cement that, when combined with water,  hardens into a solid mass.  Interspersed in an aggregate matrix it forms PCC.  As a material, portland cement has been used for well over 175 years and, from an empirical perspective, its behavior is well-understood.  Chemically, however, portland cement is a complex substance whose mechanisms and interactions have yet to be fully defined.  ASTM C 125 and the Portland Cement Association (PCA) provide the following precise definitions:

	Figure 3.43: Portland, England
	Figure 3:44: Limestone at the Portland Bill near Weymouth

4.1  Background

Although the use of cements (both hydraulic and non-hydraulic) goes back many thousands of years (to ancient Egyptian times at least), the first occurrence of "portland cement" came about in the 19^th century.  In 1824, Joseph Aspdin, a Leeds mason took out a patent on a hydraulic cement that he coined "Portland" cement (Mindess and Young, 1981).  He named the cement because it produced a concrete that resembled the color of the natural limestone quarried on the Isle of Portland, a peninsula in the English Channel (see Figure 3.43 and 3.44).  Since then, the name "portland cement" has stuck and is written in all lower case because it is now recognized as a trade name for a type of material and not a specific reference to Portland, England.

Today, portland cement is the most widely used building material in the world with about 1.56 billion tonnes (1.72 billion tons) produced each year.  Annual global production of portland cement concrete hovers around 3.8 million cubic meters (5 billion cubic yards) per year (Cement Association of Canada, 2001).  In the U.S., rigid pavements are the largest single use of portland cement and portland cement concrete (ACPA, 2002).

This section covers the following topics:

4.2  Manufacturing

Although there are several variations of commercially manufactured portland cement, they each share many of the same basic raw materials and chemical components.  The chief chemical components of portland cement are calcium, silica, alumina and iron.  Calcium is derived from limestone, marl or chalk, while silica, alumina and iron come from the sands, clays and iron ore sources.  Other raw materials may include shale, shells and industrial byproducts such as mill scale (Ash Grove Cement Company, 2000). 

The basic manufacturing process heats these materials in a kiln to about 1400 to 1600°C (2600 - 3000°F) - the temperature range in which the two materials interact chemically to form calcium silicates (Mindess and Young, 1981).  This heated substance, called "clinker" is usually in the form of small gray-black pellets about 12.5 mm (0.5 inches) in diameter.  Clinker is then cooled and pulverized into a fine powder that almost completely passes through a 0.075 mm (No. 200) sieve and fortified with a small amount of gypsum.  The result is portland cement.  The Portland Cement Association (PCA) has an excellent interactive illustration of this process on their website.

4.3  Chemical Properties

Portland cements can be characterized by their chemical composition although they rarely are for pavement applications.  However, it is a portland cement's chemical properties that determine its physical properties and how it cures.  Therefore, a basic understanding of portland cement chemistry can help one understand how and why it behaves as it does.  This section briefly describes the basic chemical composition of a typical portland cement and how it hydrates. 

4.3.1  Basic Composition

Table 3.12 and Figure 3.45 show the main chemical compound constituents of portland cement.

Table 3.12: Main Constituents in a Typical Portland Cement (Mindess and Young, 1981)

Figure 3.45: Typical Oxide Composition of a General-Purpose Portland Cement
(Mindess and Young, 1981)

4.3.2  Hydration

When portland cement is mixed with water its chemical compound constituents undergo a series of chemical reactions that cause it to harden (or set).  These chemical reactions all involve the addition of water to the basic chemical compounds listed in Table 3.12.  This chemical reaction with water is called "hydration".  Each one of these reactions occurs at a different time and rate.  Together, the results of these reactions determine how portland cement hardens and gains strength. 

• Tricalcium silicate (C₃S).  Hydrates and hardens rapidly and is largely responsible for initial set and early strength.  Portland cements with higher percentages of C₃S will exhibit higher early strength.

• Dicalcium silicate (C₂S).  Hydrates and hardens slowly and is largely responsible for strength increases beyond one week.

• Tricalcium aluminate (C₃A).  Hydrates and hardens the quickest.  Liberates a large amount of heat almost immediately and contributes somewhat to early strength.  Gypsum is added to portland cement to retard C₃A hydration.  Without gypsum, C₃A hydration would cause portland cement to set almost immediately after adding water.
• Tetracalcium aluminoferrite (C₄AF).  Hydrates rapidly but contributes very little to strength.  Its use allows lower kiln temperatures in portland cement manufacturing.  Most portland cement color effects are due to C₄AF.

Figure 3.46 shows rates of heat evolution, which give an approximate idea of hydration times and when a typical portland cement initially sets.

Figure 3.46: Rate of Heat Evolution During Hydration of a Typical Portland Cement

The result of the two silicate hydrations is the formation of a calcium silicate hydrate (often written C-S-H because of is variable stoichiometry).  C-S-H makes up about 1/2 - 2/3 the volume of the hydrated paste (water + cement) and therefore dominates its behavior (Mindess and Young, 1981). 

4.4  Types of Portland Cement

Knowing the basic characteristics of portland cement's constituent chemical compounds, it is possible to modify its properties by adjusting the amounts of each compound.  In the U.S., AASHTO M 85 and ASTM C 150, Standard Specification for Portland Cement, recognize eight basic types of portland cement concrete (see Table 3.13).  There are also many other types of blended and proprietary cements that are not mentioned here.

Table 3.13: ASTM Types of Portland Cement

4.5  Physical Properties

Portland cements are commonly characterized by their physical properties for quality control purposes.  Their physical properties can be used to classify and compare portland cements.  The challenge in physical property characterization is to develop physical tests that can satisfactorily characterize key parameters.  This section, taken largely from the PCA (1988), describes the more common U.S. portland cement physical tests.  Specification values, where given, are taken from ASTM C 150, Standard Specification for Portland Cement. 

Keep in mind that these tests are, in general, performed on "neat" cement pastes - that is, they only include portland cement and water.  Neat cement pastes are typically difficult to handle and test and thus they introduce more variability into the results.  Cements may also perform differently when used in a "mortar" (cement + water + sand).  Over time, mortar tests have been found to provide a better indication of cement quality and thus, tests on neat cement pastes are typically used only for research purposes (Mindess and Young, 1981).  However, if the sand is not carefully specified in a mortar test, the results may not be transferable.    

4.5.1  Fineness

Fineness, or particle size of portland cement affects hydration rate and thus the rate of strength gain.  The smaller the particle size, the greater the surface area-to-volume ratio, and thus, the more area available for water-cement interaction per unit volume.  The effects of greater fineness on strength are generally seen during the first seven days (PCA, 1988). 

Fineness can be measured by several methods:

• AASHTO T 98 and ASTM C 115: Fineness of Portland Cement by the Turbidimeter. 
• AASHTO T 128 and ASTM C 184: Fineness of Hydraulic Cement by the 150-mm (No. 100) and 75-mm (No. 200) Sieves
• AASHTO T 153 and ASTM C 204: Fineness of Hydraulic Cement by Air Permeability Apparatus
• AASHTO T 192 and ASTM C 430: Fineness of Hydraulic Cement by the 45-mm (No. 325) Sieve

4.5.2  Soundness

When referring to portland cement, "soundness" refers to the ability of a hardened cement paste to retain its volume after setting without delayed destructive expansion (PCA, 1988).  This destructive expansion is caused by excessive amounts of free lime (CaO) or magnesia (MgO).  Most portland cement specifications limit magnesia content and expansion.  The typical expansion test places a small sample of cement paste into an autoclave (a high pressure steam vessel).  The autoclave is slowly brought to 2.03 MPa (295 psi) then kept at that pressure for 3 hours.  The autoclave is then slowly brought back to room temperature and atmospheric pressure.  The change in specimen length due to its time in the autoclave is measured and reported as a percentage.   ASTM C 150, Standard Specification for Portland Cement specifies a maximum autoclave expansion of 0.80 percent for all portland cement types.

The standard autoclave expansion test is:

4.5.3  Setting Time

Cement paste setting time is affected by a number of items including: cement fineness, water-cement ratio, chemical content (especially gypsum content) and admixtures.  Setting tests are used to characterize how a particular cement paste sets.  For construction purposes, the initial set must not be too soon and the final set must not be too late.  Additionally, setting times can give some indication of whether or not a cement is undergoing normal hydration (PCA, 1988).  Normally, two setting times are defined (Mindess and Young, 1981):

1. Initial set.  Occurs when the paste begins to stiffen considerably.
2. Final set.  Occurs when the cement has hardened to the point at which it can sustain some load.

These particular times are just arbitrary points used to characterize cement, they do not have any fundamental chemical significance.  Both common setting time tests, the Vicat needle and the Gillmore needle, define initial set and final set based on the time at which a needle of particular size and weight either penetrates a cement paste sample to a given depth or fails to penetrate a cement paste sample.  The Vicat needle test is more common and tends to give shorter times than the Gillmore needle test.  Table 3.14 shows ASTM C 150 specified set times.

Table 3.14: ASTM C 150 Specified Set Times by Test Method

The standard setting time tests are:

• AASHTO T 131 and ASTM C 191: Time of Setting of Hydraulic Cement by Vicat Needle
• AASHTO T 154: Time of Setting of Hydraulic Cement by Gillmore Needles
• ASTM C 266: Time of Setting of Hydraulic-Cement Paste by Gillmore Needles

4.5.4  Strength

Cement paste strength is typically defined in three ways: compressive, tensile and flexural.  These strengths can be affected by a number of items including: water-cement ratio, cement-fine aggregate ratio, type and grading of fine aggregate, manner of mixing and molding specimens, curing conditions, size and shape of specimen, moisture content at time of test, loading conditions and age (Mindess and Young, 1981).  Since cement gains strength over time, the time at which a strength test is to be conducted must be specified.  Typically times are 1 day (for high early strength cement), 3 days, 7 days, 28 days and 90 days (for low heat of hydration cement).  When considering cement paste strength tests, there are two items to consider:

• Cement mortar strength is not directly related to concrete strength.  Cement paste strength is typically used as a quality control measure.
• Strength tests are done on cement mortars (cement + water + sand) and not on cement pastes.

4.5.4.1 Compressive Strength

The most common strength test, compressive strength, is carried out on a 50 mm (2-inch) cement mortar test specimen.  The test specimen is subjected to a compressive load (usually from a hydraulic machine) until failure.  This loading sequence must take no less than 20 seconds and no more than 80 seconds.  Table 3.15 shows ASTM C 150 compressive strength specifications.

Table 3.15: ASTM C 150 Portland Cement Mortar Compressive Strength Specifications in MPa (psi)

The standard cement mortar compressive strength test is:

• AASHTO T 106 and ASTM C 109: Compressive Strength of Hydraulic Cement Mortars (Using 50-mm or 2-in. Cube Specimens)
• ASTM C 349: Compressive Strength of Hydraulic Cement Mortars (Using Portions of Prisms Broken in Flexure)

4.5.4.2  Tensile Strength

Although still specified by ASTM, the direct tension test does not provide any useful insight into the concrete-making properties of cements.  It persists as a specified test because in the early years of cement manufacture, it used to be the most common test since it was difficult to find machines that could compress a cement sample to failure.

4.5.4.3  Flexural Strength

Flexural strength (actually a measure of tensile strength in bending) is carried out on a 40 x 40 x 160 mm (1.57-inch x 1.57-inch x 6.30-inch) cement mortar beam.  The beam is then loaded at its center point until failure.

The standard cement mortar flexural strength test is:

• ASTM C 348: Flexural Strength of Hydraulic Cement Mortars

4.5.5  Specific Gravity Test

Specific gravity is normally used in mixture proportioning calculations.  The specific gravity of portland cement is generally around 3.15 while the specific gravity of portland-blast-furnace-slag and portland-pozzolan cements may have specific gravities near 2.90 (PCA, 1988).  

The standard specific gravity test is:

4.5.6  Heat of Hydration

The heat of hydration is the heat generated when water and portland cement react.  Heat of hydration is most influenced by the proportion of C₃S and C₃A in the cement, but is also influenced by water-cement ratio, fineness and curing temperature.  As each one of these factors is increased, heat of hydration increases.  In large mass concrete structures such as gravity dams, hydration heat is produced significantly faster than it can be dissipated (especially in the center of large concrete masses), which can create high temperatures in the center of these large concrete masses that, in turn, may cause undesirable stresses as the concrete cools to ambient temperature.  Conversely, the heat of hydration can help maintain favorable curing temperatures during winter (PCA, 1988).

The standard heat of hydration test is:

• ASTM C 186: Heat of Hydration of Hydraulic Cement

4.5.7  Loss on Ignition

Loss on ignition is calculated by heating up a cement sample to 900 - 1000°C (1650 - 1830°F) until a constant weight is obtained.  The weight loss of the sample due to heating is then determined.  A high loss on ignition can indicate  prehydration and carbonation, which may be caused by improper and prolonged storage or adulteration during transport or transfer (PCA, 1988).

The standard loss on ignition test is contained in:

• AASHTO T 105 and ASTM C 114: Chemical Analysis of Hydraulic Cement

4.6  Summary

Portland cement, the chief ingredient in cement paste, is the most widely used building material in the world.   In the presence of water, the chemical compounds within portland cement hydrate causing hardening and strength gain.  Portland cement can be specified based on its chemical composition and other various physical characteristics that affect its behavior.  ASTM specifies eight basic types of portland cement concrete.  Tests to characterize portland cement, such as fineness, soundness, setting time and strength are useful in quality control and specifications but should not be substituted for tests on PCC.