Part 3 - Properties of Portland Cements

The chemical composition of available cements can vary widely. Since most concretes use Portland cements, we will concentrate on these. As noted before, the term "Portland" is a trade name and gives no indication of the chemical contents.

Manufacture of Portland Cement

Portland cement is simply a mixture of limestone and clay heated in a kiln to 1400 to 1600 degrees centigrade (2550 to 2990 F). Due to the high temperatures and large amounts of materials being used, considerable attention is given to each stage of the production process.

Raw materials - High quality cements require adequate and uniform raw materials. Location near sources of calcium and clay. Plant chemists analyze the material at all stages of production to ensure quality control.

Preparation of materials - quality control during mixing will produce a uniform end product. The exact procedure varies from plant to plant based on design. Some plants use wet grinding processes to better blend their mixtures, however this causes an increase in kiln cost. In a dry process the grinding and blending phase is more expensive but is offset by cheaper kiln costs. Another procedure is called a semi-dry procedure which is a dry process with 12 to 14% water added before the burning phase.

The burning process - Once the materials are ground and blended, they are ready for the kiln. The heat process is called "clinkering" or partial melting. Only about one-fourth of the material is liquid at any time. In a wet process, the material is in the kiln for 2 to 3 hours. This is reduced to 1 to 2 hours for a dry process. Some new heat exchanges only require 20 minutes.

Final processing - The hot porous material nodules leaving the kiln are known as "clinker". This clinker is ground in ball mills into a fine powder with small amounts of gypsum to avoid flash setting. Without the gypsum, the material is ground clinker and not Portland cement.

Composition of Portland Cement

In ordinary Portland cement, approximately three-fourths of the mixture is some form of calcium silicate. This material is responsible for the cementing process. The chemical composition is traditionally written in an oxide notation used in ceramic chemistry. In this notation, each oxide is abbreviated to a single capital letter. For example, lime written as CaO is denoted as C. In order to write carbonates and sulfates in shorthand a single capital letter is used with an overbar.

Determining the exact chemical composition of a cement would be a very complex procedure. However, simpler oxide analysis is generally available from most cement plants on request. With this information, the compound composition may be determined using the Bogue calculation. More sophisticated procedures have been developed, but the Bogue calculation is suitable for most purposes.

Hydration of Cement

When Portland cement is mixed with water, it undergoes a chemical reaction which leads to the hardening of the material. This process is called hydration and the results are the hydration products. The hydration process can be quantified by two characteristics; (1) the rate of reaction, and (2) the heat of reaction.

ASTM Types of Cements

  • Type I - most common, no special properties
  • Type II - good strength with lower heat of hydration
  • Type III - rapid setting; used in precast work or at low temperatures -- possible tensile cracking due to thermal stress if sections are large
  • Type IV - Used in mass concrete applications-low heat of hydration
  • Type V - provides protection from damage due to exposure to sulfates
    (seawater, some groundwater supplies, and particularly wetting and drying processes).

*** The ASTM standards are really performance specifications. There are few limitations on compound composition. Generally only Types I and III are available throughout the United States, Types IV and V in areas where a market exists. Type II is common in western states because of available materials.

Impurity Oxides

Oxides such as alumina, ferric oxide, and magnesia are composed of calcium silicates, about 3% by weight. These forms are more reactive than pure silicates and hydrate faster. Magnesia gives Portland cement its gray color and also can contribute to unsoundness, or the characteristic of cracking after hardening.

Modified Portland Cements

  1. Portland-Pozzolan Cements - blended with pozzolan (reactive silica); reduces both the heat of hydration and early strength, but provides resistance to sulfate attack and high ultimate strength.
  2. Slag Cements - Use blast-furnace slag, a by-product of the iron and steel industry. Slag is composed of lime, silica, and alumina. Requires an activator for hydration. Generally not popular in the United States.
  3. Supersulfated Cements - Slag cement activated with calcium sulfate; lower heat of hydration and better sulfate resistance than Portland blast-furnace cements. Used in Europe.

Expansive Cements

Portland cement concrete experiences high shrinkage during drying which can cause tensile cracking if restrained. Expansion during moist curing does not offset the contraction. Several cements with expansive properties have been developed. Steel reinforcement is used to control the expansion and convert it to a prestress force. Some additional restraint may occur through subgrade friction or from formwork. Lack of restraint will cause the concrete to self-destruct.

Expansion is controlled by adjusting the admixture material in the cement compound and by the amount of water available to the curing mixture. Physical properties of expansive cement is assumed to be that of Type I cement concretes.

Several areas of applications: parking structures, eliminating water damage to cars; pavements, elimination of shrinkage-control joints; structures where watertightness is important; and in tilt-up construction to help in the stress during lifting.

Rapid Setting and Hardening Cements

Two special types of cement are regulated-set (jet cement) and VHE (very high early strength). Regulated-set cement is not available in North America, but is used in Japan and Germany. VHE is sold in the United States.

  1. Regulated-set cement -- modified Portland cement where the tricalcium aluminate is replaced with calcium fluoroaluminate. The result is more reactive than with water. Care has to be taken to avoid flash setting. By controlling flash setting, the handling time can range from 2 to 40 minutes. Strength develops very rapidly. There is a balance between handling time and early strength development. High heat of hydration, more than Type II cement. Applications: lightweight insulation of roof decks, pavement and bridge deck repair, shotcreteing, and slip forming.
  2. VHE cement -- Basically a dicalcium aluminate compound with a high percent of calcium sulfoaluminate, similar to a Type K expansive concrete. Handling time and early strength development is similar to regulated-set except VHE will develop higher strength than regulated-set cement concretes. Durable, with creep and shrinkage lower than with Type III cement.

Miscellaneous Cements

  1. Other rapid-hardening cements -- extra-rapid-hardening cement - a modified Type III, ultra-rapid-hardening - a very finely ground Portland cement, super-high-early-strength developed in Japan.
  2. White cement -- An iron poor cement with a white surface color, popular with architects because of its ability to be colored by pigments.
  3. Masonry cement -- Type I Portland cement with finely ground limestone; workable, plastic, minimal water loss.
  4. Oil-well cements -- Slow hardening under high pressure and temperature and stable in corrosive conditions.
  5. Natural cements -- produced from natural clayey limestones burn at low temperatures containing very little tricalcium aluminate. Rarely used today.

Non-Portland inorganic cements

  1. High-alumina cement (HAC) -- also known as calcium aluminate cement, was developed as a sulfate-resisting cement. A major problem is loss of strength due to adverse chemical reactions. Unlike Portland cement, HAC undergoes a complete fusion of the raw materials. Develops strength rapidly. About 75% of ultimate strength is developed in the first seven days. The major disadvantage of HAC is the potential conversion problems if the cement is exposed to hot, moist conditions. The temperature causes an increase in porosity and a disruption of the original microstructure. This leads to an extreme loss of strength. Strength loss is also severe at high water/cement ratios. Several structural failures in Europe have lead to its ban in structural use. Some structural applications are possible if the strength can be accurately predicted after conversion has taken place. However, it is now used in refractories. At high temperatures, the cement forms a ceramic bond stronger than the original hydraulic bond.
  2. Gypsum plaster -- used as a surface finish on interior walls or in the production of drywall products. Quick-setting with rapid strength development, however very soluble in water. Also, leachates from the plaster are rich in sulfates, which can attack any surrounding concrete.

Specifications and Test for Portland Cement

To maintain quality control on Portland cement, a set of ASTM specifications for both the chemical and physical requirements have been established. A series of "standard" tests have been developed to ensure that these specifications are met. However, since results from different tests for the same property can vary widely, direct comparison of these tests is difficult.

  • Chemical requirements -- These specifications are not very strict since cements with different chemical compounds can have similar physical behavior.
  • Physical requirements -- These specifications are more important than chemical requirements
    • Fineness -- Grinding of the clinker is the last step in Portland cement production. The degree that the material is ground is the fineness.
      • Rate of hydration increases with fineness, leads to high strengths and heat generation.
      • Hydration takes place on the cement particle surface. Finer particles will be more completely hydrated.
      • Increasing fineness decreases the amount of bleeding but also requires more water for workability which can result in an increase in dry shrinkage.
      • High fineness reduces the durable to freeze-thaw cycles.
      • Increased fineness requires more gypsum to control setting.

        • ** The most important properties are: specific surface of the particles, and particle-size distribution. Fineness was originally measured using a sieve analysis, but this method is very awkward and really gives no information about the distribution of fine particles. In general, fineness is measured by a single parameter, specific surface area. This parameter is considered the most useful measure of cement fineness even though it does not measure particle distribution.
      • There are two ASTM tests for fineness:
        • Wagner Turbidimeter -- measured specific surface area from a suspension of the cement in a tall glass container. The test is based on Stoke's Law that states a sphere will obtain a constant velocity under the action of gravity.
        • Blaine air permeability apparatus -- This test is based on the relationship between the surface area in a porous bed and the rate of fluid flow (air) through the bed. The test is compared to a standard sample determined by the U.S. Bureau of Standards.
      • ** The Blaine method is used more often in practice and is generally 1.8 times larger than the Wagner method. However, in cases of dispute, the Wagner method governs.
    • Test on Cement Paste
      • Two of the common physical requirements for cement 1) time of setting, and 2) soundness depend on the water content of the cement. This is measured in terms of normal consistency. A cement paste is said to be of normal consistency when a 300 gram, 10-mm-diameter Vicat needle penetrates 10 + 1 mm below the surface in 30 seconds. The plasticity of the cement is sensitive to environmental conditions.
      • Time of setting -- Two arbitrary points of no real significance are used to develop general relationships between addition of water and strength gain. Used mainly for quality control.
        • Initial set - paste begins to stiffen (2-4 hours)
        • Final set - ability to withstand load (5-8 hours)
        • ** Time of setting by Vicat needle -- Initial setting occurs when a 1-mm needle penetrates 25mm into cement paste. Final set occurs when there is no visible penetration .
        • ** Time of setting by Gillmore needle -- Less common than Vicat needle test. Initial set occurs when a 113.4 gram Gillmore needle (2.12 mm in diameter) fails to penetrate. Final set occurs when a 453.6 gram Gillmore needle (1.06mm in diameter) fails to penetrate. Gillmore times tend to be longer than Vicat times.
      • Early stiffening -- Two measures of early stiffening are:
        • False set - rapid rigidity without much heat generation, plasticity can be regained by further mixing with no additional water.
        • Flash set - rapid rigidity with considerable heat generation, plasticity cannot be regained.
        • A cement paste is mixed such that Vicat needle penetrates 32 +/- 4mm after 20 seconds. The final penetration is measured at 5 minutes. The result is a percentage of (final penetration/initial penetration) X 100%.
      • Unsoundness -- is the characteristic of excessive volume change after setting. It may appear many months or years after setting. Therefore any test for unsoundness must detect the potential for this type of failure. Two standard tests are:
        • Le Chatelier test - Designed to test for expansion due to excessive lime. The device is filled with cement of normal consistency, covered with glass plates, and immersed in water at 20 + 1 degree C for 24 hours. The distance between the indicator points is measured and the device is returned to the water and brought to a boil in 25-30 minutes, and boiled for 1 hour. The device is cooled and the indicator points are measured again. The difference in the readings cannot exceed 10 mm.
        • Autoclave expansion -- More severe test than Le Chatelier. A cement paste of normal consistency is molded and cured for 24 hours. Then it is measured and placed in an autoclave and the temperature is increased for 45-75 minutes until a pressure of 295 psi is achieved. It remains for 3 hours and then is cooled in the autoclave for 1 1/2 hours, then 15 minutes in water, and 15 minutes in the air and then its length is measured. The change in length must be less than 0.80% to be acceptable.
      • Heat of Hydration -- Determined by the heat of solution method. The heat of solution of dry cement is compared to partially hydrated cement at 7 and 28 days. The heat of hydration is the difference between the dry and the partially hydrated cements.

Test on Mortar

Mortar testing depends on the sand used for the test. Therefore, a standard ASTM sand is used, natural silica sand from Ottawa, Illinois. Mortar test provides a more reliable indication of quality than do neat pastes.

  • Mortar flow -- Consistency of mortar is expressed as mortar flow. A mix of 2.75 parts Ottawa sand to 1 part cement (by weight) is compacted into a cone-shaped mold. The sample is placed on a flow table, a table whose top can be mechanically raised and lowered about 1/2 inch 25 times in 15 seconds. The flow is the increase in the base diameter as a percent of the original diameter.
  • Strength test
    • Compression is the most common measure of strength. A 2-in. mortar cube using a 2.75:1 sand/cement ratio with a water/cement ratio of 0.485 - 0.460 is tested. After a certain procedure is followed the specimens are failed.
    • Tensile strength is determined by a direct tensile test. The results are not of much value.
    • Flexural strength is determined by a flexural test of a small rectangular-shaped prism on simple supports with a center load. The flexural strength is directly calculated. This mortar strength does not necessary relate to concrete strength using the same cement; used for quality control.
  • Air Content of Mortar -- Test for air content to determine the air entraining potential of a given cement.
  • Sulfate Expansion -- Not a true measure of sulfate resistance, more of a measure of expansion. Useful for Type V cements.

This web site was originally developed by Charles Camp for his CIVL 1101 class.
This site is maintained by the Department of Civil Engineering at the University of Memphis.
Your comments and questions are more than welcome.