Concrete Mix Design: Understanding Powders

Unraveling the mystery of cementitious materials.

By Paul Ramsburg

Editor’s Note: This is the third article in a year-long series that explores the science of concrete to provide a better understanding of mix design. The series will be collaboratively written by Paul Ramsburg, technical sales specialist at Sika Corp., and Frank Bowen, business development representative with Rosetta Hardscapes. Click here for the second article in the series or here to read the fourth article.

Before we discuss cementitious materials, it’s important to first recognize that concrete doesn’t dry, it cures. The reason is that all portland cements are hydraulic cements, composed of hydraulic calcium silicates and calcium aluminates. As a result, it sets and hardens by a chemical reaction with water – a process called hydration. The mixture of cement and water is referred to as paste. This paste is the adhesive that binds the fine and coarse aggregates together.

Hydration: How does it occur?

The hydration process starts immediately upon contact between cement and mix water. During hydration, calcium silicates in cement form calcium hydroxide and a gel-like calcium silicate hydrate. CSH gel is the most important cementing component of concrete since it is responsible for setting and hardening and strength development. As hydration proceeds, cement particles and water react to form hydration products in the form of crystals that grow from the cement particles into the space originally occupied by the mix water. As the particles continue to grow, the crystals converge and the paste solidifies. The concrete gradually stiffens, loses workability, sets and develops mechanical strength.

Once again, concrete does not harden from drying. A cement paste will set and harden even when submerged in water. If sufficient moisture and appropriate temperature are available, hydration will theoretically continue indefinitely, albeit increasingly slow. You’ve probably heard the Hoover Dam is still curing. I believe this is true, but every time I attempt to core drill a cylinder specimen to prove the theory, I’m chased away.

Cement nomenclature

Before we get too far, let’s consider the nomenclature of cements as there are many key terms that are often used but may not always be fully understood.

Clinker

Before becoming cement, the quarried and processed mineral components are known as clinker. Cement clinker is composed of four principal phases that make up about 90% by mass of cement. These phases are:

1. Tricalcium silicate, C₃S. Hydrates rapidly and is responsible for setting and early strength development. The rapid hydration of C₃S is accompanied by heat evolution, which may cause concrete’s temperature to increase. In general, increasing the C₃S content results in increased early strength.
2. Dicalcium silicate, C₂S. Hydrates slower and contributes to later strength development. The slower hydration of C₂S is accompanied by the slower evolution of heat.
3. Tricalcium aluminate, C₃A. Reacts very rapidly with water and gives off a large amount of heat. It contributes to setting behavior and early strength gain.
4. Tetracalcium aluminoferrite, C₄AF. Reacts like C₃A but much slower, with lower heat development.
5. Calcium Sulfate (Gypsum), CaSO₄. Calcium sulfate is added during grinding to control C₃A hydration by forming ettringite. Without sulfate, the cement will set very quickly with rapid heat evolution, known as flash set.

Alkalis

Cement is manufactured from natural raw materials and, as such, trace amounts of other substances and materials are present besides the principal mineral phases. The most important of these minor components are the alkalis. Cement’s alkali content can affect concrete’s setting time, strength development and durability. Alkalis originate from the argillaceous or recycled material components of the cement raw mixture. Alkali content is often noted as equivalent sodium oxide, Na2Oeq, and is calculated as:

%Na2O_eq = %Na2O+0.658(%K₂O)

Fineness and particle size distribution

The reactivity of cements is directly related to the fineness. Grinding clinker to higher fineness results in smaller cement particles. Smaller particles have a larger surface area where hydration can take place. The fineness of cement can be measured following ASTM C204, “Standard Test Methods for Fineness of Hydraulic Cement by Air-Permeability Apparatus,” which gives the cement fineness as surface area of cement particles per unit mass (m²/kg).

Setting

The initial and final setting time for cement defines its setting characteristics. ASTM C150, “Standard Specification for Portland Cement,” has minimum requirements for initial setting time and maximum requirements for final setting time for the different cement types. It also has minimum requirements for compressive strength at given ages between 3 and 28 days for Types I, II and V cements. For Type III, a minimum 1-day strength is also required.

Heat of hydration

Portland cement hydration is an exothermic reaction, meaning it develops heat. The heat given off during curing is called heat of hydration. Increased temperature – whether of the fresh concrete or the ambient temperature – speeds up hydration and decreased temperature slows down hydration.

Cement’s effects on concrete

Now that we have the terminology down, we will consider the effects of these cement characteristics on concrete properties, if everything else is equal.

Workability

Cement properties that affect workability the most are those related to the early stages of hydration. Increased cement fineness, increased alkali content and increased C₃A content will increase the cement’s reactivity, making the mix less workable as it would cure faster. In addition, changes in sulfate might change set times and, thus, mix workability since calcium sulfates are added to clinker to control reactive C₃A.

Water requirement

Any changes to cement properties (chemical or physical) that increase the air content of a concrete mixture would lower the water demand of that mixture, unless mitigated by other factors.

Slump

Slump loss increases as cement alkali content increases. Increased cement particle fineness will increase slump loss as well.

Rheology

The yield stress value and plastic viscosity of concrete decrease with the fineness of cement. The chemical composition of cement has little effect on the rheological properties of concrete.

Setting time

Increased fineness without an increase in the sulfate content will result in decreased setting times. Increases in C₃A and/or C₃S content will decrease setting times.

Heat of hydration

The rate of heat production and the rate of hydration are closely related. Hence, all characteristics affecting hydration will affect the heat evolution. This includes fineness as well as C₃A and C₃S content. Alkalis also increase the rate of hydration.

Strength

Cements containing large amounts of C₃S may be expected to gain strength quickly, with a more gradual gain in long-term strength. Finer cements also gain strength more quickly.

Understanding the composition of cement

The compounds that make up cement are estimated with Bogue’s equations. A closer look at these can help us develop some predictability with our concrete mixes.

Tricalcium Silicate C₃S, or alite, is typically 50% to 63% by mass of any given cement and contributes to both early and late strength development. Cements with higher alkali levels typically have lower C₃S content. The C₃S content can help to predict early strengths based on previous data. For example, if C₃S is higher than normal, and everything else remains the same, then one would expect higher early strengths.

Before we move forward, it’s important to note that other contributing factors associated with concrete strength are alkali levels, Blaine fineness and loss of ignition (LOI), as well as the water-cement (w/c) ratio. Cements vary in the way they react, or hydrate, due to differing chemistry. The shape, size, solubility, form, etc., of compounds can differ based on raw materials, rate and length of heating and cooling, and much more. Certain levels of C₃S, alkalis or Blaine fineness don’t guarantee good or bad strengths, just different than what you’re likely used to.

Dicalcium Silicate C₂S, or belite, is typically 10% to 22% by mass of cement and contributes to late strength development (28 days and beyond).

Tricalcium Aluminate C₃A, or aluminate, is typically 5% to 12% by mass of cement and contributes to very early strength gain (1 to 3 days). It’s the first compound to hydrate and will react quickly when in contact with water. This increases the heat of hydration.

Manufacturers incorporate gypsum (CaSO₄ • 2H₂O) into cement when they grind the clinker into a fine power at the cement manufacturing facility and use it to control this reaction. When the gypsum comes in contact with water it disassociates (CaSO₄ • CaO + SO₃^-2) and the free sulfate ion reacts with aluminates to form a coating around the compound so that water can’t penetrate. The coating breaks down within hours and hydration proceeds. It also impairs the resulting concrete’s resistance to sulfate attack. Type II cement, which has moderate sulfate resistance, has a maximum limit of 8% C₃A by mass. Remember, sulfate ions preferentially attack aluminates.

Tetracalcium Aluminoferrite C₄AF, or ferrite, is 5% to 12% by mass of the cement and has little effect on cement’s behavior. It is responsible for cement’s color by the contribution of ferrite or iron. The higher the C₄AF content, the darker the cement.

Sulfur Trioxide SO_3, or sulfate, is responsible for controlling early hydration of C₃A and is influential in controlling the set time of concrete – primarily through the addition of gypsum. There are two forms of gypsum: Gypsum that is CaSO₄ • 2H₂O (calcium sulfate di-hydrate) and plaster that is CaSO₄ • ½H₂O (calcium sulfate hemi-hydrate). If a cement manufacturer switches between gypsum types, this will alter the set time of concrete using this cement.

Mill certificate properties

Next, let’s explore a few properties of cement that you can find on a mill certification which should be understood and monitored.

You can calculate LOI by finding mass loss when cement is heated to a very high temperature (900 degrees Fahrenheit). LOI is specified in ASTM C150 to have a maximum of 3%. This is an indication of the extent of the hydration and carbonation of free lime and magnesia due to exposure of cement to the atmosphere. If cement or clinker is older and has been exposed to weather, then the LOI will be higher. High LOI will have a detrimental effect on set time, strength and air entrainment. If the LOI changes on a mill certificate, expect these properties to change along with it.

Blaine fineness is the measurement in surface area per unit weight (m²/kg or cm²/g) of cement and is an indication of the fineness of a powder. The higher the Blaine, the finer the particles. This relates to reactivity, set time, early strength and final strength. Higher Blaines – as with Type III cements – typically mean faster set times, higher earlier strength, lower long-term strength and stickier fresh concrete. More air entrainment will be necessary to achieve any given air content and more heat is generated early on. Notice that finer cements can result in higher early strengths but also lower 28-day strengths. This is a principle of concrete mix design that should be highlighted, and that transcends just cement fineness. The faster a concrete gains strength, the less strength it will gain overall. Anything we do to speed up strength gain will lower the ultimate strength we would have otherwise achieved.

Another important factor to watch on a mill certification is the -325 Mesh. This is a measurement of the amount of material remaining on a 325 sieve. The coarse particles that are retained on the mesh play a very small role in hydration and strength development, but in conjunction with Blaine fineness, this value can give you a better indication of the particle size distribution of the cement. Cement particle sizes should be evenly distributed. If the Blaine is high (4,800 cm²/g) and 325 mesh is low (82 percent passing), it means there is a large number of super fines, which is an indication of clinker that has already hydrated. Since this clinker has already reacted, their presence is detrimental to concrete and you could see both early and late strengths decrease even though the cement content and w/c ratio has not changed.

A very important factor to watch on a mill certificate is the total alkalis. This is a weighted average of all the alkalis in a cement. On the certificate you will find it as NaEq, which is equal to %Na₂O + .658 %K₂O. The equation assumes that since sodium and potassium are in the same chemical family, they will react the same. Cements that are under .60 Na₂O_eq are considered low alkali. Alkalis aid in cement hydration, so higher alkali contents mean higher earlier strengths but slightly lower ultimate strengths. Very fast hydration is detrimental to concrete strength. In addition, a change in alkalis will change how various admixtures perform. For example, higher total alkali content in a cement will require a higher polycarboxylate – a high-range water reducer – dosage to achieve a given self-consolidating concrete, or SCC, slump flow.

Applying your knowledge

Cement performance is a balancing act of chemical and physical properties. It is important to monitor mill certificates as we receive cement deliveries to our batch plants. Mill certificates are not meaningless papers that we are required to file away. Watch for these attributes that we’ve discussed here and correlate them to your concrete. Even single-source cement properties can differ at times. Also, keep in mind mill certificates represent about a month’s average of tests at the cement mill. An even better indicator, perhaps, of your cement’s performance is an ASTM C917, “Standard Test Method for Evaluation of Variability of Cement from a Single Source Based on Strength,” report. Be sure to ask your cement supplier for this detailed report.

Experiment and seek help

In addition to understanding your cement’s properties, we encourage you to experiment with the cement contents and blends of powders within your mix designs in a laboratory environment. Study your mill certificates over time, seek help from your local suppliers and National Precast Concrete Association’s materials and courses, and consider using Portland Cement Association’s bookstore for study materials. The more you read and get your hands dirty in the lab, the more you’ll remove the mystery from cement and, as a result, the performance of your concrete. PI

Paul Ramsburg has worked in the prestressed concrete industry since 1988 and is currently a technical sales specialist at Sika Corp