Cement & Concrete Chemistry Primer

Now, those of you who know me, know that I'm a big proponent of empowerment through education. One particular topic that comes up a number of times a year is cement chemistry and what's happening in concrete. Although concrete is a flexible and robust building material, we need to have a basic understanding of the fundamentals and to respect and treat our product accordingly. While concrete chemistry is an incredibly complex beast with whole volumes written about it and new information coming out almost daily, it is definitely worth having a simplified understanding of the underlying mechanisms - whether you're a Batcher, QC Manager, Tester, Inspector, Driver, or Finisher.

What is Concrete?

At its simplest, concrete is a mixture of cementitious powders, water, aggregates, and a small amount of entrapped, and sometimes purposefully entrained, air. We basically end up with two different phases:

• Aggregates, which form the strong and stable skeleton of the concrete
• Paste, which is the mixture of cement and water that will eventually harden and bind the aggregate together into a strong and durable component

Ideally, we want to generate an optimal combine gradation of aggregate to balance the workability requirements and the economy of the mix, and then fill the leftover empty space with the optimal quantity of cement paste, with the correct composition to perform according to the strength and durability requirements. The quality of the concrete depends on the quality of the aggregates, the quality of the paste, and the quality of the bond between those two phases.

What is Cement?

Portland cement (or Portland Limestone Cement) is a hydraulic powder (meaning that it chemically reacts with water) consisting primarily of:

• Calcium silicates (C3S & C2S)
• Calcium aluminates (C3A & C4AF)
• Calcium sulphates (typically added in the form of Gypsum)
• Other minor and trace components

The reactivity of the cement is governed by a number of factors including: i) the amount, quality, and dispersion of the minerals, ii) the fineness of the cement grains, iii) the alkali level, iv) the mineral-to-water interface, and v) the temperature.

What is Hydration?

Hydration is an exothermic (heat releasing) series of chemical reactions that begin as soon as cement comes in contact with water. The reaction produces cement hydrates which form on the surface of each particle and gradually grow, spread, and interlock with each other and adhere to adjacent surfaces.

The minerals in the cement react with water and each other to form brand new compounds. Bear in mind that the reactions presented below are a major simplification - last I heard at the NIST seminar a few years back, I believe they had identified over 36 different chemical reactions taking place!

• C3S and C2S both react with water to form Calcium-Silicate-Hydrate Gel, or CSH, and Calcium Hydroxide, or Ca(OH)2. The C3S reacts quicker than the C2S and produces more Calcium Hydroxide than the C2S. For a typical cement, you're producing around 75% CSH and 25% Calcium Hydroxide. CSH is "the good stuff" which gives us our strength and durability, while Calcium Hydroxide contributes little to concrete performance, although it does stabilise the hydrate product matrix and will absorb some CO2 over time to form calcium carbonate.
• The C3A and C4AF will react with water to form Calcium Aluminate Hydrates and Calcium Aluminoferrite Hydrates. When we add sulphate into the mix, we also generate Ettringite (calcium sulphoaluminate) which regulates the setting characteristics, strength development, and drying shrinkage, further densifying the product matrix.

The hydration reactions can generally be broken down into 4 different stages:

I) Mixing

Some of the cement minerals are very soluble, thus there is a short period of fast reaction and heat output as the cement dissolves. The mix water is no longer pure H2O, but an aqueous solution of ionic species. The C3S, aluminates, and gypsum are highly soluble and dissolve quickly. These ions create an inter-particle charge difference that results in agglomeration due to a silica rich interior and calcium rich exterior layer in a hydroxyl (OH) rich pore solution.

II) Dormancy

An amorphous layer of ettringite forms around the cement particles which acts like a temporary barrier to slow down the dissolution rate. This lasts for a few hours to allow the plastic state of concrete while the cement silicates continue to dissolve forming calcium and hydroxyl ions until CSH and calcium hydroxide eventually grow enough to break through the barrier in the Hardening Stage. There is an optimum point for sulphate addition that balances setting time, strength, volume stability, and admixture compatibility.

III) Hardening

As the cement dissolves, the pore solution becomes supersaturated and precipitates begin to form into new solid phases (primarily the C-S-H gel and calcium hydroxide). Some of these compounds are fibrous or crystalline in nature and they interweave and mesh together causing the mixture to stiffen. 

IV) Cooling & Densification

As those hydration products precipitate out of solution, it relieves the saturation to allow further dissolution of cement minerals. The diffusion of cement ions into pore solution becomes slower and slower as the layers of hydration product become thicker and thicker. The reactions continue slowly generating little heat. Continued growth and meshing of hydration products will result in a strong solid mass and increasing the strength and reducing permeability.

What about SCM's?

Supplementary Cementitious Materials (such as slag, fly ash, silica fume, and pozzolans) are materials that, when used in conjunction with cement, contribute to the properties of the hardened concrete through hydraulic and/or pozzolanic activity. These are typically byproducts from other industries where we have found a use for them in improving our product performance and improving our product sustainability.

SCM's reacts with, or in the presence of, the relatively inert calcium hydroxide produced during the cement hydration reaction to form more CSH gel. Essentially, the SCM converts the natural "filler" reaction product into more "good stuff", resulting in more "good stuff" produced overall. On top of this, when the particle size distributions of both the cement and SCM are optimised, the particle packing is improved. The CSH produced from this reaction actually takes up more volume per unit of SCM than per unit of cement, thus helping to fill in the pore space which results in a stronger, denser, less permeable matrix of reaction products. Early strengths are generally lower as the initial cement reactions have to take place first for the secondary reactions to occur and plastic workability is generally improved due to this same effect.

What are the Admixtures doing?

When looking at your basic chemical admixtures (water reducers, super-plasticizers, accelerators and retarders, and air entraining agents), most of the action is happening during the first few stages of hydration. We're imparting surface charges (to cause cement grains to repel each other to free up trapped water and also reduce its surface tension), stabilising bubbles with a calcium hydroxide film, and absorbing onto the hydrated surfaces to promote or impede the dissolution of certain cement ions. There's a lot going on here, and some mechanisms and effects may be competing with each other, which is why its important to ensure that concrete batching sequence and mixing procedures are consistent and in accordance with our friendly neighbourhood admixture representative's recommendations.

Batching, Mixing, Placement, and Curing

All the materials need to be thoroughly mixed to become on homogenous mass. If there is one thing to stress here it's that consistency is key! The concrete must be properly mixed and not over or under mixed. This required mixing time will depend on the volume of concrete in the mixer as well as the mix design itself. With proper mixing the materials will all have intimate contact with each other and admixtures will be activated to get us to the proper slump and air content. Continuing to over-mix will cause a decrease to the slump, air, and ultimately strength. Generally, you have about a two hour window from the time of batching to when the concrete must be placed (which is of course affected by the ambient conditions and use of accelerators or retarders). Past this time, the concrete is beginning to set where further manipulation will break those bonds that are starting to form and result in a lower strength and lower durability product.

A special word about the Water-to-Cement Ratio

Water serves to primary functions in concrete:

• A certain amount of the water reacts with cement and SCM's to create the hydrate gel than hardens and gains strength (w/cm ratio around 0.24-0.28)
• Additional water over this contributes to the workability of the concrete, but dilutes and weakens the gel formation.

It was Duff Abrams, Director of the Research Laboratory at the Portland Cement Association who in 1918 said:

“For given materials the strength of the concrete depends solely on the relative quantity of water as compared with the cement, regardless of mix or size and grading of aggregate.” 

The reasoning behind this is simple if we consider the diagram below. With more water in the mix, the cement grains are further away from each other and less point-to-point contact between them will be developed during the hydration process.

We need to consider that the water in concrete is not just the mix water added into the batch, but also the moisture contained within the aggregates, the water content of the admixtures (if in significant volume), wash water left in the drum or mixer, and of course, the water added by the driver on site. This is why it's critical to adhere to the mix design, the target slump, and to document water addition if necessary.

A special word about Curing

Hydration continues indefinitely as long as moisture and temperature conditions are favourable and results in progressive stiffening, hardening, and strength development. Concrete will self dessicate and water will evapourate from it over time. Strength development will basically stall if the internal moisture (relative humidity) on the concrete drops to less than around 80%. The lower the water-to-cement ratio (or the better we want to concrete to perform), the more sensitive it will be to proper curing. The point of curing is to prevent a loss of moisture from the concrete so that hydration does continue. Curing should begin immediately after finishing when concrete is sufficiently set and strong enough that the surface won’t be damaged. Does curing make a difference? Heck yes!

The Concrete Cheat Sheet

Factors affecting concrete (per cubic meter/yard) and typical impacts

Concrete Troubleshooting

For concrete troubleshooting, I can't recommend enough taking a look at Lehigh Hanson's free i.check app - a handy to have guide to diagnosing and solving real life concrete issues in the field.

In Closing

There's definitely a lot going on with cement and concrete chemistry, but it is important to have that basic understanding to ensure consistency, product performance, and proper design and handling. You can spend a lifetime in this industry and still not know everything there is to know. But I encourage you to keep reading, keep experimenting, and keep learning. Remember... knowledge, like concrete, is strength!