A closer look at three broad topics better explained at the microscopic level.
NPCA staff report
In the classic 1957 science-fiction film “The Incredible Shrinking Man,” the lead actor’s diminutive stature leads to a fight for survival. Stairsteps become towering cliff faces. Patches of grass transform into dense forests. Harmless spiders become terrifying beasts. While you likely aren’t planning to shrink down anytime soon, thinking about concrete at the microscopic level will change the way you look at it.
Cement chemistry
Like any great recipe, special care must be taken with the ingredients that go into cement. Typical Type I portland cement is composed of five main components: calcium, iron, silica, alumina and sulfate. These can come from a variety of natural sources such as limestone, clay, iron ore, shale and gypsum. However, industrial byproducts such as fly ash or slag may also be used. The ancient Romans and Egyptians even crushed seashells and snail shells and added them to concrete mixes to provide sufficient calcium and lime.
What really matters is that the necessary amounts make it into the mix. ASTM C150, “Standard Specification for Portland Cement,” lists cement composition requirements and physical requirements along with applicable test methods for different types of cement. For example, Type I cement does not have a maximum aluminum oxide content; however, Type II limits the content to 6% by mass. By varying the proportions and ingredients, cements are designed to perform differently for different concreting applications and environments.
The selected cement materials are crushed and ground into the appropriate size, and then proportioned and blended. After this, the components are heated in a cement kiln at an extremely high temperature. When the materials reach 2,550-to-2,800 F, a chemical reaction occurs, and the calcium carbonate decomposes into two new compounds: carbon dioxide and calcium oxide. The calcium oxide then combines with other materials in the kiln to form the compounds of clinker: tricalcium silicate (alite), dicalcium silicate (belite), tricalcium aluminate and tetracalcium aluminoferrite.
All the reactions taking place inside the kiln are important, but for precasters especially, it is the alite and belite that are key. Alite, or C3S, makes up roughly 50-to-70% of the clinker and is a light-colored, hexagonal crystal. It reacts with water to produce calcium silicate hydrate, or CSH, which is the compound that binds paste together. Thanks to its quick reaction with water, alite is largely responsible for concrete’s early strength gain. Belite, or C2S, constitutes 10-to-25% of the clinker. Like alite, belite reacts with water to create CSH. In addition to the formation of CSH, alite and belite reactions also produce calcium hydroxide, or CH, which can contribute to some durability issues if exposed to harmful elements.
Calcium hydroxide magnified. Credit: Farnam’s Research Group at Drexel University, Drexel Advanced and Sustainable Infrastructure Materials Lab
Besides the alite and belite, the tricalcium aluminate undergoes several reactions and can create ettringite, calcium monosulfoaluminate and tetracalcium aluminate hydrate. These products contribute little to strength, but can have a significant impact on setting times.
Alkali-silica reactivity
While not exceedingly common, alkali-silica reactions have contributed to some field durability issues. ASR occurs when alkali hydroxides in concrete react with certain reactive silicas in aggregates and form an alkali-silica gel. The gel tries to soak up as much water as it can, causing it to expand inside the concrete. The uneven expansion caused by ASR can have several negative effects on concrete, foremost of which is cracks in the aggregate and the surrounding cement paste. This will immediately lower the compressive and tensile strengths of the concrete and result in other deleterious effects on the structure.
For example, the Seabrook Station Nuclear Power Plant in Seabrook, N.H., opened in 1990. By 2012, ASR was observed in some of the concrete at the plant. Testing revealed the concrete lost 22% of its original strength.1 Thankfully, steps have been taken to combat the ASR at Seabrook, but how can we prevent it from occurring in the first place?
First and foremost, aggregates should be carefully selected to limit reactive silica content. High levels of reactive silica are usually found in types of volcanic rock or in siliceous sand. You can avoid this issue by ensuring your aggregates comply with the requirements of ASTM C33, “Standard Specification for Concrete Aggregates.” This ASTM standard will indicate whether the aggregate supplied has reactivity potential and whether additional ASR testing is required.
It may sound counterintuitive, but one of the best ways to combat ASR is to actually introduce some silicas into the mix. Fly ash is a common supplementary cementitious material that will react with the calcium hydroxide in the concrete to create extra CSH. This helps prevent a future ASR and can also increase the strength of the concrete.
Delayed ettringite formation
Similar to ASR, delayed ettringite formation, commonly known as DEF, refers to expansion and cracking and is caused when early-age ettringite formation is slowed. Ettringite is a long, thin mineral that forms when tricalcium aluminate reacts with gypsum and water, and it is a normal part of cement-hydration reactions. Ettringite formation usually occurs in the first few hours of hydration and is spread uniformly throughout the paste. This early-age ettringite is important because it will consume sulfates in the cement and also aids in stiffening the mix.
Ettringite formation magnified. Credit: Farnam’s Research Group at Drexel University, Drexel Advanced and Sustainable Infrastructure Materials Lab
When concrete is cured at temperatures between 158 F and 212 F, the primary ettringite formation is disrupted and monosulfoaluminate forms instead.2 As the concrete cools, the monosulfoaluminate will slowly begin to transform into ettringite. Over the next few years, pressure in the paste will build as the ettringite begins to expand outward from the monosulfoaluminate and cause the paste to separate from the aggregates.
Delayed ettringite formation is not normally caused by curing on a hot day; rather it is attributed more to accelerated curing procedures that can reach these high ambient temperatures. Be sure to follow the curing procedures carefully – including beginning the accelerated curing procedures after the concrete has reached initial set, as well as following the guidelines for ambient temperature, concrete temperature and curing duration.
Under the microscope
From raw materials to the final product, few people in the precast industry get a chance to see what happens under a microscope. Becoming familiar with some of the challenges that aren’t immediately visible can help provide macro-scale answers and solutions to issues that arise on the microscopic level.
Resources:
1 http://archive.boston.com/news/local/new_hampshire/articles/2012/02/09/local_leaders_question_safety_of_seabrook_power_plant/
2 PCA Design and Control of Concrete Mixtures, 15th Edition, pg. 221
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