Cement Hydration Compounds: The Avengers of Concrete

By Erin McArtor 

This article is part of our student collaboration series. 

​Have you ever wanted to know what cement does after water has been added to it? Well, this is the video for you! In this video, Dr. Tyler Ley delves deep into the cement hydration process, with a step-by-step break down of what goes on inside the concrete.Concrete is made of rock, sand, cement, and water. All aspects of concrete are important; however, the cement and the water are what make the magic happen. In Portland Cement, there are compounds present before hydration, and other compounds that are formed after hydration. As you may already know, un-hydrated Portland Cement contains C3S (Alite), C2S (Belite), C3A (Aluminate), C4AF(iron), Gypsum, and Limestone. This video introduces the more unknown compounds that are found in hydrated Portland Cement,what each of them provide to the cement, and how each of them are related to the Avengers. Crazy, right? The first compound Dr. Ley introduces is Calcium Silicate Hydrate, also known as C-S-H (C-S-H! C-S-H!). It is formed from the reaction of C3S and water, and it gives hydrated Portland Cement its strength.C-S-H can also be developed in other ways. Pozzolans, such as Class F Fly Ash consume CH (another hydration compound) and convertit to C-S-H. The relationship between concrete and C-S-H. It is very similar to the relationship between Thor and his hammer; without his hammer, Thor has none of his “God of Thunder” powers and is basically just a normal Asgardian. You might ask yourself, “hey, what about in Thor Ragnarok where he basically becomes lightning, right?” Well we are not considering that for this example. However, great action films aside, without C-S-H, the concretewill lackstrength.It also keeps the “outside nasties” out of the concrete. C-S-H is also known for its shape-shifting abilitiesalso very hard to characterize because its structure and chemical composition changes depending on when, how, and under which situation it is formed and looked at. It does not have an ordered structure, so it is always changing its structure and chemical composition depending on when and how it is looked at. Shown below is an image of magnified C-S-H.

Figure 1 –Image of magnified C-S-H

The next hydration compound isCalcium Hydroxide, also known as CH. It is formed from the reaction of C3S and water, and is a very weak portion of the concrete. Unlike amorphous C-S-H, CH has an ordered crystals structure that looks like hexagonal plates. At the early stages of hydration, it forms within the C-S-H, and then forms in the water-filled spaces at later stages. CH helps to prevent corrosion by playingsa large part in keeping the pore solution pH of the concrete high. CH keeps outside fluids and chemicals away from the reinforcing steel by forming a layer of material around the reinforcing steel, in concrete called the passive layer. This is why some people call CH the “human shield” of concrete. Think of it as Captain America’s shield; his shield protects him from his enemies and ensures that he will hold up in a fight. CH acts as a shield for the reinforcing steel to protect it from corrosive fluids and chemicals, and in return makes the concrete durable and long-lasting. Even though CH is not necessarily the strongest or most important hydration product, the concrete would not last without it. It is easy to tell when concrete is missing CH by how well it is performing. If a concrete structure it is starting to break down, it could be because it is missingCH, or its “shield.” Captain America’s shield seems like such a small object, but he is pretty weak without it. Adding Class F Fly Ash to concrete has an impact on the amount of CH inside. As I already talked about before, Class F Fly Ash consumes the CH inside the concrete and creates C-S-H. Even though the C-S-H adds strength to the concrete, it depletes the CH and has an impact on its protective abilities. Shown below is an image of magnified CH.

Figure 2: Image of Magnified CH

The last two hydration compounds Dr. Ley discusses in the video are Ettringite and Calcium Monosulfoaluminate. Ettringite, or Aft, is formed from the reaction of C3A, Gypsum, and water, and is responsible for some initial stiffening and strength gain. Ettringite crystals are needle-shaped and absorb water as they grow. This growth causes a significant volume change in the cement particle. At later stages in hydration, the Ettringite is consumed to produce Calcium Monosulfoaluminate. Calcium Monosulfoaluminate, or Afm, is formed from the reaction of C3A, Ettringite, and water, and it is shaped like a flower or a rose rock.Afm forms after about a day of hydration, and its formation depends on the amount of Sulfur in the solution when the C3A reacts. In a way, the formation of Afm is very similar to Hulk’s transformation. The Ettringite grows rapidly and causes a large volume change in the cement particle. In the same way, Hulk rapidly grows into a massive, voluminous creature. From then, the Ettringite transforms into Afm. Similarly, Hulk transforms back into Bruce Banner, the normal human scientist; however, his transformation is very situation-dependent. He cannot necessarily change from Hulk at will, he has to calm down and control his rage first. It is worth noting that “normal scientist” for Bruce Banner is six PhDs, but doctorates aside, the same goes for Afm formation because it is very dependent on the amount of Sulfur in the solution. Another thing to note is that Afm and Ettringite transform back and forth, depending on what the cement particle needs. This transformation process alternates throughout hydration. Similarly, Bruce can transform to and from Hulk depending on the situation he is in.This is especially dependent on if the Hulk takes over ashe did in Thor Ragnarok, which once again is a great action movie. Shown below areimagesof magnified Ettringite crystals and magnified Afm.

Figure 3 –Image of Magnified Ettringite Crystals

Figure 4 - Image of Magnified Afm

​Asthe C-S-H, CH, Ettringite, and Afm are formedon the cement particles, porosity decreases. This is important for the concrete because as porosity decreases, permeability decreases. As the cement is hydrated, it shrinks the pores within the concrete, which prevents outside fluids and chemicals from getting in. In conclusion, ensuring proper hydration is very important for creating long-lasting concrete. Similar to the Avengers, the cement hydration products act as the team that adds strength, protection, and durability to concrete. Thor, Captain America, and Hulk are some of the most important members of the Avengers,and they all work together to protect the world from outside enemies. Hydrationproducts act as the Avengers for concrete; without them, the concrete would lack strength, durability, and would not be able to resist outside chemicals. If you want to learn more about cement hydration,or about concrete in general, “like” and “subscribe” to his page, and don’t forget to hit the bell button so you get notified every time he uploads a new video! #TheBell ​#CSH #HydrationProducts #Avengers #ThorRagnaro