Individual Question set 1: Cement hydration and heat generation. Find and summar
ID: 1001739 • Letter: I
Question
Individual Question set 1: Cement hydration and heat generation.
Find and summarise data for the heats of reactions (Hreaction) for the main three reactions occurring on the hydration of cement in the concrete production process, producing calcium silicate hydrate, calcium hydroxide and calcium aluminate hydrate. Find information on the varying timing of these reactions. Briefly discuss the implications of heat production from these reactions in terms of (a) rate (speed) of reaction and (b) relative timing and (c) the importance for dissipation of heat in the formed concrete structure.
Reflection: What was the most outstanding outcome of this investigation for you in terms of thinking about chemical and material processes?
Explanation / Answer
Concrete is prepared by mixing cement, water, and aggregate together to make a workable paste. It is molded or placed as desired, consolidated, and then left to harden. Concrete does not need to dry out in order to harden as commonly thought.
The concrete (or specifically, the cement in it) needs moisture to hydrate and cure(harden). When concrete dries, it actually stops getting stronger. Concrete with too little water may be dry but is not fully reacted. The properties of such a concrete would be less than that of a wet concrete. The reaction of water with the cement in concrete is extremely important to its properties and reactions may continue for many years. This very important reaction will be discussed in detail in this section.
Portland cement consists of five major compounds and a few minor compounds. The composition of a typical portland cement is listed by weight percentage in Table 2.
Cement Compound
Weight Percentage
Chemical Formula
Tricalcium silicate
50 %
Ca3SiO5 or 3CaO.SiO2
Dicalcium silicate
25 %
Ca2SiO4 or 2CaO.SiO2
Tricalcium aluminate
10 %
Ca3Al2O6 or 3CaO .Al2O3
Tetracalcium aluminoferrite
10 %
Ca4Al2Fe2O10 or 4CaO.Al2O3.Fe2O3
Gypsum
5 %
CaSO4.2H2O
When water is added to cement, each of the compounds undergoes hydration and contributes to the final concrete product. Only the calcium silicates contribute to strength. Tricalcium silicate is responsible for most of the early strength (first 7 days). Dicalcium silicate, which reacts more slowly, contributes only to the strength at later times. Tricalcium silicate will be discussed in the greatest detail.
The equation for the hydration of tricalcium silicate is given by:
Tricalcium silicate + Water--->Calcium silicate hydrate+Calcium hydroxide + heat
2 Ca3SiO5 + 7 H2O ---> 3 CaO.2SiO2.4H2O + 3 Ca(OH)2 + 173.6kJ
Upon the addition of water, tricalcium silicate rapidly reacts to release calcium ions, hydroxide ions, and a large amount of heat. The pH quickly rises to over 12 because of the release of alkaline hydroxide (OH-) ions. This initial hydrolysis slows down quickly after it starts resulting in a decrease in heat evolved.
The reaction slowly continues producing calcium and hydroxide ions until the system becomes saturated. Once this occurs, the calcium hydroxide starts to crystallize. Simultaneously, calcium silicate hydrate begins to form. Ions precipitate out of solution accelerating the reaction of tricalcium silicate to calcium and hydroxide ions. (Le Chatlier's principle). The evolution of heat is then dramatically increased.
The formation of the calcium hydroxide and calcium silicate hydrate crystals provide "seeds" upon which more calcium silicate hydrate can form. The calcium silicate hydrate crystals grow thicker making it more difficult for water molecules to reach the unhydrated tricalcium silicate. The speed of the reaction is now controlled by the rate at which water molecules diffuse through the calcium silicate hydrate coating. This coating thickens over time causing the production of calcium silicate hydrate to become slower and slower.
icalcium silicate also affects the strength of concrete through its hydration. Dicalcium silicate reacts with water in a similar manner compared to tricalcium silicate, but much more slowly. The heat released is less than that by the hydration of tricalcium silicate because the dicalcium silicate is much less reactive. The products from the hydration of dicalcium silicate are the same as those for tricalcium silicate:
Dicalcium silicate + Water--->Calcium silicate hydrate + Calcium hydroxide +heat
2 Ca2SiO4 + 5 H2O---> 3 CaO.2SiO2.4H2O + Ca(OH)2 + 58.6 kJ
The other major components of portland cement, tricalcium aluminate and tetracalcium aluminoferrite also react with water. Their hydration chemistry is more complicated as they involve reactions with the gypsum as well. Because these reactions do not contribute significantly to strength, they will be neglected in this discussion. Although we have treated the hydration of each cement compound independently, this is not completely accurate. The rate of hydration of a compound may be affected by varying the concentration of another. In general, the rates of hydration during the first few days ranked from fastest to slowest are:
tricalcium aluminate > tricalcium silicate > tetracalcium aluminoferrite > dicalcium silicate
Cement Compound
Weight Percentage
Chemical Formula
Tricalcium silicate
50 %
Ca3SiO5 or 3CaO.SiO2
Dicalcium silicate
25 %
Ca2SiO4 or 2CaO.SiO2
Tricalcium aluminate
10 %
Ca3Al2O6 or 3CaO .Al2O3
Tetracalcium aluminoferrite
10 %
Ca4Al2Fe2O10 or 4CaO.Al2O3.Fe2O3
Gypsum
5 %
CaSO4.2H2O