Cement Hydrations
Cement hydration is the name given to the chemical reaction that occurs when cement and water come into contact. This reaction releases heat during its course, which is known as the “heat of hydration.” The reaction is exothermic in nature.
Cement typically generates 90–100 cal/gm of heat in 28 days and 89–90 cal/gm in 7 days. The full hydration of cement particles can take years; it is not an immediate process.
Cement has four main types of compounds that participate in the chemical reaction: C3A, C4AF, C3S, and C2S, also referred to as Bogue’s compounds.
When these compounds hydrate, certain products are formed, which causes the cement to set and harden and eventually transform into a solid, strong mass.
Principal Products of Hydration
1. Calcium Silicate Hydrate
Water reacts with C3S and C2S in cement particles to generate calcium hydroxide (Ca(OH)2) and calcium silicate hydrate (C-S-H) as soon as water is applied.
The following illustrates the chemical reaction:
2C3S + 6H —— C3S2H3 + 3Ca(OH)2
2C2S+4H3 —— C3S2H+ Ca(OH)2
Tobermorite gel is another name for C-S-H.
Since the ratio of CaO to SiO2 is not well defined, the word C-S-H is hyphenated. In the hydrated product, it makes up between 50 and 60 percent. Of all the goods, this one is the most crucial and is in charge of giving concrete its positive qualities, such as strength and durability.
It has been discovered that when C3S is hydrated, less C-S-H is produced than Ca(OH)2 when C2S is hydrated. Moreover, C3S produces C-S-H of lower quality and density than C2S.
2. Calcium Hydroxide
Another byproduct of the hydration of C3S and C2S is calcium hydroxide, or Ca(OH)2. It makes around 20–25% of the hydrated cement paste’s solidified volume. The presence of calcium hydroxide aids in preserving a pH of 13 surrounding the reinforcement, serving as a passive protective layer that stops the reinforcement from corroding. The only benefit of calcium hydroxide being present in concrete mass is this. It causes concrete to lose its durability for the following three reasons:
(i). Concrete mass becomes porous due to the easy solubility of Ca(OH)2 in water and its leaching out, which reduces the material’s strength and durability.
(ii). When Ca(OH)2 combines with sulfates found in soil or water, it produces CaSO4, which then reacts with C3A to produce higher volume products that cause fractures and cause concrete to deteriorate. Another name for this occurrence is “sulfur attack.”
(iii). When atmospheric CO2 and Ca(OH)2 combine, CaCO3 is created. The reaction occurs on the concrete’s surface at first, but it eventually seeps into the bulk. Corrosion of the reinforcement can occur if the concrete mass is slightly porous and lowers the pH of the passive protective layer. Concrete carbonation is the term for this kind of deterioration
As a result, it is clear that while Ca(OH)2 is generally unwanted in concrete, its utility cannot be entirely discounted because it can shield the reinforcement
3. Hydrate Calcium Aluminate
The aluminates found in cement, C3A and C4AF, hydrate to become Calcium Aluminate Hydrate. Relatively stable molecules that result from the hydration of C3A and C4AF are C3AH6 and C3FH6, respectively.
These hydration compounds damage concrete instead of giving it strength or other special qualities. This is especially true when the concrete is vulnerable to sulfate attack.
4. The Ettringite
As is widely known, flash setting of cement happens as a result of the quick hydration of C3A and C4AF, which can cause cracks to form in concrete. Gypsum (CaSO4) is added to cement during the manufacturing process to prevent this problem by reducing the solubility of C3A and C4AF and regulating flash setting behavior.
When this gypsum sulfate combines with C3A and C4AF, it produces calcium aluminate sulfate, or ettringite as it is commonly called.
Notably, this compound is forming prior to the concrete hardening, hence there are no detrimental impacts on the concrete from this primary ettringite creation. If the same compound is created after the concrete has solidified, it will harm the concrete.
This is commonly referred to as Delayed Ettringite Formation (DEF), because it causes microcracks to form in concrete, making it less resilient and porous.