The reaction of silicates (C3S, C2S)

Upon hydration C2S, as C3S, there are uncertainties associated with finding stoichiometric phase C-S-H; hydration two-calcium silicate following equation can be written:

2 (2CaO-Si02) + 5H20- ^ 3CaO-2Si02- • 4H20 + Ca (OH) 2.

The amount of Ca (OH) 2, generated as a result of such a reaction is less than C3S hydration. The phase of dicalcium silicate hydrates are much slower than the phase of tricalcium silicate.

At 1.2 compared with each other hydration rate of C3S and C2S. The absolute values ​​of the velocities vary in different samples; This individual characteristic, but the C3S well above the average rate of the reaction.

Increased reactivity of C3S explained by the following reasons: C3S in the coordination number of Ca 2+ is greater than 6; coordination of Ca2 + is irregular; C3S in the crystal lattice are void; there are differences that occur in the positions at the Fermi level. Some preliminary work to determine the relative reactivity of Ca2 + CaO, Ca (OH) 2, C3S and by mixing each with known amounts AgN03 [9]. It was found that during the subsequent heating AgN03 reaction with CaO, Ca (OH) 2 and relative stoichiometry C3S hydrated Ca2 +. Only 27% of Ca2 +, present in C3S, and 6% of Ca + C2S react with AgN03. It is possible that the activity of C3S and C2S are those due to defects in their structure. There is evidence that the hydration C3S, obtained by attaching a molecule to a labeled Ca C2S, the initial reaction products mostly contain labeled Ca2 + ion. Further work will be done before definitive conclusions.

Hydration C3S

Hydration of C3S

Alite reacts with water to form Ca (OH) 2 - calcium hydroxide) of calcium hydrosilicates (C-S-H-phases). Possible reaction products C6S2H3, C2S3H2 or C2SH, but generally forms a very fine-grained and therefore X-ray amorphous silicate. Composition C-S-H-phases depends on the water cement ratio. In general, the higher the water / cement ratio, the higher the calcium deficiency of calcium silicate hydrate formed. When the ratio of water / cement equal to 0.45, for example, there C3S2H3 according to the following equation:

 

2C3S + 6H2O → C3S2H3 + 3Ca (OH) 2 Equation 16

 

C3S Hydration can generally be formulated [118]:

 

3CaO · SiO + xH2O → yCaO · SiO2 · (y- (3-x)) H2O + (3-y) Ca (OH) 2 Equation 17

(= C-S-H) portlandite

 

Immediately after the addition of water is removed from C3S crystals of calcium hydroxide. Within their solubility Ca (OH) 2 dissociates to Ca2 + cations and anions OH -. OH- ions cause a decrease in concentration of H + ions in vode mixing and thus a significant increase in the solution pH. During the first few minutes, there is a rapid increase in the pH in the cement paste to 12-13,5. Alkalinity is responsible for the protection against corrosion of steel in concrete. High pH and the presence of Ca (OH) 2ostayutsya prerequisite for latent hydraulic reaction properties of materials, such as granulated blast furnace slag, pozzolans (such as fly ash, highway). At the same time increases the concentration of Ca2 + in the pore solution is the liberated heat of reaction. Thus, it is an exothermic reaction. The rate of hydration S3S not particularly fast compared to other clinker minerals. However, it is much faster than comparable C2S reaction with water [121, 120, 119].

The degree of heat is characteristic for each particular stage of hydration and is already being used at an early stage in order to classify the hydration on separate stages. They are referred to as the induction period, a period of rest, during acceleration, retardation period and the final period. During these periods, different reactions occur, which are described in Table 1.20. Figure 1.27 shows the timing chart for the heat generating five periods, and the content of Ca2 + in the pore water and the formation of C-S-H - phases.

Formed during hydration C-S-H-phases are X-ray amorphous or crystalline submicron. Therefore, their formation in X-ray crystallography is not observed. The hydration process can be traced in the X-ray intensity to reduce the peaks characteristic of the C3S.

 

As described above, the hydration of calcium silicates is formed of certain chemical compounds, phases, which differ in their chemical composition within a certain range. How to "limit" can form mineral tobermorite 1.4 nm (C5S6H9, named after the Scottish town of Tobermory) and Jenny (C9S6H11). Tobermory is a natural mineral. Jenny unlike him not found in nature. Both minerals are formed in the hardened cement paste. It can be prepared

Hydration C2S

The reaction of pure dicalcium C2S water is very similar to the above-described reaction of C3S. The primary products of hydration are also C-S-H-phases and Ca (OH) 2. Since C2S CaO content is less than C3S, is formed smaller calcium hydroxide produced during the initial hydrolysis (see. Eq 18). Reaction of dicalcium silicate and water is exothermic, but it is much slower than the reaction with water C3S.

Composition of C2S gidratsionnyh products depends on the temperature and water content. In general, the C2S hydration expressed as follows:

 

2CaO · SiO2 + xH2O uSaO → · SiO2 · (y (2)) H2O + (2-y) Ca (OH) 2 Equation 18

(= C-S-H) Portland

example:

2 C2S + 5H → C3S2H4 + CH