Reactions of hydration aluminate without participation of sulfates

According to studies, calcium aluminates have pronounced astringent properties, hardens quickly and reach a large mechanical strength. Very quickly it reacts with tricalcium aluminate water. In the presence of excess water formed numerous hexagonal plate-like crystals form. As a result of hydration is formed hydroaluminate

Rehbinder notes that the re-crystallization in the initial stage of structure formation plays a positive role, as a result of this process determines the dependence of the solubility of the size of the nuclei of the new phase, based on the coagulation structure is formed, the crystallization structure of tumors. Measurements have shown that the specific surface area of ​​tricalcium aluminate hydration at first increases, passes through a maximum and then decreases due to the predominance of the recrystallization process, which is expressed in the consolidation of the particles, ie. E. A decrease in the specific surface. Recrystallization leads to the disappearance of most small particles grow larger and that also experimentally proved by the example gypsum samples.

Reactions hydration aluminate with participation sulphates

To adjust the timing of cement clinker grinding with added 4-5% gypsum. Otherwise, it will be the so-called "false setting" when cement powder is mixed with water once it starts to thicken and set. According to GOST 10178-85 start setting cement paste should occur no earlier than 45 minutes, setting an end must occur not later than 10 hours. Setting time of cement is determined to Vicat apparatus.

As a regulator of cement setting time using dihydrate calcium sulphate CaSO4 · 2H2O (short formula CsHx). Regulator is mainly dihydrate or hemihydrate or a mixture of both types of calcium sulphate. When grinding clinker with additives in tube ball mills there is strong heating of cement, as expended in the process of grinding energy is used directly for grinding only partially. Thus, at high temperature gypsum dihydrate can be fully or partially dehydrated to hemihydrate. Total input sulfate dihydrate and attitudes entered and hemi to be agreed with the reactivity of the clinker and the requirements relating to the establishment of cement behavior (setting time). The high content of CaSO4 · nH2O delay the starting date and the end of the cement setting up the necessary limits. Depending on the concentration of the regulator setting can be various reactions: if the content of CaSO4 · nH2O the high C3A reacts with water and CaSO4 to form a compound, which is called "ettringite" (the title of the same name in the locality Ettringen Eifel)

Hydration ferrite (C4AF)

C4AF Hydration is one of today's least understood processes in the cement hydration. For example, it is unclear why the fine net C4AF reacted with water while C4AF in the cement composition reacts very slowly with water. The latter is difficult to explain the process. It is still unclear how the hardware is included in the hydration products. Here is a description of the two models, which are offered by the authors. The first of these describes Taylor developed the idea to ferriferous hydration ettringite (AFt = Alyuminatferrit - thiosulphate) and bisulfate; the second model is based on the latest research findings on the ESEM FA Finger - Institute for Materials Science in Weimar. In general, it was observed that with increasing iron content in the alumina ferrite (as C6AF2 in the mixed crystal series C2A.... C2F) hydration is slower

 

Model Taylor

However, hydrated C4AF products are kaltsievoalyuminatnoferritnymi hydrates. Analog C3A hydration composition depends on the content of sulfate. In the absence of sulphate carrier initially formed in accordance with the equation below gidroalyumoferrit calcium as an intermediate product and a mixture of iron and aluminum:

2S4AF + 32H → C4 (A, F) H13 + 2C2 (A, F) + (A, F) H3 Equation

Model Stark

According to this model, the relatively slow hydration C4AF explained by leaching aluminum from C4AF grains. This takes place in such a way that responds to an aluminate solution as well as with pure ettringite C3A via sulfate and Ca (OH) 2. It is formed of Al-depleted and, thus, Fe-enriched grains C4AF, which after a long hydration time were still visible in the microstructure. Since leaching is relatively slow, sulphate carrier, usually goes into solution quickly, it can lead to the temporary formation of a secondary microstructure in the solidified gypsum cement paste. This plaster disappears again as soon as further needs sulfate to form ettringite leached aluminate. As a result, hydration C4AF initially leads to the formation of Fe-free ettringite, which is converted to the bisulfate salt and a secondary gypsum later. Supported, among other things, we have the fact that, contrary to the idea of ​​Taylor - is not iron hydroxide. To fully clarify C4AF hydration process certainly needed further research.

Taylor Model

C4AF Hydration is one of today's least understood processes in the cement hydration. For example, it is unclear why the fine net C4AF reacted with water while C4AF in the cement composition reacts very slowly with water. The latter is difficult to explain the process. It is still unclear how the hardware is included in the hydration products. Here is a description of the two models, which are offered by the authors. The first of these describes Taylor developed the idea to ferriferous hydration ettringite (AFt = Alyuminatferrit - thiosulphate) and bisulfate; the second model is based on the latest research findings on the ESEM FA Finger - Institute for Materials Science in Weimar. In general, it was noted that an increase in iron content in the alumina ferrite (as a mixed crystal C6AF2 series C2A.... C2F) is slower hydration [5, 121, 120, 133].

Model Taylor

However, hydrated C4AF products are kaltsievoalyuminatnoferritnymi hydrates. Analog C3A hydration composition depends on the content of sulfate. In the absence of sulphate carrier initially formed in accordance with the equation 22 gidroalyumoferrit calcium as an intermediate product and a mixture of iron and aluminum:

2S4AF + 32H → C4 (A, F) H13 + 2C2 (A, F) + (A, F) H3 Equation 22

The unstable intermediate products are converted in accordance with the equation 23 into crystals lamellar phases:

C4 (A, F) H13 + 2C2 (A, F) → 2C3 (A, F) H6 + 9H Equation 23

In the presence of sulphate carrier in a ferrous form ettringite during hydration initially (thiosulphate ferrite aluminate, AFt) and possibly later (depending on the listing sulfate) aluminate ferrite bisulfate (AFm) reaction occurs:

3C4AF + 12CsH2 + 110H → 4C3 (A, F) · 3CsH32 + 2 (A, F) H3 Equation 24

Eisenettringit, Aft

4C3 (A, F) · 3Cs · H32 + 3C4AF + 12CsH2 + 14H → 4C3 (A, F) ∙ CsH12 + 2 (A, F) H3 Eq th 25

Alyuminatferrit bisulfate, afm

Model Stark

 

The cement hydration

During the hydration of cement, all operations in varying degree occur partly simultaneously, but partially and at different times. So we have a complex system with several partially interdependent individual reactions. During hydration content of clinker phases in the reaction mixture decreases independently from each other and formed hydrate phase, some of which are metastable and are converted in the process of hydration in the thermodynamically most stable hydrate phase. During the hydration process hydrate recrystallization continues to occur which alters the morphology of the hydration products without further reaction. Figure 1.37 shows the change in the content of alite, portlandite (Ca (OH) 2), and the evaporation of water aluminate cement with and without the use of gypsum as a moderator timing skhvatyvaniya.Posle achieve supersaturation to crystallize from solution of Ca (OH) 2 and ettringite (Figure 1.38). Calcium hydroxide is released in the form of thin hexagonal plates that can rapidly grow to large sizes. Ettringite from a supersaturated solution crystallizes in the form of short prisms, while reducing the concentration of the solution to a saturation condition - in the form of elongated prismatic and needles. The resulting solution crystals of Ca (OH) 2 and ettringite form precipitates as a dense cake on the cement particles of fortified C3A and C4AF, that impede the diffusion of water to the unhydrated parts and thereby slow the hydration process. The speed of the hydration process (expressed heat release rate) depends on the quantity of the injected calcium sulfate. When a small amount of SO3 heat release rate is high in the beginning, but the cement grains once formed the shell of ettringite crystals, further retarding the flow of the hydration process. With the increasing amount of heat SO3skorost lowered and stretched in time, which is associated with the formation and destruction of cyclic membranes from crystal ettringite cement grains.