2.4 Technological ways of controlling the conditions of formation of gypsum cement gypsum stone
Introduction to hardening of the aluminosilicate system increased amounts of hemihydrate of calcium sulphate leads to the formation of low-strength cementitious stone, unstable to water. Negative effect of calcium sulfate can be eliminated only if in hardening the system additionally introduced amorphous silica in amounts corresponding to a molar ratio SiO2/C3A≥4,8 (when the specific surface area of silica of 300 m2/g). This means that the amount of amorphous silica must match the mass С3А included in the composition of the used cement. This flow rate of amorphous silica in practice unacceptable due to the high cost and scarcity of this substance.
Since the formation of the low-strength cementitious stone with unstable structure is the result of mutual influences of sulfate and cement components in their joint structure, there is a need to identify and eliminate those interactions that generate the most negative consequences.
A major role in the formation of low-strength stone with a fragile structure play a transformation occurring in aluminate phase of hardening of the composition. Therefore, the main technical influence on the hardening of gypsum cement gypsum system should be the management of processes of interaction between sulfate and aluminate phases. When you do this, you first need to prevent involvement degidrirovaniya particles in the structure of crystal structure of gypsum cement gypsum stone, since the resumption of processes of hydration of the particles give rise to the destruction of this structure.
2.5 Influence of pozzolanic additives
According to the current standard for GCPV main technological factor that will provide the possibility of combining gypsum binder with Portland cement in a single knitting composition is the introduction of active mineral additives (diatomaceous earth, silica clay, etc.).
The most typical active mineral additive — Tripoli, selected in strict accordance with the requirements of the standard, reduces the concentration of calcium hydroxide in gidratirovana gypsum cement gypsum mixture (1:1 by weight) after only 4 days of hydration (figure 26).
Such an effect is caused by the fact that the original diatomaceous earth promotes hydrolysis of the products of hydration of clinker minerals, not showing the necessary skills for linking the released calcium hydroxide. As a result, the initial stages of formation of the structure of the composition effect of diatomaceous earth is actually cannot be seen.
Figure 26 – Role of supplementation of diatomaceous earth in the change in the concentration of calcium hydroxide, DSO in the liquid phase gidratirovana gypsum cement gypsum slurry
and without diatomaceous earth in the system. (b) in the presence of 42% of diatomaceous earth to the weight of clinker;
in — in the presence of 72% of diatomaceous earth to the weight of clinker
Figure 27 – Kinetics of deformation processes of expansion at strukturoobrazovatelja reference (1) and GCPV (2)
From figure 27 it follows that the addition of diatomaceous earth is not able to resolve the interaction of Portland cement with gypsum binder at the stage of joint formation. In all cases there is a more than twofold acceleration of the process, as well as additional expansion of the system caused by formation of ettringite. This confirms that the addition of diatomaceous earth is not able to prevent involvement degidrirovaniya or slabomineralizovannaja of cement particles in crystallization, struc-ture and gypsum cement gypsum stone.
Introduction of diatomaceous earth does not provide the formation of gypsum cement gypsum stone, resistant to continuous exposure to water. As in the presence of diatomaceous earth, and gypsum cement gypsum stone, curing for 28 days at 20° C with relative humidity of 100%, with subsequent movement into water with a temperature of 10°C collapsed after 49 to 56 days.
Thus, the introduction of gypsum cement gypsum composition of active mineral additives like diatomaceous earth, the quality and number of relevant standard requirements on GCPW, is not sufficiently effective technological method, providing optimal conditions for the formation of stone.