Physical and chemical bases of gypsum binders

Gypsum binders - one of the few binders, expanding hardening: the increase in the volume of up to 0.2%. Plaster - a mineral from the group of sulfates calcium sulfate.

Properties of gypsum and gypsum concrete materials are determined by their structure. Most gypsum materials produced from a mixture of gypsum binder (GW) with water, i.e. structure is formed as a result of hydration GW. The process of hydration - the subject of extensive research. A distinctive feature of gypsum binders when mixing with water is their ability to quickly grasp and harden, which distinguishes GW other binders. Due to the rapid setting and hardening, manufacture of gypsum binders has a short molding cycle, high turnover board equipment, the possibility of using a conveyor or rolling technology. Setting and hardening of the GW is based on the reaction of addition of water to the calcium sulfate hemihydrate to convert it to the dihydrate: CaSO4 • 0,5N2O 1,5N2O + = CaSO4 • 2N2OS thermodynamic point of view of the hydration process of hardening is associated with a decrease in the Gibbs energy, so it goes spontaneously. This is evidenced by the thermodynamic characteristics of the reaction, obtained VI Babushkin and OP? Mchedlova-Petrosyan: the enthalpy H ° 298 = -19.4 kJ / mol; Gibbs energy? G ° 298 = -5.54 kJ / mol.

Gypsum binders contain particles with different degrees of non-uniformity of energy, due to the presence of various kinds of structural defects. Wherein the active centers are unsaturated groups with oxygen valence bonds, making them highly reactive. Physico-chemical nature of the reaction surface (the plane quietly) binder makes it capable of binding water molecules to produce hydroxide ions (Fig. 1). Under the influence of surface forces formed an ordered structure of intermolecular hydrogen bonds.

Thermodynamic analysis proved that the hydrophilic surface of the water molecules adsorbed localized. Consequently, the crystallization centers dihydrate occur locally.

Transition hemihydrate dihydrate is carried out in the capillary condensation at a relative humidity above 80%.

Even a slight hemihydrate dilution water (10-50 times) the high degree of supersaturation is attained after 1 min.

In practice, always work with the dough, characterized by the ratio of water / hemi ≈ 0,5 ÷ 0,7, thereby limiting the supersaturation is achieved even earlier.

Thus, fine crystals not only have a high dissolution rate (in accordance with equation Thompson-Kelvin), but may provide a substantial supersaturation and:

where s∞ - solubility of large crystals; ck - solubility of small crystals of the hemihydrate; σ - phase between the specific energy, accepted approximately equal to 12 erg / cm2; M - molar mass; R0 - gas constant, equal to 8.3 • 107 ergs / (mol • K); T - the absolute temperature; ρ - density hemihydrate, which is equal to 2.7 g / cm3; r * - critical radius of small crystals. Three-four times or more the degree of supersaturation created crystals of a size 0.0038 micron. Despite the fact that these calculations are approximate, they give a clear picture of the relation of a degree of supersaturation, the equilibrium value of the embryos and their solubility.

where C - constant (calculated took C = 1, which is quite acceptable); NA - Avogadro's number, equal to 6.02 • 1023.

The probability of formation of equilibrium of a new phase - dihydrate - can be calculated by Volmer equation

The hemihydrate by dissolving the solution becomes supersaturated with respect to dihydrate crystallized and the last. This solution leads to depletion of Ca2 + ions and SO42-, whereby it becomes possible dissolving new portions hemihydrate again to form the supersaturated solution, and then separating the dihydrate thereof. Massive nucleation dihydrate leads to the fact that the plastic plaster mixture thickens and thickens. This phenomenon corresponds to the beginning of setting.

During this period, due to intermolecular attraction energy produced coagulation contacts between the gypsum crystals. A characteristic feature is the presence of these contacts between the aqueous layer crystals resulting structure has arisen elasticity. The strength of coagulation contacts about several orders of magnitude weaker than the crystallization contacts.

The processes of dissolution of binder particles and vykri- stallizovyvaniya dihydrate continue to fully hydrate the hemihydrate.

Lack of water between layers of crystals of dihydrate due to hydration leads to the formation of quasi-coagulation fusion contacts. Gypsum slurry increasingly loses plasticity and compacted. This corresponds to the end setting gypsum mixture.

With the growth of dihydrate crystals and water consumption for the most durable hydration crystallization contacts accretion formed at the grain boundaries and provide a rigid crystalline framework of gypsum.

Formation of gypsum stone structure takes place in two stages. In the first stage of crystallization formed frame, while the second - the frame additionally acquires dihydrate crystals, ie, there are two fundamentally different mechanism of growth - layered and normal. Stratified crystal growth presupposes on an atomically smooth surface crystallization steps, to which atoms are joined atoms crystallizing dihydrate. Crystal growth is performed by successive layers of encrustation, ie tangential movement stupeney.Pri normal growth of gypsum crystals the atoms crystallizing substance attached to the atoms of the crystal almost anywhere in the surface. This is possible when the surface has a lot of places energetically favorable fixing of atoms, ie, when the surface is atomically rough. In this case the surface is moved into the growth process by the normal to ney.Ukazannye processes lead to an increase in the strength of gypsum. However, as shown by the PA Rehbinder, EE Segalova and AF Polak et al., The internal stresses that reduce the strength of the structure can develop a hardening structure. When the solidification of cast plaster mixes wet hardening structure resists quite well (relaxes) increasing internal stresses due to residual coagulation and shareware coagulation contacts accretion crystals gipsa.Sovershenno differently shaped plaster system, if the mixing water gypsum binder is not enough, that there is, for example, when compressed gypsum boards "marble." In this case, as shown by A. Volzhensky, due to an insufficient amount of pore space to deposit crystals tumors plaster structure will disintegrate during further hydration. Through quantitative calculation can determine the parameters of a stable structure kamnya.Takim gypsum, as a result of physical and chemical processes form strong capillary-porous structure of artificial gypsum, which is the basis of gypsum products.