Study On Hydration And Hardening Of Natural Anhydrite And Its Activation

Anhydrite is the main gypsum resourse in China, with high grade, widely distributed, which reserves ranks first in the world. The solubility of anhydrite is more than dihydrate gypsum and the hydration reaction of anhydrite is spontaneous thermodynamic process, in other words, anhydrite has the gelatinization property. But the structure of anhydrite is compact, it dissolves slowly, and its hydration activity is low, so most of them are discarded as waste, which not only occupies the land, increases costs, but also causes environmental pollution. Activity excitation is the key to the effectiveness of anhydrite resource utilization.The hydration and hardening process of anhydrite binder, pH, temperature and impurity effects on its performance, grinding, calcining for the physical activation, grinding aids for dispersion, the activating effect and the mechanism of sulfate salt typed activator, the activating effect of non-sulfate salt typed activator, the modifying effects of anhydrite with inorganic active material and properation of anhydrite binder were systematically and deeply studied by the determination of hydration ratio, hydration temperature-rise, grading analysis, SEM, XRD microstructure analysis and ion concentration in liquid phase in combination with the investigation of the macro-performance testing.The hydration and hardening of natural anhydrite are slow and its hydration is always carried out under low saturation, which is the main reason of low level of anhydrite hydration, crystal thickening, contact points less, setting time long and low strength. The degree of supersaturation of acidic solution is the greatest, which is beneficial to the hydration and crystallization of anhydrite, and its hydration ratio and strength are both higher. Improving hydration temperature decreases the solubility of anhydrite, which leads to supersaturation degree and hydration ratio decreased especially in the middle and later periods. The dihydrate gypsum contained in anhydrite can dissolve quickly in the early hydration to increase the initial supersaturation degree of the slurry and to play the role of crystal seed, which is good for crystallization, but little effect in later period. Na, Al and other soluble salts impurities rapidly dissolved, the cationic solvation effects and the increased concentration of the SO42- in liquid phase and supersaturation accelerate the hydration of anhydrite. As the specific surface area of anhydrite increases, the grain refined, crystal morphology improved, the extent of hydration increased, and the strength of the hardened increased. However, the greater fineness, the greater the grinding energy consumption and its hydration water demand increased, strength decreased. Therefore, the fineness of anhydrite should not be too large, it preferably is limited between 450~500 m2/kg. FDN has better aided grinding and dispersive function and water-reducing and reinforcing effect, its suitable dosage is 0.5%. FDN was suggested as an essential component (grinding aids) use.Calcination can significantly increase solution and hydration performance of anhydrite, the CaSO4·2H2O in calcined anhydrite dehydrates to beβ-CaSO4·1/2H2O, the generated dihydrate gypsum after the rapid hydration of the anhydrite have seed crystal induction effect to hydration of anhydrite, while CaSO4·2H2O, CaCO3 and MgCO3 and other minerals are decomposed , so that the structure of anhydrite is loosed, the grindability is raised, the specific surface area increases, the dissolution of the anhydrite is promoted, the supersaturation degree in liquid phases is increased, the hydration of anhydrite is clearly accelerated, the hydration ratio increases and the strength is also increased. Suitable calcination temperature is 150℃.Sulfate salt typed activator, which can markedly accelerate the dissolution rate of anhydrite, significantly shorten the setting time, significantly increase the rate of hydration, concentrate the hydration heat evolution, accelerate the hydration process and promote the strength of the hardened paste multiply, is a highly efficient activator of anhydrite hydration. Sulfate salt typed activator promotes the dissolution of anhydrite, inhibits the dissolution of dihydrate gypsum; increases the supersaturation degree of dihydrate gypsum crystallization, and thus accelerates its nucleation and growth, leading to refinement of the crystal size, enhancement of the laps among crystals and significantly increased the strength of hardened paste; produces a liquid environment rich in SO42- which is beneficial for the formation of gypsum crystal structure unit, increases gypsum crystal nucleation cores and changes its growth habit; weakens the hydrogen bonding of water solvent and then improves its dissolution ability. Sulfate salt will increase volume expansion of anhydrite, make the volume stability of anhydrite drop and will bring efflorescence, foaming and other adverse effects. Most complex activator is more effective than the single one, particularly the compound of 3% of the calcined alum and 1% Na2SO4 has the best effects, but there still exists the negative impact of expansion in the latter period. Sodium oxalate is a highly efficient activator of anhydrite, has a certain inhibitory effect on expansion of anhydrite and its suitable dosage is within 2%. Sodium oxalate, potassium dichromate, sodium silicate and other non-sulfate salts have the cationic solvation effects, in addition, the anion can react with calcium ion and generate insoluble salts, and then the liquid environment rich in SO42- is produced to promote the dissolution of the anhydrite, shorten the setting time of anhydrite, concentrate the hydration heat, increase the hydration rate, accelerate the hydration process, accordingly leading to the compactness of the structure of hardened paste, refinement of its crystal and increase of its strength.The influence of SO42- on the nucleation and growth of the dihydrate gypsum crystal is significantly greater than Ca2+. Those soluble sulfate salts which can produce a liquid phase rich in SO42- and increase the supersaturation degree in liquid phase are all the highly efficient activation of anhydrite.The hydration process of anhydrite and the hydration action of slag can mutually promote. Cement clinker plays a role of alkali-activate on slag, while it also has a good activating effect on anhydrite. The setting time of anhydrite is shortened, hydration rate is increased, the line expansion rate is decreased, the strength and water resistance are increased by combined admixture of the cement clinker and the slag. Anhydrite binder material activated by sulfate salt and modified with slag has good characteristics such as higher strength, good water resistance and low shrinkage rate.