1. 1 physical adsorption
The main technical idea of controlling chloride ion by physical adsorption is to add additives to generate substances that can adsorb chloride ion, thus inhibiting the migration of chloride ion to gypsum surface and improving the quality of gypsum mortar.
Zeolite and hydrated lime are two typical additives. Zeolite has a strong adsorption capacity, which can realize the adsorption and aggregation of chloride ions. At the same time, hydrated lime will react with the activity in zeolite to form a network-like Ca5Si6O 16 (OH) 4H2O (C-S-H) gel (formula (1)). C-S-H gel has the characteristics of large specific surface area and developed network structure, which can adsorb free chloride ions in gypsum and effectively inhibit the migration of chloride ions in gypsum mortar.
Calcium hydroxide+silica +H2O→C-S- hydrogen (1)
In addition, adding fly ash and cement can also effectively control chloride ion migration. The content of active silicon and aluminum in fly ash is high. Ultrafine treatment can increase the specific surface area, destroy the original molecular structure, expose active reaction sites, improve their pozzolanic reactivity, promote the dissolution of active Si and al and their reaction with cement hydration products, and strengthen the formation of (C-S-H) gel. At the same time, the high content of CaO in alkaline fly ash can promote pozzolanic reaction, enhance the physical adsorption of chloride ions, and finally improve the quality of gypsum mortar. However, the actual effect of adding fly ash on chloride ion control is closely related to the chemical composition of fly ash, raw coal species and operating environment, and its specific influence mechanism on chloride ion migration needs further study.
1.2 physically blocks chloride ion migration.
In addition to physical adsorption, the migration of chloride ions to the surface of gypsum mortar can be blocked by changing the physical structure of gypsum mortar. In the preparation of gypsum mortar, adding physical fillers to improve the density of gypsum products is the most important technical means to inhibit chloride ion migration. Adding fly ash in the preparation of gypsum mortar can effectively reduce the porosity of gypsum crystals and make them dense, thus blocking the migration of chloride ions in gypsum slurry, reducing the content of calcium chloride on the surface of gypsum mortar and improving the quality of gypsum mortar.
Reducing the water-cement ratio can also inhibit the physical migration of chloride ions. By comparing the chloride ion content on the surface of gypsum products with different water-cement ratios, it is found that the smaller the water-cement ratio, the lower the chloride ion concentration. The reduction of water-cement ratio can reduce the number of pores in gypsum particles, reduce porosity, improve the diffusion resistance of chloride ions in gypsum, and play a role in curing chloride ions. At the same time, the effect of adding waterproof agent in the preparation of gypsum mortar is similar to that of fly ash. By filling between gypsum crystals, the porosity of gypsum is reduced, the migration channel of chloride ions is blocked, and the migration rate of chloride ions to the surface of gypsum mortar is reduced.
2. Chemical methods
In addition to physical methods, it is also an important technical means to improve the performance of chlorine-containing desulfurization gypsum mortar by chemical methods to transform chloride ions into stable substances and inhibit their dissociation and migration. Chloroaluminate is insoluble in water, so adding active substances into gypsum mortar to react with chloride ions to generate chloroaluminate, which can stabilize chloride ions and inhibit their migration. Aluminum ions in fly ash, especially high-alumina fly ash, can be dissolved into slurry by pozzolanic effect, and react with chloride ions to form chloroaluminate, which strengthens the solidification of chloride ions. In addition, calcium aluminate cement can also be used to solidify chloride ions in chlorine-containing desulfurization gypsum. When calcium aluminate cement is added to gypsum, it is found that it can combine well with chloride ions to form chloroaluminate (formula (2)). The results also show that even if gypsum begins to hydrate at an early stage, chloroaluminate will be formed.
3cao Al2O3 6H2O+calcium chloride (4 ~ 6) H2O →
Calcium trioxide, aluminum trioxide and calcium chloride (10~ 12)H2O (2)
To sum up, the core idea of controlling chloride ions in high chlorine content desulfurization gypsum is: on the one hand, stabilizing chloride ions in desulfurization gypsum particles can be achieved mainly by physical adsorption or chemical stabilization; On the other hand, chloride ion enrichment on the surface of desulfurization gypsum can be suppressed by blocking chloride ion migration channels, which is mainly achieved by increasing the density of gypsum and reducing the water-cement ratio through physical fillers.