TYPES AND FUNCTIONS OF DISPERSANTS FOR REFRACTORY CASTABLES

Dispersants are widely used in refractory castables. Generally, the construction modes of refractory castables include vibration molding and gunning molding. By improving the rheological properties of the refractory castable, the construction time of the refractory castable and the labor intensity of the workers can be reduced. The most direct and effective way to improve the rheological properties of the castable and reduce the water demand of the castable is to select the appropriate dispersant in the refractory castable of the corresponding system.
The application of dispersants in refractory castables was initially based on the experience in the field of concrete. In the 1950s, lignosulfonate and sodium polyphosphate water reducing agents began to be used in refractory castables for vibration construction. 10%~15%. From the 1960s to the 1980s, polysulfonate compounds began to be used as second-generation water reducing agents in self-flowing castables, and the water reduction rate could reach 20%-25%. The third generation of special superplasticizers are mainly polycarboxylate compounds, which can make the water reduction rate reach 20%-30% through the steric hindrance effect after being adsorbed on the surface of the particles. Wang et al. studied the effects of three dispersants, naphthalene sulfonate, sodium tripolyphosphate, and acrylic polymer, on the hydrate morphology and castable properties of CMA cement.
He found that dispersants in cement-bonded castables can not only disperse cement particles, but also affect the morphology of cement hydration products, thereby affecting the mechanical properties of castables. Lopes et al. used sodium polyphosphate and citric acid as dispersants to prepare self-flowing phosphoric acid combined castables. He believed that the hydrolysis of small molecule long chains of phosphates in sodium polyphosphate could accelerate and improve the self-flowing value of the castable. Badiee and Otroj et al. tried to improve its rheological properties by controlling the content of silica sol in the castable. The results showed that adding 9%-11% mass fraction of silica sol can significantly improve the self-flow value of the castable by 80-110%. Anjos et al. studied the effect of different dispersants (polyethylene glycol-based polymer, citric acid (CA), and sodium tripolyphosphate (STPP)) on the rheological properties of castables for aluminum-silica sol systems. He found that these four dispersants All additives can reduce the viscosity of the system; and through the shock test, it is found that only FS10 can reduce the storage modulus and loss modulus of the system, thereby improving the construction performance of the sample. Zhu et al. believed that the sol-combined castable also did not need to use a dispersant. At pH=10, the silica sol could act as a dispersant to disperse alumina particles through electrostatic action to form a typical Newtonian fluid behavior.
1. Classification of dispersants
There are many classification methods of dispersants, among which, based on the type of hydrophilic group, they can be divided into five types: anionic dispersants, cationic dispersants, zwitterionic dispersants, nonionic dispersants, and mixed dispersants.
Anionic dispersants mainly rely on their own negative charges to provide electrostatic effects. The dissociated ion groups are adsorbed on the surface of the charged particles, changing their original double electronic layer structure, increasing the zeta potential value of the colloidal particles, and finally improving the stability of the solution. For example, sodium tripolyphosphate (STPP), citric acid (CA), carboxylates and sodium naphthalene sulfonate formaldehyde condensate (FDN).
a Sodium tripolyphosphate:
STPP is an inorganic anionic dispersant with a density of 0.3-0.9g/cm3 and a chemical formula of Na5P3O10. Both ends are terminated by Na2PO4. The structure of the entire dispersant is linear. Its solubility is large, the pH value of the aqueous solution is between 8-10, and it is easily hydrolyzed, and the hydrolyzed products are sodium pyrophosphate, sodium monohydrogen phosphate, sodium dihydrogen phosphate and sodium phosphate.
b Citric acid:
CA is a tricarboxylic acid compound, the chemical formula is H3C6H5O7, there are three H+ that can be ionized, and it contains one molecule of crystal water. Citric acid is relatively strong.
Polycarboxylate is a kind of dispersant with “comb” structure formed artificially by molecular design. In the main chain of polycarboxylate, there are many branched chains with certain length and rigidity, and some sulfonate groups that can charge the particles. It mainly achieves the dispersion effect of the castable by causing the steric hindrance effect between the particles. The advantage of using polycarboxylate as a dispersant is that the water-reducing effect is obvious, and the water-reducing effect is strong.
The pure product of FDN is a white powder, which is obtained by the sulfonation of naphthalene and the neutralization salting out of sodium hydroxide. The molecular formula is C10H7SO3Na, and the molecular weight is 230.22. In contrast to anionic dispersants. After dissociating in water, cationic dispersants can generate positively charged groups with strong activity. The two groups in the amphoteric dispersant are both hydrophilic groups, one of them is positively charged (amino group) and the other is negatively charged (carboxyl or sulfonic acid group), because different groups are at different pH. It exists in different ionic forms at the lower value, so there is an isoelectric point for this type of active agent. Non-ionic dispersants do not dissociate in aqueous solution, the hydrophilic groups are mainly polyethylene glycol groups, and the polarity of the active agent is controlled by the number of hydrophilic groups.