Abstract
Coating is one of the most important high-value-added application cases in geopolymer materials. However, efflorescence can easily cause discoloration and reduce the esthetic impression of the coating surface, thus limiting its application; hence, inhibition of efflorescence is one of the most important techniques in the application of geopolymer coatings. Efflorescence is a spontaneous behavior in a Na-based geopolymer, involving the migration of soluble alkalis. Alkalis are dissolved by water and diffuse to the material surface through nocuous pores, and then react with CO2 to produce white carbonate products. To inhibit efflorescence in geopolymer coating, this article reports a structure modification method using polydimethy siloxane (PS) and mica. To explore the inhibition mechanism, the effects of PS and mica on the pore structure, water absorption, alkali leaching, and efflorescence product were investigated. The experimental results showed that a harmful pore structure and instinctive water absorption of the geopolymer strongly contributed to efflorescence. PS and mica could reduce the pore size distribution and porosity and are helpful to establish a waterproof structure, leading to water absorption and the alkali leaching rate being significantly suppressed. Both high water glass and water content play a critical role in the increase of efflorescence, but even under a high content of water glass and water used in geopolymer coating, this method shows an 80–90% efflorescence reduction, which is much higher than that of other studies. In practical engineering, when the geopolymer coating is applied after modification, even if it is exposed to the field environment for a long time, there is no efflorescence deposit on the coating surface. It is feasible to limit water ingression in a geopolymer, which effectively blocks the efflorescence reaction process. This method is simple and practical and can be applied in practical engineering applications of geopolymer coatings conveniently.
Highlights
Inorganic coatings made of film-forming materials, pigments, and fillers have good permeability, durability, and environmental protection properties.[1]
A broad bump of 22−35° can be seen both in the sample before (RS) and after (O4) polydimethy siloxane (PS) addition, which indicates that PS in the geopolymer networks did not take part to form new chemical compounds, as the broad amorphous peak appears in the same position
These results suggested that the use of PS and mica can effectively reduce the water absorption of geopolymer coating, which might be attributed to the fact that PS, as a hydrophobic material, effectively endows the geopolymer coating with waterproof performance, and the other reason is that mica is most likely to fill and cover the pore space of the geopolymer coating
Summary
Inorganic coatings made of film-forming materials (for example, silicate and silica sol), pigments, and fillers have good permeability, durability, and environmental protection properties.[1] It is important that the inorganic coating does not contain any organic solvents, volatile organic compounds (VOCs), and toxic emissions of harmful substances. It is widely concerned due to its high-temperature resistance, strong bonding force, excellent durability, good chemical corrosion resistance, environmental friendliness, etc.[1−4] In recent decades, a number of studies have been focused on the practical application process of geopolymers;[5−7] the geopolymer used as a coating is one such practical application case with promising commercial potential.[1,8−11] Compared with traditional organic coatings, geopolymer coatings have the advantages of excellent aging resistance, good chemical− physical corrosiveness, no volatile organic compound emissions, as well as low cost. Surface efflorescence is considered to be one of the most important problems encountered during the application of geopolymer coating.[1,12]
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