Relationship between Moisture Transportation, Efflorescence and Structure Degradation in Fly Ash/Slag Geopolymer

Abstract

The relationship between moisture transportation and efflorescence in sodium hydroxide- or sodium silicate-activated fly ash/slag geopolymers was investigated. The results show that the efflorescence products are sodium carbonate hydrates, mainly composed of natron, heptahydrate, trona and sodium carbonate. The efflorescence induces compressive strength loss, water absorption increases and pore structure degradation in the geopolymer. When the curved surface of a geopolymer cylinder is covered with plastic film, the moisture transportation drives the free alkalis to the top surface to initiate efflorescence. In comparison, the efflorescence occurring on the curved surface of an uncovered geopolymer cylinder results in a more intensive alkalinity loss. For the uncovered geopolymers prepared with sodium hydroxide activator, efflorescence deposits are formed on the lower half of cylinder. A low capillary absorption capacity developed in the pore structure can only drive the moisture to the middle of cylinder, which is confronted with the drying front. More efflorescence products are formed on the upper half of the uncovered geopolymer cylinder prepared with sodium silicate activator. A relatively higher capillary absorption capacity, developed in the more compact pore structure, transports the moisture from the bottom to the top of cylinder, so no drying line is observed in the cylinder.

Chemical Distributions of Different Sodium Hydroxide Molarities on Fly Ash/Dolomite-Based Geopolymer

“…The low water absorption is a strong indication that open porosity decreases and impedes the heavy flow of water into the geopolymer portion. The lower water absorption by the geopolymer samples means that the samples are durable [ 25 ].…”

Mosses on Geopolymers: Preliminary Durability Study and Chemical Characterization of Metakaolin-Based Geopolymers Filled with Wood Ash

“…Water evaporation on the exterior of geopolymer specimens induces moisture transportation, which moves the free alkalis from the matrix to the surface to react with the atmospheric CO 2 [ 32 ]. According to Zhou et al (2020), when a geopolymer cylinder’s curved surface is covered with plastic film, the efflorescence products are mainly formed on the top surface [ 33 ]. Another factor that influences the formation of efflorescence is the pore structure that correlates with moisture transportation.…”

Effect of Carbonate Source on the Dehydrofluorination Process in Polyvinylidene Fluoride/Alkali Metal Carbonate Composites

“…The reasons for such kinds of distinct variations in the degree of PVDF dehydrofluorination for the investigated percolative composites might also be associated with the property of hygroscopy: for all present alkali metal carbonates, because of the strong affinity for water, hygroscopy can occur to attract moisture via either absorption or adsorption from the surrounding environment at room temperature if exposed to it, and alkali bicarbonate and hydrated alkali carbonate can be generated on the surfaces . However, it should be noted that, in the case of hydrated sodium carbonate, efflorescence can occur for a loss of water with an impeded formation of NaHCO 3 in the raw mixture . Accordingly, the reduced amount of generated water upon thermal treatments and the inhibited formation of NaOH might cause insufficient dehydrofluorination reactions and the occurrence of the side reactions of eqs and .…”

“… 26 However, it should be noted that, in the case of hydrated sodium carbonate, efflorescence can occur for a loss of water with an impeded formation of NaHCO 3 in the raw mixture. 27 Accordingly, the reduced amount of generated water upon thermal treatments and the inhibited formation of NaOH might cause insufficient dehydrofluorination reactions and the occurrence of the side reactions of eqs 7 and 8 . Since carbon rings could not be generated by these reactions, a lower I D / I G value and the remaining CF 2 on the carbon chains could be found.…”