Importance of Cation Species during Sulfate Resistance Tests for Alkali-Activated FA/GGBFS Blended Mortars

Abstract

In this study, the changes in mass, compressive strength, and length of blended mortars were analyzed to investigate their sulfate resistance according to the ground granulated blast furnace slag (GGBFS) blending ratio and type of sulfate solution applied. All alkali-activated mortars showed an excellent sulfate resistance when immersed in a sodium sulfate (Na2SO4) solution. However, when immersed in a magnesium sulfate (MgSO4) solution, different sulfate resistance results were obtained depending on the presence of GGBFS. The alkali-activated GGBFS blended mortars showed a tendency to increase in mass and length and decrease in compressive strength when immersed in a magnesium sulfate solution, whereas the alkali-activated FA mortars did not show any significant difference depending on the types of sulfate solution applied. The deterioration of alkali-activated GGBFS blended mortars in the immersion of a magnesium sulfate solution was confirmed through the decomposition of C–S–H, which is the reaction product from magnesium ions, and the formation of gypsum (CaSO4·2H2O) and brucite (Mg(OH)2).

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“…This can be attributed to the low solubility of the resulting magnesium hydroxide, which decreases the pH of the saturated solution, causing the decomposition of CÀ SÀ H gels. [55] Bhutta et al [56] assessed the sulfate resistance of geopolymers prepared from pulverized fuel ash and palm oil fuel ash (POFA) blends. They exposed the prepared samples to 5 % Na 2 SO 4 for a maximum of 18 months to arrive at the results.…”

“…The OPC specimens exhibited a continuous decrease in strength with exposure age, with an ultimate reduction of 63 %. Cho et al [55] studied the effects of different cations (Na + and Mg 2 + ) on fly ash and fly ash/slagblended mortars. The mass and compressive strength of all the samples increased when the samples were exposed to 10 % Na 2 SO 4 for one year.…”

“…This is primarily attributed to the absence of portlandite, CÀ SÀ H gel, and ettringite in the geopolymer matrix. [48,49,55] The addition of CBNs to the geopolymer matrix has been reported to improve the durability of the materials. It has been noted in section 4.3.2 that the incorporation of MWCNTs increased the resistance of the samples to drying shrinkage and helped reduce the extent of water absorption.…”

“…The presence of Mg 2+ significantly lowers the resistance provided. This can be attributed to the low solubility of the resulting magnesium hydroxide, which decreases the pH of the saturated solution, causing the decomposition of C−S−H gels [55] . Bhutta et al [56] .…”

Special Issue: “Microstructures and Durability of Cement-Based Materials”

“…It is particularly useful to predict the effects of slag/binder, alkali solution/binder, silicate modulus and the NaOH concentration of the activator. Cho et al [ 12 ] investigated the effect of cations on the sulfate resistance of alkali-activated fly ash and slag blended mortar, concluding that this binder tends to experience deleterious damage when magnesium sulfate solution is used. Kim et al [ 13 ] reported that carbonation leads to a substantial decrease in the abrasion resistance of alkali-activated slag cements.…”