Evaluation of Sulfuric Acid-Induced Degradation of Potassium Silicate Activated Metakaolin Geopolymers by Semi-Quantitative SEM-EDX Analysis

Abstract

Geopolymers are synthesized by mixing powdery solids, rich in amorphous silicon and aluminum species, with an alkaline solution, which leads to the formation of an inorganic alumosilicate network. Their acid resistance is affected by the composition, the porosity, and pore size distribution of the hardened binder as well as the type and concentration of the acidic solution. In the present study, two geopolymer mixtures with varying liquid-to-solid ratios and Si/Al ratios were exposed to a sulfuric acid solution (pH = 1) and analyzed after different durations of exposure (7, 14, 28, 56, and 70 days) by using a light microscope and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX). SEM-EDX elemental mapping was used to evaluate the degradation from depth profiles of silicon (Si), aluminum (Al), and potassium (K) leaching. The results clearly show the leaching kinetics of potassium and the dealumination of the network. The separate consideration of specific reaction steps in the course of degradation, namely the depth of erosion (DE), the depth of deterioration (DD), and the depth of reaction for certain elements (DR(e)), indicate a combination of chemical and diffusion controlled degradation mechanisms.

Simple Model for Alkali Leaching from Geopolymers: Effects of Raw Materials and Acetic Acid Concentration on Apparent Diffusion Coefficient

“…In future work, more advanced (numerical) modeling approaches are to be employed on the leaching results, in order to better separate the physical nature of the effective diffusion coefficient from the chemical binding isotherms, such as in [ 17 ]. Research so far has covered only empirical geopolymer tests on acid resistance [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 ], thus neglecting the fundamental chemical aspects behind it. Analogue mechanisms involved in dissolution of zeolites [ 35 ] demonstrate that decomposition of a network aluminosilicate in acid gradually shifts from initially congruent Si and Al dissolution to progressively preferential dissolution of Al, resulting in amorphous silica-rich gels, with gradually increasing Si/Al ratios.…”

“…Prepared initial solutions had pH values of 2.8, 3.4, 3.9 and neutral to mimic various (acidic) exposure conditions in agricultural (pH~3 [4]) and underground constructions (e.g., see durability exposure classification in fib 2020, CIA Z7/02 2018 and standard EN 206-AS/NZS 4058). Specimens were immersed in 100 mL of 0, 1, 10 and 100 mM acetic acid maintained at 21 • C. After 1-56 days of exposure, in 10 time intervals (1,3,7,14,21,28,35,42, 49, 56 days), the acetic acid solution was replaced with a new one (meaning free of leaching species). During renewals of the leaching solution, the specimen was exposed to air for as short a time as possible and put into a new leaching solution.…”

“…Both leaching of alkalis and the solubility of the gel framework play a crucial role in the leaching [16][17][18][19][20], acid attack [5,7,10,15] and waste stabilization [20,21] performance of geopolymers. Moreover, they may reduce the alkalinity of the pore solution [7,11,14,17], thus inducing possible corrosion of reinforcing steel in concrete structures [1,19]. Concrete infrastructures can be exposed to organic acids in many industrial environments such as agricultural (animal feed, animal waste, anaerobic digestion and crop storage), wine, sugar, dairy and waste management.…”

“…Their composition and reactivity have a major influence on the properties of the final product. One of the most decisive parameters is the (reactive) Si/Al ratio of the raw material composition which affects, among other things, the geopolymerization rate [25], the strength [26], the alkali leaching [20] and the acid resistance [4,9,11,14] of a geopolymer. Koening et al [4] studied the effect of slag (i.e., calcium) dosage to fly ash-based geopolymers on degradation induced by organic acids at pH 3, as a representative exposure condition in agricultural constructions.…”

“…Geopolymers are upcoming inorganic binders with great potential for niche applications in concrete structures [ 1 , 2 , 3 ], particularly where strong resistance against organic acids [ 4 , 5 , 6 , 7 ] as well as mineral acids [ 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 ] is required. A low calcium content in geopolymers represents a vital feature [ 4 ] and separates them from a broader class of alkali-activated binders.…”