Geopolymer leaching in water and acetic acid

Abstract

Elemental leaching of metakaolin based geopolymers was investigated by immersing hardened paste specimens in a solution. For this, pure water and 0.1 molar acetic acid solutions were replenished ten times distributed over 56 days in total. Dissolution and diffusion of the elements through and from the geopolymer paste into the surrounding solutions was investigated on cross-sections of specimens by SEM-EDS microscopy, indentation, X-ray powder diffraction analysis and measuring the eluted elements by ICP-MS when replenishing the solution over time. The presented new methodological approach thus combines the dissolution rate kinetics obtained via wet chemistry (ICP) with the complementary solid state characterisation methods to gain new insights into the complex geopolymer dissolution mechanisms.
 Results indicated a relatively small leachability of geopolymers, limited only to the surface layer which is directly exposed to the aggressive solution, while the more inner parts of the geopolymer framework remain intact. Elemental maps revealed dissolution of aluminates that occurred across the outermost surface parts of the sections, while potassium leached out gradually but reached deeper inner parts. However, there was still a high portion of potassium being left bonded inside the geopolymer, even for the harsh acidic conditions, limited by the diffusion-reaction mechanism which took place within the geopolymer. The obtained experimental results represent a first approach towards feeding reactive transport numerical modeling approaches still to be developed for simulating leaching and degradation of geopolymer materials when exposed to water or acidic solutions.

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

“…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.…”

“…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.…”

“…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.…”

“…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.…”

“…The (V l /S) ratio of the acid solution volume (V l ) to specimen surface area (S) is not specified as they should include accelerated exposure conditions that are realistic for various practical field applications. Therefore, different adaptations of the testing procedures can be found in the literature, e.g., the organic acid solution can be titrated to maintain its pH [4,23] or cyclically replenished [5][6][7], where the time intervals may also vary. The total number of specimens (and even types) per container is also variable, although a single specimen would be preferential to avoid any interactions among them (and especially the different mix types).…”