Metakaolin-based geopolymers: Efflorescence and its effect on microstructure and mechanical properties
Introduction
Efflorescence formation is a visible phenomenon observed mostly on the surface of cementitious and ceramic materials, usually causing mainly aesthetic damage or superficial deterioration. In geopolymers, efflorescence formation is still not fully understood due to the different mechanisms of reaction and product formation. Geopolymer formation involves the reaction (often called “activation”) of reactive aluminosilicate materials with highly alkaline solutions (or ‘activators’) [1,2]. Thus, geopolymers contain high amounts of alkali metals. Alkalis are primarily present in the disordered reaction product, an alkali aluminosilicate hydrate gel denoted M-A-S-(H) gel, with M representing alkali metals, most commonly Na+ or K+ [3]. Alkali cations in the M-A-S-(H) gel neutralize the excess negative charge resulting from Al in tetrahedral coordination in the alkali aluminosilicate gel framework [3], forming Na–O–Al(Si) linkages. Thus, at stoichiometric equilibrium the M-A-S-(H) gel will exhibit an Na/Al ratio of 1.0 [4]. However, sodium can also be bound weakly to the gel as Na(H2O)n+ in the pore solution [5,6]. Some of these forms are weakly bounded under certain conditions, resulting in free/leachable alkalis as measured by leaching, as reported in previous studies [7,8]; leachable alkali values between 1 and 25% of the total alkali content in the geopolymers were reported. In a previous analysis of leaching potential of metakaolin-based geopolymers using an ionic equilibrium method, around 55% of alkalis were observed to be weakly bounded and 45% were stable in the framework structure [9]. These high values of potentially leachable alkalis have raised concerns regarding extensive efflorescence and consequent damage in geopolymer cements.
Efflorescence formation occurs from free alkali mobility in geopolymers. Capillary pressure induces water transport and alkali movement via both diffusive and convective processes, the latter of which is accelerated when the material is exposed to wetting/drying cycles. The pore size distribution also plays an important role in this movement, as larger pores are more likely to be connected by microcracks and contribute to the faster alkali leaching [8]. Alkali leaching can also be damaging to the M-A-S-(H) gel structure due to the nanostructural transformation associated with removal of alkalis and consequent changes of the chemical environment of AlIV species [9]. After leaching, the alkali metals present in solution react with HCO3− or CO32− (resulting from dissolved atmospheric CO2) to form alkali carbonate phases. This process is commonly referred to as carbonation, and is mainly controlled by the dissolution and diffusivity of CO2. Both of these factors are a function of the concentration (or partial pressure) of CO2 in the atmosphere at the air/pore fluid interface, and the diffusivity is also related to the interconnectivity of pore structure (which is a function of porosity [10]) and exposure conditions [11]. A partially saturated moisture condition accelerates the carbonation reaction process, where relative humidity (RH) values of 65 ± 5% were observed as the pessimum in GBFS/MK-based geopolymers [12]. Depending on the porosity, alkali concentration of geopolymers, and diffusivity of CO2, the deposition of alkali carbonates can be internal (subflorescence) or external (efflorescence) [13]. Subflorescence can generate an internal pressure resulting from crystallization of the alkali carbonate phases, and this can affect the structural integrity of matrix [13].
The products formed in efflorescence are predominantly carbonates associated with the alkali used in the activator. The formation of a hydrated sodium carbonate (Na2CO3·7H2O) [8,14], sodium bicarbonate (NaHCO3) [7] and natrite (Na2CO3) [15] have all been previously observed. Visible formation of alkali carbonate crystals is also related to RH in the air. Low values of RH reduce dissolution and diffusion of atmospheric CO2, whereas high values of RH can dissolve the carbonate crystals formed. This crystallization occurs at a specific RH equilibrium, which is dependent on the type of carbonate crystal formed [16,17]. Thus, efflorescence formation is a phenomenon associated with different processes and their effects are dependent on the geopolymer properties and microstructure, exposure conditions, and magnitude/type of carbonate crystallization.
In previous studies, the effect of efflorescence formation was evaluated for some specific geopolymeric materials and conditions, with efflorescence formation observed to reduce the compressive strength of the binder [13,18]. In fly ash-slag based alkali-activated materials, alkali leaching processes have been observed to not lead to a reduction of compressive strength, but do hinder ongoing strength and microstructural development over time [18]. In the same study, shrinkage was more evident in samples with efflorescence formation, than in those subjected to alkali leaching without efflorescence formation. Other work, using metakaolin as precursor, attributed microstructural changes to excessive alkali leaching [9]. Using three different fly ashes, Zhang et al. [13] evaluated the compressive strengths of geopolymers in contact with air, partially immersed in water, and fully immersed. Their results showed an increase of compressive strength for samples in air and a reduction for samples partially or fully immersed in water. The negative influence of efflorescence formation was attributed to multiple factors including loss of alkalis from the M-A-S-(H) gel and subflorescence formation. However, compressive strength evaluation is not the best option to assess the impact of efflorescence because the crystallization of carbonates causes a internal expansion. Instead, tensile and flexural strength should be more suitable from the perspective of mechanical impact.
This study aims to evaluate the effect of efflorescence formation, air carbonation and alkali leaching on the mechanical and microstructural properties of metakaolin-based geopolymers. This is assessed under conditions relevant to the most common industrial settings for geopolymer cement use. The findings discussed are crucial to fully understand efflorescence in geopolymer cements.
Section snippets
Materials and sample preparation
The metakaolin (MK) used as precursor to make geopolymers had a mean particle size of 4.56 μm, specific surface area of 13.49 m2/g and consisted of 54.82% wt.% SiO2, 42.57 wt% Al2O3 and 0.11 wt% loss on ignition at 1000 °C. The complete characterization and more detailed description were previously reported [9].
Alkali activators used were analytical grade NaOH (∼99%) dissolved in water, and a sodium silicate solution with 29.4 wt% SiO2, 14.7 wt% Na2O and 52.7 wt% H2O, supplied by PQ Australia.
Visual efflorescence
Fig. 2 shows the visual aspect of geopolymers under different exposure conditions after 28 days of curing and then 28 days of exposure. The samples under reference conditions (RE) did not show any efflorescence formation for any of the geopolymers assessed.
When the samples were in contact with water at one end (EF), most of the systems exhibited efflorescence formation on the surface, which corresponds to carbonate-type products as was identified previously [19]. The images in Fig. 2 show that
Microstructure and nanostructure
Many of the observations of strength and dimensional changes due to the effect of efflorescence formation, air carbonation and leaching in geopolymers, were identified above as having an important nanostructural and microstructural basis. Therefore, these aspects of the samples were analyzed to elucidate the process of deterioration and identify the origins of this behavior.
Conclusions
This study evaluated separately the effects of air carbonation, efflorescence formation, and leaching in metakaolin based geopolymers on mechanicals strength and micro/nanostructure.
The condition of exposure to air carbonation induces the process of carbonation associated with the efflorescence formation. This phenomenon occurs in the first layers of the material and affects the mechanical performance of geopolymers. The main property affected is compressive strength. Geopolymers containing
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
M.A. Longhi is grateful for the financial support of CAPES and of SWE 203750/2017–9. The participation of A. P. Kirchheim Brazilian authors was sponsored by CNPq (Brazilian National Council for Scientific and Technological Development) through the research fellowships PQ2017 305530/2017–8. The participation of E. D. Rodríguez was supported by CNPq research fellowship PQ 309885/2020–5. The participation of B. Walkley was supported by the Engineering and Physical Sciences Research Council (UK)
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