Structural evolution of binder gel in alkali-activated cements exposed to electrically accelerated leaching conditions☆
Graphical abstract
Introduction
Portland cement (PC) plays a vital role in a radioactive waste repository (everything except a natural barrier should be an engineered barrier, including waste grout for low/intermediate-level radioactive waste (Ojovan, 2011; Jang et al., 2016) and cementitious backfill (Felipe-Sotelo et al., 2014, 2016; Corkhill et al., 2013)), owing to its high-pH buffering capacity, low diffusivity, and good mechanical strength (Saito et al., 1992). Corrosion of the steel reinforcement is the dominant factor when assessing the integrity of cement under ordinary service conditions, and little emphasis is placed on Ca leaching, as it is a slow process occurring over long time (Nakarai et al., 2006). Instead, Ca leaching can be a serious concern with regard to the integrity of a radioactive waste repository which is often located underground, where the groundwater contains corrosive ions at a minimum, as it coarsens the microstructure of cement, allowing easier mass transport (Nakarai et al., 2006).
Because the occurrence of Ca leaching in cement takes ∼100 years to be noticeably observed, some studies in this area employed an electrical acceleration technique in which Ca leaching is accelerated by applying a potential gradient to an electrolyte (Saito et al., 1992; Saito and Deguchi, 2000; Saito and Nakane, 1999; Hashimoto et al., 2013). This speeds up the movement of ions in the pore solution as the cathode and anode attract cations (e.g., Na+, K+, Ca2+) and anions (e.g., OH−), respectively, resulting in the accelerated dissolution of Ca from the binder matrix (Saito et al., 1992).
The dissolution kinetics of cement hydrates under an accelerated leaching condition differs according to the hydrates. The dissolution of alkalis takes place initially, then Ca(OH)2 followed by C-S-H (Saito and Nakane, 1999; Yokozeki et al., 2004; Jacques et al., 2010). This process is also influenced by a number of factors. Hashimoto et al. (Hashimoto et al., 2013) investigated the effect of the water-to-cement (W/C) ratio on the leaching characteristic of PC, concluding that a higher W/C ratio increases the initial pore volume through which the dissolution of Ca occurs abundantly. In particular, the residual content of Ca(OH)2 decreases notably after electrically accelerated leaching as the W/C ratio of the sample is increased (Hashimoto et al., 2013), implying that samples with lower W/C ratios have a slower net reduction of Ca(OH)2 given the supply of additional Ca(OH)2 through further hydration, therefore delaying the leaching process. This occurs because unreacted clinkers in the binder matrix of low-W/C cement, which is initially surrounded by hydrates, are exposed to water once the surrounding hydrates dissolve and undergo hydration, analogously facilitating self-curing and delaying the leaching process (Nakarai et al., 2006). Similarly, the incorporation of fly ash in concrete is reported to mitigate degradation due to calcium leaching, as fly ash particles surround the hydrates and slow the dissolution process (Nakarai et al., 2006).
The performance of cement for such applications can be enhanced by using PC blended with supplementary cementitious materials (e.g., blast furnace slag-blended PC (Sanderson et al., 2017)) or by employing new alternative binders (e.g., magnesium-based cement (Walling et al., 2015)), phosphate cement (Gardner et al., 2015), calcium aluminate cement (Chavda et al., 2015) and alkali-activated cements (Jang et al., 2016; Khalil and Merz, 1994; Mobasher et al., 2016)). These efforts were made to meet the wide variability in the radioactive waste compositions and surrounding environments and to ensure the stability and integrity of the disposal system. For instance, the performance of PC for the immobilization of key radionuclides such as cesium and strontium, is known to be poor, while other types of cements (e.g., alkali-activated cements) exhibit enhanced performance due to the negatively charged surface of hydrates, which favors the adsorption of problematic cations, and due to the complex pore network, making the transport of ions through a porous medium difficult (Jang et al., 2016, 2017). Moreover, Ca leaching from cement and concrete into its surrounding area in the repository can cause the degradation of the host rock and clay (Lothenbach et al., 2017).
Alkali-activated cements have been a topic of numerous studies owing to their comparable performance to PC for a number of applications, including in the cement types used in repositories. This binder system can be broadly categorized into two types with a clear distinction in the composition according to the Ca content of the precursor (Bernal and Provis, 2014; Provis and Bernal, 2014; Park et al., 2018a). Specifically, the C-A-S-H type gel forms in a high-Ca system along with hydrotalcite belonging to an Mg-Al layered double hydroxide group mineral when sufficient Mg is available (Provis and Bernal, 2014; Yoon et al., 2018). On the other hand, the N-A-S-H type gel forms in a low-Ca system and has a structural analogue of zeolite minerals (Provis and Bernal, 2014). The overall performance of the material is indeed largely influenced by the composition of the binder gel, but in either case the overall surface charge tends to be negative, making this type of binder suitable for the adsorption of some important fission product radionuclides such as cesium and strontium. Despite the increased demand and adoption of alkali-activated cements in practice, i.e., grout for solidifying radioactive spent resins (Lichvar et al., 2010), and cementitious backfill materials (Jang et al., 2018), the long-term durability performance of this binder system for use in a repository remains unexplored. The present study investigates the structural evolution of the binder gel in alkali-activated cements upon leaching. An electrically accelerated leaching test was conducted on alkali-activated fly ash, slag and a blend of fly ash and slag. The samples were characterized before and after the electrically accelerated leaching tests to assess the effect of leaching on the microstructures of alkali-activated binders, and ultimately on their likely durability performance in repository conditions, where the leaching of structural components from the binder gel upon contact with groundwater over an extended period of time is the main mode of degradation.
Section snippets
Materials and sample preparation
The binder materials used in this study are fly ash, blast furnace slag and Type I PC. The chemical compositions of the binder materials were obtained by X-ray fluorescence analysis and are summarized in Table 1. The alkali-activator was produced by a mixture of sodium hydroxide (pellets, 99%) and a sodium silicate solution (Korean Industrial Standard Grade-3; SiO2 = 29 wt%, Na2O = 10 wt%, H2O = 61 wt%, specific gravity = 1.38) to meet the activator composition described in Table 2. PC, which
Mercury intrusion porosimetry
The MIP results of alkali-activated cements exposed to accelerated leaching are shown in Fig. 2. It is often reported that the pore structure of materials is coarsened by leaching due to the loss of ions. On the other hand, the porosity and/or pore diameter of cementitious materials may not always increase after leaching due to the hydraulic activity and the hydration of the raw materials, which effectively increase the solid volume after contact with water (Nakarai et al., 2006). The highest
Accelerated leaching-induced structural evolution
The present study investigated the structural degradation of alkali-activated cement induced by accelerated leaching. While the PC sample showed no significant structural alteration at an applied current density of 5 A/m2 for a duration of two weeks, noticeable changes were observed in the alkali-activated cement samples. In particular, two binder gels with distinctive chemical compositions (i.e., C-A-S-H and N-A-S-H) exhibited different degradation pathways. Firstly, the amount of N-A-S-H
Concluding remarks
Structural changes in the binder gel of alkali-activated cements as induced by accelerated leaching are investigated. Electrically accelerated leaching tests were conducted on alkali-activated cement samples incorporating fly ash and/or slag by applying a potential gradient. The characterization of the samples after leaching provides a detailed description of the structural evolution of their binder gels over the course of their service lifetimes under repository conditions. The findings of the
Author contributions section
S.P., H.N.Y. and J.S. carried out the experiment. S.P. wrote the manuscript with supervision from H.K.L. and J.G.J., and with input from all other authors. S.P. and J.G.J. conceived the original idea. J.G.J. supervised the project.
Declaration of Competing Interest
There are no conflicts to declare.
Acknowledgments
This study was supported by the National Research Foundation (NRF) of the Korean government (MSIT) with a grant [2018R1A2A1A05076894] and [2018R1D1A1B07047233].
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Throughout the paper C-S-H, C-A-S-H, and N-A-S-H represent a structurally disordered non-stoichiometric calcium-silicate-hydrate, calcium-alumino-silicate-hydrate, and sodium-alumino-silicate-hydrate, respectively. These are the two major reaction products of alkali-activated cements.