The properties of low-pH cement used in the geological disposal of radioactive waste may change through atmospheric carbonation and degradation caused by groundwater during the long-term operation of a repository. In this study, we investigated the effects of atmospheric carbonation and groundwater contact on the chemical, microstructural, and transport properties of shotcrete made from low-pH, high-fly-ash silica-fume cement (HFSC) over a period of 16 years in an underground research laboratory. In both carbonated and degraded zones of the HFSC shotcrete, capillary porosity increased for pores of < 300 nm in diameter, and the total porosity was higher than in undegraded zones. These changes in porosity may be associated with the decalcification of calcium-silicate-hydrate and decomposition of ettringite. Such changes were minor in altered zones of OPC shotcrete, indicating that HFSC shotcrete is less resistant to atmospheric carbonation and groundwater leaching under the studied conditions. However, the hydraulic conductivity in HFSC was low enough to fulfill the specific functional requirements of low-pH cements for geological disposal.

The decommissioning of the Fukushima Daiichi (1F) nuclear power plant remains a significant environmental concern. A crucial aspect of this process involves the effective immobilization of 137Cs to reduce the volume of radioactive waste. This technique traps radioactive materials that adhere to the concrete surface by embedding in the glass, allowing only the glass to be removed during decommissioning. In this study, we first irradiated concrete mixed with 133Cs, which had the same composition as the nuclear reactor building at 1F, using a high-brightness laser beam to immobilize Cs. We then investigated the characteristics of in-situ immobilization of Cs from the aspects of distribution, migration, and elution. X-ray diffraction (XRD) results indicate that the concrete underwent vitrification. Measurements from an electron probe microanalyzer (EPMA) show that Cs exhibits aggregate-dependent heterogeneity within the fused, glass-like concrete. The experimental migration rate of 99 % is more reliable compared to the 57 % achieved through conventional thermal plasma melting of simulated low-level radioactive waste. As far as elution is concerned, the normalized mass loss of the elements is 0.06 to 0.08 g/m2, which is below the 2 g/m2 limit set by the American Society for Testing and Materials (ASTM) International. Consequently, laser-assisted in-situ immobilization of Cs has superior potential for supporting the decommissioning of 1F by effectively utilizing the hazardous materials on site.

Interactions between cement and host rock in geological repositories for radioactive waste will result in the formation of a chemically disturbed zone which may affect repository safety. The chemical evolution at the interface between cement (Ordinary Portland Cement: OPC and Low Alkaline Cement: LAC) and mudstone after 11 years of in situ reactions at the Horonobe Underground Research Laboratory is described. Various analytical techniques were used to identify the key reactions at the cement-rock interface, which included cement dissolution, precipitation of secondary minerals such as calcite and C-(A-)S-H phases, cation exchange in clay minerals, and reduction of rock porosity. The results show that the mudstone contacted by both OPC and LAC was altered to depths of a few millimetres, whereas cement alteration was observed over a wider area in LAC.Calcite formation occurred at both interfaces due to the ingress of carbonate ions in the groundwater. A relatively denser calcite layer formed at the OPC interface, suggesting a more favourable environment for calcite precipitation, as suggested by thermodynamic calculations. Furthermore, C-(A-)S-H phases were prevalent in the mudstone, suggesting complex interactions depending on porewater composition, pH, and mineral stability. The study also highlights the effects of cement-mudstone interactions on radionuclide migration, such as reduction of diffusivity due to reduced porosity and enhancement of sorption or incorporation into secondary minerals in the altered mudstone. Overall, the research provides valuable insights into long-term cement-mudstone interactions and their effects on radionuclide behaviour.

For elucidating the validity of accelerated Ca leaching tests, the chemical and physical changes of a hardened ordinary Portland cement induced by the leaching into deionized water were compared with those induced by the accelerate leachings with proton-type cation exchange resin for the continuous elimination of Ca2+, with an electrostatic electric field for the electrophoretic elimination of Ca2+, and with NH4NO3 solutions for the elimination of OH-. The use of the resin and the electrostatic field were found to be inappropriate because both the methods caused the formation of strong acid which severely attacked and etched the paste. The use of a NH4NO3 solution was an appropriate method for the accelerated test, since Ca2+ was homogeneously leached in the paste and the amount of the leaching was controllable by properly selecting the concentration and the volume of the NH4NO3 solution.

Calcium leaching from cementitious materials into bentonite is a key process for the long-term alteration of cement–clay interfaces of engineered barrier systems. Strong chemical gradients between cement and clay drive the precipitation of minerals such as calcium silicate hydrate (C–S–H) and calcite. To analyze the mineralogical and porosity evolution at the cement–clay interface, composite specimens consisting of cement paste and bentonite mixed with various amounts of sodium carbonate were subjected to immersion and chloride migrations tests and were investigated by electron probe micro-analysis (EPMA), thermogravimetry/differential thermal analysis (TG-DTA), and X-ray diffraction (XRD) after 4–20 months of immersion. The results show that adding sodium carbonate to the bentonite enhanced the formation of calcite in the form of a surface layer on the cement paste. This suggests pore clogging at the interface and implies the existence of a threshold amount of carbonate addition above which pore clogging occurs. This is the first of two papers; the accelerated evolution of the samples in the presence of an electrical field is discussed in the second paper.

The expansive behavior of cement pastes containing different admixtures due to delayed ettringite formation was investigated. Specimens consisted of high-early-strength Portland cement, substituted with fly ash, silica fume, metakaolin, aluminum hydroxide (Al2O3) or ground granulated blast furnace slag. Expansion was suppressed by additives in most cases, although low amounts instead functioned to accelerate its onset, and effectively so when they contained aluminum present as Al2O3. Al-27 and Si-29 solid-state NMR analysis revealed that heat curing transforms aluminum in additives to its third aluminum hydrate (TAH) phase, likely serving as a source for the monosulfate and ettringite phases despite its consumption by calcium aluminosilicate hydrate (C-A-S-H). Cement additives with high aluminum content appear to reduce the SO3/Al2O3 ratio to suppress the conversion of monosulfate into ettringite.

Cement-based materials used at radioactive waste disposal sites are required to possess long-term stability. However, when these materials come in contact with groundwater, calcium leaching from the solid occurs, and the material becomes porous. The use of mineral admixtures is recommended to minimize porosity. However, few studies have focused on the diffusion performance of cement-based materials blended with mineral admixtures after leaching. Therefore, in this study, the diffusion performance of such materials using blended cement after leaching was evaluated. It was found that the diffusion coefficient of the blended cement increased with leaching, and when leaching progressed considerably, the diffusion coefficient of the blended cement was close to that of ordinary Portland cement. Furthermore, the diffusion coefficient after leaching demonstrated good correlation with the pore volume when the pore diameter was 50 nm or larger.

Chemical conditions and mass transport properties of engineered barrier systems in TRU waste facilities would change with time due to the interaction of cement/bentonite materials. (‘TRU waste’ is one of categories of the radioactive wastes and contains a significant amount of alpha-emitting transuranic nuclides. In some countries, these wastes are classified into the Intermediate Level Waste (ILW).) Previous numerical model analyses to assess the long-term performance of engineered barrier systems in TRU waste repositories predicted to form Calcium Silicate Hydrate (C-S-H) species at the interface between the cementitious and bentonite materials. If C-S-H precipitates in the bentonite side of the boundary, mass transport in the bentonite buffer decreases and mineralogical alterations are expected to be restricted for a long period. The evidence of C-S-H precipitation in the bentonite side, however, still has not been identified in the former experimental studies. To improve the reliability of numerical analyses, immersion experiments were performed using contact samples of cementitious and bentonite materials, and X-ray absorption fine structure (XAFS) analysis was carried out to detect C-S-H precipitation at the contacting interface. Precipitation of C-S-H was confirmed from the obtained XAFS spectra. This result is one of the evidences to show the validity of the current numerical model analyses, which suggests that the bentonite buffer performance as an engineered barrier would be kept over a long period.

The Cement–Opalinus Clay Interaction (CI) Experiment at the Mont Terri rock laboratory is a long-term passive diffusion–reaction experiment between contrasting materials of relevance to engineered barrier systems/near-field for deep disposal of radioactive waste in claystone (Opalinus Clay). Reaction zones at interfaces of Opalinus Clay with two different types of concrete (OPC and “low-pH”/ESDRED) were examined by sampling after 2.2 and 4.9 years. Analytical methods included element mapping (SEM, EPMA), select spot analysis (EDAX), 14 C-MMA impregnation for radiography, and powder methods (IR, XRD, clay-exchanger characterisation) on carefully extracted miniature samples (mm). The presence of aggregate grains in concrete made the application of all methods difficult. Common features are a very limited extent of reaction within claystone, and a distinct and regularly zoned reaction zone within the cement matrix that is more extensive in the low-alkali cement (ESDRED). Both interfaces feature a de-calcification zone and overprinted a carbonate alteration zone thought to be mainly responsible for the observed porosity reduction. While OPC shows a distinct sulphate enrichment zone (indicative of ingress from Opalinus Clay), ESDRED displays a wide Mg-enriched zone, also with claystone pore-water as a source. A conclusion is that substitution of OPC by low-alkali cementitious products is not advantageous or necessary solely for the purpose of minimizing the extent of reaction between claystone and cementitious materials. Implications for reactive transport modelling are discussed.

タウマサイト生成による劣化は外部から浸透する硫酸塩劣化の一つであり、コンクリートを脆弱化させることが知られている。しかしながら、タウマサイト生成には時間を要し、また、その生成条件には不明な点が多い。本研究ではタウマサイト生成におけるNaの影響およびタウマサイト生成における結晶核の影響に着目し、タウマサイトの組成になるように調整した懸濁反応によって、タウマサイト生成速度の検討を行った。懸濁反応液に天然タウマサイトを少量添加することで生成は著しく促進し、Naの存在はタウマサイト生成を抑制した。結晶核として合成エトリンガイトの存在はタウマサイト生成を促進せず、反応系内で生成したエトリンガイトはタウマサイト生成を促進し、その理由を推察した。

コンクリートの劣化現象として知られるタウマサイト劣化(TSA)は，低温硫酸塩環境下で組織が脆弱化する硫酸塩劣化の一つであり，劣化原因となるタウマサイトは硫酸イオンの浸透でエトリンガイトとともに生成する。タウマサイトはエトリンガイトグループに属する鉱物であるため，粉末X線回折(XRD)では両者は近い回折線を示し，分離することは困難である。本研究では，タウマサイトとエトリンガイトの結晶水に着目し，試料を加熱処理および減圧処理により脱水させ，両者をXRDによって分別する方法について検討した。タウマサイトは減圧処理においても安定であったことからXRDによってタウマサイトが確認でき，また，セメントから実験的に生成させたタウマサイトおよび低温硫酸塩環境に暴露したコンクリート試験体を用いて本手法の適用性を検討した。

The porosity of hardened cement paste changes when it is exposed to the actual environments, and it may be expected that the chemical structure of the C-S-H undergoes changes. The C-S-H is the main component, comprising 60% or more by volume of hardened cement paste, and this study focuses on changes in the chemical structure of C-S-H. Deterioration induced by ammonium nitrate solution was accompanied by changes in the pore structure as well as structural changes in the C-S-H in the hardened cement paste. It was ascertained that when evaluating the decreases in the elastic modulus of the hardened cement paste, both the pore structure and the structure of the C-S-H must be considered.

In this report, X-ray photoelectron spectroscopy (XPS) and 29Si-MAS-NMR was used for the evaluation of deteriorated hardened cement pastes. The deterioration by ammonium nitrate solution was accompanied by changes in the pore structure as well as by structural changes in the C–S–H in the hardened cement paste. The CaO/SiO2 ratio of the C–S–H decreased with the progress of deterioration, there was also polymerization of the silicate in the C–S–H. It was confirmed that the degree of polymerization of silicate of the C–S–H in hardened cement paste can be determined by XPS. It was also shown that the polymerization depends on the structure of the C–S–H.

The interaction between compacted bentonite and hardened cement was evaluated. Cement-bentonite-cement-layered test specimens, after infiltrated with water, were subjected to various analyses. The EPMA analysis of the cross sections of the specimens indicated a homogeneous distribution of Ca leached from the cement paste in the bentonite. The quantity of Ca in the bentonite was apparently larger than the cation exchange capacity of the bentonite, suggesting the formation of secondary minerals involving Ca.