Immobilisation of iodide in alkali-activated materials.

At the Fukushima Daiichi Nuclear Power Station (FDNPS), continuous water circulation cools fuel debris, leading to the presence of radionuclides such as Sr-30, Cs-137, and I-129 in the cooling water. These radionuclides are adsorbed and co-precipitated by various materials. Among them, I-129 is a key radionuclide for safety assessment during the final disposal of adsorbent and co-precipitation materials, owing to its long half-life and poor sorption.

Cements and concretes materials characterisation using machine-learning-based reconstruction and 3D quantitative mineralogy via X-ray microscopy.

3D imaging via X-ray microscopy (XRM), a form of tomography, is revolutionising materials characterisation. Nondestructive imaging to classify grains, particles, interfaces and pores at various scales is imperative for our understanding of the composition, structure, and failure of building materials. Various workflows now exist to maximise data collection and to push the boundaries of what has been achieved before, either from singular instruments, software or combinations through multimodal correlative microscopy.

The pore structure and water absorption in Portland/slag blended hardened cement paste determined by synchrotron X-ray microtomography and neutron radiography.

The pore structures of hardened Portland/slag cement pastes (>75 wt% slag content), and the initial capillary absorption of moisture through these pores, were monitored using synchrotron X-ray computerised microtomography and quantitative neutron radiography. The pore structure becomes more constricted as the cement hydrates and its microstructure develops. This mechanism was effective even at a slag content as high as 90 wt% in the cementitious blend, where the lowest total porosity and a significant pore refinement were identified at extended curing ages (360 d).

Spectroscopic identification of Ca-bearing uranyl silicates formed in C-S-H systems.

Portland cement-based grouts used for radioactive waste immobilisation contain a Ca- and Si-rich binder phase, known as calcium-silicate-hydrate (C-S-H). Depending on the blend of cement used, the Ca/Si ratio can vary considerably. A range of C-S-H minerals with Ca/Si ratios from 0.

Synthesis of Ca(OH) and NaCO through anion exchange between CaCO and NaOH: effect of reaction temperature.

The CO released upon calcination of limestone accounts for the largest portion of the emissions from the cement, lime, and slaked lime manufacturing industries. Our previous works highlighted the possibility for a no-combustion decarbonisation of CaCO through reaction with NaOH solutions to produce Ca(OH) at ambient conditions, while sequestrating the process CO in a stable mineral NaCO·HO/NaCO. In this study, the effect of temperature was assessed within the range of 45-80 °C, suggesting that the process is robust and only slightly sensitive to temperature fluctuations.

Characterization of an aged alkali-activated slag roof tile after 30 years of exposure to Northern Scandinavian weather.

Alkali-activated materials (AAMs) have been known as an alternative cementitious binder in construction for more than 120 years. Several buildings utilizing AAMs were realized in Europe in the 1950s-1980s. During the last 30 years, the interest towards AAMs has been reinvigorated due to the potentially lower CO footprint in comparison to Portland cement.

Effect of Impurities on the Decarbonization of Calcium Carbonate Using Aqueous Sodium Hydroxide.

Decarbonizing calcium carbonate (CaCO) is a crucial step for a wide range of major industrial processes and materials, including Portland cement (PC) production. Apart from the carbon footprint linked to fuel combustion, the process CO embodied within CaCO represents the main concern for the sustainability of production. Our recent works demonstrated that it is possible to avoid both the fuel and process CO by reacting CaCO with aqueous NaOH and obtain Ca(OH) and NaCO·HO ( = 0 and 1).

Mechanisms of dispersion of metakaolin particles via adsorption of sodium naphthalene sulfonate formaldehyde polymer.

The influence of different alkali and alkaline earth cations (Na, K, Ca, and Mg), and of solution pH, on surface interactions of metakaolin particles with a sodium naphthalene sulfonate formaldehyde polymer (SNSFP) (a commercial superplasticizer for concretes) was investigated in aqueous systems relevant to alkali-activated and blended Portland cements. This study used zeta potential measurements, adsorption experiments, and both in situ and ex situ Fourier transform infrared spectroscopy measurements of the suspensions to gain a fundamental understanding of colloidal interactions and physicochemical mechanisms governing dispersion in this system. SNSFP was most effective in dispersing metakaolin suspensions in Ca-modified aqueous NaOH systems (CaCl-NaOH) at dosages of  5 wt.

Decarbonisation of calcium carbonate in sodium hydroxide solutions under ambient conditions: effect of residence time and mixing rates.

The decarbonisation of CaCO is essential for the production of lime (Ca(OH) and CaO), which is a commodity required in several large industries and the main precursor for cement production. CaCO is usually decarbonised at high temperatures, generating gaseous CO which will require post-process capture to minimise its release into the environment. We have developed a new process that can decarbonise CaCO under ambient conditions, while sequestering the CO as NaCO·HO or NaCO in the same stage.

Adsorption behaviour of simulant radionuclide cations and anions in metakaolin-based geopolymer.

Geopolymers are a class of alkaline-activated materials that have been considered as promising materials for radioactive waste disposal. Currently, metakaolin-based geopolymers (MK-GPs) are attracting interest for the immobilisation of radionuclides in contaminated water from the Fukushima Daiichi Nuclear Power Station. However, the associated chemical interaction mechanisms and the theoretical prediction of the adsorption behaviour of MK-GP in response to cationic radionuclides have not been thoroughly studied or fully understood.

Reversible Adsorption of Polycarboxylates on Silica Fume in High pH, High Ionic Strength Environments for Control of Concrete Fluidity.

Polycarboxylate-based superplasticizers are essential for production of ultrahigh-performance concrete (UHPC), facilitating particle dispersion through electrostatic repulsion and steric hindrance. This study examines for the first time the effect of changes in pH, ionic strength, and charge on the adsorption/desorption behavior of a polycarboxylate-based superplasticizer on silica fume in aqueous chemistries common in low-CO UHPC. Data from total organic carbon measurements, Fourier transform infrared and nuclear magnetic resonance spectroscopy, and zeta potential measurements reveal the silica surface chemistry and electrokinetic properties in simulated UHPC.

Spectroscopic evaluation of U-cement mineral interactions: ettringite and hydrotalcite.

Portland cement based grouts used for radioactive waste immobilization contain high replacement levels of supplementary cementitious materials, including blast-furnace slag and fly ash. The minerals formed upon hydration of these cements may have capacity for binding actinide elements present in radioactive waste. In this work, the minerals ettringite (CaAl(SO)(OH)·26HO) and hydrotalcite (MgAl(OH)CO·4HO) were selected to investigate the importance of minor cement hydrate phases in sequestering and immobilizing U from radioactive waste streams.

Thermodynamic properties of sodium aluminosilicate hydrate (N-A-S-H).

This study presents for the first time a systematic investigation of the thermodynamic properties of sodium aluminosilicate hydrate (N-A-S-H), through dissolution of pure synthetic N-A-S-H gels. Changes to the chemical composition and gel structure of N-A-S-H were determined characterisation of the solid phase before and after dissolution by multinuclear solid state nuclear magnetic resonance spectroscopy, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, and X-ray diffraction measurements. The correlations between the bulk Si/Al ratio of the N-A-S-H phase and its thermodynamic properties were studied by characterisation of the aqueous phase and calculation of solubility constants.

Towards designing reactive glasses for alkali activation: Understanding the origins of alkaline reactivity of Na-Mg aluminosilicate glasses.

Alkali-activated materials (AAMs), sometimes called geopolymers, are eco-friendly cementitious materials with reduced carbon emissions when compared to ordinary Portland cement. However, the availability of most precursors used for AAM production may decline in the future because of changes in industrial sectors. Thus, new precursors must be developed.

Mimicking Biosintering: The Identification of Highly Condensed Surfaces in Bioinspired Silica Materials.

Interfacial interactions between inorganic surfaces and organic additives are vital to develop new complex nanomaterials. Learning from biosilica materials, composite nanostructures have been developed, which exploit the strength and directionality of specific polyamine additive-silica surface interactions. Previous interpretations of these interactions are almost universally based on interfacial charge matching and/or hydrogen bonding.

Characterization of and Structural Insight into Struvite-K, MgKPO·6HO, an Analogue of Struvite.

Struvite-K (MgKPO·6HO) is a magnesium potassium phosphate mineral with naturally cementitious properties, which is finding increasing usage as an inorganic cement for niche applications including nuclear waste management and rapid road repair. Struvite-K is also of interest in sustainable phosphate recovery from wastewater and, as such, a detailed knowledge of the crystal chemistry and high-temperature behavior is required to support further laboratory investigations and industrial applications. In this study, the local chemical environments of synthetic struvite-K were investigated using high-field solid-state Mg and K MAS NMR techniques, alongside P MAS NMR and thermal analysis.

Incorporation of strontium and calcium in geopolymer gels.

Radioactive waste streams containing Sr, from nuclear power generation and environmental cleanup operations, are often immobilised in cements to limit radionuclide leaching. Due to poor compatibility of certain wastes with Portland cement, alternatives such as alkali aluminosilicate 'geopolymers' are being investigated. Here, we show that the disordered geopolymers ((N,K)-A-S-H gels) formed by alkali-activation of metakaolin can readily accommodate the alkaline earth cations Sr and Ca into their aluminosilicate framework structure.

Alkali aluminosilicate geopolymers as binders to encapsulate strontium-selective titanate ion-exchangers.

Alkali-activated metakaolin geopolymers are attracting interest in the conditioning of nuclear wastes, especially for their ability to immobilise cationic species. However, there is limited understanding of the chemical interactions between the encapsulated spent ion-exchangers, used for decontaminating waste water, and the host aluminosilicate matrix. The lack of such understanding makes it difficult to predict the long-term stability of the waste form.

Exploiting in-situ solid-state NMR spectroscopy to probe the early stages of hydration of calcium aluminate cement.

We report a high-field in-situ solid-state NMR study of the hydration of CaAlO (the most important hydraulic phase in calcium aluminate cement), based on time-resolved measurements of solid-state Al NMR spectra during the early stages of the reaction. A variant of the CLASSIC NMR methodology, involving alternate recording of direct-excitation and MQMAS Al NMR spectra, was used to monitor the Al species present in both the solid and liquid phases as a function of time. Our results provide quantitative information on the changes in the relative amounts of Al sites with tetrahedral coordination (the anhydrous reactant phase) and octahedral coordination (the hydrated product phases) as a function of time, and reveal significantly different kinetic and mechanistic behaviour of the hydration reaction at the different temperatures (20 °C and 60 °C) studied.

Blast furnace slag-Mg(OH) cements activated by sodium carbonate.

The structural evolution of a sodium carbonate activated slag cement blended with varying quantities of Mg(OH) was assessed. The main reaction products of these blended cements were a calcium-sodium aluminosilicate hydrate type gel, an Mg-Al layered double hydroxide with a hydrotalcite type structure, calcite, and a hydrous calcium aluminate phase (tentatively identified as a carbonate-containing AFm structure), in proportions which varied with NaO/slag ratios. Particles of Mg(OH) do not chemically react within these cements.

Atomistic Simulations of Geopolymer Models: The Impact of Disorder on Structure and Mechanics.

Geopolymers are hydrated aluminosilicates with excellent binding properties. Geopolymers appeal to the construction sector as a more sustainable alternative to traditional cements, but their exploitation is limited by a poor understanding of the linkage between chemical composition and macroscopic properties. Molecular simulations can help clarify this linkage, but existing models based on amorphous or crystalline aluminosilicate structures provide only a partial explanation of experimental data on the nanoscale.

Influence of slag composition on the stability of steel in alkali-activated cementitious materials.

Among the minor elements found in metallurgical slags, sulfur and manganese can potentially influence the corrosion process of steel embedded in alkali-activated slag cements, as both are redox-sensitive. Particularly, it is possible that these could significantly influence the corrosion process of the steel. Two types of alkali-activated slag mortars were prepared in this study: 100% blast furnace slag and a modified slag blend (90% blast furnace slag + 10% silicomanganese slag), both activated with sodium silicate.

Chloride binding and mobility in sodium carbonate-activated slag pastes and mortars.

This study evaluates the chloride binding capacity and the migration of chloride in sodium carbonate-activated slag cements and mortars. The effect on chloride mobility and binding of adding a calcined layered double hydroxide (CLDH) to the binder mix was also assessed. Significantly improved durability characteristics can be achieved for sodium carbonate-activated slag mortars by the addition of small fractions of CLDH, as a consequence of a higher degree of reaction, higher chloride binding capacity, and the refined pore structures present in these modified materials, in comparison with alkali-activated cements produced without CLDH.

Reproducible mini-slump test procedure for measuring the yield stress of cementitious pastes.

The mini-slump test is a fast, inexpensive and widely adopted method for evaluating the workability of fresh cementitious pastes. However, this method lacks a standardised procedure for its experimental implementation, which is crucial to guarantee reproducibility and reliability of the test results. This study investigates and proposes a guideline procedure for mini-slump testing, focusing on the influence of key experimental (mixing and testing) parameters on the statistical performance of the results.