Calcium Silicate Hydrate Phases Research Articles

Detailed Analytical EPMA Study of the OPC Pastes Prepared with the Addition of Raw and Milled Fly Ashes

Concrete and cement pastes belong to the most used building materials in human history and represent a solidified mixture of ordinary Portland cement (OPC), water, course and fine-grained aggregates. Fly ash is used as a pozzolanic additive to improve concrete properties. The fly ash property can also be modified by various treatments such as milling or size reduction with sieving. It was suggested that a beneficial pozzolanic reaction with fly ash gives high mechanical properties. Such pozzolanic reaction results in calcium silicate hydrate (C-S-H) phase formation. In this research comparative analytical EPMA study of the paste samples prepared with the addition of the milled and non-milled fly ashes has been performed to elucidate the pozzolanic reaction of the OPC with the milled and non-milled fly ashes. A higher pozzolanic reaction with CSH phase formation was observed in milled fly ash added paste.

Sorption experiments as efficient as possible: Mini-column experiments (MCE) using HPLC-ICP-MS coupling with a new data analysis approach to determine sorption parameters

To quantify retention and sorption parameters, diffusion-, batch- or column experiments are common practice. However, these are either not close to nature, extremely time-consuming or material intensive. A promising approach to eliminate these problems is the performance of mini-column experiments (MCE). Due to dynamic retention and realistic solid–liquid ratios, these are close to nature and resource-efficient. The aim of this work is to perform highly controllable MCE using a HPLC (high-performance liquid chromatography) system and HPLC-ICP-MS (inductively coupled plasma mass spectrometry) coupling. With this approach, ultra-fast sorption experiments are possible only using 100–150 mg adsorbent and 160 mL eluent. That means, sorption experiments are even more time- and cost-efficient and also more controllable than with conventional methods. The presented method is based on a newly developed MCE-HPLC-ICP-MS coupling, which was applied to investigate the sorption of Eu(III) on kaolinite and sand in 10 mM NaCl. With that it was possible to carry out dynamic sorption experiments in just 5–15 h and thus determine e.g. maximum loading capacity (qmax) in a very short amount of time. Furthermore, a quantification method for eluates of high ionic strength is presented. Here, the sorption of Cs(I) on calcium silicate hydrate (C-S-H) phases in 10 mM and 100 mM NaCl was investigated. For validation, the sorption distribution ratios (Rd) obtained were compared with those from batch experiments. These agreed very well with each other and with those from the literature. Overall, with MCE more sorption parameters can be determined much faster, more accurately and more efficient than with the conventional methods.

Uptake of Cm (III) and Eu (III) by C–S–H phases under saline conditions in presence of EDTA: A batch sorption and TRLFS study

Eu (III) and Cm (III) uptake by calcium silicate hydrate phases (C–S–H) was investigated in presence of EDTA in NaCl and CaCl2 solutions. Different experimental parameters, i.e., ionic strength (0.1 m ≤ I'm ≤ 5.05 m), ligand concentration (10−5 m ≤ [EDTA] ≤ 10−2 m), calcium-to-silicon ratio (0.6 ≤ C/S ≤ 1.3) and sorption time (7 d ≤ t ≤ 365 d) were varied in the frame of batch sorption experiments and Time Resolved Laser Fluorescence spectroscopy (TRLFS) measurements. No effect of EDTA on the retention of Eu(III)/Cm(III) by C–S–H phases in NaCl or CaCl2 systems was observed at ligand concentrations ≤ 10−3 M. In NaCl solutions with [EDTA] = 10−2 M and C/S < 1.3, low retention (log Rd = 2–3, with Rd in L∙kg−1) of Eu(III) was detected after 7 d of sorption time, while strong retention (log Rd = 5–6) was observed after 50 d. This behaviour was explained by the initial stabilization of Eu(III)/Cm(III) in the aqueous phase due to the formation of two aqueous Eu(III)/Cm(III)-(OH)n-EDTA complexes, followed by the slow incorporation of Eu(III)/Cm(III) into the C–S–H structure. In CaCl2 solutions for all C/S ratios, as well as in NaCl solutions for C/S ∼ 1.3, the presence of [EDTA] = 10−2 M led to a significant decrease of the uptake (log Rd = 2–3) after 7 and 50 d of contact time. This effect was explained by the formation of stable aqueous Ca–Eu(III)/Cm(III)-EDTA complexes triggered by the presence of moderate to high Ca concentrations. No evident effect caused by increased ionic strength conditions could be confirmed in our sorption experiments.Results obtained in batch sorption experiments are underpinned by TRLFS data, with the observation of three main aqueous species, tentatively defined as Cm(OH)(EDTA)2-, Cm(OH)x (EDTA)-(x+1) and Ca–Cm(III)-EDTA, as well as a fourth species corresponding to Cm(III) incorporated in the CaO-layer of the C–S–H phases. In spite of the thermodynamic stability of the CaEDTA2− complex, which reduces the concentration of free EDTA, the formation of ternary Ca–Eu(III)/Cm(III)-EDTA complexes is expected to result in a significant impact of EDTA on the uptake of An (III)/Ln (III) by cement at high ligand concentrations. The assumption that Ca outcompetes actinides for the complexation with EDTA in cementitious systems may need to be revisited under these conditions.

Phase formation and carbonation impact on the strength of alkali-activated demolition waste with calcium compounds

The present study evaluates the impact of calcium compounds on the reaction kinetics of alkali-activated construction demolition waste. Two materials, concrete and brick demolition waste, were milled to powders and supplemented with either carbonated cement stone or freshly ground cement stone. The mixed solids were further subjected to alkaline activation using a sodium silicate solution. X-ray diffraction, isothermal conduction calorimetry, thermogravimetric analysis, scanning electron microscopy and compressive strength tests were employed to understand the interaction of the reactants. The results indicate that cubes prepared from freshly ground cement stone exhibited higher compressive strength of around 18 MPa than those from carbonated cement stone with a maximum of 7 MPa. This superior strength was approved by subsequent analysis methods. A more significant reaction was observed in the material containing freshly ground cement stone, resulting in increased strength within the system. The presence of portlandite in freshly ground cement stone facilitated the initial formation of calcium silicate hydrate phases.

Passivation of Steel Reinforcement in Low Carbon Concrete

Both the high CO2 emissions associated with cement production and the increasing demand for concrete call for the use of binder types that can be produced in a more climate-friendly way than that of ordinary Portland cement. To ensure that these binders can also be used in reinforced concrete structures, their influence on the corrosion behavior of embedded steel reinforcement must be investigated. In the study presented here, the passivation behavior of steel in mortars made from various new types of binders is investigated. In addition to alkali-activated materials with high and low calcium contents, a calcium sulfoaluminate cement and a binder produced from calcium silicate hydrate (C-S-H) phases, synthesized in an autoclave, were investigated. While the steel clearly passivated in the alkali-activated slag and the C-S-H binder, the calcium sulfoaluminate cement showed the lowest open circuit potentials and polarization resistances, indicating a less effective level of passivation. The metakaolin geopolymer with a potassium-based activator showed an onset of passivation that was dependent on the environment of the specimens at an early age, whereas the alkali-activated fly ash with a sodium-based activator showed a delay in passivation that was not influenced by the environment of the specimens at an early age.

Effect of Iron Ore and Copper Ore Tailings on Engineering Properties and Hydration Products of Sustainable Cement Mortar

Abstract The prohibition of river sand mining has drawn the attention of researchers in finding practicable alternatives. In the approach of finding these alternatives, it is essential to ensure minimal or zero impairment to the ecological balance, which can be mainly attained by making use of industrial waste/byproducts. The wastes from the mining industry are the major contributors in causing impairment to the environment, and their influence on the stability of mortars on using as fine aggregates needs to be systematically investigated with the view of long-term performance concerns. Thus, the present study explores the applicability of mine tailings and finding the optimum dosage in cement mortars by investigating the engineering properties and microstructure development with the aid of qualitative and quantitative analysis associated with hydration products. The studies confirm that the increased consumption of portlandite for secondary hydration reactions followed by the additional formation of calcium silicate hydrate (CSH) and calcium aluminum silicate hydrate (CASH) phases in mine tailing-based mortars helped in achieving a quality microstructure. These additional formations of CSH and CASH phases are also confirmed through Fourier transform infrared spectroscopy by identifying the shift of Si-O-Si stretching vibration bands toward a lower wavenumber. The lowering of calcium/silicate atomic ratio and increased formation of mineralogical compounds related to CSH and CASH in x-ray diffraction patterns also confirms the same. Gismondine, chabazite, and hillebrandite are the additional phases formed and found to take part in refining the pore structure. This enhanced performance of mine tailing mortars was also verified with the aid of a modified Andreasen and Andersen particle packing model. The formation of high-quality microstructure is reflected in the hardened properties of optimized cement mortar in the proportion of 20 % for iron ore tailing and 30 % for copper ore tailing.

Effects of organic degradation products on the migration behaviour of radionuclides in cementitious materials

The impact of phthalates, which might be present in a cementitious repository as degradation products of superplasticisers or PVC additives, on the migration behaviour of An(III)/Ln(III) was investigated in batch sorption experiments on hardened cement paste. In the absence of organics, An(III)/Ln(III) sorb strongly on HCP with distribution ratios, Rd, of 105 to 106 L kg−1. Above a no-effect level of ~ 10–3 mol L−1 phthalate, a distinct decrease of the An(III)/Ln(III) sorption was observed with sorption reduction factors (i.e., the ratio between the Rd values without and with organic ligand) of 100 to 1000 at phthalate concentrations of 10–1 mol L−1. This sorption reduction is attributed to the effect of Ca complexation with phthalate on the stability of calcium silicate hydrate (C–S–H) phases as main sorbing phase. These results and ongoing diffusion experiments indicate an increase in the mobility of An(III)/Ln(III) in cementitious barriers with increasing phthalate concentrations.Graphical abstract

Mechanical and self-healing properties of cement paste containing incinerated sugarcane filter cake and Lysinibacillus sp. WH bacteria

Cement is the most widely used construction material due to its strength and affordability, but its production is energy intensive. Thus, the need to replace cement with widely available waste material such as incinerated black filter cake (IBFC) in order to reduce energy consumption and the associated CO2 emissions. However, because IBFC is a newly discovered cement replacement material, several parameters affecting the mechanical properties of IBFC-cement composite have not been thoroughly investigated yet. Thus, this work aims to investigate the impact of IBFC as a cement replacement and the addition of the calcifying bacterium Lysinibacillus sp. WH on the mechanical and self-healing properties of IBFC cement pastes. The properties of the IBFC-cement pastes were assessed by determining compressive strength, permeable void, water absorption, cement hydration product, and self-healing property. Increases in IBFC replacement reduced the durability of the cement pastes. The addition of the strain WH to IBFC cement pastes, resulting in biocement, increased the strength of the IBFC-cement composite. A 20% IBFC cement-replacement was determined to be the ideal ratio for producing biocement in this study, with a lower void percentage and water absorption value. Adding strain WH decreases pore sizes, densifies the matrix in ≤ 20% IBFC biocement, and enhances the formation of calcium silicate hydrate (C–S–H) and AFm ettringite phases. Biogenic CaCO3 and C–S–H significantly increase IBFC composite strength, especially at ≤ 20% IBFC replacement. Moreover, IBFC-cement composites with strain WH exhibit self-healing properties, with bacteria precipitating CaCO3 crystals to bridge cracks within two weeks. Overall, this work provides an approach to produce a "green/sustainable" cement using biologically enabled self-healing characteristics.

Freezing of water confined between calcium-silicate-hydrate layers: a coarse-grained molecular dynamics study

Freezing of water has a direct impact on the durability of cementitious materials, which include porous binding phases, like calcium-silicate-hydrate (CSH). Molecular dynamics (MD) simulations can capture behaviors of the water freezing in the pores and may give rise to insightful observations. In this work, the monoatomic water (mW) model and all-atom CSH model are combined to study low-temperature behavior of water molecules restricted within CSH layers. The developed model features good accuracy and accelerated computational rate, and is employed to study how ultra-confined water in CSH phases (≤6 nm) freezes. The threshold width of CSH gap that allows the stable existence of ice is 1.5 ± 0.1 nm. A non-icing layer with an average thickness of ∼0.65 nm exists ambient to CSH surface. The main factor affecting the melting point of confined water is the size of the confined space rather than the water filling fraction. The Gibbs-Thomson model is utilized to describe the relationship between the melting temperature and the width of the confined CSH layers. The utilization of this modeling technique presents a novel approach to replicate the freezing process of nanostructures in cementitious composites subjected to low-temperature erosion.

Time-Dependent Retention of a Mixture of Cs(I), Sm(III), Eu(III) and U(VI) as Waste Cocktail by Calcium Silicate Hydrate (C-S-H) Phases

In the context of the safe storage of high-level radioactive waste, the time-dependent retention of a waste cocktail (WC) consisting of Zr(IV), Mo(VI), Ru(III), Pd(II), Cs(I), Sm(III), Eu(III) and U(VI) was studied on the commercially available C-S-H phase Circosil®. The herein presented results focus on Cs(I), Sm(III), Eu(III) and U(VI). Precipitation and wall adsorption studies in the absence of the solid phase show only a small amount of precipitation for Sm(III) and Eu(III) (34 ± 18%) in the high-saline diluted Gipshut solution (DGS, pH 10.6, I = 2.6 M). For Cs(I) and U(VI), no precipitation was observed. In 0.1 M NaCl (pH 10.9), the measured retention could completely be attributed to wall adsorption for all four elements. The obtained Rd values for the time-dependent retention of Sm(III), Eu(III) and U(VI) on Circosil® of 105 to 106 L·kg−1 are in good agreement with the literature. For Cs(I) in the strongly saline background electrolytes, slightly higher Rd values of up to 8·102 L·kg−1 were determined for the crystalline Circosil® compared to the wet chemical C-S-H phases. Overall, the commercial product Circosil® is suitable as an alternative to synthesised C-S-H phases to observe trends in the retention behaviour of these elements. Comparison between both background electrolytes shows an increase in the amount and velocity of retention for all four elements with decreasing salinity. This confirms adsorption processes as the fastest and initial retention mechanism. Precipitation or incorporation of Eu(III), Sm(III) and U(VI) cannot be ruled out in the long term. Comparing the kinetic of this WC study to single-element studies in the literature, a longer uptake time to reach a steady state of 7 d in 0.1 M NaCl and 28 d in DGS instead of &lt;1 d was observed for Eu(III) and Sm(III). The situation for Cs(I) is similar. This indicates competing effects between the different WC elements for adsorption sites on the C-S-H phases.

Evaluation of thermal and freeze-thaw resistances of the concretes with the silica fume addition at different water-cement ratio

The preparation of concrete with improved durability is an essential subject in the development of building and construction materials. This study presents the preparation and characterization of a high-strength concrete by activating ordinary 42.5 grade Portland cement (OPC) with silica fume (SF). The effect of the SF addition and water-cement (W/C) ratio on concrete durability including freeze-thaw, water permeability, mechanical and thermal resistance properties have been determined. When the water-cement ratio (W/C) in the SF added concrete is appropriately designed, for example, W/C 0.23, the compressive strength was 92.85 MPa, water absorption 0.7 %, 420 freeze-thaw cycles and water impermeability is W20. Residual strength of this concrete after thermal explosion at 600 °C for 2 h was 82 % of the original plain concrete, indicating that the obtained value is the true dense concrete. It also shows the concrete slump flow of 630 mm, which indicates its self-compacting behavior. The concrete prepared with W/C 0.29 achieved 365 freeze-thaw cycles, had 65.1 MPa compressive strength, W14 water impermeability, 1.8 % water absorption and 69 % residual strength after the thermal explosion at 600 °C. Concrete with washed aggregates has higher mechanical properties than concrete with unwashed aggregates. Freeze-thaw resistance has a direct relationship with the percentage of residual strength after explosion at 600 °C in all W/C ratios tested. Better durability and high resistance to temperature explosion was caused by the beneficial formation of the calcium silicate hydrate phase (C-S-H) in the silica fume added concrete.

The Uptake of Actinides by Hardened Cement Paste in High-Salinity Pore Water

The interaction of the actinides Pu(III), Am(III), Np(V), Np(VI), and U(VI) with hardened cement paste (HCP) prepared from ordinary Portland cement was investigated by batch experiments in a diluted caprock solution (I = 2.5 M) as a function of the solid-to-liquid (S/L) ratio (0.5–20.0 g L−1) and pH (10–13). Independent of the oxidation state of the actinides, strong sorption was observed with Rd values between 104 and 5 × 105 L kg−1. For the hexavalent actinides U(VI) and Np(VI), a decrease in sorption was observed with increasing pH, which could be due to the formation of the AnO2(OH)42− species. CE-ICP-MS measurements of the supernatant solution from the U(VI) batch sorption experiment at pH ≥ 10 indicate that UO2(OH)3− and UO2(OH)42− dominate the speciation. Pu LIII-edge XANES and EXAFS measurements showed oxidation of Pu(III) to Pu(IV) when interacting with HCP. Calcium silicate hydrate (C-S-H) phases effectively immobilize Pu(IV) by incorporating it into the CaO layer. This was observed in a C-S-H sample with C/S = 1.65 and HCP at pH 12.7. Compared to data published in the literature on the retention of actinides on HCP at low ionic strength, the influence of high ionic strength (I = 2.5 M) on the sorption behavior was insignificant.

Comparative study of physicochemical properties of geopolymers prepared by four Moroccan natural clays

The present work concerns the development of geopolymer materials based on natural clays rich in kaolinite, muscovite and pyrophyllite. The aim was to study the effect of the nature and fraction of the clay minerals composing the raw sample on the physico-chemical properties of the prepared geopolymers, viz., the matrix geopolymer phase fraction, particle size distribution, density, compressive strength and the adsorption capacity of methylene blue (MB) as a cationic dye model. The raw starting clays, their meta-clay phases calcined at 800 °C, and the prepared geopolymers were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), particle size distribution (PSD) via laser diffraction method, and scanning electron microscopy (SEM). The matrix geopolymer fraction phase in the samples was qualitatively determined by the analysis of the spectra obtained by 27Al and 29Si solid-state NMR. The results showed that kaolinite-rich clay produced more geopolymer than clays with high phase fractions of muscovite or pyrophyllite and low kaolinite. The highest compressive strength was given by the clay that contained 26.7 % of calcite due to formation of calcium silicate hydrate phase. The maximum adsorption capacity of MB was obtained by the samples have a more heterogeneous surface morphology due to presence of unreacted muscovite and/or pyrophyllite in raw starting clay.

Bacterial Treatment of Recycled Concrete Aggregate

Microbial treatment of recycled concrete aggregate (RCA) may improve the quality of the aggregate, and enhance its use in the production of structural concrete and promote the recycling of concrete waste. The mortar phase of the RCA is responsible for the poor performance of the aggregate. Treating the old adhered mortar or removing it from the natural aggregate (NA) is an option to make RCA beneficial for the production of quality recycled aggregate concrete (RAC). Removing the adhered mortar from recycled concrete aggregate using silicate-solubilizing bacteria was investigated. The bacteria could synthesize the silicates in the calcium silicate hydrate phase of the cement paste leading to the breakdown of the old adhered mortar. Four SSB strains were tested for survivability and activity in an alkaline medium to simulate the concrete environment. The Serratia marcescens bacterial strain, which survived the environment, was inoculated into screw-cap glass vials containing recycled concrete aggregate fragments and glucose-enhanced nutrient broth and then incubated for 14 days. Partial removal of the old adhered mortar was observed based on the weight lost from the RCA. The S. marcescens bacterial strain could survive the alkaline concrete environment and solubilize the silicates present in cement paste resulting in the removal of the old adhered mortar.

Application of binders with lower carbon footprint in the AAC production

AbstractAutoclaved aerated concrete (AAC) is a building material that mainly contains calcium silicate hydrate phases, especially 11 Å tobermorite. For forming these mineral phases, calcium ions from the binders and silicon ions from the milled sand are necessary. Currently, hard‐burnt lime and Portland cement are used as binders for AAC production. Both raw materials contribute significantly to the carbon footprint of AAC products. On the way to climate neutrality of AAC production, it is thus necessary to replace hard‐burnt lime and Portland cement with binders with a lower CO2 impact. Therefore, tests with middle‐burnt lime in combination with different Portland limestone cement were carried out to investigate their influence on processing, material properties, and the mineral phase composition. It can be concluded that both these lower CO2 impact raw materials can be used for AAC production, as there are no changes in material properties and mineral phase composition.

Effect of ISA and chloride on the uptake of niobium (V) by hardened cement paste and C-S-H phases: Quantitative description and mechanistic understanding

The uptake of niobium by hardened cement paste (HCP) and calcium silicate hydrate (C-S-H) phases was investigated with 93Nb and 95Nb (t1/2 = 35.0 days). Structural materials used in nuclear reactors as well as cements contain the naturally occurring isotope 93Nb, while radioactive 94Nb with t1/2 = 2 × 104 years is relevant in the context of nuclear waste disposal. Strong uptake of Nb was observed for both materials, confirming that C-S-H is the main sink of Nb in cement. Isotopic exchange with 93Nb in cement can play a role in the uptake of 94Nb under repository conditions. The formation of complexes with isosaccharinic acid (ISA) decreases the Nb uptake, although sorption remains strong up to [ISA]tot = 0.1 M. Chloride has a negligible effect on the uptake of Nb up to [NaCl] = 2 M. This work provides a sound basis for the quantitative description and mechanistic understanding of 94Nb retention in L/ILW repositories.

Analysis of Uranium Sorption in a Laboratory Column Experiment Using a Reactive Transport and Surface Complexation Model

The transport and retardation of radioactive elements in hyper-alkaline conditions of radioactive waste repositories is a challenging field that is still poorly understood. In this study, the transport and attenuation of uranium in a column experiment was modelled by considering kinetic reactions, advection–dispersion and chemical/physical retardation processes. The modelling was first performed for three alluvium samples from Yucca Mountain in circumneutral pH to moderately alkaline conditions. Sorption of uranyl ({mathrm{UO}}_{2}^{2+} ({U}_{mathrm{VI}})) was found to strongly depend on the surface complexation model assumed, with no significant removal of {U}_{mathrm{VI}} by precipitation or ion exchange process. The surface/edge site reaction of Al-hydroxyl group in kaolinite was shown to have a high affinity for uranyl adsorption, while the hydrous ferric oxide edge on hematite adsorbed most of the uranyl ions. The model was then used to interpret uranium transport in a laboratory column filled with Hollington sandstone under hyper-alkaline (pH 13) conditions. The simulation results show that uranium adsorption on the Al-hydroxyl edge of kaolinite exceeds adsorption by the calcium silicate hydrate phase. This result may reflect the lack of surface complexation parameters for calcium silicate hydrate minerals. Hence, further studies are required in the field of surface complexation reactions for calcium silicate hydrate phases.Graphical

Atomistic simulations, meso-scale analyses and experimental validation of thermal properties in ordinary Portland cement and geopolymer pastes

Ordinary Portland Cement (OPC)- and alternative binder-based materials have been widely used in Thermal Energy Storage (TES) applications due to their excellent TES capacities, good mechanical properties and low-cost. In this attempt, this work proposes an upscaling procedure to model the TES properties of two hydrated pastes made of OPC and a Hybrid cement (i.e., an alternative H-Cement binder), the latter employed for a GEOpolymer-based composite (GEO). Firstly, an atomistic approach based on energy minimization and molecular dynamics is employed for modelling the thermal behaviors and heat storage capacities of CSH (calcium silicate hydrate) and NASH (sodium aluminosilicate hydrate) phases, being those the main phases for OPC-based paste and GEO, respectively. Then, an up-scaling optimization procedure and meso-scale FEM homogenization techniques are proposed to link the TES parameters of the atomistic main phases of OPC-based paste and GEO with the homogenized upper meso/macro scale values. To this end, the results of an experimental program on both OPC and GEO pastes have been considered as benchmark to calibrate/validate the numerical tools. Promising simulations at several scales and up-scaling procedures are demonstrated in terms of homogenized temperature-dependent heat capacity and thermal diffusivity, showing a good agreement with the experimental data of the analyzed mixtures.

Utilization of alkali-activated dolomite waste toward the fabrication of an effective adsorbent: Experimental study and statistical physics formalism for the removal of methylene blue and crystal violet

This study reports a potential valorization strategy for carbonate-rich dolomite waste (DW), which is mainly composed of calcite (CaCO3) and dolomite CaMg(CO3)2 minerals, to prepare an alkali-activated adsorbent (AADW). The activation process was easily performed by mixing sodium silicate (7.5 wt% Na2O) with DW at room temperature. The developed adsorbent was characterized using XRD, FTIR, TGA, DTG, FESEM, EDX, and zeta potential methods and employed to remove methylene blue (MB) and crystal violet (CV). The formation of a highly-crystalline calcium silicate hydrate phase with effective adsorption sites was mainly attributed to calcite. The pseudo-first-order (PFO) and the Freundlich equations were the best-fitted models used to explore the uptake of CV and MB. Theoretical calculations were considered to support the set-up/management of the CV and MB adsorption processes performed at 25, 40, and 50 °C. A double layer model with two energies was the best option to analyze these adsorption systems and it was found that the removal of CV and MB was controlled by parallel and non-parallel orientations. The adsorption capacities (Qsat) ranged from 343.0 to 442.1 mg/g for MB and 396.8–466.0 mg/g for CV, and the density of the AADW functional groups affected this parameter. Finally, the adsorption energies were <40 kJ/mol where physical interactions could be involved.

Cellulose degradation products and high ionic strength enhance the retardation of strontium in hydrated cement paste

The present concept for the disposal of some low- and all intermediate-level radioactive waste in the United Kingdom involves grouting with cement in steel drums and placement in a geological facility. The vaults would then be backfilled with low strength, high porosity cement-based materials designed to promote a pervasive, alkaline environment in which many of the radioactive species present are sparingly soluble. This work investigates the interaction of strontium with such a backfill under both diffusive and advective conditions given the potential significance of the fission product 90Sr. An important characteristic of the United Kingdom waste inventory is the abundance of organic compounds, including cellulosic materials; consequently, the experiments were repeated with products of alkaline cellulose degradation. Additional experiments were performed at high ionic strength simulating anticipated changes in the salinity of the groundwater. The effective diffusivity (De) of strontium in the absence of the organic compounds ranged between 5.5 × 10−11 and 8.5 × 10−11 m2 s−1, with a distribution coefficient (Rd) of between 2.8 × 10−3 and 3.1 × 10−3 m3 kg−1. The presence of organic compounds and/or an increase of ionic strength enhanced the retardation of strontium. The results indicate that the retention of strontium by hydrated cement paste occurs via a reversible ion exchange mechanism rather than mineralisation and is promoted by decalcification of the calcium silicate hydrate phases.