A long-term study on structural changes in calcium aluminate silicate hydrates.

Production of blended cements in which Portland cement is combined with supplementary cementitious materials (SCM) is an effective strategy for reducing the CO2 emissions during cement manufacturing and achieving sustainable concrete production. However, the high Al2O3 and SiO2 contents of SCM change the chemical composition of the main hydration product, calcium aluminate silicate hydrate (C-A-S-H). Herein, spectroscopic and structural data for C-A-S-H gels are reported in a large range of equilibration times from 3 months up to 2 years and Al/Si molar ratios from 0.001 to 0.2. The 27Al MAS NMR spectroscopy and thermogravimetric analysis indicate that in addition to the C-A-S-H phase, secondary phases such as strätlingite, katoite, Al(OH)3 and calcium aluminate hydrate are present at Al/Si ≥ 0.03 limiting the uptake of Al in C-A-S-H. More secondary phases are present at higher Al concentrations; their content decreases with equilibration time while more Al is taken up in the C-A-S-H phase. At low Al contents, Al concentrations decrease strongly with time indicating a slow equilibration, in contrast to high Al contents where a clear change in Al concentrations over time was not observed indicating that the equilibrium has been reached faster. The 27Al NMR studies show that tetrahedrally coordinated Al is incorporated in C-A-S-H and its amount increases with the amount of Al present in the solution. Supplementary Information: The online version contains supplementary material available at 10.1617/s11527-022-02080-x.

Methylamine Lithium Borohydride as Electrolyte for All-Solid-State Batteries.

Fast Li-ion conductivity at room temperature is a major challenge for utilization of all-solid-state Li batteries. Metal borohydrides with neutral ligands are a new emerging class of solid-state ionic conductors, and here we report the discovery of a new mono-methylamine lithium borohydride with very fast Li+ conductivity at room temperature. LiBH4 ⋅CH3 NH2 crystallizes in the monoclinic space group P21 /c, forming a two-dimensional unique layered structure. The layers are separated by hydrophobic -CH3 moieties, and contain large voids, allowing for fast Li-ionic conduction in the interlayers, σ(Li+ )=1.24×10-3  S cm-1 at room temperature. The electronic conductivity is negligible, and the electrochemical stability is ≈2.1 V vs Li. The first all-solid-state battery using a lithium borohydride with a neutral ligand as the electrolyte, Li-metal as the anode and TiS2 as the cathode is demonstrated.

Ammine Lanthanum and Cerium Borohydrides, M(BH4)3·nNH3; Trends in Synthesis, Structures, and Thermal Properties.

Ammine metal borohydrides show potential for solid-state hydrogen storage and can be tailored toward hydrogen release at low temperatures. Here, we report the synthesis and structural characterization of seven new ammine metal borohydrides, M(BH4)3·nNH3, M = La (n = 6, 4, or 3) or Ce (n = 6, 5, 4, or 3). The two compounds with n = 6 are isostructural and have new orthorhombic structure types (space group P21212) built from cationic complexes, [M(NH3)6(BH4)2]+, and are charge balanced by BH4-. The structure of Ce(BH4)3·5NH3 is orthorhombic (space group C2221) and is built from cationic complexes, [Ce(NH3)5(BH4)2]+, and charge balanced by BH4-. These are rare examples of borohydride complexes acting both as a ligand and as a counterion in the same compound. The structures of M(BH4)3·4NH3 are monoclinic (space group C2), built from neutral molecular complexes of [M(NH3)4(BH4)3]. The new compositions, M(BH4)3·3NH3 (M = La, Ce), among ammine metal borohydrides, are orthorhombic (space group Pna21), containing molecular complexes of [M(NH3)3(BH4)3]. A revised structural model for A(BH4)3·5NH3 (A = Y, Gd, Dy) is presented, and the previously reported composition A(BH4)3·4NH3 (A = Y, La, Gd, Dy) is proposed in fact to be M(BH4)3·3NH3 along with a new structural model. The temperature-dependent structural properties and decomposition are investigated by in situ synchrotron radiation powder X-ray diffraction in vacuum and argon atmosphere and by thermal analysis combined with mass spectrometry. The compounds with n = 6, 5, and 4 mainly release ammonia at low temperatures, while hydrogen evolution occurs for M(BH4)3·3NH3 (M = La, Ce). Gas-release temperatures and gas composition from these compounds depend on the physical conditions and on the relative stability of M(BH4)3·nNH3 and M(BH4)3.

Efficient Solar-Driven Hydrogen Transfer by Bismuth-Based Photocatalyst with Engineered Basic Sites.

Photocatalytic organic conversions involving a hydrogen transfer (HT) step have attracted much attention, but the efficiency and selectivity under visible light irradiation still needs to be significantly enhanced. Here we have developed a noble metal-free, basic-site engineered bismuth oxybromide [Bi24O31Br10(OH)δ] that can accelerate the photocatalytic HT step in both reduction and oxidation reactions, i.e., nitrobenzene to azo/azoxybenzene, quinones to quinols, thiones to thiols, and alcohols to ketones under visible light irradiation and ambient conditions. Remarkably, quantum efficiencies of 42% and 32% for the nitrobenzene reduction can be reached under 410 and 450 nm irradiation, respectively. The Bi24O31Br10(OH)δ photocatalyst also exhibits excellent performance in up-scaling and stability under visible light and even solar irradiation, revealing economic potential for industrial applications.

Hydrogenation properties of lithium and sodium hydride - closo-borate, [B10H10]2- and [B12H12]2-, composites.

The hydrogen absorption properties of metal closo-borate/metal hydride composites, M2B10H10-8MH and M2B12H12-10MH, M = Li or Na, are studied under high hydrogen pressures to understand the formation mechanism of metal borohydrides. The hydrogen storage properties of the composites have been investigated by in situ synchrotron radiation powder X-ray diffraction at p(H2) = 400 bar and by ex situ hydrogen absorption measurements at p(H2) = 526 to 998 bar. The in situ experiments reveal the formation of crystalline intermediates before metal borohydrides (MBH4) are formed. On the contrary, the M2B12H12-10MH (M = Li and Na) systems show no formation of the metal borohydride at T = 400 °C and p(H2) = 537 to 970 bar. 11B MAS NMR of the M2B10H10-8MH composites reveal that the molar ratio of LiBH4 or NaBH4 and the remaining B species is 1 : 0.63 and 1 : 0.21, respectively. Solution and solid-state 11B NMR spectra reveal new intermediates with a B : H ratio close to 1 : 1. Our results indicate that the M2B10H10 (M = Li, Na) salts display a higher reactivity towards hydrogen in the presence of metal hydrides compared to the corresponding [B12H12]2- composites, which represents an important step towards understanding the factors that determine the stability and reversibility of high hydrogen capacity metal borohydrides for hydrogen storage.

Metal borohydride formation from aluminium boride and metal hydrides.

Metal borides are often decomposition products from metal borohydrides and thus play a role in the reverse reaction where hydrogen is absorbed. In this work, aluminium boride, AlB2, has been investigated as a boron source for the formation of borohydrides under hydrogen pressures of p(H2) = 100 or 600 bar at elevated temperatures (350 or 400 °C). The systems AlB2-MHx (M = Li, Na, Mg, Ca) have been investigated, producing LiBH4, NaBH4 and Ca(BH4)2, whereas the formation of Mg(BH4)2 was not observed at T = 400 °C and p(H2) = 600 bar. The formation of the metal borohydrides is confirmed by powder X-ray diffraction and infrared spectroscopy and the fraction of boron in AlB2 and M(BH4)x is determined quantitatively by 11B MAS NMR. Hydrogenation for 12 h at T = 350-400 °C and p(H2) = 600 bar leads to the formation of substantial amounts of LiBH4 (38.6 mol%), NaBH4 (83.0 mol%) and Ca(BH4)2 (43.6 mol%).

Trends in Syntheses, Structures, and Properties for Three Series of Ammine Rare-Earth Metal Borohydrides, M(BH4)3·nNH3 (M = Y, Gd, and Dy).

Fourteen solvent- and halide-free ammine rare-earth metal borohydrides M(BH4)3·nNH3, M = Y, Gd, Dy, n = 7, 6, 5, 4, 2, and 1, have been synthesized by a new approach, and their structures as well as chemical and physical properties are characterized. Extensive series of coordination complexes with systematic variation in the number of ligands are presented, as prepared by combined mechanochemistry, solvent-based methods, solid-gas reactions, and thermal treatment. This new synthesis approach may have a significant impact within inorganic coordination chemistry. Halide-free metal borohydrides have been synthesized by solvent-based metathesis reactions of LiBH4 and MCl3 (3:1), followed by reactions of M(BH4)3 with an excess of NH3 gas, yielding M(BH4)3·7NH3 (M = Y, Gd, and Dy). Crystal structure models for M(BH4)3·nNH3 are derived from a combination of powder X-ray diffraction (PXD), (11)B magic-angle spinning NMR, and density functional theory (DFT) calculations. The structures vary from two-dimensional layers (n = 1), one-dimensional chains (n = 2), molecular compounds (n = 4 and 5), to contain complex ions (n = 6 and 7). NH3 coordinates to the metal in all compounds, while BH4(-) has a flexible coordination, i.e., either as a terminal or bridging ligand or as a counterion. M(BH4)3·7NH3 releases ammonia stepwise by thermal treatment producing M(BH4)3·nNH3 (6, 5, and 4), whereas hydrogen is released for n ≤ 4. Detailed analysis of the dihydrogen bonds reveals new insight about the hydrogen elimination mechanism, which contradicts current hypotheses. Overall, the present work provides new general knowledge toward rational materials design and preparation along with limitations of PXD and DFT for analysis of structures with a significant degree of dynamics in the structures.

Hydrogen reversibility of LiBH4-MgH2-Al composites.

The detailed mechanism of hydrogen release in LiBH4-MgH2-Al composites of molar ratios 4 : 1 : 1 and 4 : 1 : 5 are investigated during multiple cycles of hydrogen release and uptake. This study combines information from several methods, i.e., in situ synchrotron radiation powder X-ray diffraction, (11)B magic-angle spinning (MAS) NMR, Sievert's measurements, Fourier transform infrared spectroscopy and simultaneous thermogravimetric analysis, differential scanning calorimetry and mass spectroscopy. The composites of LiBH4-MgH2-Al are compared with the behavior of the LiBH4-Al and LiBH4-MgH2 systems. The decomposition pathway of the LiBH4-MgH2-Al system is different for the two investigated molar ratios, although it ultimately results in the formation of LiAl, Mg(x)Al(1-x)B2 and Li2B12H12 in both cases. For the 4 : 1 : 1-molar ratio, Mg(0.9)Al(0.1) and Mg17Al12 are observed as intermediates. However, only Mg is observed as an intermediate in the 4 : 1 : 5-sample, which may be due to an earlier formation of Mg(x)Al(1-x)B2, reflecting the complex chemistry of Al-Mg phases. Hydrogen release and uptake reveals a decrease in the hydrogen storage capacity upon cycling. This loss reflects the formation of Li2B12H12 as observed by (11)B NMR and infrared spectroscopy for the cycled samples. Furthermore, it is shown that the Li2B12H12 formation can be limited significantly by applying moderate hydrogen partial pressure during decomposition.

Effect of Water-Solid Mixing Sequence and Crystallization Water of Calcium Sulphate on the Hydration of C3A.

Tricalcium aluminate (Ca3Al2O6: C3A) is the most reactive clinker phase in Portland cement. In this study, the effect of the sequence of mixing of C3A with gypsum and water on the hydration kinetics and phase assemblage is investigated. Three mixing sequences were employed: (i) Turbula mixing of C3A first with gypsum and then with water (T-mix); (ii) Hand mixing of C3A with gypsum before mixing with water (H-mix); (iii) Pre-mixing gypsum with water and then with C3A (P-mix). The results suggest that there is a considerable difference in the hydration kinetics and hydrate phase assemblage, particularly during the initial stages of hydration. P-mix promotes a higher degree of hydration in the initial minutes and considerably influences the main peak in the calorimetry curve of C3A hydration. Effects of calcium sulphate with different amounts of crystallisation water (anhydrite, hemihydrate and gypsum) on C3A hydration are also investigated, and it is found that the water of crystallisation does not have a significant impact on the kinetics of reaction or the formed hydrate phase assemblage.

Fast Room-Temperature Mg2+ Conductivity in Mg(BH4)2·1.6NH3-Al2O3 Nanocomposites.

Design of new functional materials with fast Mg-ion mobility is crucial for the development of competitive solid-state magnesium batteries. Herein, we present new nanocomposites, Mg(BH4)2·1.6NH3-Al2O3, reaching a high magnesium conductivity of σ(Mg2+) = 2.5 × 10-5 S cm-1 at 22 °C assigned to favorable interfaces between amorphous state Mg(BH4)2·1.6NH3; inert and insulating Al2O3 nanoparticles; and a minor fraction of crystalline material, mainly Mg(BH4)2·2NH3. Furthermore, quasi-elastic neutron scattering reveals that the Mg2+-ion mobility in the solid state appears to be correlated to relatively slow motion of NH3 molecules rather than the fast dynamics of BH4- complexes. The nanocomposite is compatible with a metallic Mg anode and shows stable Mg2+ stripping/plating in a symmetric cell and an electrochemical stability of ∼1.2 V. The nanocomposite has high mechanical stability and ductility and is a promising Mg2+ electrolyte for future solid-state magnesium batteries.

Synthesis of 17O-labeled Cs2WO4 and its ambient- and low-temperature solid-state 17O MAS NMR spectra.

Following several seemingly straightforward but unsuccessful attempts to prepare a sample of (17)O-enriched Cs(2)WO(4), we here report a simple, aqueous procedure for synthesis of pure Cs(2)WO(4), if so desired, enriched in (17)O. The purpose for the preparation of (17)O-enriched Cs(2)WO(4) is to record its solid-state (17)O MAS NMR spectrum, which would allow for a determination of its quadrupole coupling and chemical shift anisotropy (CSA) parameters and thereby for a comparison with the corresponding (33)S and (77)Se parameters in the related compounds M(2)WS(4) and M(2)WSe(4). These compounds are isomorphous and crystallize in the orthorhombic space group Pnma, and Cs(2)WO(4) turns out to be the only alkali metal tungstate with the Pnma crystal structure. Therefore, it has been mandatory to use Cs(2)WO(4) and not K(2)WO(4) (space group C2/m) for which CSA data have previously been published, to achieve a reliable comparison with the (33)S and (77)Se data and thus allow assignment of the three different sets of (17)O NMR parameters to the three distinct oxygen sites (O(1,1), O(2), and O(3)) in the Pnma crystal structure of Cs(2)WO(4). Because the ambient temperature (17)O MAS NMR spectrum of Cs(2)WO(4) exhibits a dynamically broadened singlet, resorting to low-temperature (-83 °C) conditions at 21.15 T was necessary and resulted in a high-resolution (17)O MAS spectrum that allowed both (17)O quadrupole coupling and CSA parameters to be determined. As no quadrupole coupling data were obtained from the earlier investigation on K(2)WO(4), the present results for Cs(2)WO(4) prompted a reinvestigation of the (17)O MAS spectrum for K(2)WO(4), which actually also shows the presence of (17)O quadrupole couplings for all three oxygen sites. These data for Cs(2)WO(4) and K(2)WO(4) are consistent and result in unambiguous assignments of the parameters to the three distinct oxygen sites in their crystal structures.

Pair distribution function and 71Ga NMR study of aqueous Ga3+ complexes.

The atomic structures, and thereby the coordination chemistry, of metal ions in aqueous solution represent a cornerstone of chemistry, since they provide first steps in rationalizing generally observed chemical information. However, accurate structural information about metal ion solution species is often surprisingly scarce. Here, the atomic structures of Ga3+ ion complexes were determined directly in aqueous solutions across a wide range of pH, counter anions and concentrations by X-ray pair distribution function analysis and 71Ga NMR. At low pH (<2) octahedrally coordinated gallium dominates as either monomers with a high degree of solvent ordering or as Ga-dimers. At slightly higher pH (pH ≈ 2-3) a polyoxogallate structure is identified as either Ga30 or Ga32 in contradiction with the previously proposed Ga13 Keggin structures. At neutral and slightly higher pH nanosized GaOOH particles form, whereas for pH > 12 tetrahedrally coordinated gallium ions surrounded by ordered solvent are observed. The effects of varying either the concentration or counter anion were minimal. The present study provides the first comprehensive structural exploration of the aqueous chemistry of Ga3+ ions with atomic resolution, which is relevant for both semiconductor fabrication and medical applications.

Incorporation of phosphorus guest ions in the calcium silicate phases of Portland cement from 31P MAS NMR spectroscopy.

Portland cements may contain small quantities of phosphorus (typically below 0.5 wt % P(2)O(5)), originating from either the raw materials or alternative sources of fuel used to heat the cement kilns. This work reports the first (31)P MAS NMR study of anhydrous and hydrated Portland cements that focuses on the phase and site preferences of the (PO(4))(3-) guest ions in the main clinker phases and hydration products. The observed (31)P chemical shifts (10 to -2 ppm), the (31)P chemical shift anisotropy, and the resemblance of the lineshapes in the (31)P and (29)Si MAS NMR spectra strongly suggest that (PO(4))(3-) units are incorporated in the calcium silicate phases, alite (Ca(3)SiO(5)) and belite (Ca(2)SiO(4)), by substitution for (SiO(4))(4-) tetrahedra. This assignment is further supported by a determination of the spin-lattice relaxation times for (31)P in alite and belite, which exhibit the same ratio as observed for the corresponding (29)Si relaxation times. From simulations of the intensities, observed in inversion-recovery spectra for a white Portland cement, it is deduced that 1.3% and 2.1% of the Si sites in alite and belite, respectively, are replaced by phosphorus. Charge balance may potentially be achieved to some extent by a coupled substitution mechanism where Ca(2+) is replaced by Fe(3+) ions, which may account for the interaction of the (31)P spins with paramagnetic Fe(3+) ions as observed for the ordinary Portland cements. A minor fraction of phosphorus may also be present in the separate phase Ca(3)(PO(4))(2), as indicated by the observation of a narrow resonance at delta((31)P) = 3.0 ppm for two of the studied cements. (31)P{(1)H} CP/MAS NMR spectra following the hydration of a white Portland cement show that the resonances from the hydrous phosphate species fall in the same spectral range as observed for (PO(4))(3-) incorporated in alite. This similarity and the absence of a large (31)P chemical shift ansitropy indicate that the hydrous (PO(4))(3-) species are incorporated in the interlayers of the calcium-silicate-hydrate (C-S-H) phase, the principal phase formed upon hydration of alite and belite.

Thermal polymorphism and decomposition of Y(BH(4))(3).

The structure and thermal decomposition of Y(BH(4))(3) is studied by in situ synchrotron radiation powder X-ray diffraction (SR-PXD), (11)B MAS NMR spectroscopy, and thermal analysis (thermogravimetric analysis/differential scanning calorimetry). The samples were prepared via a metathesis reaction between LiBH(4) and YCl(3) in different molar ratios mediated by ball milling. A new high temperature polymorph of Y(BH(4))(3), denoted beta-Y(BH(4))(3), is discovered besides the Y(BH(4))(3) polymorph previously reported, denoted alpha-Y(BH(4))(3). beta-Y(BH(4))(3) has a cubic crystal structure and crystallizes with the space group symmetry Pm3m and a bisected a-axis, a = 5.4547(8) A, as compared to alpha-Y(BH(4))(3), a = 10.7445(4) A (Pa3). Beta-Y(BH(4))(3) crystallizes with a regular ReO(3)-type structure, hence the Y(3+) cations form cubes with BH(4)(-) anions located on the edges. This arrangement is a regular variant of the distorted Y(3+) cube observed in alpha-Y(BH(4))(3), which is similar to the high pressure phase of ReO(3). The new phase, beta-Y(BH(4))(3) is formed in small amounts during ball milling; however, larger amounts are formed under moderate hydrogen pressure via a phase transition from alpha- to beta-Y(BH(4))(3), at approximately 180 degrees C. Upon further heating, beta-Y(BH(4))(3) decomposes at approximately 190 degrees C to YH(3), which transforms to YH(2) at 270 degrees C. An unidentified compound is observed in the temperature range 215-280 degrees C, which may be a new Y-B-H containing decomposition product. The final decomposition product is YB(4). These results show that boron remains in the solid phase when Y(BH(4))(3) decomposes in a hydrogen atmosphere and that Y(BH(4))(3) may store hydrogen reversibly.

Probing the validity of the spinel inversion model: a combined SPXRD, PDF, EXAFS and NMR study of ZnAl2O4.

Spinels are of essential interest in the solid-state sciences with numerous important materials adopting this crystal structure. One defining feature of spinel compounds is their ability to accommodate a high degree of tailorable point defects, and this significantly influences their physical properties. Standard defect models of spinels often only consider metal atom inversion between octahedral and tetrahedral sites, thereby neglecting other defects such as interstitial atoms. In addition, most studies rely on a single structural characterization technique, and this may bias the result and give uncertainty about the correct crystal structure. Here we explore the virtues of multi-technique investigations to limit method and model bias. We have used Pair Distribution Function analysis, Rietveld refinement and Maximum Entropy Method analysis of Powder X-ray Diffraction data, Zn edge Extended X-ray Absorption Fine Structure, and solid-state 27Al Nuclear Magnetic Resonance to study the structural defects in ZnAl2O4 spinel samples prepared by either microwave hydrothermal synthesis, supercritical flow synthesis, or spark plasma sintering. In addition, the samples were subjected to thermal post treatments. The study demonstrates that numerous synthesis dependent defects are present and that the different synthesis pathways allow for defect tailoring within the ZnAl2O4 structure. This suggests a pathway forward for optimization of the physical properties of spinel materials.

Site preferences of fluoride guest ions in the calcium silicate phases of Portland cement from 29Si{19F} CP-REDOR NMR spectroscopy.

A reduction in CO(2) emission from Portland cement production can be achieved by energy savings associated with a lowering of the temperature at which the high temperature alite (Ca(3)SiO(5)) and belite (Ca(2)SiO(4)) silicates form. This can be accomplished by fluoride mineralization where a small amount of fluorine (e.g., CaF(2)) is added to the raw mix of starting materials. This work provides the mechanism for incorporation of fluoride ions in the calcium silicate phases of Portland cements which is important in the optimization of the fluoride mineralization. It is demonstrated by double-resonance (29)Si{(19)F} CP/MAS NMR experiments that the fluoride ions are exclusively incorporated into the alite phase of the two calcium silicates. The fluoride ions substitute for oxygen by a coupled mechanism that also involves replacement of Si(4+) by Al(3+) to achieve charge balance. Most importantly, (29)Si{(19)F} REDOR NMR experiments reveal that the fluoride ions are incorporated in alite with a site preference for the "interstitial" oxygen sites and thus not the covalently bonded oxygens of the SiO(4) units. This implies that only one-fifth of the oxygen sites in alite are available for substitution by fluoride ions which limits the gain in entropy of mixing that is a key factor for the reduction in upper temperature of the cement kiln.

Site preferences of NH4+ in its solid solutions with Cs2WS4 and Rb2WS4 from multinuclear solid-state MAS NMR.

Solid solutions of NH(4)(+) in Cs(2)WS(4) and Rb(2)WS(4) are obtained by precipitation/crystallization from aqueous solutions. By means of (14)N, (87)Rb, and (133)Cs magic angle spinning NMR, compositions and extraordinarily accurate NH(4)(+)-site preferences are established for these materials.

Improved quantification of alite and belite in anhydrous Portland cements by (29)Si MAS NMR: effects of paramagnetic ions.

The applicability, reliability, and repeatability of 29Si MAS NMR for determination of the quantities of alite (Ca3SiO5) and belite (Ca2SiO4) in anhydrous Portland cement was investigated in detail for 11 commercial Portland cements and the results compared with phase quantifications based on powder X-ray diffraction combined with Rietveld analysis and with Taylor-Bogue calculations. The effects from paramagnetic ions (Fe3+) on the spinning sideband intensities, originating from dipolar couplings between 29Si and the spins of the paramagnetic electrons, were considered and analyzed in spectra recorded at four magnetic fields (4.7-14.1T) and this has led to an improved quantification of alite and belite from (29)Si MAS NMR spectra recorded at "high" spinning speeds of nu(R)=12.0-13.0kHz using 4 or 5mm rotors. Furthermore, the impact of Fe3+ ions on the spin-lattice relaxation was studied by inversion-recovery experiments and it was found that the relaxation is overwhelmingly dominated by the Fe3+ ions incorporated as guest-ions in alite and belite rather than the Fe3+ sites present in the intimately mixed ferrite phase (Ca2Al(x)Fe(2-)(x)O5).

Optical Sensing of pH and O2 in the Evaluation of Bioactive Self-Healing Cement.

Leakage from cementitious structures with a retaining function can have devastating environmental consequences. Leaks can originate from cracks within the hardened cementitious material that is supposed to seal the structure off from the surrounding environment. Bioactive self-healing concretes containing bacteria capable of microbially inducing CaCO3 precipitation have been suggested to mitigate the healing of such cracks before leaking occurs. An important parameter determining the biocompatibility of concretes and cements is the pH environment. Therefore, a novel ratiometric pH optode imaging system based on an inexpensive single-lens reflex (SLR) camera was used to characterize the pH of porewater within cracks of submerged hydrated oil and gas well cement. This enabled the imaging of pH with a spatial distribution in high resolution (50 µm per pixel) and a gradient of 1.4 pH units per 1 mm. The effect of fly ash substitution and hydration time on the pH of the cement surface was evaluated by this approach. The results show that pH is significantly reduced from pH >11 to below 10 with increasing fly ash content as well as hydration time. The assessment of bioactivity in the cement was evaluated by introducing superabsorbent polymers with encapsulated Bacillus alkalinitrilicus endospores into the cracks. The bacterial activity was measured using oxygen optodes, which showed the highest bacterial activity with increasing amounts of fly ash substitution in the cement, correlating with the decrease in the pH. Overall, our results demonstrate that the pH of well cements can be reliably measured and modified to sustain the microbial activity.

Potassium octahydridotriborate: diverse polymorphism in a potential hydrogen storage material and potassium ion conductor.

Octahydridoborate, i.e. [B3H8]- containing compounds, have recently attracted interest for hydrogen storage. In the present study, the structural, hydrogen storage, and ion conductivity properties of KB3H8 have been systematically investigated. Two distinct polymorphic transitions are identified for KB3H8 from a monoclinic (α) to an orthorhombic (α') structure at 15 °C via a second-order transition and eventually to a cubic (ß) structure at 30 °C by a first-order transition. The ß-polymorph of KB3H8 displays a high degree of disorder of the [B3H8]- anion, which facilitates increased cation mobility, reaching a K+ conductivity of ∼10-7 S cm-1 above 100 °C. ß-KB3H8 starts to release hydrogen at ∼160 °C, simultaneously with the release of B5H9 and trace amounts of B2H6. KBH4 and K3(BH4)(B12H12) are identified as crystalline decomposition products above 200 °C, and the formation of a KBH4 deficient structure of K3-x(BH4)1-x(B12H12) is observed at elevated temperature. The hydrogen-uptake properties of a KB3H8-2KH composite have been examined under 380 bar H2, resulting in the formation of KBH4 at T≥ 150 °C along with higher metal hydridoborates, i.e. K2B9H9, K2B10H10, and K2B12H12.