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The formation of energetically favorable and metastable mineral phases within the Fe–H2O system controls the long-term mobility of iron complexes in natural aquifers and other environmentally and industrially relevant systems. The fundamental mechanism controlling the formation of these phases has remained enigmatic. We develop a general partial equilibrium model, leveraging recent synchrotron-based data on the time evolution of solid Fe(III) hydroxides along with aqueous complexes. We combine thermodynamic considerations and particle-morphology-dependent kinetic rate equations under full consideration of the aqueous phase in disequilibrium with one or more of the forming minerals. The new model predicts the rate of amorphous 2-line ferrihydrite precipitation, dissolution, and overall transformation to crystalline goethite. It is found that the precipitation of goethite (i) occurs from solution and (ii) is limited by the comparatively slow dissolution of the first forming amorphous phase 2-line ferrihydrite. A generalized transformation mechanism further illustrates that differences in the kinetics of Fe(III) precipitation are controlled by the coordination environment of the predominant Fe(III) hydrolysis product. The framework allows modeling of other iron(bearing) phases across a broad range of aqueous phase compositions.

ABSTRACT Ternary blends are currently used to improve the properties of Portland cement (PC). In this paper, thermodynamic modelling has been used to analyze the influence of the PC composition [...]

ABSTRACT The effect of the kind of calcium sulfate on the hydration of CSA cements was investigated. Thermal anhydrite, gypsum and a combination of anhydrite with gypsum were used at [...]

The transformation of 2-line ferrihydrite to goethite from supersaturated solutions at alkaline pH ≥ 13.0 was studied using a combination of benchtop and advanced synchrotron techniques such as X-ray diffraction, thermogravimetric analysis, and X-ray absorption spectroscopy. In comparison to the transformation rates at acidic to mildly alkaline environments, the half-life, t1/2, of 2-line ferrihydrite reduces from several months at pH = 2.0, and approximately 15 days at pH = 10.0, to just under 5 h at pH = 14.0. The calculated-first order rate constants of transformation, k, increase exponentially with respect to the pH and follow the progression log10k= log10k0+ a·pH3. Simultaneous monitoring of the aqueous Fe(III) concentration via inductively coupled plasma optical emission spectroscopy demonstrates that (i) goethite likely precipitates from solution and (ii) its formation is rate-limited by the comparatively slow redissolution of 2-line ferrihydrite. The analysis presented can be used to estimate the transformation rate of naturally occurring 2-line ferrihydrite in aqueous electrolytes characteristic to mine and radioactive waste tailings as well as the formation of corrosion products in cementitious pore solutions.

ABSTRACT The present study compares the cement hydration process and the mortar properties of samples containing calcium sulphoaluminate clinker and gypsum combined with quartz filler or limestone filler. Two different [...]

ABSTRACT The use of superplasticizers (SP) is widespread in today's concrete production. Usually, SP are used when specific demands in regard to workability, strength or durability have to be met. [...]

The production of cement is responsible for about 8% of man-made CO2emissions. CO2fixation by mineral carbonation in Ca- and Mg-rich raw materials such as cement-based concrete in various stages of its lifetime and magnesium silicate-based rocks (e.g. olivine) can provide a significant and long-lasting sink of CO2. Carbonated material can be used in novel construction materials, which potentially could even be CO2negative. Despite first applications beyond pilot stage, further research is needed to reach economically and environmentally friendly processes. The properties of the novel construction materials, their reaction mechanisms, suitable mix designs, mechanical properties, and durability need to be explored further.

The hydration of different blast-furnace slags with sodium metasilicate or NaOH as activator was investigated. Sodium metasilicate shows poor strength after 1 day, but high strength after 7 days and beyond, whereas NaOH activation leads to high strength after 1 day , but only moderate strength development at later ages. At the same hydration degree, sodium metasilicate activation gives a much higher compressive strength than NaOH activation. This is due to the formation of a dense hydration product in the NaOH-activated system at the early age, which leads to a more porous microstructure than in the case of sodium metasilicate. A higher MgO content in the slags is beneficial with respect to faster hydration kinetics and a more rapid strength development. Increasing Al2O3contents lead to slightly slower hydration kinetics and slightly lower compressive strengths. The best correlation between compressive strength and slag composition was found using the extended basicity (CaO + MgO)/SiO2index.

The early-age impacts of CO2activation (using a dose of 0.3% CO2by weight of cement) on two binder systems (tricalcium silicate and cement) were studied, principally across the first 3 h of hydration. The investigation included calorimetry, ICP-OES, TGA, and SEM. The in-situ mineralization of CO2accelerated the hydration of both the systems, with the effect being more pronounced in cement. During the first 30 min of hydration, the CO2addition impacted the solution phase pH and certain elemental concentrations (Ca, Mg, S). At later times of observation, the values were comparable to the level of the reference sample. Thermodynamic modeling revealed that the presence of CO2resulted in stronger undersaturation with respect to the binder phases, which implies a strong driving force for their dissolution. The carbonates were observed in SEM micrographs as a multitude of rhombohedral-shaped calcite crystals while TGA confirmed an increased quantity of carbonates in both binder systems.

•Test device for determining effective CO2diffusion coefficient was developed.•CO2and water vapour diffusion coefficients determined in concrete made from cement with 0 to 60 wt.% limestone.•Limestone content and CO2concentration (2% and 20% CO2) affect the porosity.•CO2and water vapour diffusion coefficients in concrete decrease with increasing compressive strength.

The partial replacement of Portland cement by aluminosilicate-rich supplementary cementitious materials (SCMs) represents a valuable approach to lower the CO2footprint of concrete. However, SCMs alter the composition and structure of the principal binding component, calcium-alumino-silicate-hydrate (C-(A)-S-H), a less-crystalline aluminum-substituted calcium silicate hydrate. The uptake and structural impact of alkalis and aluminum are investigated for synthesized C-(A)-S-H phases with different Ca/Si and Al/Si ratios and alkali (Na+) contents, using solid-state 23Na, 27Al, and 29Si NMR as principal analytical tools. 29Si{1H} CP/MAS NMR data propose that the −87 ppm resonance originates from a second type of Q2pairing sites most likely in the middle of octameric or longer silicate units. This assignment along with constraints induced by the silicate dreierketten structure results in reliable simulations of the 29Si NMR spectra. A quantitative 1:1 relation between the Al/Si ratios from 29Si NMR and the content of fourfold-coordinated Al from 27Al NMR is observed, which reveals that the 29Si Qp2(1Al) resonance is only associated with Al(IV) in C-(A)-S-H. Combined analysis of the 27Al and 29Si NMR spectra can account for sixfold-coordinated Al being present in the bridging sites of the silicate chains as proposed very recently. The sodium uptake is found to decrease with increasing Ca/Si ratio. For C-(A)-S-H phases with high Ca/Si ratios (Ca/Si ≳ 1.0), sodium promotes the incorporation of Al(IV) in the silicate chains but not at low Ca/Si ratios (Ca/Si ≲ 1.0). The results indicate that the mutual uptake of alkali and Al ions is governed by more than one mechanism and depends on the composition/structure of the C-(A)-S-H phase.

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