4D nanoimaging of early age cement hydration.

Despite a century of research, our understanding of cement dissolution and precipitation processes at early ages is very limited. This is due to the lack of methods that can image these processes with enough spatial resolution, contrast and field of view. Here, we adapt near-field ptychographic nanotomography to in situ visualise the hydration of commercial Portland cement in a record-thick capillary. At 19 h, porous C-S-H gel shell, thickness of 500 nm, covers every alite grain enclosing a water gap. The spatial dissolution rate of small alite grains in the acceleration period, ∼100 nm/h, is approximately four times faster than that of large alite grains in the deceleration stage, ∼25 nm/h. Etch-pit development has also been mapped out. This work is complemented by laboratory and synchrotron microtomographies, allowing to measure the particle size distributions with time. 4D nanoimaging will allow mechanistically study dissolution-precipitation processes including the roles of accelerators and superplasticizers.

Effect of Boron and Water-to-Cement Ratio on the Performances of Laboratory Prepared Belite-Ye'elimite-Ferrite (BYF) Cements.

The effect of superplasticiser, borax and the water-to-cement ratio on BYF hydration and mechanical strengths has been studied. Two laboratory-scale BYF cements-st-BYF (with ß-C2S and orthorhombic C4A3S¯) and borax-activated B-BYF (with α'H-C2S and pseudo-cubic C4A3S¯)-have been used, and both show similar particle size distribution. The addition of superplasticiser and externally added borax to BYF pastes has been optimised through rheological measurements. Optimised superplasticiser contents (0.3, 0.4 and 0.1 wt % for st-BYF, B-BYF and st-BYF with externally added 0.25 wt % B2O3, respectively) result in low viscosities yielding homogeneous mortars. The calorimetric study revealed that st-BYF is more reactive than B-BYF, as the values of heat released are 300-370 J/g and 190-210 J/g, respectively, after 7 days of hydration; this fact is independent of the water-to-cement ratio. These findings agree with the higher degree of hydration at 28 days of ß-C2S in st-BYF (from 45 to 60%) than α'H-C2S in B-BYF (~20 to 30%). The phase assemblage evolution has been determined by LXRPD coupled with the Rietveld method and MAS-NMR. The formation of stratlingite is favoured by increasing the w/c ratio in both systems. Finally, the optimisation of fresh BYF pastes jointly with the reduction of water-to-cement ratio to 0.40 have allowed the achieving of mortars with compressive strengths over 40 MPa at 7 days in all systems. Moreover, the st-BYF mortar, where borax was externally added, achieved more than 70 MPa after 28 days. The main conclusion of this work does not support Lafarge's approach of adding boron/borax to the raw meal of BYF cements. This procedure stabilises the alpha belite polymorph, but its reactivity, in these systems, is lower and the associated mechanical strengths poorer.

Quantitative disentanglement of nanocrystalline phases in cement pastes by synchrotron ptychographic X-ray tomography.

Mortars and concretes are ubiquitous materials with very complex hierarchical microstructures. To fully understand their main properties and to decrease their CO2 footprint, a sound description of their spatially resolved mineralogy is necessary. Developing this knowledge is very challenging as about half of the volume of hydrated cement is a nanocrystalline component, calcium silicate hydrate (C-S-H) gel. Furthermore, other poorly crystalline phases (e.g. iron siliceous hydrogarnet or silica oxide) may coexist, which are even more difficult to characterize. Traditional spatially resolved techniques such as electron microscopy involve complex sample preparation steps that often lead to artefacts (e.g. dehydration and microstructural changes). Here, synchrotron ptychographic tomography has been used to obtain spatially resolved information on three unaltered representative samples: neat Portland paste, Portland-calcite and Portland-fly-ash blend pastes with a spatial resolution below 100 nm in samples with a volume of up to 5 × 104 µm3. For the neat Portland paste, the ptychotomographic study gave densities of 2.11 and 2.52 g cm-3 and a content of 41.1 and 6.4 vol% for nanocrystalline C-S-H gel and poorly crystalline iron siliceous hydrogarnet, respectively. Furthermore, the spatially resolved volumetric mass-density information has allowed characterization of inner-product and outer-product C-S-H gels. The average density of the inner-product C-S-H is smaller than that of the outer product and its variability is larger. Full characterization of the pastes, including segmentation of the different components, is reported and the contents are compared with the results obtained by thermodynamic modelling.

Multiscale understanding of tricalcium silicate hydration reactions.

Tricalcium silicate, the main constituent of Portland cement, hydrates to produce crystalline calcium hydroxide and calcium-silicate-hydrates (C-S-H) nanocrystalline gel. This hydration reaction is poorly understood at the nanoscale. The understanding of atomic arrangement in nanocrystalline phases is intrinsically complicated and this challenge is exacerbated by the presence of additional crystalline phase(s). Here, we use calorimetry and synchrotron X-ray powder diffraction to quantitatively follow tricalcium silicate hydration process: i) its dissolution, ii) portlandite crystallization and iii) C-S-H gel precipitation. Chiefly, synchrotron pair distribution function (PDF) allows to identify a defective clinotobermorite, Ca11Si9O28(OH)2.8.5H2O, as the nanocrystalline component of C-S-H. Furthermore, PDF analysis also indicates that C-S-H gel contains monolayer calcium hydroxide which is stretched as recently predicted by first principles calculations. These outcomes, plus additional laboratory characterization, yielded a multiscale picture for C-S-H nanocomposite gel which explains the observed densities and Ca/Si atomic ratios at the nano- and meso- scales.

Differences in antimicrobial activity of four commercial 0.12% chlorhexidine mouthrinse formulations: an in vitro contact test and salivary bacterial counts study.

AIM: To evaluate the in vitro and in vivo antimicrobial activity of four commercial 0.12% chlorhexidine mouthrinses. MATERIAL AND METHODS: The in vitro antimicrobial activity test consisted in a modified contact test where 20 selected bacterial species were tested during 1 min with each test product. After the contact, the inoculum was cultured, and the results were expressed in terms of survival/resistance and the percentage of survival as compared to a saline control. The in vivo test consisted of a double-blind, randomized, crossover salivary bacterial counts study. 10 volunteers rinsed during 1 min with each tested product. Saliva samples were obtained before rinsing, and after 5 min, and 1, 3, 5 and 7 h. These samples were cultured both aerobically and anaerobically. Percentages of survival, in regard to baseline, were calculated for each time point. Comparisons among products were tested using anova and selected paired t-test. RESULTS: The in vitro contact test showed no survival in any tested species with CHX+CPC, while three species (Lactobacillus casei, Streptococcus mitis and Peptostreptococcus micros) were resistant to the other three products. CHX and CHX+NaF demonstrated additional resistant species (three and four species, respectively). The in vivo salivary bacterial counts test showed higher reductions of CHX+CPC and CHX+ALC in aerobic and anaerobic bacteria, lasting for 5 h. Significant differences were detected at multiple time points, when these two products were compared both with the control and the other tested products. CONCLUSION: Important differences in activity, among 0.12% CHX products, were detected by both in vitro and in vivo tests. The formulation with alcohol was more active than those without alcohol, excepting the formulation with CHX+CPC, in which the reformulation and addition of CPC not only compensate but rather increase the antimicrobial activity.

Ansiedad generalizada, ansiedad por separación y fobia escolar: el predominio de la terapia cognitivo-conductual / Generalized anxiety, separation anxiety and school phobia: the predominance of cognitive behavioural therapy

Se presentan los resultados de un meta-análisis sobre la eficacia del tratamiento para la ansiedad generalizada, ansiedad por separación, y fobia/rechazo escolar, de niños y adolescentes. Se analizaron 10 informes científicos, del período 1980-2002, que cumplieron los criterios de selección, originando 25 estudios independientes. Participaron 743 niños y adolescentes, de 5 a 17 años, con edad media de 11 años, y una proporción de género de 3 chicas por 2 chicos. El tratamiento, de diez semanas de duración media, alcanzó una eficacia global media-alta en el postest (d+= 0,78) y alta en un seguimiento medio de diez meses (d+= 1,06). Excepto un estudio, se aplicó siempre terapia cognitivo-coductual, sola o en combinación con intervención familiar o imipramina, resultando altamente eficaz (d+ = 0,87). Los componentes de los programas se mostraron muy eficaces, destacando el manejo de contingencias (d. = 1,53), la relajación (d+ = 1,29), la exposición (d. = 1,27), y las autoinstrucciones (d* = 1,07). Se discuten las implicaciones clínicas de estos hallazgos (AU)

Tratamientos psicológicos bien establecidos y de elevada eficacia: terapia de conducta para las fobias específicas / Highly effective well established psychological treatments :behaviour therapy for specific phobias

Se realizó un meta-análisis sobre la eficacia del tratamiento psicológico para las fobias específicas de niños y adolescentes. Se localizaron 14 informes científicos, entre 1960 y 2002, que originaron 42 estudios independientes. Participaron 662 niños y adolescentes, de 3 a 17 años, con edad media de 8 años y duración media del trastorno de 3,6 años. El tratamiento psicológico para las fobias específicas fue altamente eficaz (d+ = 0,97), manteniéndose la mejoría en el seguimiento (d+ = 1,21). Los procedimientos eficaces coincidieron con los más utilizados y fueron modelado (d+ = 1,99), usado principalmente en muestras subdínicas con fobias animales, exposición (d+ = 1,21), empleada más con muestras clínicas y mayor gama de fobias (animales, ambientales, dentales, etc.), y programas combinados (d+ = 1,47), aplicados a muestras de ambos tipos que presentaban sobre todo fobia a la oscuridad.La respuesta al tratamiento fue mejor en los más jóvenes, en el género femenino, en fobias animales, en casos sin problemas asociados y en fobias de aparición reciente. Se discuten las implicaciones clínicas de estos hallazgos. (AU)