Calcium silicate hydrate (C-S-H) is the main constituent of hydrated cement paste and determines its cohesive properties. Because of the environmental impact of cement industry, it is more and more common to replace a part of the clinker in cement by secondary cementitious materials (SCMs). These SCMs are generally alumina-rich and as a consequence some aluminum is incorporated into the C-S-H. This may have consequences on the cohesion and durability of the material, and it is thus of importance to know the amount and the location of Al in C-S-H and what the parameters are that control these features. The present paper reports the (29)Si and (27)Al MAS NMR analyses of well-characterized C-A-S-H samples (C-S-H containing Al). These samples were synthesized using an original procedure that successfully leads to pure C-A-S-H of controlled compositions in equilibrium with well-characterized solutions. The (27)Al MAS NMR spectra were quantitatively interpreted assuming a tobermorite-like structure for C-A-S-H to determine the aluminum location in this structure. For this purpose, an in-house written software was used which allows decomposing several spectra simultaneously using the same constrained spectral parameters for each resonance but with variable intensities. The hypothesis on the aluminum location in the C-A-S-H structure determines the proportion of each silicon site. Therefore, from the (27)Al NMR quantitative results and the chemical composition of each sample, the intensity of each resonance line in the (29)Si spectra was set. The agreement between the experimental and calculated (29)Si MAS NMR spectra corroborates the assumed C-A-S-H structure and the proposed Al incorporation mechanism. The consistency between the results obtained for all compositions provides another means to assess the assumptions on the C-A-S-H structure. It is found that Al substitutes Si mainly in bridging positions and moderately in pairing positions in some conditions. Al in pairing site is observed only for Ca/(Si+Al) ratios greater than 0.95 (equivalent to 4 mmol.L(-1) of calcium hydroxide). Finally, the results suggest that penta and hexa-coordinated aluminum are adsorbed on the sides of the C-A-S-H particles.

No abstract

Structural characterization of calcium silicate hydrate (C-S-H) is of major importance, as it is the main constituent of Portland cement and is responsible for its principal cohesion and durability properties. In this paper we apply homonuclear and heteronuclear solid-state NMR methods for the investigation of C-S-H. We use double quantum homonuclear 29Si−29Si correlation using the BAck to BAck recoupling scheme to gain information about connectivity between silicates and 2D 1H−29Si HETeronuclear chemical shift CORelation (HETCOR) to characterize the proton environment of the silicate chains. Our observations are in agreement with the tobermorite model and with previous results, but we get evidence of a continuous decrease of the silicate chain length with increasing Ca/Si ratio rather than a bimodal distribution of chain length.

NMR cross-polarization (CP) measurements are usually analyzed assuming that the cross-polarization time T IS of magnetization transfer from the abundant I spins to the rare S spins is shorter than the relaxation time T 1F in the rotating frame of the I spins (fast CP regime). Here, it is shown that the reverse situation (T IS > T 1F I , slow CP regime) may occur, for instance, for the 1 Hf 29 Si transfer in commonly encountered inorganic materials and that analyzing the experimental data in this case under the usual fast CP assumption will, beside leading to erroneous dynamic parameters, underestimate the number of spins by a factor ∼T IS /T 1F . Experimental ways to distinguish between the two situations are presented, the most efficient being to resort to the TORQUE experiment. Examples are given on silica gel and calcium silicate hydrate (CSH) samples for which the proper analysis allows a deeper insight into the nature of the protonated surfaces. This slow CP regime may be expected to occur also in some organic materials, for instance in 13 C NMR of polyaromatic compounds.

No abstract

The production of molecular hydrogen in the radiolysis of dried or hydrated nanoporous controlled-pore glasses (CPG) has been carefully studied using 10 MeV electron irradiation at high dose rate. In all cases, the H2 yield increases when the pore size decreases. Moreover, the yields measured in dried materials are two orders of magnitude smaller than those obtained in hydrated glasses. This proves that the part of the H2 coming from the surface of the material is negligible in the hydrated case. Thus, the measured yields correspond to those of nanoconfined water. Moreover, these yields are not modified by the presence of potassium bromide, which is a hydroxyl radical scavenger. This experimental observation shows that the back reaction between H2 and HO* does not take place in such confined environments. These porous materials have been characterized before and after irradiation by means of Fourier-transform infrared (FT-IR) spectroscopy, electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) techniques, which helps to understand the elementary processes taking place in this type of environment, especially the protective effect of water on the surface in the case of hydrated glasses.

No abstract

Ag19 cluster ions are mass selected and deposited on a Pt(111) surface covered by five monolayers of Kr. Almost monodispersed hexagonal shaped Ag islands are observed after Kr evaporation at 125 K. The identification of the island shape and the exact number of atoms has been successful by decorating the clusters with Kr atoms which can be counted by high resolution scanning tunneling microscopy.