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Not surprisingly, with the increased awareness of environmental issues, construction with raw (crude, unbaked) earth (subsoil) is gaining renewed interest. However, it suffers from a poor image and from the difficulty to meet modern productivity standards and to pass some durability tests designed for industrial materials. The recent trend is to overcome these drawbacks by “stabilizing” the material most often with Portland cement (PC). Here we show that stabilization with PC is in general neither technically nor environmentally advisable. It brings only moderate mechanical improvement at a high environmental cost. Rather than massively transforming crude earth into a low quality concrete, it would be more appropriate to adapt the architectural practice and/or to look for milder ways to improve properties. In this respect, the recent successful attempts to improve the workability and the strength of raw earth by controlling the dispersion of its fine fraction seem to be particularly promising.

Reply to the discussion of the paper “Earth Concrete. Stabilization Revisited” by A.T.M. Marsh, A. Heath, P. Walker, B.V. Venkatarama Reddy, and G. Habert.

A thermoresponsive copolymer solution is used to control the mechanical properties of a cement paste suspension. We use a comb copolymer consisting of a polymethacrylic acid (PMAA) backbone grafted with a polypropylene oxide (PPO) and polyethylene oxide (PEO) copolymer that possesses a low critical solution temperature (LCST). When the temperature crosses the LCST value, microphase separation occurs and the adsorption of the copolymer onto the cement particles is modified. We show that a control of the grafting ratio and of the graft chain composition allows monitoring of the transition temperature as well as the viscosity of the paste in the low and high temperature phases.

International audience; The study presented in this paper is devoted to improve the knowledge on the influence of cellulose ethers (CE) on the freshly-mixed mortars water retention. Indeed, this crucial property is the most important imparted by these polysaccharides. One of the assumptions proposed to explain this phenomenon is that CE acts as diffusion barrier to the water. To test this hypothesis, the CE effect on the self-diffusion coefficient of water in solution and on the water mobility between two fresh cement pastes was studied by Nuclear Magnetic Resonance. CE does not significantly modify the water self-diffusion coefficient in CE solution or in admixed cement pastes. Moreover the interdiffusion imaging experiments demonstrated that the water diffusion at the paste/paste interface is not affected by the presence of cellulosic admixture.

The influence of secondary phases (illite, quartz) on the geopolymerization reaction of metakaolin has been investigated by comparing two metakaolins, one prepared from a pure kaolinite and the other from illite- and quartz-containing Algerian kaolin from the Tamazert region, respectively. Geopolymerization was achieved by mixing the metakaolins with an alkaline sodium silicate solution at room temperature and curing at 50 °C. The products were characterized by X-ray diffraction and 29 Si and 27 Al MAS-NMR. The results show that the secondary phases, at the concentration used in this work, do not prevent the geopolymerization reaction.

We analyze, in the light of experiments, a few typical aspects of the rheological or mechanical behavior of pastes, defined as dense and liquid-saturated assemblies of grains, with or without physical–chemical interactions. Emphasis is placed on the consequences of jamming, which distinguishes pastes from concentrated suspensions, contact lubrication, which distinguishes pastes from dry granular media, and cohesion. The points addressed are the following: the onset of flow, the contribution of friction in the yield stress, solid–liquid segregation and compaction, adhesion (tackiness) and cohesive rupture. To cite this article: H. Van Damme et al., C. R. Physique 3 (2002) 229–238.

The current research in herbicide application aims to develop formulations that reduce leaching of the herbicide to deep layers of the soil and to concentrate its biological activity at the top layers. Adsorption of metolachlor on clay minerals, their organic derivatives or pillared forms provides the best possibility to develop slow-release formulations. Metolachlor is a selective pre-emergence herbicide widely used in irrigated crops to control annual weeds. It is adsorbed by bentonites and montmorillonites, but the amount adsorbed strongly depends on the type of bentonite and possible pretreatment reactions. Wyoming bentonites adsorbed considerable amounts of metolachlor but other bentonites did not bind this herbicide. An acid-activated pillared montmorillonite was also an effective adsorbent of metolachlor. Modification of this sample by preadsorbing different amounts of benzyl trimethylammonium ions did not influence the level of herbicide adsorption. The biological efficiency of the formulations was tested with bioassay soil columns. Slow-release formulations could be prepared with raw bentonites and the acid-activated pillared montmorillonite. A formulation, prepared by adsorbing metolachlor from aqueous solution on the acid-activated pillared montmorillonite, showed high herbicide activity at the top 10 cm, and did not diffuse significantly to greater depths. This formulation should allow a better weed control than the commercial formulations.

We show experimentally that a vertically vibrated granular packing can be densified, even when saturated by a Newtonian fluid which reduces the effective buoyancy. Densification is slower than in a dry packing, due to viscous drag, but the final density is higher, due to lubrication. The evolution of the packing density follows the global logarithmic time dependence observed with dry packings, but an additional density-dependent parameter had to be introduced.

Interactions between an acid-activated pillared clay and several organic cations including dyes (methylene blue, MB; crystal violet, CV; acriflavin, AF) and benzyl derivatives (benzyltrimethylammonium, BTMA; benzyltriethylammonium, BTEA) were studied by adsorption measurements and X-ray diffraction. When the dyes were adsorbed from low ionic strength solutions, adsorption was irreversible but saturated at levels below the cation exchange capacity (CEC) of the clay (0.6 meq/g). The difference with CEC value was largest for CV. This mode of adsorption was interpreted in terms of interlayer adsorption with steric hindrance in the pillared galleries. On the other hand, when the dyes were adsorbed from high ionic strength solutions, adsorption levels well beyond the CEC of the clay could be reached, in particular for MB and CV. This was interpreted in terms of a second adsorption mode, involving formation of molecular aggregates on the outer surface of the clay, as evidenced by X-ray diffraction. The behavior of the cationic benzyl derivatives was markedly different, with an adsorption level always below the CEC and a decrease of adsorption as the ionic strength was increased, as expected for non-complex-forming cations. Copyright 1999 Academic Press.

Six silica gels, where each gel had a characteristic calibrated pore size (40-4000 Å), have been characterized both by water vapor and nitrogen adsorption isotherms. From these results, it was concluded that two-thirds of the silica surface is hydrophobic. The selection of wide pores (300 Å) has enabled the determination of both the water and nitrogen t-curves for porous silica. It was observed that the development of the multilayer was identical in the wide pores irrespective of their size between 300 and 4000 Å. The porous silica t-curve for water coincided exactly with the t-curve of nonporous adsorbents provided that their BET C constants were similar. For nitrogen t-curves, a matching BET C constant ensured similarity only in the monolayer region, above which divergence progressively increased, becoming important close to saturation. The effect of the t-curve choice on the pore size distribution was established. A t-curve displaying reduced adsorption had a tendency to shift the pore-size distribution to lower dimensions, toward the micropore region. As a consequence, the cumulated surface area obtained from the BJH model gave increasingly higher values than the BET nitrogen surface area. However, the pore-size distribution was shifted to higher pore sizes when the selected t-curve was above the natural t-curve. Errors as much as 25% were detected for the mean pore radius and cumulated surface area for certain literature t-curves. A comparison of the water and the nitrogen t-curves indicated a crossover point when the water multilayer thickness became greater than the nitrogen thickness. Such behavior lends support to the cooperative adsorption mechanism for water vapor once a fixed number of water molecules (two layers) are present on the surface. Copyright 1998 Academic Press.

Resume Nous montrons experimentalement que l'on peut fracturer, par injection d'un liquide ou d'un gaz, des fluides visco-elastiques possedant des proprietes elastiques extremement faibles. La vitesse de propagation des fractures peut etre tres proche de la vitesse de propagation des ondes de cisaillement et conduire a un mode de fracturation en fractures multiples et branchees. Ces experiences pourraient servir de modeles pour l'etude des instabilites de fracturation hydraulique dans les roches.

The oxidation of water to oxygen by one-electron oxidants such as Ce4+, M( bpy)33” camp lexes (M = Fe, Ru, OS; bpy = 2,2’-bipyridine) or IrC16- is a mechanistically difficult reaction which, to occur at an appreciable rate, requires very active catalysts. Supported and unsupported noble metal oxides (PtOZ, IrO,, RuOz and their mixed oxides) have been used exten- sively for this purpose [ 1 - 71. Homogeneous catalysts, such as simple Co(I1) salts [ 4,8] or amine complexes [ 91, diaq~obis(bpy)Ru~II) [lo] or a diaquo- ~-0x0 dimer of Ru(III) [ll], have also been used with variable success. We wish to report here an OS-based molecular catalyst, covalently linked to a mineral surface. This type of catalyst is clearly of interest for the photo- chemical cleavage of water or for any energy-storing photoreaction which requires the transformation of oxidizing equivalents into oxygen.

The fluorescence decay of the phenoxy group of two monodisperse and one polydisperse (Triton X-100) polyoxyethylene alkyl phenols has been studied in aqueous solution, and in the adsorbed state on a hydrophilic (hydroxylated) or hydrophobic (methylated) silica surface. In aqueous solution, below the CMC, one single exponential decay is observed with a lifetime of 4.5 nsec. Above the CMC, a second exponential component with a lifetime of 37 nsec appears with the monodisperse compounds, but not with Triton X-100. This long decay is assigned to dimeric species and its amplitude is quantitatively correlated to the fraction of molecules involved in micellar aggregates. On the hydrophilic silica surface, the same long decay appears from the very beginning of the adsorption isotherm and its amplitude remains constant up to the plateau of the isotherm, close to the value observed in the pure liquid surfactant. This is taken as evidence that, from the outset of adsorption, the majority of molecules is involved in molecular aggregates. On the hydrophobic surface, the constant amplitude regime is observed only at high surface coverage. At low surface coverage, the amplitude increases linearly with θ, suggesting that, in this regime, the adsorbed phase can be described as a dilute gas.