Raquel Vigil de la Villa

Zeolite formation during the alkaline reaction of bentonite

The cement-bentonite interface is a highly dynamic region in engineered barrier systems (EBS) designed to isolate radioactive wastes. Two very different chemical environments are held together: a near-neutral clay mineral-calcite equilibrium-dominated system (hydrated bentonite system) and a thermodynamically unstable hyperalkaline one (cement). The interface is, therefore, in permanent disequilibrium due to the conditions of changing pH that are related to the different degradation steps of the cement. The formation of phillipsite-(Na,K), (Si/Al ratio of 1.8-2.4), has been found in a series of closed-system hydrothermal tests at 35-90 °C when the resulting pH was within the 11.7-12.6 range. The chemistry of the equilibrium solutions, rather than the crystallization substrata, controls the Si/Al atomic ratio and the type of zeolite that has been formed. This is based on the decrease of Si/Al at higher pHs and the predominance of phillipsite-Na. The formation of phillipsite-Na in these experiments is in agreement with the available thermodynamic data on zeolites.

Alteration of compacted bentonite by diffusion of highly alkaline solutions

Compliance of concrete – bentonite barriers is a key issue in the performance assessment of long-term underground storage of radioactive waste. The reaction of synthetic alkaline solutions (K–Na–OH and Ca–(OH) 2 ) interacting by diffusion with a Mg-saturated compacted FEBEX-bentonite column of 2.1 cm thickness was investigated through closed-system experiments at 60 °C over 6 and 12 months. The first few millimeteres near the surface were grated to resolve mineralogical changes at sub-mm resolution determined by XRD, SEM, BET and the analysis of exchangeable cations. Alkaline cations diffused beyond the mineralogical alteration zone (2–2.5 mm), and were exchanged by Mg 2+ in the interlayer region of montmorillonite, but no K-silicates were formed. The presence of minor zeolites in the alteration assemblage was only detected by XRD on samples treated with ethylene-glycol. A cemented rim mixed with poorly ordered clay materials, mainly brucite, a chlorite-like phase and Mg-smectite was found at the interface. Montmorillonite was partially dissolved and a part of it remained shielded by the newly-formed cementation crusts. Cation diffusion through the interlayer of montmorillonite is inferred to be the dominant transport pathway in compacted bentonite equilibrated with external high pH solutions. The high pH front diffused at a much slower rate due to buffering of mineral reactions.

CHEMISTRY OF Mg SMECTITES IN LACUSTRINE SEDIMENTS FROM THE VICALVARO SEPIOLITE DEPOSIT, MADRID NEOGENE BASIN (SPAIN)

The chemical and structural properties of Mg smectites in the Vicálvaro sepiolite deposit have been studied in detail. The characterization was performed on different size-fractions of selected smectitic samples (5−2 µm; 2−1 µm; 1−0.5 µm; <0.5 µm and <0.1 µm). The chemical compositions of individual particles (5−1 µm) and of bulk undifferentiated fine fractions (1−<0.1 µm) were determined by energy dispersive spectroscopy-scanning electron microscopy and interpreted with the aid of X-ray diffraction (XRD) and infrared spectroscopy (IR) methods. The XRD and IR data demonstrate that all of the Mg smectite materials studied are mainly composed of a complex mixture of stevensite, saponite and mica-type minerals. Although the presence or absence of saponite cannot be confirmed absolutely, stevensite is a significant component of these Mg smectites. This is proven by the calculated layer charge reduction after the Hofmann-Klemen effect. The results are in close agreement with the suggested mechanism of topotactic overgrowth of stevensite on pre-existing phyllosilicate templates. This characterizes clay diagenesis in saline-lake systems.

Recycling of Waste Paper Sludge in Cements: Characterization and Behavior of New Eco-Efficient Matrices

The pulp and paper industry, in Europe, generates 11 million tons of solid waste each year (Monte et al., 2009). Paper waste covers a diverse range of non-hazardous waste streams, prominent among which are different types of sludge, boiler ash, combustion furnace ash and organic and inorganic rejects. Manufacturing processes to produce new paper from the deinking of recycled paper account for 70% of these waste products. Following its reception, sorting and storage, the recycled paper is transformed into an aqueous suspension of fibers, while inappropriate materials are eliminated in different cleaning processes. Following this initial treatment, the resultant paper sludge is subjected to deinking in a froth flotation process, which produces waste known as de-inked sludge. This waste sludge is fundamentally composed of water, fiber, ink and a mineral load. In addition, various paper manufacturing processes have water treatment plants that generate sludges with high humidity contents. The deinked paper sludge and the sludge from the water treatment process have a high humidity content (≈ 50%), and are roughly composed of organic material with their origin in paper fibers (≈ 25%) and mineral loads such as calcium carbonate, kaolin, talc and titanium oxide (≈ 25%). A similar composition highlights the wealth of energetic and mineral resources saturating the paper sludge. Thus, the most advanced techniques for the use of paper sludge are intended to take full advantage of the saturated biomass and the recovery of the mineral constituents in the inorganic fraction. The most common options for the processing of paper industry sludge range from their exploitation for agricultural purposes, composting, or use as a primary material in the manufacture of ceramics and cement (Moo-Young & Zimmie, 1997; Ahmadi & Al-Khaja, 2001; Lima & Dal Molin, 2005; Conesa et al., 2008), to energy recovery in biomass boilers or fluidized bed systems. Thus, the Dutch CDEM process (International Patent, 2006) represents a pioneering recovery system, where the paper sludge is treated at temperatures of around 730oC, in a fluidized bed combustion system, so as to activate the latent

Fly ash and paper sludge on the evolution of ternary blended cements: mineralogy and hydrated phases

AbstractNatural pozzolans are defined as either raw (volcanic material, limestone) or calcined natural materials (burnt shale, calcined kaolinite) with pozzolanic properties. Historically, they are among the oldest materials that have been used in combination with lime for construction purposes. For example, Santorin earth is a natural pozzolan from a volcanic eruption around 1500 BCE on an island of the same name in Greece. Over the last decades, research into cement matrices has been changing direction, owing to deeper environmental concerns over the impact of the cement industry and its consequences for climate change. Global environmental policies are increasingly strict, prioritizing the re-use rather than the disposal of industrial wastes. In this context, one of the biggest challenges is the search for strategies that promote the industrial rotation of high volumes of wastes in their productive cycles. The main purpose of this research is to widen the existing knowledge base on the behavior of new ...

Forced Aging and Ionic Mobility of Ternary Cements Exposed to Aggressive Saline Marine Environments and Cryoclastic Processes

The most common supplementary cementitious materials used in concrete, in addition to Portland cement, are fly ash and silica fume; metakaolin, produced by controlled thermal treatment of kaolin, is also used in cementitious composites on account of its pozzolanic properties. This study forms part of an extensive research into the degradation of uncommercialized pozzolanic ternary Portland cement (TPC) exposed to different saline marine environments and cryoclastic processes. In marine environments, CSH gels from ordinary Portland cement fill the gaps between the grains and cover the surfaces. However, the porosity of ternary Portland cement is notably higher and permits the retention of ions. Cracking as a consequence of cryoclastic processes is found in the CSH gels and ettringite of ordinary Portland cement in continental climates. However, when ternary Portland cement is studied under similar conditions, the cracks are partially sealed by the gels without further rupturing due to the aforementioned processes.