Nature of C-(A)-S-H Phases Formed in the Reaction Bentonite/Portlandite

Abstract

Reactions between bentonite/montmorillonite and portlandite have been studied in the context of the engineered barriers of a purpose built repository for the deep geological disposal of spent fuel and high-level radioactive wastes. Portlandite was selected as the representative material of cement leaching in the early alkaline stage expected in a repository when conventional Ordinary Portland Cement (OPC) is used. Eight different batch experiments were performed for a reaction time near to two months, including bentonite or montmorillonite at montmorillonite/portlandite molar ratios of 2 : 1 and 3 : 1 under hydrothermal conditions. Temperatures of reactions were maintained constant at either 60 or 120°C. Calcium silicates hydrates with limited substitution of Al for Si (C-(A)-S-H phases with Al/Si <0.3) were formed with different structures and compositions as a function of the reaction conditions. Orthorhombic 11 Å-tobermorite-type phase was detected in experiments at 120°C while a more disordered monoclinic tobermorite formed at 60°C. These results are useful for the interpretation of experimental data in more complex experiments using concrete or cement pastes and bentonite, where C-(A)-S-H phases of variable compositions can precipitate, in addition to the characteristic cement hydrates and other secondary minerals carbonates.

The strength and microstructure of cemented sand-gravel (CSG) mixture containing fine-grained particles

“…Therefore, the hydration process of cement is weakened by presenting the bentonite particles. The phenomenon of a low Ca/Si ratio due to montmorillonite interaction with cement hydration in cemented materials was previously reported by Fernández et al [36].…”

“…The influence of bentonite plates on the cement hydration changes the participant of basic elements in the mixture. The participant ratio of calcium to silicon elements in the cemented admixture (denoted by the Ca/Si ratio) accounts for an indication for the hydration of cement and the ratio of Ca/Si increases by completing and intensifying the cement hydration in the mixture [, , [36][37][38]. From the EDX analysis, the ratio of Ca/Si in the S-C5-B10 mixture is reduced to 0.639 and, in turn, an increase in the participant of Na and Mg elements occurs.…”

Sorption mechanism(s) of orthophosphate onto Ca(OH)2pretreated bentonite

“…Additionally to that, the XRD pattern of CaT-B compared to the starting material, is characterized by three new peaks at 26.88 (3.314 Å), 30.30 (2.948 Å), 39.36 (2.287 Å) and a small shoulder at 29.37 (3.041 Å); a possible explanation for these peaks may tentatively related with the formation of a CSH phase like tobermorite with limited substitution of Al for Si (see Section 3.1.1), even if the most intense reection at 7.86 (11.253 Å) is missing. Tobermorite, has been commonly described as a reaction product between Ca(OH) 2 and aluminosilicate mineral like montmorillonite, especially under hydrothermal conditions within a wide range of temperatures, from 120 C to 300 C. [25][26][27] However, just recently it has been veried the formation of tobermorite at lower temperatures (60 C), which according to 28 may ascribed to selective silica (quartz or cristobalite) dissolution in bentonite and precipitation of C-(A)-S-H either in the porous areas or as thin coating lm (Fig. 2).…”

El ciclo hidrosocial : hacia un abordaje relacional y dialéctico del agua

“…Las dimensiones políticas del conocimiento hidrológico se están explorando cada vez más en la literatura. Algunos trabajos recientes en ecología política han demostrado la naturaleza parcial o el manipulado de los datos hidrológicos (Bakker, 2000;Budds, 2009;Kaika, 2003;Swyngedouw, 1995), y han revelado cómo los conceptos y estudios hidrológicos se influyen por las visiones específicas de qué es «la naturaleza» (Bouleau, 2014;Budds, 2009, Cohen y Davidson, 2011Fernández, 2014;Linton, 2004). Esta literatura emergente muestra cómo la hidrología se basa en las visiones «occidentales» de la naturaleza que reducen el agua a su composición material (H 2 O) (Linton, 2010), la homogeneización de diferentes aguas (Orlove y Caton, 2010) y la caractefización de procesos hidrológicos como ordenados y universales (Brown;Walker, 2005).…”