Showing papers by "Ángel Palomo published in 2012"

Effect of relative humidity on the reaction products of alkali activated fly ash

Abstract: The alkali activation of fly ash (AAFA) is a chemical process in which the ash is mixed with an alkaline activator and cured at a mild temperature to generate compact solids. Both the curing conditions (temperature, time, relative humidity, etc.) and the nature and concentration of the alkali activator play a key role in the development of AAFA micro- and nanostructure, and consequently the properties of these materials. In the present study, fly ash was activated with a 15% water-glass (Na 2 SiO 3 ) + 85% 10-M NaOH solution at 85 °C for 12 h, 7 and 30 days. Two curing methods were used, in which the variable was the relative humidity (RH).

C–S–H Gels: Interpretation of 29Si MAS‐NMR Spectra

Abstract: The detection of bridging tetrahedra, or Q2(L), in the silica chains constituting C–S–H gels is controversial. While some authors maintain that 29SiMAS-NMR can be used to distinguish between Q1 and Q2 units but not between the silicon environments associated with Q2 (intermediate position) and Q2(L) (intermediate and bridging position) units, others claim the contrary. The present article addresses this issue with a report on 29SiMAS-NMR studies of a number of calcium silicate hydrate gels. The findings showed that nuclear magnetic resonance can differentiate between Q2 and Q2(L) units in highly polymerized C–S–H gels with mean chain lengths (MCL) of over 12 and Ca/Si ratios of <1. At higher Ca/Si ratios, however, the MCL declined, with the concomitant rise in Q1 units. Under these conditions, in which it was practically impossible to distinguish between Q2 and Q2(L), the presence of a single signal for all Q2 units was assumed.

Alkaline Hydration of Tricalcium Aluminate

Abstract: This study explored the behavior of laboratory-synthesized tricalcium aluminate (C3A) in alkaline media. In all the trials, the C3A was hydrated with water containing a certain percentage of alkali activators (4% Na2CO3, 4% Na2SO4, or 8% Na2SO4 by weight of C3A). Two- and 28-day mechanical strength values were determined and the products were characterized with XRD, DTA, and NMR. The compressive strength of the C3A pastes rose in the presence of the alkaline activators used in this study. The alkalis induced the formation of calcium monocarboaluminate (C4ACH11) as the main hydration product in all cases. In addition, when Na2SO4 was added, a sodium calcium sulfoaluminate, phase U (C4ASNH16), formed. The presence of this mineral, whose formation is associated with high pH values, furnished indirect information on the alkalinity in the medium.

"Alkaline Activation" as a procedure for the transformation of fly ashes into cementitious materials. Part II. Immobilisation of toxic and hazardous elements

Abstract: In this study, the potential of alkali activated fly ash cements to immobilise metals that are generally not well immobilised in Portland cements was examined. A class F fly ash was activated with a solution of 8M NaOH to form a hardened cementitious paste. Prior to activation, fly ashes were doped with either: i) 5000 mg/kg (0.5%) Hg 2+ as HgCl2, ii) 10000 mg/kg (1.0%) Cs + as CsOH.H2O or iii) 10000 mg/kg (1.0%) As 3+ as NaAsO2. Mechanical strengths of the pastes were measured and the degree of metal immobilisation in pastes was determined by TCLP leaching tests. Finally, a study of the crystalline phases and paste microstructure was carried out to determine the fate of immobilised metals in the AAFA matrix. Microstructural analysis of Hg doped pastes showed a strong correlation between Hg and S, implying that formation of highly insoluble HgS or Hg2S precipitates is an important immobilisation mechanism. However, the coexistence of HgO could not be entirely ruled out. In Cs doped pastes, it is proposed that Cs + ions are associated with the N-A-S-H gel and zeolites formed in AAFA cements, suggesting that Cs is mainly chemically bound rather than physically encapsulated. With As doped pastes, generally poor performance was observed, both with and without the addition of Fe2O3 in an attempt to improve immobilisation.