In this study, the heterogeneous reactivity of nitric acid on oxide particles of some of the most abundant crustal elements is investigated at 296 K. The oxide particles are used as models for mineral dust aerosol found in the atmosphere. Transmission FT-IR spectroscopy is used to probe changes in the spectrum of the oxide particle surface following adsorption of HNO 3 on SiO 2 , R-Al 2 O 3 , TiO 2 , γ-Fe 2 O 3 , CaO, and MgO. It is found that HNO 3 molecularly and reversibly adsorbs on SiO 2 . For the other oxides investigated, HNO 3 dissociatively and irreversibly adsorbs to form surface nitrate. There is also a small amount of molecularly adsorbed nitric acid (<10% of the adsorbed nitrate) on the oxide particle surface in the presence of gas-phase nitric acid. Because adsorbed water may play a role in the heterogeneous uptake of nitric acid in the atmosphere, transmission FT-IR spectroscopy is used to investigate H 2 O adsorption on SiO 2 , R-Al 2 O 3 , TiO 2 , γ-Fe 2 O 3 , CaO, and MgO particles as well. Uptake of water on the oxide particles can be described by a multilayer adsorption isotherm. Water uptake on nitrate-coated oxides remained similar for R-Al 2 O 3 and TiO 2 compared to the uncoated surface; however, for γ-Fe 2 O 3 , CaO, and MgO, the shape of the adsorption isotherm changed for the nitrate-coated particles with an increased amount of water adsorption at a given relative humidity. The infrared spectrum of the surface nitrate shows that water adsorbed on the particle surface can solvate the nitrate ion. The rate of nitric acid uptake on R-Al 2 O 3 and CaO is found to increase by nearly 50-fold when going from conditions near 0 to 20% relative humidity, indicating that the nitric acid dissociation kinetics on the wetted particle surface is significantly enhanced. In the case of MgO and CaO, the amount of nitric acid uptake is increased in the presence of water and is not limited to the surface of the particles, producing saturated solutions of Mg(NO 3 ) 2 and Ca(NO 3 ) 2 . From the studies presented here, atmospheric implications of heterogeneous reactions of HNO 3 with mineral dust aerosol are discussed.

No abstract

No abstract

No abstract

No abstract

The addition of MgCl 2 to synthetic calcium silicate hydrate (C-S-H) with Ca/Si=0.8 lowers the pH values below 10. Experimental observations and thermodynamic calculations show the destabilization of C-S-H and the precipitation of magnesium silicate hydrate (M-S-H). In contrast, the addition of MgO to C-S-H increases the pH values up to 11.5 and the Ca/Si in C-S-H from 0.8 to ~1.0. At high pH, the formation of M-S-H and brucite is observed experimentally. Less brucite is present at 50 than at 20°C indicating a slow dissolution kinetic of brucite at 20°C.

No abstract

Zeolite formation caused by interactions between cement hydrates and rock forming minerals was investigated by two sets of batch experiments and supported by thermodynamic modelling. The first set of batch experiments investigated the interaction between calcium silicate hydrates (C-S-H) (Ca 0.8 SiO 2.8 •32H 2 O) and ettringite (Ca 6 Al 2 (SO 4) 3 (OH) 12 (H 2 O) 26) as cement hydrate minerals and albite (NaAlSi 3 O 8) as a rock forming mineral at 20, 50 and 80°C. The dissolution of C-S-H, ettringite and albite led to relatively high calcium and low silicon and sodium concentrations and to the formation of zeolite P(Ca) (Ca 2 Al 2 Si 2 O 8 4.5H 2 O) and natrolite (Na 2 Al 2 Si 3 O 10 2H 2 O). The second set of experiments used ettringite and silica fume as cement phases and NaAlO 2 to represent a rock forming mineral. High initial sodium, hydroxide and aluminium concentrations were observed leading to the precipitation of zeolite X (Na 2 Al 2 Si 2.5 O 9 6.2H 2 O) and C-S-H gel at 20 and 50°C where only 40 to 60% of the silica had reacted after 3 years. At 80°C where more silica fume had reacted, the formation of SiO 2-rich zeolite Y (Na 2 Al 2 Si 4 O 12 8H 2 O) and chabazite (CaAl 2 Si 4 O 12 6H 2 O) was observed. Solubility products for the zeolite P(Ca), natrolite, chabazite, zeolite X and zeolite Y were obtained from the measured concentrations. Comparison with values published in the literature shows a high