Silica

Exposure to high CO2 levels in enclosed environments may result in adverse health impacts. To provide a safe breathing environment, the exhaled gases must be removed. Currently, NASA uses a multi-bed system known as the Carbon Dioxide Removal Assembly (CDRA) for CO2 removal. The process involves cyclic adsorption-desorption using zeolite-5A molecular sieves. Owing to the presence of a wet gaseous mixture and the hydrophilic nature of zeolite-5A, the removal of CO2 and water vapor must be conducted in two separate vessels, resulting in additional costs. Therefore, the objective of this study was to integrate and intensify the process utilizing amine-grafted silica. Adsorbent performance was gauged on equilibrium CO2 uptake and kinetics, activation temperature, CO2 desorption temperature, and consecutive cycling in the presence of 1 vol.% CO2 in N2 at 25 °C. Aminosilica outperformed 5A and achieved similar equilibrium CO2 uptake while exhibiting faster kinetics, and lower desorption and regeneration temperature requirements.

In previous research, cements with high alkali content (EqA 1.0-1.2 percent) extended the
corrosion initiation time of reinforcing steel in concrete. During this study, laboratory
tests were performed to determine the suitability of high alkalinity cements to improve
concrete durability without modifying physical properties and to control the risk of
alkali-aggregate reaction (AAR).
A mix design for the FOOT-Class V concrete served as base material for this study. On a
cubic meter basis the cementitious material in this concrete included 363 kg of Type l/ll
Portland cement and 83 kg of Class F fly ash. The water-to-cementitious material ratio of
the concrete was 0.40. The fine aggregate used in the experimental concretes was quartz
sand from a Florida source with no history of alkali-silica reactivity (ASR) susceptibility. A number of cement alkali contents were prepared by different additions of sodium
hydroxide to the concrete mix (3.42 - 4.57 kglm\ in some cases, and by using different
cements in others. Thus, effects on concrete susceptibility to ASR, electrical resistivity,
and strength were studied. Pore water alkalinity was measured by ex-situ leaching and
pore water extraction methods. It was concluded that leaching procedures were not
appropriate to determine concrete pore water alkalinity in the presence of fly ash.
Results suggested that it is feasible to use high alkali cement without the risk of ASR or
the loss of strength for two of the seven coarse aggregates studied, given that
supplementary cementitious materials and lithium nitrate admixtures are utilized. Criteria
for qualification of a concrete as being ASR resistant was based on dimensional stability
(less than 0.01% average specimen length change) and the absence of cracking over the
one and two year exposure periods according to ASTM Cl293.
Based on the fundamentals of the electric double layer theory, the incidence of bivalent
cations adjacent to the surface of cement hydrates and reactive silica particles was
proposed to provide an explanation for the effects of alkali addition on the electrical
resistivity of concrete and the source of the expansive nature of the ASR gel.