Transformation of meta-stable calcium silicate hydrates to tobermorite: reaction kinetics and molecular structure from XRD and NMR spectroscopy

Understanding the integrity of well-bore systems that are lined with Portland-based cements is critical to the successful storage of sequestered CO2 in gas and oil reservoirs. As a first step, we investigate reaction rates and mechanistic pathways for cement mineral growth in the absence of CO2 by coupling water chemistry with XRD and NMR spectroscopic data. We find that semi-crystalline calcium (alumino-)silicate hydrate (Al-CSH) forms as a precursor solid to the cement mineral tobermorite. Rate constants for tobermorite growth were found to be k = 0.6 (± 0.1) × 10-5 s-1 for a solution:solid of 10:1 and 1.6 (± 0.8) × 10-4 s-1 for a solution:solid of 5:1 (batch mode; T = 150°C). This data indicates that reaction rates for tobermorite growth are faster when the solution volume is reduced by half, suggesting that rates are dependent on solution saturation and that the Gibbs free energy is the reaction driver. However, calculated solution saturation indexes for Al-CSH and tobermorite differ by less than one log unit, which is within the measured uncertainty. Based on this data, we consider both heterogeneous nucleation as the thermodynamic driver and internal restructuring as possible mechanistic pathways for growth. We also use NMR spectroscopy to characterize the site symmetry and bonding environment of Al and Si in a reacted tobermorite sample. We find two [4]Al coordination structures at δiso = 59.9 ppm and 66.3 ppm with quadrupolar product parameters (PQ) of 0.21 MHz and 0.10 MHz (± 0.08) from 27Al 3Q-MAS NMR and speculate on the Al occupancy of framework sites by probing the protonation environment of Al metal centers using 27Al{1H}CP-MAS NMR

The SFA Economist Vol. 1 No. 2

https://scholarworks.sfasu.edu/economist/1001/thumbnail.jp

Forestry Bulletin No. 20: A Short History of Forest Conservation in Texas, 1880-1940

This short history of the development of forest conservation in Texas is intended primarily for the general reader. The study has concentrated on the early history of the conservation movement, dating from the beginning of commercial lumbering in the state, approximately 1880, to about 1940 and World War II.https://scholarworks.sfasu.edu/forestrybulletins/1015/thumbnail.jp

Driver Preference for Automatic or Manual Transmission Systems for Vehicles: A Case Study in Ghana

Systems in vehicles are being modified due to current technological trends. One of the commonest modified systems in vehicles was the transmission system. This study presented    views of drivers on preference for automatic or manual transmission vehicles in Ghana.   In all, data was collected from 1260 drivers selected randomly through survey, in-depth interviews and six focus group discussions. The study indicated that manual transmission vehicles were more in Ghana with the automatic now gaining relative patronage. However, automatic transmission vehicles were preferred by females than males. The first three factors that determined preference for automatic transmission were shifting quality, easy to use and power (engine) performance. However, for manual transmission vehicles repairability (easily reparable), reliability and better fuel economy were the first three factors that determined driver’s choice for such vehicles. The results further showed that most respondents believed that manual transmission vehicles had strong advantages over the automatic ones. The study concludes that more people prefer manual transmission vehicles than the automatic ones although the automatic ones are now gaining popularity. Key words:Automatic transmission, Manual Transmission, Vehicles, Preference, Ghan

A Methodology for the Indirect Determination and Spatial Resolution of Shear Modulus of PDMS-Silica Elastomers

ABSTRACT: A methodology is described that allows for the spatial resolution of shear modulus in silica-filled PDMS elastomers via 1 H relaxation measurements and stray-field imaging (STRAFI) techniques. Traditional Hahn echoes provide a simple, robust route to the extraction of a proton residual dipolar coupling constant (RDC), a direct measure of chain mobility and a parameter that can be corollated to numerous mechanical properties. Defining a dimensionless RDC eliminates any artifacts associated with low-field measurement and allows the RDC to become independent of field strength. A direct correlation between the NMR determined dimensionless RDC and results from dynamic mechanical analysis are presented, then employed via STRAFI to determine spatial variations in moduli associated with irradiated elastomeric materials. Reliable performance, despite poorly optimized STRAFI conditions, is demonstrated with an error of no more than 22% between the calculated shear modulus and the measured value via DMA