New Publication on Kaolinite Morphology under Dissolution

A new publication titled “Evolution of kaolinite morphology upon exfoliation and dissolution: Evidence for nanoscale layer thinning in metakaolin” was published in Applied Clay Science in March 2022.

Kaolinite and metakaolin dissolving under the optical microscope.

When kaolinite is calcined, it transforms into metakaolin, which can dissolve and react under alkaline conditions, making it a potential low-CO2 alternative to Portland cement. The reactivity of kaolinite has been studied in terms of dissolution kinetics, but not much is known about the evolution of clay morphology upon dissolution. Here, we apply quantitative imaging approaches to quantify the extent of morphological changes that occur in dissolving kaolinite and metakaolin at multiple scales. At the micro-scale, we successfully capture in situ exfoliation of clay particles while dissolving in NaOH. We find a noticeable difference in the pattern in which these clays break apart. Raw kaolinite would expand along its length, while layers in metakaolin were not as well defined as in kaolinite. At the nano-scale, when comparing the layer thickness of metakaolin and dissolved metakaolin, an evident thinning of ~20 nm (from 95 nm to 75 nm) is found. These results explain how the dissolution process takes place on these layered structures: by breaking the bonds in-between layers and then dissolving these individual layers leading to a reduction in thickness. These new results pave the way towards a morphological understanding of calcined clay dissolution.

Illustration of a kaolinite particle with reduced layer thickness after calcination and dissolution. These results bring clarity about the nanoscale changes in this clay when subjected to dissolution.

This is the first article from our group’s M.S. candidate Pablo Romero. Congratulations Pablo!

The article can be accessed here.

New Publication on Alkali-Activation of Metakaolin

A new publication titled “Impact of Na/Al Ratio on the Extent of Alkali-Activation Reaction: Non-linearity and Diminishing Returns” was published in Frontiers in Chemistry in January 2022.

A snapshot of results from 27Al MAS NMR of various AAM mixtures, illustrating a direct effect of the overall Na/Al ratio on the local Al environments.

To address the high CO2 footprint associated with cement production, many alternative, sustainable binders are now gaining worldwide attention, including alkali-activated materials (AAM). The alkali-activation reaction of metakaolin is a fairly complex process involving transformation of one amorphous reactant (precursor metakaolin) into another amorphous product or products (N-A-S-H gel and/or disordered zeolite type phases). In spite of this complexity, researchers in the past 2 decades have gained significant knowledge on the nature of this reaction at multiple scales. Understanding and developing a clear relationship between the alkalinity of the mix and the extent of reaction is of high interest for practical applications. However, detailed and thorough investigations on this important relationship are limited. Here, in this study, we address this gap by systematically investigating a series of alkali-activated materials samples with a wide range of Na/Al ratios (0.5–1.8) using seven different yet complementary analytical techniques (isothermal calorimetry, FTIR, XRD, TGA, 27Al and 23Na NMR, and Raman imaging). Applied in tandem, these tools reveal a clear but non-linear relationship between the Na/Al ratio and the extent of alkali-activation reaction indicating diminishing returns at higher Na/Al ratios, where higher Na/Al ratios cause an increase in the degree of reaction until a certain point at which the increase in Na/Al ratio does not significantly affect the reaction kinetics, but may affect the gel polymerization. These findings could potentially aid decision making for commercial applications of AAMs where alkalinity of the mix is an important parameter for performance as well as safety.

This is the first article from our group’s MS candidate Omar Abdelrahman. Congratulations Omar!

The article can be accessed here.

New Publication on US Waste Streams

A new publication titled “National and Regional Waste Stream in the United States: Conformance and Disparity” was published in Environmental Research: Infrastructure and Sustainability in November 2021.

An illustration of the spatial distribution of current Waste-to-Energy (WTE) facilities along with the US population density plotted on a common map.

Accurate estimation of material classes – paper, food, plastic, yard, metal, and glass waste – present in the municipal solid waste stream is critical for efficient waste management. The generation estimates for these material classes (both composition and quantity) are estimated via two approaches, the material-flow-based estimates and site-specific estimates. In the United States, the U.S. EPA’s material flow-based predictions yield MSW generation estimates for the entire nation, whereas site-specific estimates yield MSW generation estimates on a regional scale, i.e., states and counties. In the past, several studies had indicated that the U.S. EPA’s material-flow-based predictions differ substantially from the aggregated tonnage of MSW managed by waste handling facilities in the United States. However, the material-class-specific factors that led to these discrepancies are not apparent. In this study, we uncover the basis of these discrepancies by comparing national MSW generation estimates with the site-specific MSW general estimates. Specifically, our analysis suggests that the material-flow-based estimates are accurate for food, plastic, and glass material classes. In contrast, we find that the material-flow-based predictions underestimate paper waste disposal by at least 15 million tons annually. Based on these insights, the material-flow-based MSW estimation framework can be refined to yield better MSW generation estimates. A thorough estimation of waste is the key to efficient waste management.

This is the second article from our group’s Ph.D. candidate Vikram Kumar. Congratulations Vikram!

The article can be accessed here.

New Publication on Reactivity of Incineration Ashes

A new publication titled “The Chemical and Physical Origin of Incineration Ash Reactivity in Cementitious Systems” was published in Resources, Conservation and Recycling in October 2021.

Incorporating industrial byproducts and waste in concrete is the key to reducing landfill usage as well as lowering the environmental footprint of cement industry. An emerging industrial byproduct which can partly replace cement is the Municipal Solid Waste Incinerator Ash (MSWI ash: residue that is left after incineration of municipal solid waste in a Waste-to-Energy facility). These ashes are predominantly calcium-rich; however, they also contain additional elements whose speciation is not known. These elements can significantly alter the hydration characteristics of a cementitious system. Our initial foray into cementitious matrices including these ashes, reveals that these ashes can  accelerate as well as retard  cement hydration. Specifically, Pb, Br, S, Ca, and Cl appear to accelerate cement hydration, whereas Cu, Fe, Al, Ti, Si, K, Zn, and Sr appear to retard cement hydration. Changes in hydration characteristics can have a strong bearing on the physical characteristics of cementitious systems incorporating incineration ashes. Thus, to selectively screen ashes that synergistically enhance the physical characteristics, we introduce a novel “Incineration Ash Coefficient (IAC),” which shows a strong correlation with the compressive strength (R2=0.79) of cement-ash binary mixtures.

This is the first article from our group’s Ph.D. candidate Vikram Kumar. Congratulations Vikram!

The article can be accessed here.

New Publication on Raman Imaging of Anhydrous Cements

A new publication titled “Enabling Phase Quantification of Anhydrous Cements via Raman Imaging” was published in Cement and Concrete Research in September 2021.

Quantifying the mineral phase composition of an anhydrous cement is essential in determine/predicting the hydrated phase assemblage which consequently governs the overall performance of hardened concrete. Traditional techniques such as X-ray diffraction, optical microscopy, and electron microscopy are well suited to quantify phases in anhydrous cements but they may have some sample-specific limitations in certain scenarios. Here, we demonstrate Raman imaging as a complementary tool for quantitative phase analysis on 11 different cements. Using sufficient statistics (250,000 spectra per image, 5×5 mm area scans with 10 μm/pixel in each image), we were able to accurately quantify the 4 principal phases (alite, belite, aluminate, and ferrite) as well as (up to) 8 secondary phases (gypsum, anhydrite, bassanite, syngenite, dolomite, calcite, quartz, and portlandite) in a broad variety of cements. These results pave the way for future application of Raman imaging for phase quantification in other complex mixtures and systems.

This is the second article from our group’s Ph.D. candidate Krishna C. Polavaram. Congratulations Krishna! 

The article can be accessed here.

New Publication on Raman Imaging of Granites

A new publication titled “High-fidelity and high-resolution phase mapping of granites via confocal Raman imaging” was published in Scientific Reports in April 2021.

Granites are one of the most abundant silicates on Earth’s crust, and they can often be found in concrete mixtures where siliceous aggregates have been used. Understanding the mineral phase composition of these complex rocks is a key requirement to predict their tolerance to long-term radiation in a nuclear power plant. However, obtaining accurate phase maps from traditional petrographic methods as well as newer elemental mapping methods has a series of limitations. Here, we report a methodology that allows direct mineralogical mapping and fingerprinting using Raman spectroscopy and imaging. Our results enable high-resolution and high-fidelity spatial mapping of minerals at the sub-micron scale, opening up pathways to rapidly assess and quantify the mineralogical composition of samples that require minimal sample preparation.

This is the first article from our group’s Ph.D. candidate Krishna C. Polavaram. Congratulations Krishna!

The article can be accessed here.

New Publication on C-S-H

A new publication titled “Nanoscale Ordering and Depolymerization of Calcium Silicate Hydrates in the Presence of Alkalis” was published in The Journal of Physical Chemistry C in September 2019.

Sustainable cements like alkali-activated materials often contain non-negligible amounts of alkalis (Na or K) which significantly influence the resulting material’s performance. However, the precise role of these alkalis is not fully understood. In this publication, using a combination of X-ray PDF and NMR, we present evidence on the silicate polymerization and the structure of the CNASH gel. Additionally, we also report novel data on the long-range atomic ordering of a series of 45 synthetic gels.

The article can be accessed here.

New Publication on Clay Dissolution

A new publication titled “Dissolution Kinetics of Calcined Kaolinite and Montmorillonite in Alkaline Conditions: Evidence for Reactive Al(V) Sites” was published in Journal of the American Ceramic Society in July 2019.

Using solid-state Nuclear Magnetic Resonance spectroscopy and Inductively-coupled Plasma – Optical Emission Spectroscopy, we followed the dissolution kinetics of calcined kaolinite and montmorillonite. It was shown that dissolution kinetics are correlated with pozzolanic reactivity and dissolution rates are strongly influenced by the presence of reactive Al(V) sites.

The article can be accessed here.

New Publication on C-S-H

A new publication titled “Symmetry-Induced Stability in Alkali-Doped Calcium Silicate Hydrate” was published in The Journal of Physical Chemistry C in May 2019.

Using first-principles quantum chemistry calculations on the model crystalline phase clinotobermorite, it was shown that there is a strong interplay between the thermodynamic stability of alkali-doped C-S-H and the symmetry of the alkali atoms in the structure. The article can be accessed here.