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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Abstract for CRADA between NETL and Rivalia Chemical Co

The National Energy Technology Laboratory (NETL) will assist Rivalia Chemical Co (Participant) with system analyses to aid the Participant with setting priorities and securing funding. The Participant is an early-stage startup pioneering new chemical extraction technology to solve two important problems: critical mineral scarcity and coal ash waste management. The U.S. generates coal fly ash from coal combustion for power generation and has amassed over two billion metric tons of ash, which contains valuable rare earth elements. The ash is often stored in unlined ponds and frequently contaminates the local environment. The Participant’s patent-pending process harvests rare earths from ash, then transforms the residual ash for use in green concrete, providing an economic pathway for utilities to empty and remediate the ash ponds. Founder Laura Stoy created the company in July 2022 after developing the core intellectual property during her Ph.D. at the Georgia Institute of Technology. Stoy is currently participating in the Chain Reactions Innovations (CRI) program at the U.S. Department of Energy (DOE) Argonne National Laboratory (ANL) where she will be optimizing the core technology and scaling from bench scale to demonstration scale. The technology has been validated at bench scale (TRL3); the goal is to approach TRL5 by the conclusion of the CRI program and prepare to raise capital investment for a pilot facility. NETL and the Participant will develop an updated technoeconomic analysis (TEA) and identify key sustainability performance parameters to inform ’s research and development priorities and de-risk the technology. NETL will also provide the Participant with guidance to prepare a more detailed life cycle assessment (LCA), and a narrative on the Participant’s product markets. These tools will aid the Participant with the next steps of commercialization.

01 COAL, LIGNITE, AND PEAT↗
DOE OSTI2020

Sorption of rare-earth elements onto a ligand-associated media for pH-dependent extraction and recovery of critical materials

Toward recovering rare-earth elements, we prepared a solid-phase media that binds and elutes metals for extraction and enrichment. We report the synthesis and methodology for a solid-phase media that exhibits pH-dependent binding of aqueous rare-earth elements. The media shows pH-dependent binding of Nd III and retains efficiency over at least six cycles of binding and elution. Mixed solutions of rare-earth elements demonstrated a preference for mid and heavy rare-earth elements based on thermodynamic binding preferences of the ligand, and selectivity for rare-earth elements over iron and aluminum were observed with coal fly ash leachate. In conclusion, we expect this new recovery method to be a significant step towards aqueous-based extraction and enrichment of critically important rare-earth elements.

01 COAL, LIGNITE, AND PEAT↗
DOE OSTI2022

Rare earth elements from waste

Rare earth elements (REEs) are critical materials in electronics and clean technologies. With the diminishing of easily accessible minerals for mining, the REE recovery from waste is an alternative toward a circular economy. Present methods for REE recovery suffer from lengthy purifications, low extractability, and high wastewater streams. Here, we report an ultrafast electrothermal process (~3000°C, ~1 s) based on flash Joule heating (FJH) for activating wastes to improve REE extractability. FJH thermally degrades or reduces the hard-to-dissolve REE species to components with high thermodynamic solubility, leading to ~2× increase in leachability and high recovery yields using diluted acid (e.g., 0.1 M HCl). The activation strategy is feasible for various wastes including coal fly ash, bauxite residue, and electronic waste. The rapid FJH process is energy-efficient with a low electrical energy consumption of 600 kWh ton –1 . The potential for this route to be rapidly scaled is outlined.

01 COAL, LIGNITE, AND PEAT↗
DOE OSTI2021

Long-term cement hydration studies with isothermal calorimetry

The purpose of this study was to investigate for how long the thermal power of hydration can be measured with isothermal calorimetry, which is a general measurement technique with stable properties that, in many cases, can detect low rates of processes. Measurements were conducted for 365 days on cement paste with Portland cement, fly ash blended Portland cement and different water-to-binder ratios. The results showed that all the tested systems still produced a measureable signal after one year. The samples with Portland cement were also analyzed by XRD after 365 days, and phase composition calculations were made with GEMS. Together with the calorimetric results, these results were used to determine the degree of hydration by five methods, the results of which showed good agreement.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Hydration behavior of cements with reduced clinker factor in mixture with sulfoaluminate binder

CO{sub 2} regulation and raw material availability will strongly influence the future cement market and a wider use of composite cements containing limestone and pozzolan or slag is expected. In the present paper, the possibility to combine Portland cements (PCs) with reduced clinker factor with sulfoaluminate cement (CSA), is explored. Low Portland clinker cements are prepared in laboratory using different supplementary cementitious materials (limestone, slag, pozzolan and fly ash) and mixed with a sulfoaluminate cement: the hydration behavior of the resulting binders is investigated up to 90 days through a multi-technique approach involving X-ray diffraction, differential scanning calorimetry and nuclear magnetic resonance spectroscopy. Understanding the interaction mechanisms between SCMs and CSA/PC blends will support the development of innovative high performing binders based on CEM III or CEM V, or even on new standardized CEM II. Results highlight that slag is particularly suitable promoting late strength development.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Efficacy of SCMs to mitigate ASR in systems with higher alkali contents assessed by pore solution method

This study demonstrates the efficacy of supplementary cementitious materials (SCMs) to mitigate alkali-silica reaction (ASR) even when used in conjunction with cement of higher alkali contents (up to 1% Na{sub 2}O{sub eq}). The expansion of concrete prisms was studied immersed in simulated pore solution in order to address the limitations of conventional ASR testing methods, accelerated mortar bar test (AMBT) and concrete prism test (CPT). Expansion results demonstrate that 25% fly ash and 50% slag are both sufficient to mitigate ASR even with cements with alkali content up to 1% Na{sub 2}O{sub eq} and that the pore solution method is a viable alternative ASR testing method. Massive amounts of ASR products (~20 μm thickness) were observed in concretes without SCMs consistent with high degree of expansion and extensive cracking. Small amounts of ASR products (≤5 μm thickness) were also observed in concrete with SCMs despite absence of significant expansion.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Presence of aromatic-rich organic matter and its characterization in grout materials: Implications for radionuclide immobilization

Grout materials are commonly used to immobilize low-level radioactive waste. Organic moieties can be unintentionally present in common ingredients used to make these grout waste forms, which may result in the formation of organo-radionuclide species. These species can positively or negatively affect the immobilization efficiency. However, the presence of organic carbon compounds is rarely considered in models or characterized chemically. Here, we quantify the organic pool of grout formulations with and without slag, as well as the individual dry ingredients used to make the grout samples (ordinary Portland cement (OPC), slag and fly ash), including total organic carbon (TOC) and black carbon, followed by aromaticity evaluation and molecular characterization via Electro Spray Ionization Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry (ESI-FTICRMS). All dry grout ingredients contained significant amounts of organic carbon, ranging from 550 mg/kg to 6250 mg/kg for the TOC pool, with an averaged abundance of 2933 ± 2537 mg/kg, of which 60 ± 29% was composed of black carbon. The significant abundance of a black carbon pool implies the presence of the aromatic-like compounds, which was further identified by both phosphate buffer-assisted aromaticity evaluation (e.g., >1000 mg-C/kg as aromatic-like carbon in the OPC) and dichloromethane (DCM) extraction with ESIFTICRMS analysis. Besides aromatic-like compounds, other organic moieties were also detected in the OPC, such as carboxyl-containing aliphatic molecules. While the organic compound only consists of minor fractions of the grout materials investigated, our observations of the presence of various radionuclide-binding organic moieties suggests the potential formation of organo-radionuclides, such as radioiodine, which might be present at lower molar concentrations than TOC. Here, evaluating the role of organic carbon complexation in controlling the disposed radionuclides, especially for those radionuclides with strong association with organic carbon, has important implications for the long-term immobilization of radioactive waste in grout systems.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Overview of Upcoming Process Improvement Efforts at the Saltstone Facilities at the Savannah River Site - 20474

The Saltstone Facilities at the Savannah River Site (SRS) process and dispose of low activity salt solution waste generated by other liquid waste facilities. Today in the Liquid Waste System (LWS), approximately 125 million liters of salt waste remain to be processed. The Saltstone Production Facility (SPF) receives the decontaminated salt waste, which is mixed with dry feeds consisting of cement, fly ash and slag to produce the saltstone. The mixed slurry is pumped to the Saltstone Disposal Units (SDUs) for safe disposal in the Saltstone Disposal Facility (SDF). To support continuous Salt Waste Processing Facility (SWPF) operations at annual processing volumes many times higher than present, significant modifications are required to ensure that the SPF has the capacity to support the throughput of salt waste. These changes include several infrastructure upgrades that are being worked through the Enhanced Low Activity Waste Disposal (ELAWD) project. In addition to infrastructure upgrades, Savannah River Remediation (SRR), the Savannah River Site's Liquid Waste Contractor for the U.S. Department of Energy, has identified a flowsheet improvement opportunity of implementing a cement-free saltstone formulation to eliminate cement from the current three component formulation. This two-component formulation would simplify the procurement, transport, off-loading, and storage of the premix materials, and increase the storage capacity for the individual components. This flowsheet, when implemented, will reduce operational risk and improve throughput necessary to achieve disposal rates necessary for SWPF operations. Research has been completed to confirm the feasibility of this formulation, with the new cement-free flowsheet expected to be implemented in the next few years. Improvements have also been identified downstream of the SPF to increase SDU utilization. Specifically, a concentrated effort has taken place to raise the fill height in SDU 6 to the full disposal height, 13 meters, to maximize the usage of each SDU in support of the liquid waste disposal mission. This improvement has been made through additional thermal modeling and improved process understanding to create a SDU flammability model and control strategy more representative of facility operations. This work will allow for the completion of construction and operational readiness for SDU 7 while filling SDU 6 to ensure there is no lapse in available space once SWPF starts up. A summary of these improvements as they relate to increased throughput will be provided along with associated status. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2021

A review of rare earth elements and yttrium in coal ash: Content, modes of occurrences, combustion behavior, and extraction methods

Rare earth elements and yttrium (REY) have attracted considerable attention over the last decade because of their vital roles in clean energy, consumer product, national defense and security applications, among other uses. Due to the retention of REY during coal burning, coal combustion ash is considered as potential alternative sources for REY. Understanding the content, speciation, retention and/or transformation behavior of REY during coal combustion not only expands our knowledge of the combustion behavior of the trace elements in coal, but also provides basis for modeling REY partitioning during coal combustion and for developing economically viable REY recovery technologies. This review makes a critical summary of recent progress in the study of REY in coal ash. The contents and the extraction potentials of REY in coal ash derived from 15 major coal-producing countries worldwide were summarized and evaluated. Various analytical methods for determining REY bulk contents and speciation, together with the solid sample pretreatment, analytical accuracy and precision, advantages and disadvantages were summarized and compared. Modern analytical approaches combined indirect methods (e.g., sequential extraction) shed light on the physical distribution, mineralogy, and the chemical state of REY in coal ash. Three types of REY occurrences in coal ash, including Si-Al glassy association, discrete minerals or compounds, and organic association (bound with unburned carbon) were defined in the review. The glassy association can be further divided into REY minerals closely bound to glass phases and dispersed throughout the glassy structure. REY partitioning in various emission streams, the size distribution, and their enrichment behavior in coal ash were discussed. Additionally, thermal behavior and transformation of various REY forms in coal during combustion process, including organic-associated REY, REY phosphates, REY carbonates, clay-bound REY and among others were summarized. Two possible retention mechanisms of REY by aluminosilicate glass at boiler temperature were proposed: the incorporation of the individual REY phases into the glass as inclusions and the diffusion of REY phases throughout Si-Al glass structures in the melting process. Feed coal mineral types, mineral-mineral associations, boiler conditions, and other factors control the retention process. After coal combustion, the speciation of REY in fly ash may be modified by the reactions of REY phases with flue gas components. Further, an overview of REY transformation mechanisms during coal combustion was deeply discussed. Finally, current extraction techniques for REY recovery from coal combustion ash were introduced. Future outlooks and research problems were also identified.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Concentrating Rare Earth Elements in Coal mine drainage Using Coal Combustion Products through Abandoned Mine Land Reclamation

Rare earth elements (REEs) (including scandium, yttrium and a group of 15 lanthanides) are often considered to be critical components in the productions of renewable energy hardware, electric vehicles, health care and military equipment, and consumer electronic products. The demand of REEs has been projected to be growing at an annual rate of 5-9% in the next 25 years. In 2011, the global demand of total rare earth oxides (REOs) was estimated to be approximately 105,000 tons, which is expected to grow to 210,000 tons by 2025. China overwhelmingly dominates the current worldwide rare earth productions but has strategically restricted its exports, causing significant instability for the global market. In response to the increasing demand for REEs and the supply dominance of China, identifying alternative sources of REEs has become a critical issue for the United States and other countries. Coal, coal ash, and coal mine drainage (CMD) are considered to be the alternative sources of REEs. In the U.S., high REE concentrations have been reported to be closely associated with coal deposits, including the Appalachian Basins. When surface and/or groundwater come in contact with geologic strata containing sulfide minerals exposed by coal mining, the accelerated oxidation of sulfide minerals in the presence of ferric iron and/or oxygen can produce sulfuric acid. The process promotes the weathering of REE-bearing rocks and minerals in the host geologic strata. Compared to average river water and seawater, the concentrations of REEs can be orders of magnitude higher in CMD. In this study, we demonstrated a trap-extract-precipitate (TEP) process that can effectively recover REEs from CMD. The three-stage TEP process uses alkaline industrial by-products to capture REEs from CMD and then applies an extraction/precipitation procedure to produce a feedstock that can be economically processed to produce marketable rare earth oxides. The alkaline industrial by-products tested in this study include the residual from a water softening process (DRWP sludge) and two types of stabilized flue gas desulfurization materials (sFGDs). sFGD material is a mixture of lime (CaO) and two coal combustion by-products, calcium sulfite FGD by-product and fly ash. The objectives of this study are to (1) validate the effectiveness and feasibility; (2) determine mechanisms controlling the rare earth recovery, (3) quantify the associated economic and environmental benefits, and (4) evaluate the full-scale application. To achieve these objectives, tasks to be carried out in this proposed project are organized into three phases. In the first phase, the research team collaborated with Ohio Department of Natural Resources, American Electric Power, The Wilds (a nonprofit wildlife conservation organization), and a private landowner to carry out field investigations aimed to screen and evaluate the seasonal changes of rare earths in the CMD discharges that have high recovery potentials. Next, the recovery of REEs from CMD was tested using a series of lab-scale column and batch tests under, respectively, percolation and completely mixed conditions. Results obtained from these lab-scale studies show that all three tested solids are very effective in retaining REEs. Over 98% of the CMD REEs that contacted the solids were captured before the solids exhausted their neutralization capacities. We also determined an extraction process using a non-acid, organic ligand extraction solution that can effectively remobilize the retained REEs from the spent solids (over 90%). The REE concentrate (>7.5 wt. % of total REEs) is then formed in an aeration process. The TEP process uses environmentally benign industrial by-products and a naturally-occurring organic ligand to mitigate CMD and recover REEs. Techno-economic analysis (TEA) and life-cycle assessment (LCA) was carried out in the third phase. The engineering-economic costs and net energy, net CO2 emissions, and water and other requirements were investigated to understand the economic and environmental implications of this process. This work uses mass and energy balances from laboratory-scale experiments to estimate the economic costs and environmental impacts. The results suggest that passive treatment systems that use DRWP sludge are preferred over those that use sFGD material, because of lower economic costs ($89,300/yr with a unit cost of $86/gT-REE vs. $89,800/yr, or $278/gT-REE) and improved environmental performance across all indicators from two different impact assessment methods. These differences are largely attributable to the larger capacity of DRWP sludge in the passive treatment application. We envision this TEP process can be integrated with abandoned mine land (AML) reclamation to create an approach that can add economic incentives for AML reclamation, remediate CMD discharge, and eliminate public safety hazards and threats to local environment and ecological systems posed by AMLs. It can restore lands and communities that are adversely impacted by legacy mining.

01 COAL, LIGNITE, AND PEAT↗
DOE OSTI2023

Mechanisms of Zn removal from water by amorphous geopolymer: Molecular-level insights from X-ray absorption spectroscopy, isotope fractionation, and surface complexation modeling

Porous geopolymers have attracted widespread attention as promising heavy metal adsorbents that can be synthesized from aluminosilicate solid wastes. However, the precise microstructural evidence for adsorbed heavy metals on geopolymers remains unclear due to the insensitivity of conventional characterization techniques on minerals with amorphous structure and surface disorder. Batch adsorption and column experiments coupled with X-ray absorption spectroscopy (XAS), Zn stable isotope, and surface complexation model (SCM) were employed to reveal the Zn removal mechanisms with coal fly ash porous geopolymer (CFAPG) at a molecular scale. The macroscopic kinetic and isothermal adsorption of Zn on CFAPG were well described by the pseudo-second-order model and Bi_Langmuir equation, respectively, indicating the presence of abundant heterogeneous active sites on the CFAPG surface. Further, two types of active sites on the CFAPG surface were identified by XAS coupled with Zn isotopes in batch experiments at pH <= 6.0: one is pH-dependent and associated with tetrahedral zinc coordi-nation, and the other is pH-insensitive and associated with octahedral zinc coordination; these sites were confirmed by the bidentate SCM as the variable charge site (surface complexation, >S-OH) and the permanent negative charge site (cation exchange, >X - ), respectively. Furthermore, the important contribution of surface co -precipitation besides surface complexation and cation exchange to the Zn adsorption on CFAPG was identified by XAS coupled with SCM in a flow-through column experiment at pH >6.0. These investigations provide a systemic understanding of the Zn adsorption mechanisms on CFAPG and an SCM reference for the application and prediction of geopolymers in heavy metal-contaminated water remediations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Unravelling chloride transport/microstructure relationships for blended-cement pastes with the mini-migration method

A chloride mini-migration method is proposed to estimate effective diffusion coefficients at the scale of the cement paste and to investigate mechanisms with complementary microstructure analyses on the same material. Blended-cement pastes with a wide range of properties were investigated in this study: systems at w/b = 0.3–0.5 including Portland cement, white Portland cement, slag-Portland cement, fly ash, glass powder, and/or limestone and calcined clay. Comparisons showed the relative and combined importance of three main parameters on the effective diffusion coefficient: the porosity, the pore connectivity parameter and the conductivity of the pore solution (low values of the latter two are generally key aspects of the high resistance of blended-cement systems against chloride ingress). Notably, a general correlation for all the investigated systems was established between the effective diffusion coefficient and the bulk conductivity (whereas no simple correlation was observed between the effective diffusion coefficient and the formation factor or the pore connectivity parameter).

36 MATERIALS SCIENCE↗
DOE OSTI2021

Temperature transformation of blended magnesium potassium phosphate cement binders

In this study, a multi-technique approach was utilised to determine the high temperature performance of magnesium potassium phosphate cement (MKPC) blended with fly ash (FA) or ground granulated blast furnace slag (GBFS) with respect to nuclear waste immobilisation applications. Conceptual fire conditions were employed (up to 1200 °C, 30 min) to simulate scenarios that could occur during interim storage, transportation or within a final geological disposal facility. After exposure up to 400 °C, the main crystalline phase, struvite-K (MgKPO{sub 4}·6H{sub 2}O), was dehydrated to poorly crystalline MgKPO{sub 4} (with corresponding volumetric and mass changes), with MgKPO{sub 4} recrystallisation achieved by 800 °C. XRD and SEM/EDX analysis revealed reaction occurred between the MgKPO{sub 4} and FA/GBFS components after exposure to 1000–1200 °C, with the formation of potassium aluminosilicate phases, leucite and kalsilite (KAlSi{sub 2}O{sub 6} and KAlSiO{sub 4}), commensurate with a reduced relative intensity (or complete elimination) of the dehydrated struvite-K phase, MgKPO{sub 4.} This was further supported by solid-state NMR ({sup 27}Al and {sup 29}Si MAS), where only residual features associated with the raw FA/GBFS components were observable at 1200 °C. The high temperature phase transformation of blended MKPC binders resulted in the development of a glass/ceramic matrix with all existing porosity infilled via sintering and the formation of a vitreous phase, whilst the physical integrity was retained (no cracking or spalling). This study demonstrates that, based on small-scaled specimens, blended MKPC binders should perform satisfactorily under fire performance parameters relevant to the operation of a geological disposal facility, up to at least 1200 °C.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Development of a stoichiometric magnesium potassium phosphate cement (MKPC) for the immobilization of powdered minerals

Ordinary Portland Cement (OPC)-based materials are not systematically adapted for immobilizing industrial hazardous waste, e.g. for aluminium powder or plutonium waste sludge. In such case, Magnesium Potassium Phosphate Cements (MKPC) represent an interesting alternative. }The originality of this research is to develop a formulation of a MKPC paste for hazardous waste immobilization, which incorporates a maximum amount of such waste, preferably in powdered form. To this purpose, a stoichiometric MKPC paste is selected, and its properties are improved by powdered waste addition. }Firstly, the physico-chemical mechanisms generating expansion in stoichiometric MKPC paste are analyzed. Swelling is attributed to a pH gradient in the paste, due to the progressive sedimentation of MgO particles in the fresh mix. }Secondly, over-stoichiometric MgO is replaced by varying amounts of minerals simulating the waste, of different mineralogy and granulometry, in order to achieve sufficient workability and no swelling. An optimal formulation is proposed, which incorporates powdered fly ash at a fine-to-cement mass ratio (F/C) of 1. Its mechanical performance and endogenous dimensional changes are comparable to typical over-stoichiometric pastes, and they stabilize between 7 and 28 days.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effect of supplementary cementitious materials on carbonation of cement pastes

Supplementary cementitious materials (SCM) are increasingly used in concrete for economical and environmental reasons. However, the durability of reinforced concretes against, for example, corrosion induced by carbonation varies. Here, the phase assemblage of various cement pastes with/without SCM (slag, fly ash and metakaolin), carbonated in accelerated conditions (1.5% CO{sub 2} and 65% RH) or not, has been investigated by various technics (XRD, TGA/DTA and {sup 29}Si as well as {sup 27}Al nuclear magnetic resonance spectroscopy) and compared. Results show that, after carbonation, anhydrous phases are less decalcified than hydrated phases. In cement pastes with slag, most of the calcium remains in the non-hydrated part of the slag. In contrast, the C-A,S-H phase is deeply modified and results show a coupling between C-A,S-H and hydrated aluminate phases during carbonation. In all carbonated materials, these phases tend to become an aluminosilicate gel, a very amorphous/disordered phase, containing less water than the original hydrates.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Early age hydration and application of blended magnesium potassium phosphate cements for reduced corrosion of reactive metals

Highlights: • Magnesium potassium phosphate cements were studied for nuclear waste encapsulation. • The optimised formulation in this study was determined to be 0.24 w/s ratio. • Different reaction kinetics were discovered for the FA/MKPC and BFS/MKPC binders. • Struvite-K was the only crystalline reaction product observed via in-situ SXPD. • Uranium trials revealed MKPC corrosion was reduced compared to a UK nuclear grout. Magnesium potassium phosphate cements (MKPC) were investigated to determine their efficacy towards retardation of reactive uranium metal corrosion. Optimised low-water content, fly ash (FA) and blast furnace slag (BFS) blended MKPC formulations were developed and their fluidity, hydration behaviour, strength and phase assemblage investigated. In-situ time resolved synchrotron powder X-ray diffraction was used to detail the early age (~60 h) phase assemblage development and hydration kinetics, where the inclusion of BFS was observed to delay the formation of struvite-K by ~14 h compared to FA addition (~2 h). All samples set within this period, suggesting the possible formation of a poorly crystalline binding phase prior to struvite-K crystallisation. Long-term corrosion trials using metallic uranium indicated that MKPC systems are capable of limiting uranium corrosion rates (reduced by half), when compared to a UK nuclear industry grout, which highlights their potential application radioactive waste immobilisation.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Towards ternary binders involving limestone additions — A review

The review summarises literature to examine the transition from portland limestone cement system to composite ternary binder systems involving limestone. Interest in limestone addition as an ideal component in multicomponent binder systems has surged as evident from the large volume of literature published in the recent past. A ternary blended system, with co-substitution of limestone, has the potential to complement the reaction of the supplementary cementitious materials (SCMs). The direct addition of limestone powder helps to attain higher substitution levels of portland cement clinker, improve early age properties, and supplement SCM's reactivity. However, the dilution of hydrates could hamper the long-term benefits. In this review, the interaction of fine limestone is classified and elaborated under two broad umbrellas: physical and chemical interactions. The physical interactions can manifest in three ways, namely, filler action, shearing action and improved packing, which alters reaction rate and extent at early ages. The chemical interactions also modify the reaction kinetics and phase assemblage due to nucleation of C-S-H on calcite surfaces, preservation of the ettringite phase and formation of carboaluminates. Two different forms of carboaluminate hydrates — hemicarboaluminate and mono-carboaluminate can be present in the hydrate matrix depending on the balance between carbonate ions and aluminates in the pore solution. Several factors such as replacement level, particle size, choice of SCM, its reactivity and reactive aluminates content, sulphate levels, curing temperature, and duration of curing can control the carboaluminate formation, reaction degree of SCMs and chemical interaction from limestone additions. A combination of physical and chemical effects makes fine limestone a potential material for co-substitution with aluminosilicate based SCMs, mainly fly ash, slag, and calcined clay. In this review, the factors affecting limestone-SCM composites are summarised based on a detailed literature survey. The effects of SCM-limestone cement composites on hydration kinetics, reaction chemistry, the reactivity of SCMs, the stability of hydrated phases, and contribution to the physical structure development and macroscopic properties by evaluating hydration and mechanical properties are discussed. The importance of AFm (Al{sub 2}O{sub 3}–Fe{sub 2}O{sub 3}-mono) phases in various deterioration mechanisms in concrete and their influence on performance characteristics in different exposure environment is critically appraised.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Influence of supplementary cementitious materials on microstructure and transport properties of spacer-concrete interface

Reinforcement spacers are a critical component of concrete structures. Their presence affects microstructure and transport properties of concrete cover though this is not widely appreciated. This paper presents the first study to determine whether the negative effects of spacers can be mitigated through the use of supplementary cementitious materials such as silica fume, fly ash and blast-furnace slag. Concrete samples (>200) with different spacers, binders, curing and drying regimes were prepared and tested for diffusion, permeation, absorption, electrical conductivity, carbonation and microstructure. It was found that spacers increase all transport properties, the extent depending on type of spacer, drying regime and transport mechanism. The spacer-concrete interface is weak, porous and micro-cracked, and this lowers the resistance of concrete to ingress of aggressive agents. The beneficial effects of SCMs (strength enhancement and densification) and prolonged curing (120-day) are insufficient to overcome the negative effects of spacers. Implications for durability are discussed.

36 MATERIALS SCIENCE↗