A pinboard by
Brant Walkley

Postdoctoral Research Associate, The University of Sheffield


Controlling molecular structure and properties of low-CO2 ultra-high-performance cements

In the face of an overwhelming need to mitigate climate change, rapidly increasing population growth, urbanisation and ever-advancing applications in the infrastructure and energy industries have created tremendous demand for ultra-durable, high-performance and eco-friendly low-CO2 cement and concrete technologies.

Second only to water in terms of commodity use, worldwide production of concrete exceeds 10 billion tonnes per annum. Portland cement is the main material in concrete, and is responsible for approximately 8% of human-driven CO2 emissions worldwide. A reduction in cement-related CO2 emissions by more than 80% is achievable by replacing Portland cement with low-CO2 cement, however the limited ability to control the structure and performance of low-CO2 cement has restricted widespread use within industry. This has had the flow-on effect of restricting our ability to limit dangerous climate change.

Through my research I create innovative ways to design new combinations of engineered low-CO2 cements, introducing new formulations capable to meet industry demands for ultra-high-performance and ultra-high-durability while maintaining an extremely low CO2 footprint. Particular focus is centred on understanding the relationships between material composition, structure and properties and using this knowledge to design smart, ultra-high performance low-CO2 cements.

Understanding (and hence controlling) the structure and properties of low-CO2 cement is achieved by using advanced spectroscopic and microscopic analytical techniques, including solid state nuclear magnetic resonance (NMR) spectroscopy. These provide information about the fundamental interactions occurring during cement reaction, formation and hardening.

Advanced solid state NMR is ideally suited to study complex and disordered phases such as cements, and is capable of determining and quantifying the environment surrounding each atom, and therefore the atomic structure, of the material. This is essential for understanding, modelling and controlling reaction mechanisms, nanostructural development and performance in low-CO2, high-performance cements, equipping us with the ability to reduce CO2 emissions and mitigate climate change.


Dissolution behaviour of source materials for synthesis of geopolymer binders: A kinetic approach

Abstract: Controlling the initial release rate of alumina and silica from source materials is known to have a significant effect on the nanostructure of geopolymer gel and its final mechanical properties. However, most of the studies regarding the solubility of source materials take an equilibrium approach, and there is a gap in understanding of the release rates at far-from-equilibrium conditions. In the present study, the initial dissolution rate of some geopolymer precursors is characterised. The liquid to solid ratios are designed to be sufficiently high to minimise precipitation of hydration products, and the effects of solution alkalinity and milling on dissolution rates are investigated. While fly ash and blast furnace slag particles seem to release Si and Al at approximately similar rates, metakaolin shows a distinctively higher release of Si from the very early time of dissolution. Increasing solution alkalinity increases the dissolution of source materials up to some point, and the greatest effect is observed on fly ash particles. The most interesting result of milling is observed on fly ash particles where the release rate of silica has become higher than alumina, while contrasting behaviour is observed in the non-milled fly ash system. The opposite behaviour is observed in the slag system where milling rapidly increases the release rate of Al while the release rate of Si is increased slowly.

Pub.: 21 May '16, Pinned: 18 Aug '17

Attenuated total reflectance fourier transform infrared analysis of fly ash geopolymer gel aging.

Abstract: Structural changes in fly ash geopolymers activated with different sodium hydroxide and silicate concentrations are investigated using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy over a period of 200 days. A strong correlation is found between the concentration of silicate monomer in the activating solution and the position of the main Si-O-T stretching band in the FTIR spectrum, which gives an indication of the relative changes in the gel Si/Al ratio. The FTIR spectra of geopolymer samples with activating solution concentrations of up to 1.2 M SiO2 indicate that an Al-rich gel forms before the final gel composition is reached. The time required for the system to reach a steady gel composition depends on the silicate activating solution concentration and speciation. Geopolymers activated with solutions containing predominantly high-order silicate species rapidly reach a steady gel composition without first forming an Al-rich gel. A minimum silicate monomer concentration of approximately 0.6 M is required to shift the geopolymer synthesis mechanism from hydroxide activation to silicate activation. Silicate speciation in the activating solutions also affects zeolite formation and geopolymer microstructures, with a more homogeneous microstructure and less zeolite formation observed at a higher SiO2 content.

Pub.: 26 Jun '07, Pinned: 18 Aug '17

In situ ATR-FTIR study of the early stages of fly ash geopolymer gel formation.

Abstract: The kinetics of geopolymer formation are monitored using a novel in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopic technique. Reaction rates are determined from the intensity variation of the bands related to the geopolymer gel network and the unreacted fly ash particles. Comparison with deuterated geopolymer samples provides critical information regarding peak assignments. An initial induction (lag) period is observed to occur for hydroxide-activated geopolymers, followed by gel evolution according to an approximately linear reaction profile. The length of the lag period is reduced by increasing the concentration of NaOH. An increase in the rate of network formation also occurs with increasing NaOH concentration up to a maximum point, beyond which an increased NaOH concentration leads to a reduced rate of network formation. This trend is attributed to the competing effects of increased alkalinity and stronger ion pairing with an increase in NaOH concentration. In situ analysis also shows that the rate of fly ash dissolution is similar for all moderate- to high-alkali geopolymer slurries, which is attributed to the very highly water-deficient nature of these systems and is contrary to predictions from classical glass dissolution chemistry. This provides for the first time detailed kinetic information describing fly ash geopolymer formation kinetics.

Pub.: 31 Jul '07, Pinned: 18 Aug '17

Geopolymers for immobilization of Cr(6+), Cd(2+), and Pb(2+).

Abstract: Alkali activation of fly ash by sodium silicate solutions, forming geopolymeric binders, provides a potential means of treating wastes containing heavy metals. Here, the effects on geopolymer structure of contamination of geopolymers by Cr(VI), Cd(II) and Pb(II) in the forms of various nitrate and chromate salts are investigated. The addition of soluble salts results in a high extent of dispersal of contaminant ions throughout the geopolymer matrix, however very little change in geopolymer structure is observed when these materials are compared to their uncontaminated counterparts. Successful immobilization of these species will rely on chemical binding either into the geopolymer gel or into other low-solubility (silicate or aluminosilicate) phases. In the case of Pb, the results of this work tentatively support a previous identification of Pb(3)SiO(5) as a potential candidate phase for hosting Pb(II) within the geopolymer structure, although the data are not entirely conclusive. The addition of relatively low levels of heavy metal salts is seen to have little effect on the compressive strength of the geopolymeric material, and in some cases actually gives an increase in strength. Sparingly soluble salts may undergo some chemical conversion due to the highly alkaline conditions prevalent during geopolymerization, and in general are trapped in the geopolymer matrix by a simple physical encapsulation mechanism. Lead is in general very effectively immobilized in geopolymers, as is cadmium in all except the most acidic leaching environments. Hexavalent chromium is problematic, whether added as a highly soluble salt or in sparingly soluble form.

Pub.: 04 Mar '08, Pinned: 18 Aug '17

Mechanical and thermal characterisation of geopolymers based on silicate-activated metakaolin/slag blends

Abstract: This article assesses the effect of mix design parameters on the compressive strength and thermal performance of alkali silicate-activated blends of metakaolin (MK) and granulated blast furnace slag (GBFS). A strong interrelationship between the effects of activator composition and the GBFS/(GBFS + MK) ratio is identified through statistical analysis of compressive strength data. Pastes formulated with higher SiO2/Al2O3 molar ratios show improvements in mechanical strength with increasing GBFS addition, associated with the formation of a structure comprising coexisting aluminosilicate ‘geopolymer’ gel and Ca-rich Al-substituted silicate hydrate (C-(A)-S-H) reaction products. The inclusion of GBFS in MK-based geopolymers seems also to improve their performance when exposed to high temperatures, as higher residual compressive strengths are reported for these mixtures compared to solely MK-based systems. Only slight differences in shrinkage behaviour are observed at temperatures of up to 600 °C with the inclusion of GBFS; however, slag-blended pastes exhibit enhanced stability at temperatures exceeding 800 °C, as no variation in the compressive strength and no additional shrinkage are identified. These results suggest that nanostructural modifications are induced in the gel by the inclusion of GBFS into MK-based geopolymers, improving the overall performance of these materials.

Pub.: 01 Apr '11, Pinned: 18 Aug '17

Acid resistance of inorganic polymer binders. 1. Corrosion rate

Abstract: The resistance to acid-induced corrosion of inorganic polymer (including “fly ash geopolymer”) binders is examined, by exposing specimens to nitric and sulphuric acids at pH values between 1 and 3, and measuring the corroded depth as a function of exposure time. The inorganic polymer binders are shown to be affected by acid attack by surface corrosion, which contradicts some previous claims of extremely high acid resistance in such binders. Corroded depth is shown to be a more sensitive measure of the performance of inorganic polymer binders than change in mass, because acid attack on the highly-connected aluminosilicate network of an inorganic polymer binder leads to the formation of an apparently intact, but physically weak and porous, reaction product layer on the sample surface, rather than complete disappearance of the binder as is often the case for other binder types. A strong correlation between permeability and resistance to acid attack is noted across a wide range of inorganic polymer formulations, including samples based on fly ash, ground granulated blast furnace slag, and mixtures of the two. The presence of calcium (supplied either by a Class C fly ash or by slag) and of high alkali concentrations each show a positive influence on acid resistance, which is attributed to the reduction in mass transport rates through the finer and more tortuous pore networks of such binders.

Pub.: 12 May '11, Pinned: 18 Aug '17

Activation of Metakaolin/Slag Blends Using Alkaline Solutions Based on Chemically Modified Silica Fume and Rice Husk Ash

Abstract: This study describes the use of alkaline silicate solutions produced by mixing silica fume (SF) or rice husk ash (RHA) with aqueous NaOH, as alternative silica-based activators for metakaolin (MK)/slag (GBFS) blended binders. Pastes prepared with these activators show similar trends in mechanical strength development as a function of activation conditions compared with the pastes obtained using commercial silicate solutions as activator. All activating solutions promote higher compressive strength development with increased contents of GBFS in the binders, which promotes the coexistence of aluminosilicate reaction products along with calcium silicate hydrate gel. Higher-silica binding systems prefer a higher GBFS content for optimal strength development compared to those with a lower overall SiO2/Al2O3 ratio. SF-derived activators give reaction products which are very similar to those obtained using commercial silicate solutions, as a consequence of the high reactivity of this precursor, supplying high concentrations of Si to the systems since the early stages of reaction. RHA-derived activators appear to have slightly delayed Si availability due to the less-reactive character of this precursor, which influences the relative rates of formation of the two types of gel in blended systems. These results show that activation of GBFS/MK blends with by-product derived silicate-based activators can generate mechanical strengths and structures comparable to those obtained using commercial silicate solutions.

Pub.: 07 Oct '11, Pinned: 18 Aug '17

In situ X-ray pair distribution function analysis of geopolymer gel nanostructure formation kinetics.

Abstract: With the ever-increasing environmentally-driven demand for technologically advanced structural materials, geopolymer cement is fast becoming a viable alternative to traditional cements due to its proven engineering characteristics and the reduction in CO2 emitted during manufacturing (as much as 80% less CO2 emitted in manufacture, compared to ordinary Portland cement). Nevertheless, much remains unknown regarding the kinetics of reaction responsible for nanostructural evolution during the geopolymerisation process. Here, in situ X-ray total scattering measurements and pair distribution function (PDF) analysis are used to quantify the extent of reaction as a function of time for alkali-activated metakaolin/slag geopolymer binders, including the impact of various activators (alkali hydroxide/silicate) on the kinetics of the geopolymerisation reaction. Quantifying the reaction process in situ from X-ray PDF data collected during the initial ten hours can provide an estimate of the total reaction extent, but when combined with data obtained at longer times (128 days here) enables more accurate determination of the overall rate of reaction. To further assess the initial stages of the geopolymerisation reaction process, a pseudo-single step first order rate equation is fitted to the extent of reaction data, which reveals important mechanistic information regarding the role of free silica in the activators in the evolution of the binder systems. Hence, it is shown that in situ X-ray PDF analysis is an ideal experimental local structure tool to probe the reaction kinetics of complex reacting systems involving transitions between disordered/amorphous phases, of which geopolymerisation is an important example.

Pub.: 02 Mar '13, Pinned: 18 Aug '17

Determination of particle size, surface area, and shape of supplementary cementitious materials by different techniques

Abstract: The particle size distribution, surface area and shape are fundamental characteristics of supplementary cementitious materials (SCMs). Accurate measurement of these properties is required in computational efforts to model the hydration process, and the characterization of these parameters is also an important practical issue during the production and use of blended cements. Since there are no standard procedures specifically for the determination of physical properties of SCMs, the techniques that are currently used for characterizing Portland cement are applied to SCMs. Based on the fact that most of the techniques have been developed to measure cements, limitations occur when these methods are used for other materials than cement, particularly when these have lower fineness and different particle shape and mineralogical composition. Here, samples of fly ash, granulated blast furnace slag and silica fume were tested. Different results obtained using several methods for the determination of specific surface area are presented. Recommendations for testing SCMs using air permeability, sieving, laser diffraction, BET, image analysis and MIP are provided, which represent an output from the work of the RILEM Technical Committee on Hydration and Microstructure of Concrete with Supplementary Cementitious Materials (TC-238-SCM).

Pub.: 04 Oct '14, Pinned: 18 Aug '17

Structure and properties of binder gels formed in the system Mg(OH)2-SiO2-H2O for immobilisation of Magnox sludge.

Abstract: A cementitious system for the immobilisation of magnesium rich Magnox sludge was produced by blending an Mg(OH)2 slurry with silica fume and an inorganic phosphate dispersant. The Mg(OH)2 was fully consumed after 28 days of curing, producing a disordered magnesium silicate hydrate (M-S-H) with cementitious properties. The structural characterisation of this M-S-H phase by (29)Si and (25)Mg MAS NMR showed clearly that it has strong nanostructural similarities to a disordered form of lizardite, and does not take on the talc-like structure as has been proposed in the past for M-S-H gels. The addition of sodium hexametaphosphate (NaPO3)6 as a dispersant enabled the material to be produced at a much lower water/solids ratio, while still maintaining the fluidity which is essential in practical applications, and producing a solid monolith. Significant retardation of M-S-H formation was observed with larger additions of phosphate, however the use of 1 wt% (NaPO3)6 was beneficial in increasing fluidity without a deleterious effect on M-S-H formation. This work has demonstrated the feasibility of using M-S-H as binder to structurally immobilise Magnox sludge, enabling the conversion of a waste into a cementitious binder with potentially very high waste loadings, and providing the first detailed nanostructural description of the material thus formed.

Pub.: 03 Apr '15, Pinned: 18 Aug '17

Role of Microstructure and Surface Defects on the Dissolution Kinetics of CeO2, a UO2 Fuel Analogue

Abstract: The release of radionuclides from spent fuel in a geological disposal facility is controlled by the surface mediated dissolution of UO2 in groundwater. In this study we investigate the influence of reactive surface sites on the dissolution of a synthesized CeO2 analogue for UO2 fuel. Dissolution was performed on the following: CeO2 annealed at high temperature, which eliminated intrinsic surface defects (point defects and dislocations); CeO2-x annealed in inert and reducing atmospheres to induce oxygen vacancy defects and on crushed CeO2 particles of different size fractions. BET surface area measurements were used as an indicator of reactive surface site concentration. Cerium stoichiometry, determined using X-ray Photoelectron Spectroscopy (XPS) and supported by X-ray Diffraction (XRD) analysis, was used to determine oxygen vacancy concentration. Upon dissolution in nitric acid medium at 90 °C, a quantifiable relationship was established between the concentration of high energy surface sites and CeO2 dissolution rate; the greater the proportion of intrinsic defects and oxygen vacancies, the higher the dissolution rate. Dissolution of oxygen vacancy-containing CeO2-x gave rise to rates that were an order of magnitude greater than for CeO2 with fewer oxygen vacancies. While enhanced solubility of Ce3+ influenced the dissolution, it was shown that replacement of vacancy sites by oxygen significantly affected the dissolution mechanism due to changes in the lattice volume and strain upon dissolution and concurrent grain boundary decohesion. These results highlight the significant influence of defect sites and grain boundaries on the dissolution kinetics of UO2 fuel analogues and reduce uncertainty in the long term performance of spent fuel in geological disposal.

Pub.: 29 Mar '16, Pinned: 18 Aug '17