Key Neutron and X-ray Characterization Techniques
For amorphous materials such as alternative cements and amorphous carbonate phases, conventional powder diffraction techniques yield limited information regarding their atomic bonding arrangements. Total scattering (which contains both Bragg [crystalline information] + diffuse [disorder information] scattering) is an experimental technique that is ideal for studying the atomic structure of disordered and amorphous materials. The requirements for a wide Q-range (scattering angle) and low and stable background have historically led to these experiments being conducted at spallation neutron sources and synchrotron facilities. However, due to recent developments in technology this type of analysis can now be performed on specialized laboratory X-ray diffraction instruments (typically with Mo or Ag radiation).
(Image: In situ X-ray pair distribution function analysis of the formation of an alkali-activated cement binder. Data were collected on 11-ID-B at the Advanced Photon Source, Argonne National Laboratory.)
(Image: Laboratory X-ray diffractometer in SCCSG for pair distribution function analysis)
Small-angle X-ray and/or neutron scattering provides information on the nanoscale morphology of materials and changes occurring over time. Applications for cements include in-situ measurements of the morphology changes occurring during formation and accurate density measurements. Neutron small-angle scattering enables contrast-matching (using hydrogen/deuterium) to pin-point information from solid, voids and surfaces within the material. More information can be found here.
(Image: In situ neutron small-angle scattering analysis of the formation of an alkali-activated cement binder. Data were collected on LQD at the Lujan Neutron Scattering Center, Los Alamos National Laboratory.)
Inelastic Neutron Scattering
Discovering the role of water in cement systems and during CO2 storage is of importance for these technologies. Inelastic neutron scattering analysis is a key tool that can shed light on this area of research due to the large incoherent neutron scattering cross-section of hydrogen. By analyzing the vibrations of hydrogen in the materials, the location and bonding of H-atoms can be elucidated. Once the role of water is discovered for these systems, the materials can be optimized to obtain superior properties and performance characteristics. Read more about inelastic neutron scattering here at the ISIS webpage.
(Image: Inelastic neutron scattering spectra of kaolinite and kaolinite previously heated to the denoted temperatues. Data were collected on TOSCA at ISIS, Rutherford Appleton Laboratory.)
Micro- and Nano-Tomography
X-ray micro- and nano-tomography are two techniques used to probe the micro- and nano-scale morphology of materials. Micro-tomography is used as a non-destructive technique to analyze the pore structure of alkali-activated cements, extracting out key parameters including porosity and tortuosity. Nano-tomography is used to study the binder phase of the cement gels at the nanoscale. When combined with fluorescence, nano-tomography can resolve the spatial distribution of elements in the cement binders and other materials. The wikipedia page for micro-tomography can be found here. X-ray nano-tomography is available at the nanoprobe beam line at the Advanced Photon Source and other synchrotron facilities around the world.
(Image: X-ray nano-tomograph of a fly ash particle. Data were collected on the Nanoprobe, Sector 26 at the Advanced Photon Source, Argonne National Laboratory, in collaboration with Prof. John Provis at the University of Sheffield.)
Some Laboratory Experimental Tools
X-ray Powder Diffraction
Identification and quantification of crystalline phases present in composite materials can be carried out using laboratory-based X-ray powder diffraction. This standard form of analysis is a component of research for alkali-activated cements and associated materials.
One experimental tool capable of quantifying the amount and type of water in a sample (i.e, bound vs. free water) is thermogravimetric analysis (TGA). This is achieved by measuring the weight loss of a sample as a function of temperature. Among other things, TGA is also used to measure weight changes associated with decomposition or oxidation of a material. Complementary thermal analysis instruments include thermal mechanical analysis (TMA), which measures changes in dimensions and/or mechanical properties as a function of temperature, and differential scanning calorimetry (DSC), which measures heat gain/loss of a sample (endothermic vs. exothermic) as a function of temperature.
Without the compressive strength characteristics of concrete, this material would not be used as extensively throughout the world as it is today. However, understanding which mix designs provide superior compressive strength and associated mechanical properties is a necessary component of cement materials research. Other testing methods that are necessary include creep and fatigue.
Theoretical Modeling Tools
Molecular and Solid-State Density Functional Theory Calculations
Electronic structure calculations based on Schrödinger's equation and subsequent density functional theory are used extensively in solid-state materials science research including disordered crystalline systems. Recently this theoretical modeling technique has been shown to be capable of revealing important atomic structural information for complex highly-disordered systems such as conventional cementitious materials and alkali-activated cements. Accurate comparisons with experimental data are required to ensure that these models are a true representation of reality. This is possible by combining these calculations with key experimental studies including total scattering (as mentioned above) to ensure the models are experimentally valid. Density functional theory has also been used to determine the Gibb's free energy of reaction for hydrolysis and condensation reactions that occur during formation of alkali-activated binders.
Multiscale Monte Carlo Simulations
Recently, coarse-grained Monte Carlo simulations have been employed to model the formation of alkali-activated cements at the nanoscale, revealing important structural information regarding the nanoscale morphology and changes occurring during formation. This information is almost impossible to measure experimentally, and therefore these simulation techniques play a crucial role in the development of new cement materials. By modeling the nanoscale morphology development during formation of cement systems, we can pin-point which mix designs produce more stable and denser configurations at this length scale, and correlate these results with larger length scale properties.
By combining modeling and experiments across length and time scales it becomes possible to successfully "engineer" materials to meet the needs of society around the globe whilst reducing the impact we have on the environment.