This EArly-concept Grant for Exploratory Research (EAGER) project advances the use of X-ray computed tomography (CT) to view cement microstructure like never before: three-dimensional images of undisturbed wet samples as a function of time. While CT is emerging as a powerful tool that is available to more and more researchers, the literature remains sparse. Proof of concept experiments have demonstrated the power of these new tools to provide profound new insights into cement hydration and microstructure development. The primary objectives of this project are the development of experimental methods for time studies of hydrating Portland cement. Such experiments require high resolution, high precision sample holders to assure spatial registration over time, special sealed sample holders that prevent evaporation, and special sample preparation techniques to assure that samples are representative of bulk cement, and imaging protocols to assist in segmentation of phases. The proposed study will use university-based nano-scale X-ray computed tomography (funded by an NSF large equipment grant) and Argonne National Lab synchrotron x-ray computed tomography. The synchrotron CT offers high resolution, but is not ideal for long term studies by students. The university CT offers superior access for students for long term studies, but suffers from being lower x-ray flux. This EAGER project aims where little work has been conducted, little precedent has been developed for experimental methods, but great potential exist to advance the science of understanding the complex microstructure of porous solids like hydrated portland cement.
Study of microstructure and material modeling have broad impacts in helping us understand cement and concrete properties, durability, life cycle performance, and sustainability. There is no question that improved understanding of materials has potential to improve building materials and extend the service life of the built infrastructure. The high resolution 3-dimensional images that will be acquired in this project provide important insight into the pore network and distribution of phases that control material properties of concrete. Such knowledge will lead to new products and processes that will improve concrete materials in the future.
X-ray computed tomography (X-ray CT) is a powerful new technology for investigating the 3D structure of hydrating Portland cement. No other technique can obtain high resolution images of internal features and pore structure without drying, sectioning, and polishing specimens as required for microscopic observation. MicroXCT has potential to study a wide array of cementitious material characteristics including cement paste microstructure, durability, fracture properties, concrete porosity, and behavior of steel fiber reinforced concrete. The current study was motivated by our interest in documenting changes in microstructure that occur in the first days of hydration when the mechanical and durability properties are established. Special specimen handling and preparation techniques are needed to acquire high resolution images of wet cement using X-ray CT. The specimen size needs to be very small to reduce attenuation of the X-ray signal, yet small samples are very fragile and vulnerable to drying. This project developed several new methods for preparing and mounting samples for X-ray CT to maximize resolution and high quality images. X-ray CT techniques were first used for the study of cement based materials in 1980. During the 1990s CT scans were often performed in high energy industrial facilities in order to improve the image quality and achieve higher resolution. Synchrotron radiation facilities were used in the 2000s in the never ending quest to increase image quality and achieve higher resolution. Voxel sizes as small as 0.44 mm have been achieved. A new generation of X-ray CT equipment has become available in recent years that brings high resolution imaging to local university laboratories, thus overcoming logistical challenges of off-site national laboratories with synchrotron facilities. This project used a Xradia nanoXCT system at the Beckman Institute of the University of Illinois. This research project led to several important outcomes. First, new methods were developed for packaging very small wet samples of Portland cement. Among the techniques of this study were: a) bare sample; b) high relative humidity shroud; c) fluid filled shroud; d) polymer encased wafer; and e) aerosol deposition. Second, new knowledge about image reconstruction was developed, particularly related to registration of images taken at different times and post-processing algorithms that can improve image resolution. Registration of images taken at different times– with perhaps days of time between those images – required very precise strategies for aligning the stage and marking the sample for registering images. Control of grey scale and segmentation of phases was difficult, and methods were developed to make phase identification more accurate. Third, this work served as a demonstration that X-ray CT could indeed provide images that could be used for identifying early phases of cement hydration and quantifying developing pore structure over time. Images of samples at 3, 7, 14, and 28d of age were acquired. The images were registered so that precise comparison of microstructural detail could be achieved. Images from X-ray CT can be used to track degree of hydration and volume of solid phases over time to study the evolution of microstructure. X-ray CT is a powerful technique that will open the door for researchers of cement based materials to better understand early hydration and microstructure development. Such knowledge can improve our understanding of how concrete materials develop properties and how we can better engineer the performance of concrete for civil infrastructure.
