Abstract
The production of cement is a key indicator of a region's level of development. As such, its use is essential for any society aiming to create healthy, comfortable, safe and secure living and working environments. However, these benefits come at a price; Portland cement production accounts for ≈8% of the total anthropogenic CO(2) emissions. If cement fabrication was considered a country, it would rank as the third largest emitter, after China and the United States. Consequently, reducing the CO(2) footprint of the construction industry is a societal need. Numerous low-carbon cement alternatives have been proposed, primarily involving the partial substitution of Portland clinker with materials that possess much lower CO(2) footprints. However, these cements have not been widely adopted because they exhibit reduced mechanical strength at 1 day of hydration, failing to meet current practices for formwork stripping. Therefore, a primary objective is to elucidate the mechanisms of early age cement hydration to accelerate their hydration rates. Portland cement and low-carbon cements are complex, multimineral materials comprising at least seven crystalline components. Additionally, during the hydration process, new hydrate phases - both crystalline and amorphous - are formed, resulting in the development of intricate, time-dependent microstructures. The compositional and spatial complexity, along with the inherent heterogeneity, underscores the necessity for additional analytical tools such as 3D synchrotron X-ray imaging techniques. Furthermore, as dissolution and precipitation processes are time-dependent, advanced 4D (3D + time) imaging tools are essential. Many pertinent features, such as alite etch-pits, alite reaction zone, and calcium silicate hydrate (C-S-H) gel shells and needles, are submicrometric in size, necessitating the use of 4D synchrotron X-ray nanoimaging. Consequently, various synchrotron X-ray imaging techniques are presented, with a particular emphasis on those leveraging the coherent properties of synchrotron radiation, which are better suited for 4D nanoimaging. The five stringent requirements necessary for obtaining relevant results to investigate early age cement hydration are thoroughly discussed. Following this, examples of such studies are presented, highlighting the key data that can be obtained. Both the advantages and current limitations of these techniques are addressed. Particular emphasis is placed on the spatial dissolution rates of alite, which seem to be strongly dependent on the initial particle sizes. Additionally, descriptors related to the C-S-H gel shells, such as growth rate and densification over time, are provided. Unfortunately, to date, 4D nanoimaging lacks the temporal and spatial resolution required to measure the growth rates of C-S-H gel needles. However, optimized beamlines at fourth-generation synchrotron sources are expected to enable these types of studies in the near future. In the final section, future perspectives are presented. Initially, the technical specifications necessary to investigate the transition from the accelerated to decelerated cement hydration stages are discussed. The key requirements are a temporal resolution better than 100 min concurrent with a spatial resolution of 100 nm. Upon meeting these technical objectives, the mechanistic role of admixtures in accelerating low-carbon cement hydration could be elucidated, and the mechanical strengths at early ages are expected to be further enhanced. The accompanying image is an artistic view and it is explicitly stated that 4D nanotomography is performed on capillaries filled of commercial cement pastes and not on concrete samples.