Abstract
Time-resolved in-situ synchrotron X-ray microtomography reveals new levels of detail about the chemical and physical processes that take place as Portland cement hardens. The conversion of a fluid paste into a hardened product can be monitored on a sub-minute time-scale, and with sample movement/settlement corrections applied to enable individual particles to be tracked as they react, hydrate, and become interconnected into a single strong monolith. The growth of the strength-giving hydrate phases surrounding cement grains, and of the fluid-filled pore network that surrounds them, is able to be directly viewed at the level of individual cement particles through the application of this tracking protocol. When cement is brought into contact with water, a layer which differs in density from the bulk of the cement grains becomes observable on the grain surfaces during the induction period (during which time the heat evolution from the paste is relatively low). As hydration continues, reaction products grow both from particle surfaces into the initially fluid-filled region, and also into the space originally occupied by the cement particles, forming a density gradient within the microstructure. As the reaction accelerates and larger volumes of solid phases precipitate, the newly-formed solid structure percolates via interconnection of agglomerated low-density outer hydrates, which then densify as hydration continues. This eventually leads to solidification of the structure into a hardened porous matrix.Graphical abstract
Highlights
Portland cement is the most widely used manufactured material on Earth, being produced in quantities of approximately 4 Gt p.a. and is ubiquitous in use in modern society
A new and strong relationship has been identified between the stages of reaction identified by isothermal calorimetry and the tortuosity of the pore network as it develops during cement hydration, which is only directly identifiable via in-situ tomographic analysis
When cement is brought into contact with water, hydrated phases are observable on cement grain surfaces from very early in the reaction process
Summary
Portland cement is the most widely used manufactured material on Earth, being produced in quantities of approximately 4 Gt p.a. and is ubiquitous in use in modern society. The strength and durability of concretes whose main binding component is Portland cement are controlled by the microstructure of the hardened cement paste, including a strong influence from the nanometre- to micron-scale porosity present within the hardened cementitious matrix [2]. This porosity results from the inability of hydrate phases to entirely fill the space within the hardened material, which is governed by the mechanism by which they form and grow to fill the initially fluid-filled spaces as cement paste hydrates [3]. The development of a fuller understanding of the early-age development of these characteristics is critical in producing materials which are durable, sustainable, and offer excellent mechanical properties at early age and throughout their service life
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