Concrete is one of the most used materials in the world, second only to water. One of the key advantages of this versatile material is its workability in the early stages before setting. Here, we use in situ underwater Raman microspectroscopy to investigate and visualize the early hydration kinetics of ordinary Portland cement (OPC) with submicron spatial and high temporal resolution. First, the spectral features of the C−S−H gel were analyzed in the hydroxyl stretching region to confirm the coexistence of Ca−OH and Si−OH bonds in a highly disordered C−S−H gel. Second, the disordered calcium hydroxide (Ca-(OH) 2 ) is experimentally identified for the first time in the mixture before setting, suggesting that Ca(OH) 2 crystallization and growth are essential in the setting of cement paste. Finally, the phase transformations of clinker, C−S−H, and Ca(OH) 2 are spatially and temporally resolved, and the hydration kinetics are studied by analyzing the spatial relationships of these phases using two-point correlation functions. The results quantitatively validate that the setting occurs as a percolation process, wherein the hydration products intersect and form an interconnected network. This timespace-resolved characterization method can map and quantitatively analyze the heterogeneous reaction of the cementitious colloidal system and thus provide potential application value in the field of cement chemistry and materials design more broadly.

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This paper investigates the mineral sequestration of carbon dioxide in circulating fluidized bed combustion (CFBC) boiler bottom ash. CFBC bottom ash, which originated from two sources, was prepared along with pulverized coal-fired (PC) boiler bottom ash as a control. These ashes were exposed to accelerated carbonation conditions at a relative humidity of 40% and 100%, in order to investigate the effects of humidity on the carbonation kinetics of the bottom ash. The obtained results showed that not only lime but other calcium-bearing phases (gehlenite, wollastonite, and brownmillerite) in CFBC bottom ash participated in the mineral carbonation reaction. In particular, these phases underwent hydration in a wet carbonation environment, whereby the carbon dioxide uptake and capacity of CFBC bottom ash are significantly enhanced. This study may have important implications, demonstrating the feasibility of carbon dioxide sequestration and recycling of CFBC boiler bottom ash.

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