Concrete is well known for its capacity to sequester carbon dioxide (${\mathrm{CO}}_2$ ) through carbonation curing. If carbonation mixing can be used to introduce ${\mathrm{CO}}_2$ during concrete mixing, the process is able to enhance the carbon capture and storage capacity of concrete and makes the technology available to both precast and cast-in-place concretes. This paper studies the effect of carbonation mixing on ${\mathrm{CO}}_2$ uptake and strength gain of cement paste, which is the key carbon-reactive ingredient in concrete. Three mixing methods were investigated: (1) normal air mixing, (2) carbonation mixing, and (3) hybrid mixing with carbonation mixing first, followed by normal air mixing. Two curing methods, air curing and carbonation curing, were also studied to examine the effect of carbonation mixing on curing. It was found that carbonation mixing could increase the ${\mathrm{CO}}_2$ uptake but decrease the strength gain. The reduction in strength by carbonation mixing was caused by the formation of carbonate coating on cement particles. The property can be improved if air mixing is introduced right after carbonation mixing.

In this paper, the mechanical and fracture properties of fly ash geopolymer concrete (FAGC) mixed with calcium aluminate cement (CAC) were explored. Fly ash was partially replaced by CAC with 2.5%, 5% and 7.5%. The results exhibit that the mechanical and fracture behaviors of FAGC are significantly influenced by CAC content. Based on the formation of more aluminum-rich gels, C-(A)-S-H and C-S-H gels, with the increase of CAC content, the compressive strength, splitting tensile strength and elastic modulus improved. Meanwhile, the peak load and effective fracture toughness show a monotone increasing trend. In addition, because C-S-H gels absorbed more energy, the fracture energy of FAGC increases. The maximal peak load, double-K fracture toughness and fracture energy reached up to1.79 kN, 4.27 MPam^0.5, 10.1 MPam^0.5 and 85.8 N/m with CAC content of 7.5%, respectively.