A mechanism proposed for the accelerated carbonation of Portland cement has shown how the reaction proceeds through gaseous, liquid and solid phases in 9 distinct sequential steps. The overall speed of reaction is thus determined by the slowest step, and we have found that solvation and hydration of CO2 in water is commonly the rate-limiting step in the carbonation process. The literature suggests that the speed of this step might possibly be increased by three different classes of chemical 'enhancers' of CO2 hydration: (1) inorganic oxy-anions such as hypochlorite (ClO– ) or sulphite (SO32–) which act as Lewis bases to CO2; (2) organic solutes which form anions at alkaline pH, such as sugars and polyhydric alcohols; or (3) amines and alkanolamines, which may exert catalytic action by producing carbamates with CO2 by either zwitterion formation or charge-transfer. This paper explores these options in detail, supporting theoretical predictions with precise measurement of the rate of CO2 uptake in a 'eudiometer', to determine whether such rates might be beneficially enhanced in the carbonation of hydraulic binders and wastes, or in CO2 capture by mineral sequestration
