Role of carbonates in the chemical evolution of sodium carbonate-activated slag binders

Multi-technique characterisation of sodium carbonate-activated blast furnace slag binders was conducted in order to determine the influence of the carbonate groups on the structural and chemical evolution of these materials. At early age (< 4 days) there is a preferential reaction of Ca2+ with the CO3 (2-) from the activator, forming calcium carbonates and gaylussite, while the aluminosilicate component of the slag reacts separately with the sodium from the activator to form zeolite NaA. These phases do not give the high degree of cohesion necessary for development of high early mechanical strength, and the reaction is relatively gradual due to the slow dissolution of the slag under the moderate pH conditions introduced by the Na2CO3 as activator. Once the CO3 (2-) is exhausted, the activation reaction proceeds in similar way to an NaOH-activated slag binder, forming the typical binder phases calcium aluminium silicate hydrate and hydrotalcite, along with Ca-heulandite as a further (Ca,Al)-rich product. This is consistent with the significant gain in compressive strength and reduced porosity observed after 3 days of curing. The high mechanical strength and reduced permeability developed in these materials beyond 4 days of curing elucidate that Na2CO3-activated slag can develop desirable properties for use as a building material, although the slow early strength development is likely to be an issue in some applications. These results suggest that the inclusion of additions which could control the preferential consumption of Ca2+ by the CO3 (2-) might accelerate the reaction kinetics of Na2CO3-activated slag at early times of curing, enhancing the use of these materials in engineering applications.