Effect of chemical admixtures on the expansion behavior and microstructure of K-type CSA expansive concrete

K-type calcium sulfoaluminate (K-type CSA) expansive agent concrete base their expansion on the formation of the hydrate ettringite. Although this is generally accepted, there is still controversy in the determination of the mechanism responsible for the expansion, mainly the morphology of the ettringite causing the expansion and the mechanism of formation of this ettringite. The use of chemical admixtures in concrete is a common practice to modify its initial rheological and hydration properties. However, the effects or synergy of certain chemical admixtures when they are used in K-type CSA expansive concretes are unknown. In this study, the effect of three types of chemical additives (air-entraining, setting retarder and hardening accelerator) when added to a self-stressing concrete made with a K-type CSA expansive agent has been analyzed. It was demonstrated that these chemical additives conditioned both the compressive strength and the magnitude and kinetics of the expansion in a restricted regime. Likewise, the microstructural evolution of hydrates and anhydrides related to the expansive phenomena was evaluated by BSEM microscopy, EDX analysis and thermogravimetry techniques. The possibility of the existence of a colloidal ettringite initially formed around the alumina-bearing anhydrous phase by a mechanism of topo-chemical reaction and in situ precipitation with through-solution contribution of sulfate ions was observed. The novelty of this research is based on the fact that the type of chemical additives used conditioned the expansion of reference concrete, not by modifying the morphology of the ettringite, but by varying the rigidity of the cementitious matrix. • Chemical additives can modify kinetics of the hydration, expansion and shrinkage. • The stiffness of the matrix plays a fundamental role in the expansion. • Setting-retarder admixture suppress or limit the expansion of K-type agents. • Hardening accelerator admixture increases the kinetics and limits the expansion. • Air pores introduced do not limit expansion or favor ettringite precipitation.