Formation of green strength in hydraulically bonded refractory castables at room temperature

Abstract

A sufficient green strength of refractory castables is necessary to avoid damage during demoulding and heating up of refractory linings and prefabricated parts. However, there is a lack of knowledge on the mechanisms underlying the build-up of green strength in refractory castables. This work focuses on how green strength is formed within the matrix of PCE-dispersed and CA cement bonded refractory castables during their setting.Setting was monitored using several measuring methods, such as sonic velocity, amount of hydrate phases formed from CA cement, ζ-potential (stability resp. coagulation of the matrix suspension), porosity, and specific surface area and was linked to the corresponding increase of green strength of refractory castables during setting.Considered by time after adding water, the first stiffening of the refractory castables is related to the collapse of the rheological system of their matrix suspension (coagulation), resulting in a very low increase in green strength (apparent hardening). This stiffening occurs early if Ca-precipitates are formed together with molecules of the dispersing agent (Ca-PCE, PCE with short side chains) or if the amount of pore water is being reduced below a critical value (rheologically stable matrix due to dispersion of hydrate phases by PCE with long side chains). The major increase in green strength occurs when a significant amount of CA cement reacts to hydrate phases and, correspondingly, water is being bonded. This main increase in green strength may occur directly after stiffening (if CA cement is being hydrated quickly) or several hours after stiffening (if the hydration of CA cement is retarded).Regarding the green strength, the predominant factor influencing the build-up of green strength in refractory castables is the reduction of free water in the pores due to its bond in hydrate phases (reduction of a lubricating, interparticle water film). Minor influences are the reduction in porosity (volume expansion by hydration of CA cement), the increase in the specific surface area (hydrate phases are finer than the initial CA cement particles), and the entanglement of plate-like hydrate phases.