Formation of thaumasite in synthetic cement mineral slurries: Fid-Bau Portal

Formation of thaumasite in synthetic cement mineral slurries

Purnell, P / Francis, O.J / Page, C.L

AbstractIn order to investigate the effect of alite polymorphism on the susceptibility of cements to thaumasite formation, slurries were prepared from magnesium sulphate solution and mixtures of calcium carbonate with synthetic cement preparations plus additions of 0%, 1% or 2% w/w alumina. Two synthetic cement preparations were used; a co-prepared mixture of monoclinic alite (∼C54S16AM) with β-C2S (designated A) and a ∼99% pure, primarily triclinic C3S (designated B). The slurries were prepared after the manner of Gaze [Cement Concrete Res. 27(2) (1997) 259] and maintained in a state of continual agitation at 5 °C for 100 days. After this period, the slurries were dried and characterised using X-ray diffraction. In slurries made with A, both ettringite and thaumasite (or a solid solution thereof) were detected, the apparent ratio of the former to the latter increasing with alumina addition. No gypsum was detected. In slurries made with B, the only sulphate phase detected was gypsum. This interesting observation leads to a number of avenues for discussion;• that alumina must be present in sufficient quantities within the clinker or gel structure, rather than as an addition, to initiate TF; • that TF is only initiated by C2S, not C3S; • that TF cannot be initiated by triclinic C3S.