M.T. Blanco-Varela

Formation of thaumasite in carbonated mortars

Abstract The main objective of this study was to investigate the formation of thaumasite in carbonated Portland cement mortars. Another important purpose was to study the role that ettringite (AFt) plays in the process of formation of Th and its influence in the deterioration of the mortars. Work was carried out with mortar prisms, elaborated using two different cements: one of them with low Al 2 O 3 content (exempt of C 3 A) and the other one with high Al 2 O 3 content. Additions of different amounts of gypsum and/or calcium carbonate were also done. After three months of curing at 21 °C and 100% RH mortars were submitted to an accelerated carbonation process until total transformation of Ca(OH) 2 into CaCO 3 . Then mortar prisms were partially immersed in distilled water and kept at temperature ranging from 0 to 5 °C for one year. A mineralogical, micro-structural and physical characterisation was carried out on samples at different ages. The thaumasite formation rate was much lower in those mortars elaborated with the cement exempt of C 3 A than in those mortars made with the cement having high proportion of C 3 A when they are conserved in water. The study of transversal sections of prisms reveals the presence of a white expansive product (thaumasite) not always homogeneously distributed.

Quantitative analysis of mineralized white Portland clinkers: The structure of Fluorellestadite

Fluorellestadite, Ca 10 (SiO 4 ) 3 (SO 4 ) 3 F 2 , has been synthesized as single phase. This compound crystallizes in the apatite type structure, s.g. P6 3 /m, with parameters a =9.4417(1) A, c =6.9396(1) A and V =535.76(1) A 3 . The refinement of its crystal structure converged to R WP =12.33% and R F =4.58%. The atomic parameters have been used to analyze the phase content of mineralized white Portland clinkers. These clinkers contain Ca 3 SiO 5 , Ca 2 SiO 4 , Ca 12 Al 14 O 32 F 2 and Ca 10 (SiO 4 ) 3 (SO 4 ) 3 F 2 . The agreement between the elemental composition inferred from the Rietveld phase analysis and that measured by XRF is noteworthy. This comparison does not take into account the presence of amorphous phases and unmodeled elemental substitutions in crystalline phases. Similar Rietveld studies on commercial white Portland clinkers are also shown to be feasible.