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Synthesis and crystal structure determination of hydrated potassium dawsonite from powder diffraction data

Institut de Ciència de Materials de Barcelona (CSIC), Campus de la UAB, 08193 Bellaterra, Catalunya, Spain

^* Corresponding author, e-mail: jordi.rius{at}icmab.es

During the optimisation of the synthesis conditions of non-hydrated potassium dawsonite, a compound which has been recently shown to be involved in the alkaline hydrolysis of calcium aluminate cement (CAC), a new closely related hydrated phase appearing below T = 25°C was detected by powder X-ray diffraction. To gain a more complete understanding of the crystal chemistry of CAC, this new phase was synthesised in pure form and its crystal structure solved from powder X-ray diffraction data. The crystal data are a = 8.3312(4), b = 11.2670(5), c = 5.661(2)Å, Pna2₁ (No. 33) and the resulting chemical formula is KAl(CO₃)(OH)₂.H₂O with Z = 4. From the refined positions of the non-hydrogen atoms and from the balance of bond valences, the chemical nature of the O atoms could be univocally established [R_wp= 0.132, R_B = 0.056, Chi² = 1.15]. The unit cell of this hydrated compound contains two carboaluminate chains formed by the basic unit [Al₂(OH)₄(CO₃)₂]^2– propagating along the c axis and being stabilised by a complex H-bond network. The arrangement of these chains is similar to that found in dawsonite, so that the atomic positions of K⁺ and Na⁺ in both structures are also similar. As confirmed by the X-ray diffraction study of a one-year-old sample, hydrated potassium dawsonite slowly transforms into the non-hydrated form at room temperature. Due to the more favourable formation conditions, i.e. lower temperature and higher humidity, this hydrated phase could be eventually found in CAC pastes or mortars.

Key-words: potassium dawsonite, calcium aluminate cement, alkaline hydrolysis, ab-initio structure solution, powder X-ray diffraction.