The Chemistry of Al-Tobermorite and its Coexisting Phases AT 175°C

The substitution of Al into the structure of tobermorite was investigated, along with the resultant phase assemblages that compatibly coexist with the Al-tobermorite. All experiments were conducted under hydrothermal conditions at saturated steam pressure. The temperatures of the experiments varied from 90° to 190°C and times from 1 to 28 days. Ca/Si ratios varied from 0.7 to 1.7, Al/(Al+Si) from 0.12 to 0.30. NaOH varied from none to 5 moles/mole tobermorite constituents. X-ray diffraction was used not only to identify phases but to determine degree of substitution and the rate of approach to steady state.Maximum Al substitution was found to be Al/(Al+Si) = 0.13, and the maximum interlayer spacing was 11.42 ± 0.015 Å. This is in agreement with previously published work which found a trace of hydrogarnet at Al(Al+Si) = 0.15 and a maximum interlayer spacing of 11.45 ± 0.02 A. When Al concentrations in the mix are greater than enough to make maximum Al-tobermorite, “low”-hydrogarnet (“low”-hydrogarnet has ∼0.1 SiO2 per mole). As Al concentration is raised, a variable composition hydrogamet is formed and then a “high”-garnet of constant composition (SiO2 = 0.7 moles/mole hydrogarnet) is formed.NaOH was found to have a major effect on the kinetics and the phase assemblages at a given time. Al-tobermorite, although stable at 90°C, converts to xonotlite at 175°C in the absence of NaOH. In its presence, even at a level too low to make Na-containing phases, Al-tobermorite persists at 21 days at 175°C. In the absence of NaOH, at 175°C, Al-tobermorite forms at up to 7–10 days but is converted to xonotlite at 28 days. Coexisting phases are gyrolite and cowlesite at Ca/Si ≥ 1 and hillebrandite and “low”-garnet at Ca/Si > 1. When there is enough NaOH to form Na-containing phases, Al-tobermorite coexists with reyerite and analcime at Ca/Si > 1. At Ca/Si < 1 and Al less than the composition on the tie line between Al-tobermorite and “low”-garnet, Al-tobermorite coexists with “low”-garnet and portlandite. In the compatibility triangle between Al-tobermorite, “low”-garnet, and “high”-garnet, Al-tobermorite coexists with variable Si-gamet. When Al exceeds its limiting value on the tie line, Al-tobermorite-“high”-gamet, the assemblage is Al-tobermorite-“high”-gamet-cowlesite.