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.