Impact of prolonged warm (85 degrees C) moist cure on Portland cement paste

A commercial Portland cement paste was fabricated as similar to 200-g cylinders to a water/cement weight ratio of 0.50. After similar to 30 days cure at similar to 20 degreesC, cylinders were additionally cured at similar to 20 degreesC and 85 degreesC, both +/- 2 degreesC, in sealed, vapour-saturated systems for similar to8.4 years. Thereafter, cylinders were allowed to stand, still in sealed state, at similar to 20 degrees for 1.5 to 2.0 years. The 20 degreesC cure mineralogy and microstructure is essentially normal: only a little unhydrated clinker persists and the matrix consists of relatively coarse, blocky Ca(OH)(2) crystals embedded in a groundmass of C-S-H together with some AFt (ettringite). However, prolonged 85 degreesC cure significantly alters the microstructure and mineralogy. Clinker hydration progressed only slowly between 28 days and 8.4 years, with the result that similar to 30% cement clinker persists. Subsequent prolonged storage at similar to 20 degreesC has apparently not allowed hydration to restart. Ca(OH)(2) is present in approximately unchanged amounts, comparing the two cures, provided allowance is made for the presence of unhydrated clinker. Paste porosity is, however, significantly increased at 85 degreesC relative to similar to 20 degrees cure. The 85 degreesC mineralogy consists of four solid hydrate phases: Ca(OH)(2), C-S-H gel, with a Ca/Si mole ratio close to 1.52, katoite (a siliceous hydrogarnet) and a hydrotalcite-like phase. The amounts of these phases are determined. The compositions of the C-S-K gel and hydrogarnet have been estimated by transmission electron microscopy and microprobe analysis. The amount and composition of the mineral phases can be recalculated to yield a bulk composition of the cement that agrees with a batch analysis. (C) 2001 Elsevier Science Ltd. All rights reserved.