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 cylin ders 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-saturat ed systems for similar to8.4 years. Thereafter, cylinders were allowed to s tand, 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 relati vely coarse, blocky Ca(OH)(2) crystals embedded in a groundmass of C-S-H to gether with some AFt (ettringite). However, prolonged 85 degreesC cure sign ificantly alters the microstructure and mineralogy. Clinker hydration progr essed only slowly between 28 days and 8.4 years, with the result that simil ar to 30% cement clinker persists. Subsequent prolonged storage at similar to 20 degreesC has apparently not allowed hydration to restart. Ca(OH)(2) i s present in approximately unchanged amounts, comparing the two cures, prov ided allowance is made for the presence of unhydrated clinker. Paste porosi ty is, however, significantly increased at 85 degreesC relative to similar to 20 degrees cure. The 85 degreesC mineralogy consists of four solid hydra te phases: Ca(OH)(2), C-S-H gel, with a Ca/Si mole ratio close to 1.52, kat oite (a siliceous hydrogarnet) and a hydrotalcite-like phase. The amounts o f these phases are determined. The compositions of the C-S-K gel and hydrog arnet have been estimated by transmission electron microscopy and microprob e analysis. The amount and composition of the mineral phases can be recalcu lated to yield a bulk composition of the cement that agrees with a batch an alysis. (C) 2001 Elsevier Science Ltd. All rights reserved.