STUDY OF HYDRATION PRODUCTS IN THE MODEL SYSTEMS METAKAOLIN-LIME AND METAKAOLIN-LIME-GYPSUM

Abstract

Possible preferential formation of ettringite instead of required calcium silicate hydrate (CSH) and calcium aluminosilicate hydrate (CASH) phases when aluminosilicates were added to the blended cements was investigated on the model systems comprising of metakaolin, lime and gypsum. Compressive strength, microstructure and phase composition of the samples were evaluated after 7 days of curing at 50oC, using thermal analysis, X-ray diffraction techniques and scanning electronic microscopy. Samples prepared from equal amounts of metakaolin, lime and with more than 8 wt. % of gypsum, displayed the highest compressive strength values. Development of compressive strength was correlated with the formation of ettringite. Further rising of gypsum content resulted in the decrease of compressive strength, which is notable in the samples with a different metakaolin/lime ratio. Lower content of gypsum led to the preferential formation of portlandite, CSH and CASH. Calcite was detected in all the samples and its content declined with increasing amount of ettringite. In addition to calcite, carboaluminates were detected in complementary binary metakaolin - lime system. Tendency to carbonation declined with increasing content of metakaolin and eventuated in the highest compressive strength value for the samples with equal initial content of metakaolin and lime.

Keywords

• Blended cements
• Metakaolin
• Gypsum
• Ettringite