Investigations on geopolymeric cements based on alkali activation of low lime fly ash

摘要

Geopolymers are inorganic polymers of the zeolytic type. They are amorphous in nature and also called zeolytic precursors. Synthesis of geopolymers involves dissolution of alumino-silicate source material like low calcium fly ash by concentrated alkali solution at high pH forming free SiO_4 and AlO_4 tetrahedral units, transportation of dissolved species followed by polycondensation, that results in three dimensional polymeric structures. Such structures may exist in the form of poly(sialate) (-Si-O-Al-O-), poly(sialate-siloxo) (Si-O-Al-O-Si-O), poly(sialate-disiloxo) (Si-O-AI-O-Si-O-Si-O), sialate stands for silicon-oxo-aluminate building unit. Polysialates are chain and ring polymers with Si~(4+) and Al~(3+) in 4-fold coordination with oxygen. The positive ions (Na~+, K~+ etc.) are present to balance the negative charge of AI in 4-fold coordination. The geopolymers posses high compressive strength gain, good fire resistance, along with high surface hardness and smoothness. Therefore, geopolymers are considered potential construction materials as a replacement of Portland cement in some applications.