The role of Al2O3, SiO2 and Na2O on the amorphous -> Crystalline phase transformation in geopolymer systems

The effect of initial Na2O, Al2O3 and SiO2 content on the long term stability of amorphous geopolymer phase, with respect to crystallisation, has been studied. Alkali activated metakaolin systems with varying amount of Na2O, SiO2 and Al2O3 contents (molar ratio SiO2/Al2O3 2.50-C5.01 & Al2O3/Na2O 0.60-1.70) were cured at 40 degC for 7 months. The products were tested periodically for compressive strength and analyzed for phase evolution using XRD and SEM techniques. Irrespective of the initial mix composition, all the mixtures showed an amorphous Na-Al-Si phase at early stage of. the reaction. In some of the mixtures, the amorphous phase(s) gradually transformed to crystalline phase(s) with prolonged curing. The initial SiO2, Na2O, and Al2O3 contents of mixtures appeared to be critical factors governing the observed amorphous - > crystalline transformation. In particular, SiO2/Al2O3 molar ratios lower or higher than 3.8 and increasing the Na2O content tended to favour this transformation. Well-developed crystalline zeolitic phases were identified in some of the mixtures investigated. In most cases, with prolonged curing, some correlation emerged relating compressive strength development with corresponding phase changes. The mixture formulations that developed crystalline phases after prolonged curing tended to produce low strengths. The relevance of these findings on the phase development of mild- to warm-temperature prolonged curing of geopolymer systems is discussed.