Medium-term phase stability of Na 2O–Al 2O 3–SiO 2–H 2O geopolymer systems

Abstract

The phase evolution and microstructural development of a series of geopolymer mixtures comprising SiO 2, Al 2O 3, Na 2O and H 2O prepared by alkali reaction of metakaolin, have been studied. The study also included the effects of cure duration and its impact on physical properties such as compressive strength. The characteristic molar ratios of the geopolymer mixtures were of the range SiO 2/Al 2O 3 [2.50–5.01] and Al 2O 3/Na 2O [0.60–1.70], respectively. The formulations were subjected to continuous curing at 40 °C for 7 months, and were analyzed periodically by XRD and SEM techniques. Amorphous Na–Al–Si phase(s), observed at early ages, gradually transformed to crystalline phase(s) with prolonged curing. The initial SiO 2, Na 2O, and Al 2O 3 contents of mixtures appeared to be critical factors governing the observed amorphous → crystalline transformation. Well-developed crystalline zeolitic phases, including chabazite, faujasite, zeolite A and zeolite P, 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. In essence, 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.