Replacement of alkali silicate solution with silica fume in metakaolin-based geopolymers

A metakaolin（Mk）-based geopolymer cement from Tunisian Mk mixed with different amounts of silica fume（SiO2/Al2O3 molar ratio varying between 3.61 and 4.09） and sodium hydroxide（10M） and without any alkali silicate solution, is developed in this work. After the samples were cured at room temperature under air for 28 d, they were analyzed by X-ray diffraction（XRD）, Fourier transform infrared（FTIR） spectroscopy, environmental scanning electron microscopy, mercury intrusion porosimetry, 27Al and 29Si nuclear magnetic resonance（NMR） spectroscopy, and compression testing to establish the relationship between microstructure and compressive strength. The XRD, FTIR, and 27Al and 29Si NMR analyses showed that the use of silica fume instead of alkali silicate solutions was feasible for manufacturing geopolymer cement. The Mk-based geopolymer with a silica fume content of 6 wt%（compared with those with 2% and 10%）, corresponding to an SiO2/Al2O3 molar ratio of 3.84, resulted in the highest compressive strength, which was explained on the basis of its high compactness with the smallest porosity. Silica fume improved the compressive strength by filling interstitial voids of the microstructure because of its fine particle size. In addition, an increase in the SiO2/Al2O3 molar ratio, which is controlled by the addition of silica fume, to 4.09 led to a geopolymer with low compressive strength, accompanied by microstructures with high porosity. This high porosity, which is responsible for weaknesses in the specimen, is related to the amount of unreacted silica fume.