The effect of natural zeolite on microstructure, mechanical and heavy metals adsorption properties of metakaolin based geopolymers

This work investigates the effect of clinoptilolite, a natural zeolite, as filler on the mechanical performance and heavy metal`s adsorption capacity of the metakaolin-based geopolymers. Clinoptilolite was chosen as an inexpensive additive with high adsorption capacity, replacing metakaolin (0, 25, 50 and 75%) in the synthesis of four different geopolymers (MK100, MK75, MK50 and MK25, respectively). To produce geopolymers with low environmental impact, during the geopolymerization processes the SiO ₂/Al ₂O ₃ and Na ₂O/Al ₂O ₃ molar ratios were kept constant at 1, to reduce sodium silicate and sodium hydroxide to a minimum. The final products were studied by powder X-ray diffraction, ²⁷Al and ²⁹ Si solid-state NMR and Scanning electron microscopy. Moreover, strength parameters and heavy metals Pb ^{2 +}, Zn ^{2 +}, Cu ^{2 +}, Cd ^{2 +} and Cr ^{3 +} adsorption tests were performed. The results show that geopolymerization in the presence of zeolite leads to an increase of the compressive strength of all blended geopolymers, with an optimal metakaolin precursor/zeolite filler ratio of 50:50, affording the highest strength (8.8 MPa at 28 days). The adsorption of metal cations on geopolymers was well fitted using the Langmuir model (0.97 < R ² < 0.99). The geopolymers adsorbed heavy metals in the order Pb ^{2 +} > Cd ^{2 +} > Zn ^{2 +}, Cu ^{2 +} > Cr ^{3 +}. The maximum adsorption capacity of Cu ^{2 +} and Cr ^{3 +} was highest for geopolymer with 100% of metakaolin (MK100), while for Pb ^{2 +}, Cd ^{2 +} and Zn ^{2 +} the highest adsorption capacity is for geopolymers with 75% of metakaolin (MK75), indicating that 25% zeolite addition to geopolymers has efficiently improved the adsorption capacity.