Synthesis and properties of inorganic polymers (geopolymers) derived from Cameroon-meta-halloysite

Cameroonian Balengou clay has been characterized as precursor for inorganic polymer (geopolymer) binder. The lowest possible calcination temperature for this halloysite type clay was found to be 600 °C to convert it into meta-halloysite (MH), which is reactive enough for geopolymerization. This implies an energy saving compared to the 800 °C, previously reported in literature. The inorganic polymer has been produced using sodium hydroxide or sodium silicate solutions. A maximum dry compressive strength of 27.5 MPa after 28 days was obtained with a sodium silicate solution [Na2O·1.25SiO2·9.76H2O] resulting in a material with composition: Na2O·3.78SiO2·7.22Al2O3·9.76H2O. This inorganic polymer was amorphous according to XRD, showed a compact microstructure (scanning electron microscopy), with lower values of water absorption, porosity, and higher density. In addition the same composition showed better resistance to 5% sulfuric/hydrochloric solution than the ones made with higher and lower ratio of Na/Al. Meta-halloysite based geopolymers had a 20–30% reduction in strength after immersion in 5% sulfuric/hydrochloric acids for 7 days and even up to 62.5% after 28 days, due to the depolymerisation process of the geopolymer network. The results obtained demonstrated the suitability of Cameroonian meta-halloysite for geopolymer synthesis at room temperature.