Microstructural characteristics of geopolymer materials with twenty eight days of curing and after eight years stored at room temperature

Abstract

The main purpose of this study was to perform a microstructural evaluation in geopolymer samples with 28 days of curing (GP28D) and after 8 years of storage (GP8Y). The knowledge on the microstructure of geopolymers in the long run is crucial for the use of these materials, given their great potential applicability. Equally important is the use of industrial waste as raw materials for the manufacture of new materials. In this perspective, this study employed two industrial wastes to produce the samples: bottom ash from mineral coal (BA) and residues from white paper recycling industries. The latter was treated to use the kaolin present in composition. The treatment of the paper waste was carried out by burning the cellulose. Then, the resulting material was subjected to chemical reaction in hydrochloric acid (HCl) to separate the CaCO3 from kaolin. In addition to kaolin, calcium chloride (CaCl2) and water were also obtained as reaction products. The kaolin obtained from the paper residue was dehydroxylated at a temperature of approximately 850 °C for 2 h to obtain processed metakaolin (PMK). To obtain the geopolymer samples, BA/PMK proportions of 2:1 were activated with NaOH solutions of 5, 10 and 15 M mixed with sodium silicate. After the synthesis, the samples were cured for 28 days and subjected to analysis. Then they were stored for more eight years and again underwent microstructural analysis. The results of the analyzes showed significant changes in the microstructure of the materials with 28 days of cure and after eight years. The micrographs obtained in SEM showed a denser structure and the micrographs obtained in TEM showed that the amorphous structure was partially replaced by a crystalline structure. The samples aged for eight years did not show significantly higher mechanical resistance in relation to the samples analyzed with 28 days of cure. It is important to highlight that the results did not show microstructural degradation after eight years.