Three sodium waterglass (NWG) such as commercial NWG (S1), NWG from pure rice husk ash (S2) and NWG from raw rice husk ash (S3) were applied for producing geopolymer cements using metakaolin (MK) as aluminosilicate source. Geopolymers (Geo1, Geo2 and Geo3) were prepared using each NWG with the molar ratios SiO2/Na2O and H2O/Na2O kept constant at 1.5 and 12, respectively. It could be observed that the water absorption of Geo1, Geo2 and Geo3 is 7, 9 and 13.2 % and the mass loss is 15.8, 14.7 and 12.4 %, respectively. Their compressive strength at 20 days (37.5/34.3/29.6 MPa) and 28 days (43.3/40.3/33.2 MPa) increases with increasing the aging and decreases in the course Geo1/Geo2/Geo3. Their average pore radius (6/8/20 nm) and cumulative pore volumes (155/205/245 mm3/g) increase in the course Geo1/Geo2/Geo3. It is discussed that the presence of phosphate known as corrosion inhibitors in raw rice husk ash hinders the dissolution of SiO2. It entails the formation of NaH2PO4 in S3 which reduces the soluble Si atoms. Therefore, less amount of metakaolin could be dissolved leaving thus a higher amount of unreacted metakaolin particles in Geo3. The reacted volumes and compositions of the geopolymers are different in the three cases, too. A content of approximately 20, 25 and 35 % of unreacted metakaolin was proved for Geo1, Geo2 and Geo3, respectively. Graphical Abstract: [Figure not available: see fulltext.
The aim of this work was to study the thermal behavior of metakaolin-based geopolymer cements using sodium waterglass (NWG) from rice husk ash and waste glass as alternative alkaline activators. Metakaolin-based geopolymer cements were obtained by adding freshly prepared NWG and metakaolin. The geopolymer cements obtained using sodium waterglass from rice husk ash and the one obtained with sodium waterglass from the waste glass were treated at room temperature for 28 days and then heated at 200, 400, 600 and 800 °C. The results show that the metakaolin-based geopolymer cements lose their compressive strength from room temperature to 400 °C. At 600 °C, the compressive strength of geopolymer cements increases relative up to 200 and 400 °C. At 800 °C, the reduction of compressive strength of geopolymer cements is assigned to the total evaporation of the rest of structural water. However, it is higher than that measured at ambient temperature for geopolymer cement obtained using the sodium waterglass from waste glass but lower for the sample obtained with sodium waterglass from rice husk ash. The total mass loss of geopolymer cements obtained with sodium waterglass from rice husk ash and one obtained with sodium waterglass from waste glass are about 12.57 and 15.04 %, respectively. This suggests that geopolymer cement obtained using sodium waterglass from waste glass are more condensed geopolymer structure indicating that it could a very suitable material for fire resistant application. The results indicate that NWG from rice husk ash and waste glass could be served as suitable alternative activators for producing metakaolin-based geopolymer cements with high-temperature performance
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