Comparison of metakaolin-based geopolymer cements from commercial sodium waterglass and sodium waterglass from rice husk ash

Abstract: 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.]