Early properties and chemical structure analysis of alkali-activated brick geopolymer with varied alkali dosage

Abstract

Alkali-activated brick powder geopolymer (BPG) was prepared by use of brick powder (BP) under the activation of sodium silicate-based activator with silicate modulus of 1.6 and alkali dosages of 2%, 4%, 6%, 8%, and the effect of alkali dosage on the early properties and reaction mechanism of BPG was studied in this paper. Results show that the BPG paste prepared with alkali dosage of 6% exhibits the best fluidity (180 mm). With increasing alkali dosage, the dissolution rate of Al from BP first increases and then decreases. The negative correlation exists between the setting time of BPG and the dissolution rate of Al from BP. The optimal compressive strength reaches 31.1 MPa at the alkali dosage of 6% after cured for 3 days (T = 70 °C, RH = 30%). Lower or excessive alkali dosage is both unfavorable to the enhancement of strength. The calorimetry, FTIR and NMR results show that the increase in alkali dosage (≤6% Na2O) improves the reaction degree, promotes the formation of network structures and increases the Al-rich structural units (Q4(3Al) and Q4(4Al)) due to the beneficial effect of alkali enhancement on dissolution of Al from BP, thereby increasing the polymerization degree of reaction products and enhancing the compressive strength of BPG. However, the higher alkali dosage (8% Na2O) impedes the further dissolution of Al and the formation of Al-rich structural units, which results in the lower reaction degree and compressive strength.