Mechanical properties and microstructure of ternary alkali activated system: Red brick waste, metakaolin and phosphogypsum

Nowadays, alkali activated materials are becoming more and more common due to sustainability benefits and relatively good mechanical properties. This study analyzes a three-component alkali-activated binder (AAB) system, which consists of two calcined aluminosilicate precursors (red brick waste and metakaolin waste), a calcium source (phosphogypsum) and alkali solution (sodium hydroxide). The alkali-activated binder composition consists of the main precursor, red brick waste, which is gradually substituted (20, 30, 40, 50, 60, 70, 80 and 100 wt%) by metakaolin waste. The activator of the aluminosilicate materials (NaOH) was dosed by keeping the ratio of sodium and aluminum in a range of 1.03 - 1.64. Alkali activated binders were investigated by using X-ray powder diffraction (XRD) and Fourier Transform Infrared (FT-IR) analysis for the evaluation of mineral composition, along with scanning electron microscopy (SEM) for the analysis of the microstructure. The compressive strength of the material was also determined. The results have shown that the use of different types of industrial waste materials (RBW, MKW and PG) are suitable to produce ternary AAB. The replacement level of red brick waste with metakaolin waste in the precursor had a significant influence on the compressive strength of AAB. The highest compressive strength (35.1 MPa after 28 days) was achieved by using the combination of 70% RBW and 30% MKW. In this case the compact microstructure developed, consisting of the amorphous sodium aluminosilicon hydrate gel and hydrosodalite as crystalline compound, had an influence on the compressive strength of the AAB.