Mechanism of hydration and structure formation of autoclaved aerated concrete with multiple solid wastes

Abstract

The compressive strength of autoclaved aerated concrete (AAC) mainly varies depending on the type and quantity of hydration products as well as the pore structure. Recycled concrete powder (RCP), calcium carbide slag (CCS), fly ash (FA), and Phosphogypsum (PG) were employed to prepare AAC, which is in line with the development direction of green building materials. Nevertheless, the variation rule of hydration and microstructure of AAC fabricated from the aforementioned solid wastes along with the autoclaved curing time and the mechanism need to be clarified urgently. This study explored the influence of different autoclaved curing durations on the compressive strength, hydration products, microstructure, and pore structure of AAC composed of RCP and other diverse solid wastes under the autoclaved condition of 180 ℃. The obtained findings indicate that the compressive strength of the samples attained the maximum value of 8.2 MPa at 9 h of autoclaved curing at 180 ℃, which is 127.78% higher than that of 1 h compressive strength. As the autoclaved curing time increased from 1 h to 10 h, the average pore size initially decreased from 37.047 nm to 22.54 nm and subsequently increased slightly to 23.455 nm. During this process, a significant transformation occurred where C-(A)-S-H gradually converted into tobermorite. The morphology of tobermorite evolved from sheet-like to plate-like and finally to fibrous structures. The accumulation of fibrous tobermorite not only refined the pore structure but also improved the compressive strength. However, an overly long autoclaved curing time (> 9 h) led to the transformation of tobermorite to xonotlite, resulting in a 10.67% decrease in strength and a 4.06% increase in the average pore size.