Limestone powder activated by sodium aluminate: Hydration and microstructure

Limestone is widely distributed on earth. In this study, sodium aluminate (NaAlO2) was used as an activator to prepare a cementitious material at room temperature. The hydration and microstructure of the material using one-part and two-part activation approaches were investigated. The results showed that the hydration heat release curves contained two peaks caused by the dissolution of NaAlO2 and the formation of Ca4Al2(OH)12(CO3)·5H2O. The reaction process could be divided into five steps, which are 1) NaAlO2 dissolved quickly; 2) CaCO3 dissolved gradually and partial Na+ formed Na2CO3·H2O; 3) partial Al(OH)4− transferred to Al(OH)3 in a weaker alkaline solution; 4) Ca4Al2(OH)12(CO3)·5H2O gradually formed in the solution containing Ca2+, Al(OH)4−, and CO32−; 5) Na2CO3·H2O crystallized on the surface of the hardened paste. The lamellar shape Ca4Al2(OH)12(CO3)·5H2O and elongated crystal Na2CO3·H2O were the main hydrates in the NaAlO2-activated limestone paste. These two hydration products were closely intertwined and tightly formed the cementitious matrix with the residual limestone particles, containing a small amount of Al(OH)3. Compared with the two-part group, the one-part group had a higher dissolving heat of alkali activator, which led to fast initial hydration process, short setting time, and high early age strength.