Hydration superposition effect and mechanism of steel slag powder and granulated blast furnace slag powder

Steel slag has great potential as a supplementary cementitious material, and often shows good effects when used in combination with granulated blast furnace slag (GBFS). In order to reveal the interaction law of steel slag and GBFS, the hydration superposition effect and mechanism were studied in this paper in terms of ion dissolution, hydration heat, hydration products, hydration degree, microstructure and mechanical properties. The results show that steel slag and GBFS have a significant hydration superposition effect, and the strength of the paste when the two are used in combination is much higher. The mechanical properties at the early and later ages are the best (16.5 MPa at 7 days and 31 MPa at 28 days) when the steel slag-to-GBFS ratio is 1:1 and 2:3, respectively. Steel slag-slag composite powder (SSCP) has three exothermic peaks, which are the dissolution peak, hydration peak of steel slag and pozzolanic ash reaction peak of GBFS (after 85 h), respectively. The fibrous and amorphous C-S-H gels and plate Ca(OH)2 appear in the SSCP pastes at 3 days. Furthermore, the microstructure of hydration products consists of a number of fiber-like and particle-like C-S-H gels with widths ranging from 15 nm to 20 nm and lengths ranging from 50 nm to hundreds of nanometers. The C-S-H gels wrap the unhydrated mineral phase (UMP) to form a nucleus(UMP) - membrane (C-S-H) structure. The early hydration of steel slag would increase the pH and conductivity of the pore solution, promoting the dissolution of aluminosilicate glass in GBFS and pozzolanic ash reaction, at the same time, the consumption of Ca(OH)2 by the pozzolanic ash reaction would promote the further hydration of steel slag. The synergistic hydration effect of SSCP would promote hydration and generate more hydration products to fill the pore structure, resulting in good hardening properties.