Enhancement in compressive strength of foamed concrete by ultra-fine slag

In this study, ultra-fine slag (UFS) was used to prepare foamed concrete by replacing Portland cement (PC) to enhance the compressive strength of foamed concrete. PC-UFS system was designed and the replacement ratio of UFS to cement was 10%, 20%, 30% and 40%. The compressive strength at the age of 7 d, 14 d and 28 d were measured, and the fluidity was evaluated. The hardness of PC-UFS foamed concrete was characterized by nanoindentation test. The related mechanism was researched by XRD, TGA, SEM and 29Si-NMR, and pores structure was analyzed by X-ray micro-computed tomography. Results exhibited that the compressive strength of PC-UFS foamed concrete was higher than that of PC foamed concrete, noting that the 7 d and 28 d compressive strength of S0 (0% UFS) was 4.46 MPa and 6.99 MPa, and that of S40 (40% UFS) reached to 5.59 MPa and 7.75 MPa, with an increase by 25% and 11%. The reason was that UFS showed a great filling and particle grading effect, and it was also found that UFS accelerated the formation of C-A-S-H gel and densified the microstructure. Moreover, adding UFS decreased the average diameter and the diameter distribution of the pores. Such results proposed a novel method to improve the mechanical properties of foamed concrete.