Hydration, microstructure characteristics, and mechanical properties of high-ferrite Portland cement in the presence of fly ash and phosphorus slag

Abstract

High-ferrite Portland cement (HFPC) has the advantages of excellent sulfate resistance, higher abrasion resistance, lower energy consumption, and lower CO2 emission, and is expected to be widely utilized in erosion environmental engineering. This paper presents evolution and kinetics of hydration process, quantity of unhydrated clinker and portlandite (CH), C–S–H structure, pore structure, and mechanical properties of HFPC in the presence of fly ash (FA) and phosphorus slag (PS). PS exhibits lower mechanical properties and hydration heat in comparison with FA at early age owing to its retarding effect. The Qmax order is: HFPC > PS > FA, where HFPC, PS25, and FA25 (25 indicates percentage replacement of HFPC, the same below) correspond to 345 J/g, 322 J/g, and 313 J/g, respectively, according to the Krstulovic-Dabic (K-D) model. The FA and PS retard the crystal nucleation and growth (NG) and phase boundary reaction (I), and I process of FA takes longer than PS. PS hinders the formation of CH and C–S–H at early age, and plenty of CH and high polymerization C–S–H are formed at a later time. The effects of FA on the enhancement of Al content of C–S–H and mean chain length (MCL), as well as the pozzolanic reaction are more significant than that of PS. FA and PS have a comparable total porosity at 90 days. FA has an excellent ability to refine the pore structure distribution, which reduces the prevalence of large pores (50–10,000 nm) by 17.92% and increases the proportion of small pores (2.5–20 nm) by 8.87% than PS, at 90 days. The quantity of C–S–H and pore size distribution affect the mechanical properties more than the CH content. The unhydrated C2S in HFPC, FA25, and PS25 are about 14.3%, 9.1%, and 22.1% at 90 days.