Autoclave Expansion and Compressive Strength of MgO-Admixed RCC with Partial Fly Ash Replacement by Phosphorus Slag

High-volume fly ash (FA) mitigates the expansion of magnesium oxide (MgO), and the uneven regional distributions of high-quality FA collectively limit the application of roller-compacted concrete admixed with MgO (M-RCC). This study evaluated the autoclave expansion and compressive strength of MgO-admixed cement paste and mortar, wherein phosphorus slag (PS) was used to partially or fully replace FA. The expansion mechanism within the MgO-FA-PS system was explored. Results show that the autoclave expansion of the mortar increased as the proportion of PS replacing FA rose. At a replacement ratio of 33% (i.e., 20% of the total mass of cementitious materials), the mortar maintained the same ultimate MgO dosage (8%) as the control specimen, yet exhibited a 12.7% increase in expansion and an 8.8% decrease in strength. The mechanism is that PS is less efficient than FA in reducing the pore solution alkalinity, thereby promoting the formation of more brucite. The growth pressure of brucite crystals expands the internal gaps in the matrix and coarsens the pore size, resulting in greater expansion and reduced compressive strength. The results of this study can provide theoretical and technical insights for the application of PS in M-RCC.