Influence behavior of phosphorus slag and fly ash on the interface transition zone in concrete prepared by cement-red mud

Abstract

Cement production requires a large amount of raw materials, such as clay, limestone, gypsum, and other materials, and its production process causes a certain degree of pollution and destruction of the ecological environment. Using industrial solid waste as a mineral admixture to partially replace cement in the preparation of concrete is helpful in saving cement consumption, absorbing industrial solid waste, and optimizing the structure of the interfacial transition zone (ITZ) in the concrete. Compressive and flexural strength, mercury intrusion porosimetry (MIP), scanning electron microscope (SEM), X-ray diffraction (XRD) microzone testing, and microcalorimeter were performed to investigate the influence behaviors of phosphorus slag (PS) and fly ash (FA) on the ITZ in concrete prepared with cement partially replaced by red mud (RM), respectively. PS and FA are industrial wastes produced at high temperatures, and PS and FA contain a certain amount of gelling active substances (such as amorphous SiO2). The inactive substances in PS and FA mainly play filling roles and have a microaggregate effect in the matrix and in the ITZ, while the amorphous SiO2 reacts with calcium hydroxide (CH) produced from cement hydration to generate calcium silicate hydrate (C–S–H) gel to fill the porous area near the ITZ, resulting in a reduction in pore size near the ITZ. Moreover, the C–S–H gel squeezes the growing space for CH to weaken the CH growth and its directional distribution, which is beneficial to the compactness of the ITZ in RM–cement concrete.