Microstructure and Key Properties of Phosphogypsum-Red Mud-Slag Composite Cementitious Materials

Abstract

Due to the low content of silicon and aluminum in red mud and the low reaction activity of red mud, when it was used to prepare composite cementitious materials, it was necessary to assist other aluminosilicates and improve their activity by certain methods. In this study, it was proposed to add slag to increase the percentage of silicon and aluminum in the system, and to improve the reactivity of the system through the activation effect of sulfate in phosphogypsum. The effects of slag and phosphogypsum contents on the mechanical properties and microstructures of composite cementitious materials were studied. X-ray diffraction analysis (XRD), thermogravimetric analysis (TG-DTG), and scanning electron microscopy (SEM) were used to analyze the effects of slag and phosphogypsum contents on the hydration products, microstructure, and strength formation mechanism of composite cementitious materials. The results show that with the increase of slag, the strength of the composite cementitious material increases gradually. When the slag content is 50%, the 28-day compressive strength reaches a maximum of about 14 MPa. Compared with the composite material without phosphogypsum, the composite cementitious material with 10–20% phosphogypsum showed higher strength properties, in which the 28-day compressive strength exceeds 24 MPa. The main reason for this is that the sulfate in phosphogypsum can cause the composite cementitious material to generate a large amount of ettringite and accelerate the dissolution of red mud and slag, increasing the release of aluminates, silicates, and Ca2+ to form more C-(A)-S-H and ettringite. In addition, a large amount of C-(A)-S-H makes ettringite and unreacted particles combine into a uniform and compact structure, thus improving the strength. When the content of phosphogypsum exceeds 40%, the 28-day compressive strength of the composite cementitious material drops below 12 MPa due to the presence of fewer hydration products and the expansion of ettringite.

Preparation and Micromechanics of Red Sandstone–Phosphogypsum–Cement Composite Cementitious Materials

“…In addition, the density of long needle-like or short columnar AFt [ 32 ] is significantly weakened. However, the number of fibrous amorphous C-(A)-S-H gels [ 32 , 48 ] increased in RS-25 compared with RS-0, whereas this was not observed in RS-60, indicating that the active SiO 2 in RS could react with Ca(OH) 2 by hydration to form C-S-H gels, but the amount that can participate in the reaction is limited, further confirming the low activity of RS. In addition, it was seen that when the rate of RS increased, a significant quantity of unreacted RS particles ( Figure 1 b) manifested itself in the sample, coating the surface of the hydration products and filling the pores.…”

“…This indicated that PG was not completely consumed during the formation of AFt and there was still a surplus, even though PG was present throughout the entire hydration process. According to Ma et al, the SO 4 2− produced by PG may combine with the hexa-ligand [Al(OH)6] 3− to generate AFt, and the system’s reduced concentration of Al 3+ can encourage the dissolution of reactive Al 2 O 3 and increase the production of C-A-H and AFt [ 48 ], which can increase the RS-PG-OPC’s strength. Figure 12 illustrates that after 28 days of hydration, the intensity of the diffraction peak of AFt in RPO-5 was greater than that in RS-25, which was consistent with the active Al 2 O 3 in the RS playing a role in the hydration process.…”

“…In addition, the intensity of the absorption peak at 995 cm −1 increased gradually with the hydration time, becoming especially pronounced from 3 days to 28 days, whereas the intensity of the absorption peak at 1115 cm −1 decreased, indicating that the addition of PG facilitated the hydrolysis of RS to produce more C-(A)-S-H gels. Intriguingly, even though the amount of hydration products (C-(A)-S-H and AFt) in RPO-5 was greater than that in RS-25, the mechanical strength was comparable because the PG that was not involved in the reaction acted as an aggregate filler, whereas the strength of the PG crystals was extremely low [ 48 ] and contributed little to the strength of RS-PG-OPC. Therefore, adequate quantities of PG can enhance the volcanic ash activity of RS and the later intensity of RS-PG-OPC.…”

“…These substances were detected at various timepoints. After 1 day of curing, only a few C-(A)-S-H gels and AFt had formed, and the microstructure was primarily composed of unreacted RS and PG crystals with a sheetlike structure [ 48 ]. After 3 days of curing, the system began to create increasingly fibrous C-(A)-S-H gels and needle-like AFt; however, at this stage, the AFt mostly was in the form of tiny needles with subpar mechanical qualities.…”

Effect of Lightweight Aggregates Incorporation on the Mechanical Properties and Shrinkage Compensation of a Cement-Ground Granulated Blast Furnace Slag-Phosphogypsum Ternary System

“…Hence, many researchers are exploring effective utilization ways for PG to serve as an alternative cementitious material in concrete. PG was tried to prepare high-content PG concrete, and although its mechanical properties were reduced, the permeability resistance of concrete was qualified [14][15][16][17]. This approach not only reduces the reserves of PG, but also reduces the production and consumption of cement, promoting environmental sustainability and conserving natural resources.…”