X ray fluorescence

PG was collected from a backfill plant in Guizhou, China. Considering that the pH value of PG is in the range of 1–7, three batches of PG with pH values of 1.68, 4.15 and 6.75 (measured by mixing PG and deionized water in a 1:1 ratio) were collected in this study. PG was used as an aggregate for cemented backfill. A composite cementitious agent (CCA; prepared with phosphorous slag, fly ash cement clinker and slaked lime [36 ]), 32.5R composite Portland cement (CPC) and S95 ground granulated blast furnace slag (GGBFS) were used as binders in this study. Figure 2 and Table 1 show the particle size distribution and physical properties of the raw materials, respectively, and indicate that the three batches of PG were coarser than the binders. In particular, GGBFS had the finest particle size. Table 2 and Figure 3 show the main chemical components (measured using X-ray fluorescence; Bruker, Billerica, MA, USA) and crystal phases (measured using advance D8 X-ray automatic diffractometer; Bruker, Billerica, MA, USA) of the PG samples, respectively. The results indicated that the main component of PG was gypsum. In addition, it was observed that the PG sample with a low pH value contained more P₂O₅ and F.

This study evaluated representative samples of PG and composite binder in Guizhou, China. The binder is composed of yellow phosphorous slag: fly ash: cement clinker in 4:1:1, and 16–20% lime of the yellow phosphorous slag mass ratio is added. The main chemical compositions (measured by X-ray fluorescence; Bruker, Switzerland) and physical properties (measured by a particle size analyzer; Malvern Instruments, UK)of PG with different pH values were investigated through the toxicity leaching test, as listed in Table 1.Table 1

Impurity concentrations and physical properties of PG and binder.

Impurity concentration	Raw PG					Binder
	PG-1	PG-2	PG-3	PG-4	PG-5
pH	1.75	1.99	2.63	3.52	4.99	13.16
PO43− (mg/L)	4840	4000	1510	252	20	0.04
F− (mg/L)	1641	1103	521	509	182	16
SO42− (mg/L)	10,885	10,042	4419	2313	1359	487
TDS (ppt)	7.89	3.55	2.79	1.63	0.73	4.91
Physical property
D10 (µm)	13.56	11.94	11.94	10.61	15.61	6.02
D30 (µm)	42.79	29.18	33.15	36.10	37.76	13.73
D60 (µm)	92.05	55.24	81.02	67.08	81.31	30.18
Cu = D60/D10	6.79	4.63	6.79	4.04	5.21	5.01
Cc = D302/(D60*D10)	1.47	1.29	1.14	1.17	1.12	1.04

Phosphate tailings (PT) were gathered from a phosphate mine’s (Guiyang, China) tailings pond and utilized as an aggregate. The main phase compositions of PT are dolomite and quartz. Fly ash (FA, collected from a powerplant in Guiyang, China) and 42.5 ordinary Portland cement (OPC) were used as binder. Based on the Chinese standard GB/T 1596-2017, the strength activity index (A = (R/R0) × 100, where A is the strength activity index, R0 is the UCS of hardened type 42.5 ordinary Portland cement, and R is the UCS of the hardened mixture of fly ash and type 42.5 ordinary Portland cement in a 3:7 ratio) of fly ash was more than 70% after 28 days.The particle size distribution of PT (measured by a particle size analyzer, Malvern, UK) is shown in Figure 1, and the components of PT (measured by X-ray fluorescence, Bruker, Switzerland) are listed in Table 1.