Determination of the best modified materials: in order to solving the reunion phenomenon and improving the dispersion and stability of the slag powder in the asphalt, phosphate type single alkyl oxygen base titanates TM-P and TM-931, composite aluminum titanium coupling agent A4, silane coupling agent KH-550 and KH-570 were selected as surface modifiers to modify phosphorus slag powder respectively. The reaction continued with 3% (by weight of the phosphorus slag powder) modifier at 80 ± 1 °C for 30 min. The effect of modified phosphorus   slag powder was evaluated by wetting test, oil absorption and dispersibility test. The results show that TM-P has the best effect on surface modification of phosphorus slag powder, so it was selected as the optimal surface modifier for further study.

The optimal conditions for the modification can be defined as below: to improve the modification effects of the phosphorus slag powder, it is necessary to determine the modification conditions. To determine the optimum modification conditions, different reaction time, different temperature and different content of modifier were discussed by wetting and oil absorption test in this paper. The result

shows that it is effective to modify the phosphorus slag powder with 2.5% (weight by asphalt) TM-P reacting for 35 min under 70 °C. A layer of TM-P was formed on the surface of the phosphorus slag powder particles and the dispersibility of the modified slag powder in asphalt was better according to SEM results.

The morphology of the phosphorus slag powder modified by TM-P and unmod- ified was comparative analyzed by S-3000N scanning electron microscope made by HITACHI (Hitachi) Company in Japan. The results were shown in Figs. 1 and 2.

Figs. 1 and 2 show that the agglomeration of the non-surface-modified phos- phate slag powder is more serious, and the agglomerated particles range from sev- eral micrometers to about ten micrometers, while the TM-P modified phosphate slag powder can be dispersed evenly. This is mainly because that the unmodified phosphate slag powder too small to have a great deal of specific surface area and lots of surface energy. And the powder particles have a tendency to agglomerate and reunite under the Coulomb force and van der Waals force.

2.2. Preparation of modified asphalt of phosphorus slag  powder

70# (based on Penetration) petroleum asphalts for heavy traffic road pavement (AH-70) was used as base asphalts to blend with TM-P by colloid mill for preparing the TM-P modified asphalts. The results show that with the 10% (weight by asphalt) addition of phosphate slag powder, the modified asphalt has better rutting resis- tance and anti-aging performance [13]. Therefore, the asphalt modified by phos- phorus slag powder was prepared in a content of 10%. The preparation device is shown in Fig. 3 and the sample is shown in Fig. 4.

Specific experimental steps are depicted as follows:

1) Add 10% of the prepared phosphorus slag powder after being dried in an oven at 130 °C for 4 h to the asphalt and keep stirring, and then put the modified asphalt in an oven of 130 °C to swollen for about 10 min.

2) The colloid mill was preheated for two hours at 130 °C before the  test.

3) Open the colloid mill and pour the asphalt from the inlet to cut in the col- loid mill for about 30 min to prepare the asphalt modified by phosphorus slag powder.

2.3. Preparation of asphalt mixture specimen

2.3.1. Physical  properties test

On the basis of ‘‘Asphalt & Asphalt Mixture Test Code” (JTG E20-2011). The physical characteristics of the virgin asphalts and modified asphalts were detected, for instance the penetration, ductility and softening point, respectively. The test results are shown in Table 3.

ified asphalt. Under the same experimental conditions, the perfor- mance indexes of three asphalts namely 70# asphalt, unmodified phosphate slag powder modified asphalt and TM-P modified phos- phorus slag powder asphalt were tested   respectively.