| Phosphogypsum is an industrial solid waste produced by wet process phosphoric acid,with large discharge and low comprehensive utilization rate.In order to achieve a large amount of phosphogypsum absorption,road engineering is a potential field.At present,many research data show that the low strength,poor water stability,poor volume stability and other problems of large amount of phosphogypsum pavement base material limit the large-scale application of phosphogypsum in the road field.In this paper,phosphogypsum is used as the main material to composite with other materials by combining indoor tests and theoretical analysis,and phosphogypsum cement slag and phosphogypsum sulpho aluminate cement sand pavement base material systems are established.The strength characteristics,water stability and volume stability of the materials are systematically studied.The main research results are as follows:（1）The volume deformation of phosphogypsum cement slag pavement base material is expansion under standard curing conditions and contraction under dry curing conditions.The unconfined compressive strength under standard curing conditions is higher than that under dry curing conditions.When the content of phosphogypsum increases from 60%to 80%,the unconfined compressive strength decreases and the expansion and contraction strain increases;When the cement content increases from 4%to 10%,the unconfined compressive strength and expansion strain first increase and then decrease.When the phosphogypsum content is 60%and the cement content is 6%,the maximum unconfined compressive strength for 7 d is 7.50 MPa,and the minimum expansion strain for 28 d is 840×10-6.The effect of activating slag with water glass in alkaline activator is better than that of sodium hydroxide and calcium hydroxide,with higher strength improvement and smaller change in dry shrinkage.When the water glass content is 3%,the maximum unconfined compressive strength at 28 d reaches 13.92 MPa.The addition of additives such as sodium methylsilicate,lignin fibers,silica,and soil curing agents can improve the compressive strength of the specimen and reduce its shrinkage,expansion,and deformation,the optimal dosage is 0.5%.The expansion strain of the specimen after 60 d is500×10-6 to 800×10-6,with a shrinkage strain of 2000×10-6.（2）Phosphogypsum sulphoaluminate cement sand pavement base material,with the increase of sand content,the phosphogypsum content decreases,and the expansion strain first increases,then decreases and then increases;The dry shrinkage strain increases with the increase of cement content.When the cement content is 10%,phosphogypsum:sand=3:1,the dry shrinkage strain is small.The unconfined compressive strength increases with the decrease of phosphogypsum content.When the cement content is 10%and 20%,the 7 d unconfined compressive strength of the test piece can meet the requirements of the highway base specifications of Class II and below,and the water stability coefficient of the test piece after water absorption saturation is above 75%,so the water stability is good.（3）The mineral composition and micro morphology of hydration products of phosphogypsum based pavement base materials were studied by XRD and SEM.The results showed that the hydration products of phosphogypsum cement slag and phosphogypsum sulphoaluminate cement sand pavement base material systems were C-S-H gel,a small amount of AFt and partially reacted gypsum dihydrate.As the reaction proceeds,the diffraction peak of dihydrate gypsum weakens,the AFt diffraction peak strengthens,and the total generation of hydration products increases.The C-S-H gel binds the partially hydrated phosphogypsum,and the AFt interpenetrates between the C-S-H gel and the partially hydrated phosphogypsum surface.The C-S-H gel gradually wraps other hydration products to make the structure more dense.（4）In order to control the volume stability of phosphogypsum cement slag pavement base material,the recommended content of phosphogypsum is 60%,the cement content is 6%～10%,the water glass content is 3%,the admixture content is 0.5%,and the expansion strain of the specimen is 500×10-6 to 800×10-6,with a shrinkage strain of 2000×10-6.Considering the strength and volume stability of phosphogypsum sulphoaluminate cement sand pavement base material,the recommended content of sulphoaluminate cement is 10%,the ratio of phosphogypsum to sand is 3:1～2:1,and the 28 d expansion strain of the material is 800×10-6,with a dry shrinkage strain of 300×10-6.The strength of specimens with different ratios can meet the specification requirements for road base layers of Class Ⅱ and below. |
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