Workability,Mechanical Properties And Durability Of Alkali-activated Phosphorus Slag Cementitious Material

Phosphorus slag is the by-product of the production of yellow phosphorus by electric furnace,and its annual emissions exceed 8 million tonnes in China.Phosphorus slag is mainly composed of Ca O、Si O₂、Mg O and Al₂O₃,it contains a large proportion of amorphous aluminosilicate,and it is the potential raw material of alkali-activated cement.At present,there are many studies on alkali-activated slag and alkali-activated fly ash,but alkali-activated phosphorous slag,as a new type of alkali activated cementitious material,is less studied.In this paper,Na OH and waterglass are used as alkaline activators to study the effects of alkali dose and silicate modulus on the workability,the mechanical properties and the resistance to sulfate attack and efflorescence of alkali-activated phosphorous slag,and their influence mechanism also is explored.Firstly,the influence mechanism of the alkali dose and silicate modulus on the workability evolution of alkali-activated phosphorus slag is revealed from the perspective of the hydration reaction.The mutual inhibition occurs between Al and Ca and between Al and Mg in the pore solution of alkali-activated phosphorus slag,and the mutual inhibition is an important factor affecting the formation rate of hydrates and the rate of slump loss.Soluble Si in waterglass promotes the dissolution of phosphorus slag,which can be attributed to the quick bonding of dissolved Ca,Mg and Al with soluble Si and the weakening of the passivation adsorption layer of Ca,Mg and Al on the surface of phosphorus slag particles.When there is little difference in the magnitude of mutual inhibition between Ca,Mg and Al in the pore solution,all hydrates are generated slowly;thus,the slump losses of the pastes are slow;however,when there is a large difference in the magnitudes of their mutual inhibition,some hydrates containing excess elements are quickly produced;thus,the slump losses of these pastes occurred rapidly.Secondly,the relationship among the hydration,the microstructure and the mechanical properties of alkali-activated phosphorus slag is studied.Silicate modulus has both positive and negative effects on the hydration and compressive strength.Na⁺in N-A-S-H could be replaced by Ca²⁺,so N-A-S-H is transformed into C-N-A-S-H,which is accompanied by dealumination.C-N-A-S-H（high Na）whose dry shrinkage is larger is easy to cause the crack in paste.When the alkali dose is high and the silicate modulus is low,more C-N-A-S-H（high Na）cause more cracks in paste,which is harmful to the strength development at later ages.The denser the paste is,the higher the strength;and the density of paste is related to the type,amount and formation rate of hydrates.Thirdly,the resistance of alkali-activated phosphorus slag to Na₂SO₄ and MgSO₄attack is studied,and their similarities and differences are compared.During Na₂SO₄attack,the micro strain is mainly controlled by C-N-A-S-H expansion.Na₂SO₄promotes C-N-A-S-H polymerization shrinkage,which is of benefit to decrease the micro strain.MgSO₄ attack is mainly through destroying C-N-A-S-H and producing Mg（OH）₂,C-N-A-S-H is transformed into M-N-A-S-H,and Mg（OH）₂ can both cause expansion and inhibit expansion.When the alkali dose is high and the silicate modulus is low,the dense Mg（OH）₂ protective layer on the surface of alkali-activated phosphorus slag inhibits MgSO₄ attack.Alkali-activated phosphorus slag possesses excellent resistance to Na₂SO₄ attack,but poor resistance to MgSO₄ attack.Finally,the resistance of alkali-activated phosphorus slag to efflorescence is studied.The efflorescence of C-N-A-S-H in Na OH-activated phosphorus slag is more serious than that of C-N-A-S-H in waterglass-activated phosphorus slag.The efflorescence not only causes the Na dissolution,dealumination and polymerization shrinkage of C-N-A-S-H,but also destroys the homogeneity of C-N-A-S-H distribution in paste.The crystallization growth of efflorescence and C-N-A-S-H shrinkage result in the crack of paste.Aggregates can effectively inhibit the efflorescence and crack of alkali-activated phosphorous slag paste,and also contribute to the strength development.