Research On Stability Of Hydration Products Of Phosphgypsum-blastfurnance Slag-cement Blends

Phosphgypsum-blastfurnance slag-cement blends（PPSC）is a kind of newartificial hydraulic binding materials with high strength,which is a mixture of phosphogypsum（PG）,ordinary Portland cement（OPC）,ground granulated blast-furnace slag（GGBFS）and other industrial waste residue.As the dosage of PG is too large for complete reaction,the hydrated paste of this kind binder is a dense structure of hydration product associated with un-reacted GGBFS and lots of soluble PG,which may cause hidden danger in its application.This paper investigated its hydration process quantitatively;especially focused on the reaction degree of PG and GGBFS along the hydration.On the basis of the understanding of the composition of its hydrated paste,the resistance to soft water and the dissolution behavior of SO₄^2-were evaluated through specific experiment.Using Pb（NO₃）₂ and Cr（NO₃）₃ as the supporter of Pb、Cr,influence of the PPSC on heavy metals solidification effect was analyzed either.The conclusions are as follows:1.The best curing condition for PPSC’s hydration is self-curing,which means specimens were cured covered by plastic wrap to sequester CO₂ in curing box after demoulded.Though a lot of experiment,the optimum added quantity of PG,OPC and phosphorus slag（PS）were 45%,2%,10%.2.The reaction degree of GGBFS improves continually along the hydration,although it can only achieve 25%⁴0%after hydration for 28 days as well as a greater slag dosage leads to a lower reaction degree of slag.The reaction degree of PG only gets a slight increase along the hydration which means the reactive PG has already participated in the reaction in the earlier stage of hydration process and it can only achieve 25%of the matrix approximately after hydration for 28 days.3.The release of SO₄^2-in soft water is affected by the dosage and finesse of PG,which behaves as the earlier period of leaching is dominated by diffusion and depletion occurs in the rest periods.After carbonized or damaged by load,the earlier period of leaching is dominated by dissolution and the rest is dominated by diffusion.The diffusion coefficientof SO₄^2-of the periods controlled by diffusion is 5¹0orders of magnitude higher than the diffusion coefficient of heavy metals in concrete and 2³ orders of magnitude higher than the diffusion coefficient of sulfate in concrete under sulfate attack.The coefficientis positivelycorrelative to the PG dosage and negatively correlative to the fineness of PG.4.After dry-wet circulation the strength and mass of the sample would decreased significantly.The dosage and finesse of PG affects the mass&strength loss ration either,for which a greater PG dosage leads to a higher mass&strength loss ration of the sample after dry-wet circulation and the levigation of PG could decrease the loss ration oppositely.5.After immersion in soft water,the decomposition of ettringite and decalcification of C-S-H led to the surface topography of the eroded part of the paste become loose,which manifested that the erosion by soft water would lead to the degradation of microstructure of the hardened paste and decrease in strength.The leaching depth of SO₄^2-shows great positivelycorrelation to the leaching time,which can be expressed as d=k（?）.6.PPSC has a good capacity to solidify Pb、Cr,and the results showed that the solidified rate could reach above 92%when the dosage of Pb,Cr come up to 1%.The optimum added quantity of Pb,Crought to be restricted within 0.5%.