Preparation And Investigation On The Persulfate Cementitious Materials

Phosphogypsum is the by-product of phosphoric acid, which is produced bychemical companies in wet-milling process. Nowadays, it is more difficult to processthan other kind of industrial waste in our country. In this paper, persulfatecementitious material is prepared by mixing phosphogypsum, sulphoaluminatecement clinker, slag and alkali-activator at proper ratio. Among them phosphogypsumneed to be dry and griding. Using this method, not only is the pre-treatmenttechnology of phosphogypsum simplified, but also its utilization in the buildingmaterials industry is improved.The research in this paper is focused on the quaternary system that is formed bysulphoaluminate cement clinker-phosphogypsum-slag-CaO. It shows that thesetting time of the system will be longer with the increasing proportion ofphosphogypsum and its mechanical properties will be reduced. When the proportionof phosphogypsum comes to30%, there will be a large quantity of the ettringite in3-day hydration products of the system. The relative density of the structure is higherat28-day. The sulphoaluminate cement clinker can make the setting time less,improve the early strength, and enhance the surface sugaring resistance performanceof the cementitious material system. But when the proportion of sulphoaluminatecement clinker is over5%, there will be the reduction in long-term strength.Enlarging the specific surface area of sulphoaluminate cement clinker can acceleratethe process of condensation and hardening of the system and better its mechanicalproperties. When the specific surface area is over520m2/kg, the increase ofmechanical properties will be less. The CaO in the system can make the alkalinity ofthe system higher and promote the development of its strength. Persulfatecementitious material can be prepared by30%of phosphogypsum,5%ofsulphoaluminate cement clinker (520m2/kg),2%of CaO and63%of slag, whichwould be up to13MPa at3-day and even48MPa at28-day.When using NaOH as the alkali-activator of the system, it has a similar effectwith CaO. When the proportion of NaOH ranges from2%to3%, the system hasbetter mechanical properties. When using Portland cement clinker as the alkali-activator of the system, with the higher proportion of clinker, the setting time ofthe system becoming less, the water requirement of normal consistency will increaseand the early strength will be improved. But if the proportion becomes too high, itwill be detrimental to the long-term strength. The experiment shows that the optimalrange of the proportion of clinker is from4%to5%.On account of the persulfate cementitious material which have been prepared bydifferent alkali-activators, in comparison with the pH of the system, it is known thatthere are tight connections between the early strength and the pH of the system. Theearly strength will be improved when the initial pH is higher, and the greater the pHchanges, the more the strength of the system increases. Besides, according to theresearch of the persulfate cementitious material with different ages of dryingshrinkage, the size of the drying shrinkage rate is related to the early hydration rate ofthe system. Only when the alkalinity environment is appropriate can the earlyhydration rate be controlled better and the good volume stability be ensured. Theexperiment results of carbonation resistance performance show that the persulfatecementitious material with4%proportion of Portland cement clinker has betterperformance while the one with3%proportion of NaOH has worse carbonationresistance performance. It is concluded that the carbonation resistance performancehas to do with the hydration degree and density of the system. The micro-analysismeasurements illustrate that the ettringite and C-S-H gel exist mainly in the hydrationproducts which are produced in the persulfate cementitious material with differentalkali-activators. Meanwhile, there are small amounts of un-reacted cementitiousmaterial particles in the hydration products. Only when the proportion ofalkali-activators is appropriate can the activity of slag be excited. If so, it can beensured that the system may produce more hydration products and tiny cracksreducing the strength of the cementitious material system will not appear.