Research On A Novel Process And The Strength Mechanism Of Non-fired Bricks Prepared With High Content Of Phosphogypsum

Phosphogypsum is a solid waste generated in phosphoric acid production, which is composed primarily of calcium sulfate dihydrate (CaSO4·2H2O) and partially of some impurities, such as phosphate, fluorides, sulfate ions and organic matters. In general, approximately5.0tons of phosphogypsum are generated when producing1.0ton of phosphoric acid. At present, the output of phosphogypsum is estimated to be around280million tons per year all over the world and more than22million tons per year throughout China, and the total generated phosphogypsum have been over100million tons in ChinaAt present, only15%of worldwide phosphogypsum has been used to produce building materials, agricultural fertilizers, soil stabilization amendments, Portland cement retarder, sulfate acid, etc., and the remaining85%is usually discarded in landfills, rivers, seas and lakes without any treatment. In China, the reusing proportion of phosphogypsum is even less than10%. The discarded waste phosphogypsum occupies considerable land resource and leads to serious environmental pollution.Previous studies have been carried out to use phosphogypsum in preparing building materials, such as blocks, bricks, plasters, etc., of which non-fired bricks have been a hot research topic recently. At present, there are two kinds of processes to prepare non-fired bricks from phosphogypsum. One is the autoclaving-curing process, in which the green non-fired bricks are pressed at normal pressures (20～40MPa) and then are autoclave-cured at the high pressure of0.8～1.2MPa and the temperature of100～180℃for4-8h. The major shortage of this process is the exorbitant energy consumption in course of autoclave-curing. The other is the high-pressure press-forming process, in which the non-fired bricks are press-formed at a very high pressure of about80MPa and air-dried in natural condition. The key shortages of this process are the need of using high-pressure forming equipments and the low compressive strength of the prepared brick products (generally less than10MPa). In addition, the content of phosphogypsum in the brick bodies is quite low (no more than30%) in the two processes, which limits the consumption and recycling efficiency of waste phosphogypsum.To effectively recycle waste phosphogysum and solve environmental problem, in this paper, a novel process of preparing non-fired bricks from waste phosphogypsum, named as "the Hydration-Recrystallization process", was proposed and tested. In detail, the procedures and elementary principle of the process are as follows:(1) Mixing the raw materials including phosphogypsum, river sand, Portland cement, hydrated lime and water;(2) Press-forming the mixtures into green non-fired bricks at normal pressures (about20～40MPa);(3) Wet-curing the bricks to promote the hydration reaction of Portland cement, which endows the bricks with certain early mechanical strength to avoid the crack in the following procedures;(4) Hot-drying the bricks at120～180℃and atmospheric pressure for2.0-6.0h to dehydrate gypsum (CaSO4·2H2O) into semi-hydrated gypsum (CaSO4·0.5H2O);(5) Immersing the bricks into water at room temperature for1h and then air-drying in natural condition, to recrystallize the new gypsum (CaSO4·2H2O) crystals in-situ from semi-hydrated gypsum (CaSO4·0.5H2O) and finally to form a type of interlock crystalline microstructure which endows the bricks with high mechanical strength. The key steps of the process are the steps (4) and (5), namely hot-drying for dehydration and water-immersing for hydration and recrystallization.Based on the experimental investigation of the novel process reported in this paper, the following conclusions can be drawn:(1) Using waste phosphogypsum as the main raw material, non-fired bricks can be prepared by the Hydration-Recrystallization process. The optimal mix designation composes of70.0%phosphogypsum,21.4%river sand,8.0%Portland cement and1.4%hydrated lime, and the corresponding compressive strength, water-saturated compressive strength and bending strength of the as-prepared bricks are24.1MPa,18.4MPa and6.1MPa, respectively, which can meet the property requirement of MU20grade in the Chinese standard (JC/T422-2007).(2) In this paper, the optimal operational conditions of the Hydration-Recrystallization process are as followings:a:Press-forming the mixtures into green non-fired bricks at the pressure of30MPa; b:Wet-curing the bricks for1day by intermittently spraying water onto the bricks for3times and then naturally air-dried for3days; c:Hot-drying the bricks at150～170℃and atmospheric pressure for3.0～4.0h; d:Immersing the bricks into water at room temperature for60min; e:Air-drying the bricks for more than28days in nature condition.(3) The high mechanical strength of the phosphogypsum non-fired bricks is attributed to the regular, dense and interlock crystalline microstructure of the gypsum (CaSO4·2H2O) crystals recrystallised in-situ inside the green brick bodies by hot-drying and water-immersing steps of the Hydration-Recrystallization process.Moreover, the Hydration-Recrystallization process doesn’t need high-pressure press-forming or autoclave curing, and so it is favorable to save the energy consumption and reduce the production cost. Meanwhile, due to the high proportion of phosphogypsum in the mix designation, the industrial application of the novel process can help to significantly reduce the environmental impacts of waste phosphogypsum.