Research And Development Of Phosphogypsum Based Cement

Phosphogypsum (PG) is a byproduct of manufacturing phosphate acid by wet process in fertilizer industry. Five tons PG is generated for every 1000 kg phosphate acid produced. The annual production of PG is more than 40 million tons with the rapid development of China's phosphate industry, and total accumulation of PG is more than 200 million tons in China. Due to various reasons, less than 10% PG is reused and the rest is deposited as solid waste. The deposited PG not only occupies a lot of land, but also causes serious environment pollution. It is urgent to expedite the utilization of PG.In this study, a new type of low energy consumption hydraulic binder phosphogypsum-based-cement (PBC) had been developed by mainly utilizing PG and ground granulated blast-furnace slag (GGBFS) with small addition of alkaline activator, and experiments of component design and production process optimization were conducted to improve the property of PBC. The results show that, the 28d compressive strength of PBC made with 45%PG,35-40%GGBFS and 10%steel salg or 4%portland cement as alkaline activator was over 40 MPa when cured in water. Although the setting time of PBC was slow and the early strength was low, but its strength increased continuously cured in water. The early strength was increased and the setting time was shortened by adding 1%NaOH in steel slag activated PBC. The 3d strength of PBC was up to 12 MPa by using super fine ground portland cement as alkaline activator which is close to that of 32.5 portland blended cement.The hydration products, hydration mechanism and process, and microstructure development of PBC were studied by XRD and SEM analyses. The main hydration products of PBC were C-S-H gel and ettringite. Partial PG reacts with GGBGS and alkaline activator to form ettringite, the rest PG was enclosed by the hydration products to act as filler. At the hydration of PBC, GGBFS was dissolved into pore solution by alkaline activation, which then reacted with gypsum to form hydration products ettringite and C-S-H gel. Ettringite and C-S-H gel mingled together to fill the porous space. As a result, the microstructure of hardened paste became increasingly denser, and the strength of PBC developed continuously. Ettringite formed during early hydration filled the porous space, which contributed together with C-S-H gel to shorten the setting time and to improve the early strength of cement. If large amount of ettringite formed after the density of the hardened paste reached to a certain degree, the excessive crystalline phase from hydration was harmful to density improvement of the microstructure; it may cause damage to microstructure due to the ettringite crystallization pressure, which resulted in the deterioration of strength development or even expansive cracks. Because the gypsum was excess in PBC, the damage of expansive ettringite could be avoided by controlling the dosage of alkaline activator.The durability properties of PBC, including long term strength, volume stability, carbonation resistance, water resistance and sulfate resistance were also studied. The results show that:1. The strength of PBC increased continuously when cured in water and stabilized at about one year. The final strength of PBC was higher at higher dosage of GGBFS.2. When cured in water, steel slag activated PBC showed small expansion at beginning, and its volume was stable after a certain amount of expansion. The amount of expansion was decreased with increase of PBC strength. When cured in air, the PBC shrank like portland cement, but its shrinkage was about half of that of portland cement. Portland cement activated PBC shrank if cement dosage less than 3% and swell if cement dosage more than 3% when cured in water.3. Carbonation resistance of PBC was inferior to that of portland cement. After carbonated for 28d in carbonation test box, the compressive strength of PBC was retained 65-84%. The strength decrease was related to the strength of the specimens before carbonation, the higher the strength before carbonation the less the strength decreased. Carbonate acid reacted with hydration products C-S-H gel and ettringite of PBC and formed calcite and gypsum, which made the PBC microstructure less cohesive and was the main reason for strength loss after carbonation.4. The hydration products of PBC contained large amount of residual gypsum. At early hydration, the microstructure was not dense enough and when immersed in water some gypsum was dissolved. But as hydration continued, gypsum was wrapped by hydration products of C-S-H gel and ettringite, dissolution became more difficult and finally stopped. As a result, PBC showed very good water resistance.5. PBC was also resistant to sulphate because its hydration process was under the condition of saturation of gypsum, the alkalinity of hydration products was low, and its microstructure was dense. Consequently, it was difficult for sulphate erosion media to react with the hydration products of PBC and form ettringite and gypsum.