|Journal of the Network of African Countries on Local Building Materials and Technologies - Volume 3, Number 1 (HABITAT, 1994, 44 p.)|
** Developed by the Central Building Research Institute (CBRI), Roorkee, India
Phosphogypsum is produced as a by-product in the manufacture of phosphoric acid industry by the interaction of ground phosphate rock with sulphuric acid. Approximately 3.7 million tons per annum are generated from twelve phosphoric acid fertilizer plants in India. Phosphogypsum contains impurities of P2O5, F, alkali and organic matter which adversely affect the setting and strength development of plasters/cements produced from it. These impurities exist in phosphogypsum as water-soluble, lattice bound and insoluble forms. P2O5, F and organic matter are generally found in the range of 0.40 to 1.5 per cent, 0.44 to 1.5 per cent and 0.11 to 0.60 per cent respectively. For effective utilization of phosphogypsum, it is essential to develop processes for reducing these impurities or to make them innocuous to enable proper and effective utilization of phosphogypsum in building materials.
Various methods ranging from washing, thermal to chemical treatments were evolved at CBRI to reduce/inactivate the impurities. Wet sieving of phosphogypsum through 300 micron IS sieve helps in removing the major part of impurities through the rejection of coarse particles in phosphogypsum.
The purified phosphogypsum has been utilized in the production of calcined gypsum such as hemihydrates which have a compressive strength of 8.1 to 13.1 N/mm2 or anhydrate plasters which have a compressive strength of 30.0 to 33.3 N/mm2.
These hydrates have been used for the production of:
(i) Fibrous plaster boards which have the following characteristics:
specific thermic value:
K = 0.14 - 0.17 cal/m/h/°c
(ii) Building blocks with the following characteristics:
900 - 950 kg/m3
2.5 - 3.5 N/mm2
(iii) Slotted tiles which have the following characteristics:
425 - 490 kg/m3
(iv) Super sulphated cement, and
(v) High strength alpha-plaster which has a compressive strength of 2.5-35.0 N/mm2 for use in false ceilings, non-load bearing partition walls, masonry and plastering works, etc.
Moreover, CBRI has recently succeeded in developing a durable and water-resistant gypsum binder using calcined phosphogypsum and other additives. Having a compressive strength of 20-30 N/mm2 (at 28 days) and a water-absorption capacity of 5.5-10.5 per cent, this material could be used in masonry work, block production, fibre reinforced boards and external and internal plastering.
Figures 1, 2 and 3 show three different types of building materials in which phosphogypsum has been used as basic raw material.
Figure 1. Fibrous gypsum plaster board
Figure. 2. Gypsum blocks
Figure 3. Slotted gypsum tiles