Editing Gypsum (section)

=== Refinery waste ===
The production of [[phosphate]] fertilizers requires breaking down calcium-containing [[phosphate rock]] with acid, producing calcium sulfate waste known as [[phosphogypsum]] (PG). This form of gypsum is contaminated by impurities found in the rock, namely [[fluoride]], [[silica]], radioactive elements such as [[radium]], and heavy metal elements such as [[cadmium]].<ref name=Taylor>{{cite journal|doi=10.1016/j.jenvman.2009.03.007|pmid=19406560|title=Environmental Impact and Management of Phosphogypsum|journal=Journal of Environmental Management|volume=90|pages=2377–2386|year=2009|last1=Tayibi|first1= Hanan|last2=Choura|first2=Mohamed|last3=López|first3=Félix A.|last4=Alguacil|first4=Francisco J.|last5=López-Delgado|first5=Aurora|issue=8|bibcode=2009JEnvM..90.2377T |hdl=10261/45241|hdl-access=free}}</ref> Similarly, production of [[titanium dioxide]] produces titanium gypsum (TG) due to neutralization of excess acid with [[calcium hydroxide|lime]]. The product is contaminated with silica, fluorides, organic matters, and alkalis.<ref name="pmid26495867">{{cite journal |last1=Zhang |first1=Y |last2=Wang |first2=F |last3=Huang |first3=H |last4=Guo |first4=Y |last5=Li |first5=B |last6=Liu |first6=Y |last7=Chu |first7=PK |title=Gypsum blocks produced from TiO2 production by-products. |journal=Environmental Technology |date=2016 |volume=37 |issue=9 |pages=1094–100 |doi=10.1080/09593330.2015.1102329 |url=https://www1.cugb.edu.cn/uploadCms/file/20600/papers_upload/20161008164505966187.pdf |archive-url=https://web.archive.org/web/20220325023932/https://www1.cugb.edu.cn/uploadCms/file/20600/papers_upload/20161008164505966187.pdf |archive-date=2022-03-25 |url-status=live |pmid=26495867|bibcode=2016EnvTe..37.1094Z |s2cid=28458281 }}</ref>

Impurities in refinery gypsum waste have, in many cases, prevented them from being used as normal gypsum in fields such as construction. As a result, waste gypsum is stored in stacks indefinitely, with significant risk of leaching their contaminants into water and soil.<ref name=Taylor/> To reduce the accumulation and ultimately clear out these stacks, research is underway to find more applications for such waste products.<ref name="pmid26495867"/>