They prevent the use of top soil for manufacturing of clay bricks
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/action-plan-mooted-boost-usage-fly-ash-bricks
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/hindu-hyderabad
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/andhra-pradesh
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/fly-ash-bricks
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/industrial-pollution
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/centre-science-and-environment-cse
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/environment