Gypsum = CaSO4 * 2H2O
but what kind of plaster do you mean?
Gypsum-Plaster, Lime-Plaster, Cement-Plaster, Bitoumenious-Plaster????
best regards
Andreas Rohatsch
Dear Said,
I’m agree with answers written before. Maybe should be more useful if you explain better your need. Probably you aren’t interested at natural gypsum CaSO4•2 H2O, because isn’t reactive so it cannot be used for plasters. For these products is used hemi hydrate gypsum CaSO4 • 0,5 H2O , very reactive (20 minutes of setting time) and workability will be adjusted using additives to obtain the necessary open time.
Normally plasters can be done in several ways but the most traditional are : lime based (hydrate or hydraulic lime) , “bastard” so based on blend lime / Portland cement and gypsum or anhydrite (CaSO4) based.
For this reason chemistry of plasters is more complexed than gypsum and if you use Portland cement or lime, in any case, is necessary to add some amount of natural or artificial pozzolana (metakaolin, zeolite, coccio pesto , slag and so on ) to stabilize binder and increase the mechanical strength development.
Finally to answer at your last question, yes of course reaction with waste brick , that is called “coccio pesto” because was discovered as very useful by Romans as artificial pozzolana , will be very different depending on the system you select.
The most reactive raw material for coccio pesto is hydrate lime Ca(OH)2 that creates CSH with coccio pesto , but you must use a correct ratio between them that depends on granulometry curve and mineralogy of coccio pesto (for example ancient bricks are different to new and coccio pesto obtained from roof tiles is still more different because has been cooked at higher temperature) .
Hoping to be useful .
Best regards
Alex Reggiani
the formula of gypsum is CaSo4 2H2O. the formula of plaster of Paris is Caso4 1/2H2O
Gypsum is CaSO4.2H2O. But the plaster of Paris is CaSO4.1/2H2O
After heating gypsum to 170 C we find the plaster of Paris (Plaster).
