Table 1 Independent variables and their coded levels for CCD of geopolymer synthesis.

From: RSM-CCD design of volcanic ash/ rice husk ash based phosphate geopolymer for crystal violet adsorption: kinetics and isotherms

Variables

Code

Unit

Coded variables levels

-1

0

+ 1

VA mass

X1

g

2

3.5

5

RHA mass

X2

g

0

1.5

3

Phosphoric acid concentration

X3

M

5

7.5

10

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