PERFORMANCE EVALUATION OF GYPSUM OF MULTIPLE
ORIGIN - AN INITIATIVE TOWARDS CEMENT SUSTAINABILITY
Dr. B.N Mohapatra*, Neeraj Palriya*, M.L. Jat* and Chander Shekhar*
Dr. J. M. Sharma**and Dr. Pinky Pandey**
* Vikram Cement Works (Unit of Ultratech Cement Ltd.)
** Central R&D, Ultratech Cement Ltd.
1.0
Introduction
Mineral Gypsum is most common cement setting retarder used in all cement plants which is a
dihydrated product of calcium sulfate (CaSO4.2H2O). The need for gypsum in cement increases
with increasing amount of C3A, alkalis and fineness of the cement (Ref-1,2). Theories on the
hydration process (Ref-3,4) suggest that upon contact with water, the CaSO4 bearing materials give
sulfate anions, which react with the C3A grains forming ettringite, as given below.
C3A + 3CSH2 + 26H → C3A. 3CS.H32
(1)
The formation of ettringite crystals create a thin coating around the anhydrated cement grains,
hence preventing the quick reaction of C3A with water (Flash Set):
C3A + 21H→ C4AH13 + C2AH8 → 2C3AH6 + 9H
(2)
Various calcium sulfate bearing materials is being utilized as setting retarder which may be proved
as set retarder and can be used in place of mineral gypsum. Chemical gypsum such as Phospho and
Fluoro gypsum are one of the cheap and easily available gypsum. Phosphogypsum (PG) and
Fluorogypsum are produced as a by-product of the phosphoric acid and hydrofluoric acid industries
respectively. PG contains some impurities such as P2O5 and F which retard the hydration of cement
to a large extent and adversely affect cement setting time and strength if present more than 0.7 % .
Manjit Singh has reported that the impurities of P2O5 and F exist on the surface of gypsum crystals
as Ca(H2PO4) H2O and H3PO4 as CaHPO4 built in gypsum crystals in place of SO42- and CaF2
compounds (Ref-5). The retardation of setting time can be attributed to the protective coating of
Ca3(PO4)2 and CaF2 compound as inert & inactive substances formed by the impurities of
phosphates and fluorides on the hydrating cement particles (Ref-9), thereby causing temporary
suppression of the hydration of cement grain.
Marine gypsum is obtained as a by-product of common salt by solar evaporation. It is also used as
the substitute of mineral gypsum but it contains chloride as the impurity which has get deleterious
impact on the concrete structure (Ref-6). Mould gypsum is one kind of waste gypsum which is
produced by the hydration of Plaster of Paris. Different types of waste gypsum contain different
amount of dihydrate, hemihydrate and anhydrite (Ref-7).
CaSO4.2H2O → CaSO4.0.5H2O + 1.5H2O
(3)
CaSO4.0.5H2O + 1.5H2O → CaSO4.2H2O
Synthetic gypsum is calcium sulfate which is produced by the treatment of limestone with
commercial grade of sulphuric Acid (Ref- 8,9).
In the CFBC boiler of thermal power plant, high sulphur pet coke is used as the main fuel. To trap
the sulphur content lime stone powder is continuously quenched & the bed ash generated out of it
has been identified to have appreciable amount of gypsum anhydrite alongwith other cementitious
phases like anhydrite (CaSO4), aphthitalite (3K2SO4.Na2SO4), arcanite (K2SO4), Calcium
langbeinite (K2SO4.2CaSO4) and thenardite (Na2SO4) (Ref-10). Vikram Cement Works successfully
using anhydrite gypsum along with Mineral gypsum, Chemical gypsum since last four years and
incurring significant amount of cost benefits by maximizing the use of Chemical gypsum.
2.0
Experimental
In the present investigation, Vikram Cement Works in association with the Central R & D have
initiated the work to explore the possibility of using different types of gypsum such as Mineral
gypsum, Chemical gypsum, Marine gypsum, Mould gypsum, Imported gypsum and Synthetic
gypsum. The complete chemical and mineralogical investigation have been carried out in the
Central R & D and physical testing of cement of lab ground cement have been carried out in both
VC lab and Central R & D.
100% gypsum of different types has been used in lab ball mill maintaining identical SO 3 & Blaine
fineness. Similarly blending of Mineral, Marine and Synthetic gypsum have also been carried out to
get the required setting time and compressive strength. On the basis of lab trial results different
blend of gypsum also used in plant during manufacturing of both OPC & PPC. Four different
gypsums in different proportion have been tried in laboratory scale & finally used in plant scale
production.
2.1 Chemical Characteristics of different types of gypsum:
Presently Vikram Cement Works is using Chemical Gypsum, Marine Gypsum, Mould Gypsum,
Imported Gypsum, by product gypsum anhydride along with Mineral Gypsum. All the above
gypsums have been analyzed for the complete chemical characteristics with respect to their major
and minor elements following the relevant IS. The test results are shown here in Table no.1.
Table 1: Chemical characteristics of different types of Gypsum
Sample ID
Combined
water
SiO2+IR
Al2O3
Fe2O3
CaO
MgO
SO3
Na2O
K2O
TiO2
P2O5
Cl
F
3.0
Mineral
Gypsum
Chemical
Gypsum
Marine
Gypsum
Mould
Gypsum
Imported
Gypsum
Synthetic
Gypsum
13.10
12.52
13.16
16.10
16.42
18.69
16.31
0.77
0.61
29.37
0.68
32.50
0.23
0.34
0.16
0.07
0.004
0.005
4.69
0.99
0.76
30.22
0.86
43.76
0.62
0.06
0.43
1.53
0.05
0.25
12.21
1.95
1.90
29.23
0.72
32.50
3.83
1.22
0.32
0.06
2.58
0.15
6.29
0.77
0.64
31.45
0.61
40.93
0.22
0.14
0.01
0.06
0.06
0.02
3.42
0.74
0.44
33.37
0.46
40.33
0.06
0.13
0.03
0.35
0.04
0.03
5.96
0.85
0.84
30.22
0.29
42.61
0.07
0.15
0.12
0.05
0.004
0.001
Result and Discussion:
Chemical analyses done by wet conventional method shows that chemical gypsum contains high
concentration of P2O5 & F while marine gypsum possesses higher chloride content. Other gypsum
types have characteristics more or less similar to mineral gypsum.
XRD results presented in Fig.-1, indicated that all types of gypsum have prominent peaks of
dihydrate & Quartz. Chemical gypsum also has small peak of bassanite while marine gypsum has
bassanite as well as halite as expected. Calcite is present in mineral & imported gypsum.
From the results reported in Table-3, it is found that marine gypsum is showing higher early
strength at 1 & 3 days and lower strength at 7 & 28 days. Phosphogypsum exhibits lower early
strength and longer setting time due to higher amount of P2O5 & F but the strength improvement
from 3 to 28 days is significantly higher compared to other type of gypsum.
Mould gypsum accelerates the setting of cement and reduced the setting time. It is also increasing
early strength of cement. However no negative impact on later strength has been observed. It is
evident that Mould gypsum has some quantity of hemihydrate which has an important role in
decreasing the setting time of cement without negative effect on the cement compressive strength
(Ref-7). Solubility of calcium sulfate has an important role for cement setting and hemihydrates has
less solubility than mineral gypsum (Ref-11,12).
Synthetic gypsum reduced the initial and final setting time and increased 1 & 3 days strength. It is
evident that each CaSO4 has different solubility. The absolute solubility for both gypsum and
anhydrite is similar, while the dissolution rate of gypsum is faster than that of anhydrite. Thus in
anhydrite SO42- availability is less for reacting with C3A and due to this less formation of ettringite
occurs and reduction in setting observed (Ref-11,12). All other gypsums are found to exhibit more
or less similar trend at all ages.
From the result reported in Table-4, it is found that marine gypsum is displaying higher strength at
all ages whereas chemical gypsum & synthetic gypsum are showing slightly lower 1 & 3 days
compressive strength. But interestingly it is found that chemical gypsum used here was having P 2O5
0.7 % & F 0.18 % hence it has relatively lower impact at early strength development compared to
earlier trials where P2O5 was 1.53 & F 0.25 %. This finding is also reported by Mr. Manjit Singh
(Ref-5). The % increase of use of chemical gypsum depends upon its impurity content.
From Table-5, it is found that lab scale trial results with the blend of mineral & marine gypsum
have identical setting time & compressive strength. Hence marine gypsum as the replacement of
mineral gypsum can be used but due to high chloride content it cannot be used alone as it will
exceed the chloride limit in finished cement to be used in pre stressed concrete.
Experiment was designed with the use of multi blend gypsum like mineral, chemical, marine &
mould gypsum taking in to consideration the negative impact of chemical & marine gypsum both
on early & later ages respectively.
Mineral gypsum was replaced successively by chemical gypsum from 70 to 30 % with some
proportion of marine & mould gypsum as indicated in Table-6.
By replacing mineral gypsum with incremental dose of chemical gypsum, 1 & 3 days compressive
strength were reduced marginally without affecting 7 & 28 days compressive strength. At the same
time, no significant impacts on setting time were noticeable.
On the basis of successful lab scale trial, similar blend was maintained by mixing homogeneously
& used during grinding of OPC & PPC and the results were reported in Table -7 & 8 respectively.
Plant scale trial results are also found to be in line with the similar trend to that of lab scale trial.
From Table-8, it is also found that both the strength development is in the similar trend to that of
OPC.
4.0
Conclusion
1. Blends of mineral gypsum, chemical gypsum, marine gypsum & mould gypsum can be
developed to be used as the replacement of mineral gypsum.
2. There is a significant reduction in cost on the use of blended gypsum compared to use of
mineral gypsum.
3. The blending proportion is decided on the basis of climatic condition in different season &
taking the purity & impurity of gypsum.
4. Conservation of mineral gypsum by use of other type of gypsum towards the initiative of
cement sustainability.