AC'I'IVA'~ION OF PIIGH-CALCIUM FLY ASH : PRELIMITVARY INVESTIGA'I'IONS 4.1 INTRODUCTION This chapter reports and discusses the experimental study in which lime and gypsum have been used as 'activators' of high-calcium fly ash, in mortar and concrete. Compressive strength attained at normal and at later-ages has been chosen as the principal parameter for comparing the effect of the above activators on the various fly ash-based mortars and collcretes. The effect of three different 'curing regimes' on the strength characteristics have also been studied and reported. 4.2 PLAIN FLY ASH 4.2.1 Plain Fly ash Paste Consistency and setting times were determined for plain fly ash paste. It was observed that the consistency was 23.5% and the initial and final setting times were 9 and 17 minutes, respectively. Plain fly ash paste specimens were cast with water content equal to its consistency. The above quantity of water was added to fly ash and the mixture was hand mixed and kneaded well to obtain a homogenous mix and the paste was cast into 70.7mni cubes in three layers with 25 tampings for each layer, as specified in IS: 1727-1967. The specimens were then kept under moist jute cloth untiI the time of testing. Compressive strength of the above paste specimens evaluated at 3,7,14 i r E 1 1.2, 12.0, and 12.0 MPa, respectively 4 4.2.2 Plain Fly ash Mortar As the fly ash is of self-hardening type, the first study was made with plain fly ash mortar without addition of extra lime. As it was also proposed to compare the strength of these mixes with that of fly ash mortars activated with lime and gypsum, it was decided to adopt the same test procedure as that of lime reactivity test for fly ash as per IS 1727-1 967, except curing, which was done at the laboratory temperature. Binder to standard sand ratio was maintained as 1 :3 (by wt.) and the required quantity
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AC'I 'IVA'~ION OF PIIGH-CALCIUM FLY ASH : PRELIMITVARY INVESTIGA'I 'IONS
4.1 INTRODUCTION
This chapter reports and discusses the experimental study in which lime and gypsum
have been used as 'activators' of high-calcium fly ash, in mortar and concrete.
Compressive strength attained at normal and at later-ages has been chosen as the
principal parameter for comparing the effect of the above activators on the various fly
ash-based mortars and collcretes. The effect of three different 'curing regimes' on the
strength characteristics have also been studied and reported.
4.2 PLAIN FLY ASH
4.2.1 Plain Fly ash Paste
Consistency and setting times were determined for plain fly ash paste. It was
observed that the consistency was 23.5% and the initial and final setting times were 9
and 17 minutes, respectively. Plain fly ash paste specimens were cast with water
content equal to its consistency. The above quantity of water was added to fly ash and
the mixture was hand mixed and kneaded well to obtain a homogenous mix and the
paste was cast into 70.7mni cubes in three layers with 25 tampings for each layer, as
specified in IS: 1727-1967. The specimens were then kept under moist jute cloth untiI
the time of testing. Compressive strength of the above paste specimens evaluated at
3,7,14 i r E 1 1.2, 12.0, and 12.0 MPa, respectively 4
4.2.2 Plain Fly ash Mortar
As the fly ash is of self-hardening type, the first study was made with plain fly ash
mortar without addition of extra lime. As it was also proposed to compare the strength
of these mixes with that of fly ash mortars activated with lime and gypsum, it was
decided to adopt the same test procedure as that of lime reactivity test for fly ash as
per IS 1727-1 967, except curing, which was done at the laboratory temperature.
Binder to standard sand ratio was maintained as 1 :3 (by wt.) and the required quantity
)f water was dctcrrnincd based on the 'flow test'. 'She specimens after dc~noulding
vere transf'errcd lo a humid~ty cabmet. After 7 days of 'hulnld curing' at 27* C and
,O% RI-I. the compressive strcngth of thc above specimens. was determined by the
,tandard procedure and presented in Table 4.1. Crushed satnples from the specimens
vere kept soaked in acetone in order to arrest hrther hydration and preserved for
{RD and SEM analysis.
<RD d'ffiactogram of the hydrated fly ash mortar sample is shown in the previous t* Zhaptercn Pig. 3.P It is observed that, C ~ A C H I I , c~A'SIHX, MgO, CSH, c ~ A % .
-
:ttringite, and C5S2S, could be identified. Hydrated of fly ash particles are seen in
3EM micrographs, shown in Figs. 4.1 and 4.2.
1.3 FLY ASH - LIME
1.3.1 Fly ash-Lime Paste
2onsistency and setting tests were carried out on the fly ash-lime blends with the lime
;ontent varied between 10% to 30% (by wt.). It was observed that the corresponding
;onsistency values of the F-L blends, varied between 37.5% to 76.67% and the initial
md final setting times, 6 to 9 minutes and 9 to 15 minutes, respectively. Fly ash-lime
~ a s t e specimens were cast for various lime contents and their co~npressive strength ma
determined as detailed in Section 4.2. It was found thatkmaximum strength was
3btained at a lime content of 18.5% (by wt. of total binder). Compressive strength of
F-L pastes at 3,7,14 and 28 days arei8.4, 9.6 11.5 and 11.6 MPa, respectively.
Estimafion of Tlieoretical Lime Required
The actual quantity of lime required for the complete hydration of fly ash is usually
difficult to estimate. which is due to the fact that complex compounds are formed
during hydration. However, an approximate estimate of the lime required to hydrate
completely a unit quantity of a given fly ash is possible. The procedure outlined by
Anne Roja (1996), which is very simple and provides reasonably a good estimate of
lime required, has been followed. The above procedure is described below:
The main reactionary compounds due to the hydration of fly ash-lime blend are:
&ydrates of calcium silicate, calcium aluminate, and calcium ferrite. The reactions can
6e represented as follows:
3C'aO I 2SiO2 -t 3 1-120 3Ca0.2Si02.3 1-120 (C-S-[I) . . . 4.1 ( a )
I ' ! 0 1 - 0 I A 9 1 I ( - 1 j . . . 4. i t b j
3CaO t- Fe20.7 + 6 H z 0 3Ca0 . Fe203.6 H20 (C-F-H) ... 4.1 (c)
The molecular weights of the diffcrcnt compounds are :
From equation 4. ](a), one can obtain that one gram of SiOz requires 1.4 grams of
CaO to forni C-S-H. Similarly from equation 4.1 (b), it can be seen that, one gram of
A1203 requires 2.2 grams of CaO to form C-A-H. Equation 4.1 (c) gives that one gram
of Fez03 requires 1.05 grams of CaO. Thus, one gram of fly ash requires
(1.4+2.2+ 1.05) = 4.65 granis of CaO, for its complete hydration.
As the fly ash sample used in this study, has the following chemical composition,
namely, SiOz = 34.18%; Al203= 24.09% and Fe203= 10.78%, the lime required per
gram of fly ash has been computed as, equal to 1.18 gm igm of fly ash. The above
quantity is valid only, when the components participating in the reactions are 100%
'reactive'. Since, most of the Indian fly ashes have only 27-34 % of 'reactive silica? as
reported by Sharma (1990), the lime required for various reactivity levels of fly ash
(5% to 40%), was computed proportionately and presented in Table 4.2.
Corresponding to the reactivity of the fly ash used in this study, (i.e. 16 %), the
theoretical quantity of lime required for the hydration of fly ash-lime blend is 0.1 89
gm 1 gm of fly ash. The CaO content already present in the fly ash sample is 16.9 gm /
gm. llcnce, thc additional quantity of linlc rcquircd will only bc vcry small. In
addition to the above 'major' compounds participating in the hydration of fly ash-
lime, other 'minor' compounds present in fly ash, may also participate in the reactions
with lime. Hence, the actual lime required is likely to be somewhat higher than the
quantity theoretically s
~eterrniilatioll of Actual Qzrarrtity of Lime Rey ztired
In order to determine the actual quantity of lime to be added to the fly ash sample,
compressive strength of fly ash-lime mortars was determined, using the procedure
detailed in Section 4.2. Taking into consideration the theoretical quantity of lime
required for the fly ash and the small additional quantity of lime required for the
hydration of minor compounds in fly ash, it was decided to add 4 - 16 % lime (reagent
grade) to the mix (by weight). The compressive strength of the such fly ash-lime
mortars are presented in Table 4.3. Lumps of the crushed specimens were powdered,
soaked in acetone and preserved for conducting XRD and SEM analysis.
Compressive strength results obtained indicate that there is only a marginal increase in
strength with increase in lime and that the maximum strength obtained is about 15%
higher than the reference mortar strength at an additional lime of 16%. Corresponding
XRD ( Fig. 4.3) also does not indicate much variation with respect to that obtained
without additional lime (Fig. 3.3). Only notable factor is the pronounced formation of
C4AH3 and CSH. SEM micrographs (Figs. 4.4 and 4.5) indicate the formation of
relatively denser hydrated products in the system with probably a few calcium
hydroxide cubical crystals. As the increase in the compressive strength is not
significant, considering the quantity of lime added, it is inferred that the fly ash
sample has the potential to impart good strength, without addition of (external) lime.
4.4 FLY ASH - GYPSUM (F-G)
4.4.1 Fly ash -Gypsum Paste
Gypsum content was varied between 10/o to 13% in fly ash - gypsum (F-G) blends.
Consistency of the F-G blends corresponding to the above gypsum content, showed
water content variation between 22.5% to 29%; initial and final setting times of the
above F-G blends showed a variation of 10 to 12 minutes, and 18 to 30 minutes,
respectively. F-G paste specimens cast with various gypsum contents and tested as
detailed in section 4.2, attained the maximum strength at a gypsum content of 6.25%
,(by wt. of total binder). Compressive strength of the above blends observed at 3,7,14
and 28 days are: 14.0, 19.2,24.0 and 24.0 MPa, respectively.
4.4.2 Fly ash - Gypsum Mortar
Studies ca t~ lcd out cullier tiad indicated that there was no dimensional instability of
the matrix, ~ ~ p t o a gypsLlm content of about 15% in F-L-G system [Karasimha and
~thers (1998, 1998, 2001 ); Jagannathan and others (1996)] . As, the focus at this stage
of the study was on F-G systems, it was decided to vary the gypsum content between
0 - 28 Oh, in order to understand better, the role of gypsum, in the system. Mortar cube
specimens (70.07 11irn) wcrc prepared and their compression strength determined, as
outlined in Scction 4.2. Thc results obtained are presented in Table 4.4 and in Fig. 4.6.
Lumps of the crushed specimens were, powdered and soaked in acetone and preserved
for carrying out XRD and SEM analysis. Compressive strength results (as given in
Table 4.4) indicate that the addition of gypsum to fly ash increases the strength upto
8% of it and that the maximum strength attained is 60 % higher than the strength of
reference fly ash mortar. But, when the gypsum content is increased beyond 8%,
strength reduction with respect to the maxiniuni value is observed. This indicated that
all the gypsum added could not be utilized for the formation of hydrated (i.e, strength-
giving) products, when its content is very high. Moreover, some of the gypsum added
may act as 'soft intrusions' in the rigid skeleton of the hydrated products.
From the compressive strength results of the fly ash-gypsum mortars, it can be
inferred that addition of gypsum to the high-calcium fly ash is advantageous, in
preference to the addition of lime, as substantial increases in compressive strength
can be obtained, provided, gypsum is not in excess quantities. Addition of gypsum
beyond 8% to the high-calcium fly ash, is not expected to contribute to maximizing
the strength.
XRD diffractogram, for 8% of gypsum addition (Fig. 4.7), shows the formation of
I I I I - (*) - k t e r content adjusted to obtain a constant flow as specif.ied in IS: 1727. (**) - t fe rence mortar strength of fly ash using standard sand.
Cotnpressive Strength c4 10 days (M Pa) 4.10 11.3 11.9
Compresc;ive Strength @ 10 days (M Pa) 4.70 7.50 8.50
9.00 9.90 8.30
8.00
I ime
26.60 20.00 28.50
7.50 13.30 26.60
28.00 53.40 11.90
Gypsum
6.60 13.30 14.50
19.00 20.00 20.00
32.00 13.30 33.30
'J'ablc .t.O(a): Propertics of fine atid coarse aggregates
Dcscr~ptior~ Obscrved Val~lc F ~ n e aggregate --
- - --
2 3 4
Table 4.9(b): Sieve analysis of aggregates
Coarse aggregate
L- i
5 6
2.37 --- Specific gravity
Bulk density Water absorption
Table 4.10: Combination of binders and their ranges
Rodded density Grading
S ~ e v e size, mrn
2 0 10 4.75 2.36 1.18 0.6 0.3 0.15
2.64 1.59glcc
1 (YO
1 Total numb; of mixes studied 28 1 Note: (i) B1 - only fly ash; B2-fly ash-lime; B3-fly ah-gypsum; B4-fly ash-lime-gypsum
1.4 1 glcc 0.5% I
1.73gJcc Zone I1
Cumulative percentage passing
/Type Range of values (%) No. of mixes
(ii) All the above mlxes were cast and tested for their compressive strength. Rut only those mixes which gave the highest strength values for a given type of binder have been chosen for reporting.
1.55gIcc --
Coarse aggregate 100 13 2 0 0 0 0 0
Fine aggregate 100 100 100 97.8 87.8 46.7 4.9 1.3
studied F L G
'I'ahlc 4. I I : ('ompressive strength of 1:-I .-G concretes ( 111 M Pa)
- - - - -- - - . Age in days
bincicr .- -
Table 4.12: Consistency and setting time of F - G blends
- LB3 e4 {'. 8.8 -- --
Table 4.13: Compressive strength of F - G mortar cubes [ W/(F + G) = 0.38; Sand / (F + G ) = 0.67; Curing = "MOIST"]
Note: ( I ) 'l'he values given above are the maxlmuni In each of the blend ( 1 1 ) 'The correspond~ng llme or 1 and gypsum are also ~ndicated.
12.4
10.5
r-
24.8
11.2
Blend Designation
R 0
S1. No. 1 2
13 4 5 6 7
25.7
13.7
Fly ash (%)
1 00
Note (i) C* - Tested immediately after demould (ii) ~ * j dtrengths indicated above are based on the Average of five cubes (iii) BO refers to the use of only fly ash and it is taken as the reference mortar.
Blend
B0 B1 B2 B3 B4 B5 BG -.
Age at testing (in days) 0* 3.6 4.2 4.8 5.2 5.7 3 3.4
26.3
12.5
Gypsum (%)
0
12% gypsum
2'%L and 8% G
Consistency (%)
44
1
4.1 6.4 6.0 6.5 6.3 7.0 7.5
Setting Time (min) -
Initial / Final 4 5 / 210
7
5.4 7.7 8.0 12.3 9.4 7.7 8.0
3 4.4 6.8 7.2 8.0 7.3 7.3 7.8
28 6.5 7.9 10.8 16.3 9.8 10.9 8.7
56 9 10.9 11.9 23.7
9 0 9.4 14.8 15.3 23.2
120 9.6 '
13.9 17.5 25.6
I able 4.14: ~ ' o 1 ~ 1 i x c s s s l v ~ strength I; - (; mot-tar. cubes [ A 8 ! + : ~ j ? , y , can4 , :,; 1;()7,( ~ , , , , , g - ' ' !~;~~!i, i<,$I~~;' 1
\,, -7 ' ,-u.-.. ....... I......"., U I I V . U"I.."UIU
( 1 1 ) @*I -strengths mdicated above are based on the Average of five cubes (111) BO refers to the use of only fly ash and ~t IS taken as the reference mortar
Table 4.1 5 Average compressive strength F - G mortar cubes [w/(I'+ G) ~ 0 . 3 8 ; Sand 1 (F; G) = O.67,Curing = "BOll.lNc; WA.I ER**"]
I SI. No. / Blend / Compressive strength I
Note: (1) i*I - Strength Indicated above are based on average offifteen cubes I I I
(11k4 - Spec~rnens were demould after 24 hours of moist curing and Immersed In boiling water for 3% hours. They are then alr cured for one hour and then thelr compressive strength evaluated
Table 4.1 6: Comparison of the compressive strength of F -- G mortars (under various curing conditions)
S1. No. 1 2 3 4 5 6 7 -
116
Blend
BO B1 B2 B3 B4 B5 B6
28 days Compressive strength ( M Pa) Accelerated Curing 7.6 9.7 13.9 14.8 9.7 9.4 9.1