CHAPTER 4: RESULTS AND DISCUSSIONS
4.2 MECHANICAL PROPERTIES AND PERFORMANCE OF MORTAR
4.2.1 Water Requirement for Constant Workability
Sample of rice husk ash (RHA 3) obtained from 60 hours retention time in
4
accumulation chamber was whitish in colour. The silica content in RHA is 9 1.43%.Whereas, the percentage of LOI of RHA was about 3.86. represents the loss of volatile organic compounds and adsorbed water on the ash. Several researchers reported that RHA also consists of trace elements like Fe2O3, CaO, MgO, Na2O, K20, and MnO (Chen and Yeoh, 1992 James and Subba. 1986). Some others elements except SiO2 was observed in this RHA and shown in Table 4.5.
According to ASTM C618-89 requirements for a Type N Pozzolan for using as a mineral admixture in Portland cement concrete describe that the minimum amount of sum of SiO2 Al2O3, and Fe2O3 is about 70%. From Table 4.5 it is observed that the sum of SiO2, A1203 and Fe2O3 is 93.47%. This value is more than the minimum amount which confirm the material can act as a pozzolanic one.
60
55
50 45 40 A3-25 A3-30 A3-0 A3-10 A3-I5 A3-20
Sample ID
ei)
115
0 Li. 110
105 100
130 -
O— Flow Value. RHA I
---- Flow Value, RHA2 125 I — w/cration,RHAl
I—zr--w/cratio.RHA2 I
120 115
>
.2 110 105 100
80 75 70 65 ' 60 55
Ce .)u G)
- cr I..
45
Ulm
AI,2-0 AI,2-I0 AI,2-15 Al,2-20 AI,2-25 AI,2-30 Sample ID
Figure 4.11: Water requirement for standard flow value of RHA I and RHA 2 Regarding the retention time and fineness have already presented in Table 4.3. The ashes of 36, 48 and 60 hours retention time with 90 minutes laboratory grinding were selected for finding out the water requirement to maintain the constant flow value. The water requirement for the standard flow value is shown in Figure 4.12 using the RHA 3 with retention time of 36, 48 and 60 hours.
—0— Flow value, 361i
X Flow value, 48h
0 Flow value, 60h
--- w/cratio,36h
---A-- w/c ratio. 48h w/c ratio. 60h
80
75 70 65.
130
125 E 120
Figure 4.12: Water requirement for standard flow value of RHA 3
From the Figure 4.11 and 4.12 it was observed that the RI-IA 1, RI-IA 2 and RI-IA 3
4
of 36 hours of retention exhibited cracks at the time of flow table test with the replacement level of 25 and 30% (More results are presented in Annexure-A: Table A-I, A-2 and A-3). The Blai ne fineness of ash RHA I and RHA 2 was very close to the cement fineness of 3264 cm2/gm. The losses on ignition of these two ashes were also higher. Nehdi, et al. (2003) stated that the RHA is finer than cement and should be expected to play not only a pozzolanic role but also a microfiller effect to enhance the particle packing density of concrete.In Figure 4.12, it is observed that water requirements of 48 and 60 hours ash are quite similar up to 20% replacement levels. The water requirement of 48 hours ash shows higher than the 60 hours ash at 25 and 30 % replacement levels. The ash of 36 hours of retention shows quite similar water requirement of RHA I and RHA 2. The flow value in all cases of retention times are considered within the standard range of 110±5 mm for RHA 3. From the above discussion it can be summarized that the ashes with retention time of 48 and 60 hours exhibit better results compared to other ashes.
The Blaine fineness is one of the important parameter for ash quality. It was observed that at 90 minutes grinding time the fineness of 60 hours ash was 25%
higher than that of 481i ash. Therefore, the ash with 60 hours retention time is more suitable as cement replacing material in context of Blaine fineness, Loss on Ignition and workability. The Rice Husk Ash of 60 hours retention time designated as A3' was used for finding out the rheology of cementitious materials.
Rheology of cementitious materials is mostly defined by means of its flow characteristics, plastic viscosity (pt) and yield stress required to make flow (I-lu and Larrard, 1996). It was considered enough to use an engineering approach for characterizing rheological properties of cementitious materials such as (flow time, slump and compacting density) to categorize the mixtures in terms of workability and to find out the amount of energy requisite to get a definite degree of compaction (Daczko, 2000; I-lu and Larrard, 1996). Therefore, the rheology of OPC-RHA matrix
in various levels of applied external stresses was analyzed in this study by means of flow table apparatus. This was done by measuring the instant flowability of OPC-
RFIAmixtures at definite uninterrupted number of blows. The number of blows were generated by the flow table was considered as an indicative parameter for the applied external stress on the samples to make it flow. The outcomes of this investigation are shown in Figure 4.13 in which the relationship between the number of blows and the corresponding immediate flowability of OPC along with rice hush ash, A3 having a range of 0 to 30% is plotted.
114
._
112
Er-
110
A3-0: R2
=0.9119 A3-10:
12 =0.9613 A3-20: R2
=0.989 A3-30: R2
=0.9729
101
-
108
-
106
c-I-
104 102
Er
1001 98
Lel