EFFECT OF RICE HUSK ASH PARTICLE SIZE IN LIME BASED
MORTARS
João Carlos Duarte Tiago
October, 2011
EFFECT OF RICE HUSK ASH PARTICLE SIZE IN LIME BASED MORTARS
João Carlos Duarte Tiago
Instituto Superior Técnico
E-mail address: [email protected]
Abstract This paper presents some results achieved by a research program with the main
objective of evaluating the effect of rice husk ash particle size in lime based mortars. A
Portuguese commercial RHA used was sieved and grounded in laboratory to get different
particle size distributions. The influence of the RHA particle size was evaluated on fresh and
hardened properties of lime based mortars. The results show: the reactivity of the rice husk ash,
the increasing of the pozzolanic reactivity with the reduction of the RHA particle size and the
possibility of improving lime mortars properties.
Key-Words: lime mortar, rice husk ash, pozzolanic reactivity, particle size.
1 INTRODUCTION
Lime based mortars are presented as a compatible solution for the rehabilitation of ancient
masonry. However, associated with them there are several characteristics that affect their
application in this area. The main complexity concerns the difficulties presented by these local
hardening in poor contact with the carbon dioxide in the atmosphere or in high humidity
environments.
In this context, the lime mortar with added pozzolanic components seems to be an interesting
alternative. The possibility of hardening of these mortars being to occur also by hydration
reaction. Therefore, by controlling the amount of pozzolan and depending on the purpose,
mortars can be made with different properties, bearing in mind the need for compatibility in
mechanical, physical and chemical.
Considering the need to use the previously mentioned products and the adoption of solutions
that may involve a reduction of energy consumption, several studies have been made on
formulations of lime mortars with incorporation of industrial by-products with pozzolanic
characteristics. The rice husk ash is presented as a material with enough potential in this
context. It then becomes essential to understand the factors affecting the pozzolanic reactivity
of the ashes, so as to maximize its potential use in the formulation of mortars.
1
Thus, this paper aims to study the pozzolanic action conferred by the addition of rice husk ash
in lime mortars and evaluate the influence of its particle size in the performance of mortars.
2 EXPERIMENTAL SECTION
The aim of the present work is the study of the pozzolanic reactivity of rice husk ash in lime
mortars and the evaluation of rice husk ash particle size effect in lime based mortars. For this
purpose, four mortars were analyzed with three fixed parameters, namely the rice husk ash type
(commercial brand), the ratio lime/ash (1:2) and the consistency (165 ± 5 mm). Firstly, the
commercial rice husk ash was previously prepared in order to obtain samples with different
particle sizes. As a reference, a pure lime mortar sample was also formulated. The latter was
subjected to dry cure whereas the lime mortars with rice husk ash were subjected to saturated
environments. Mortars characterization tests included: determination of consistence of fresh
mortar, bulk density, exudation, water retention, flexural resistance, compressive resistance,
ultrasonic propagation velocity, superficial hardness, capillarity water absorption, open porosity,
karsten tube penetration test, dry test and carbonation depth.
2.1
Materials
For the production of mortars were used river sand, aerial hydrated lime in powder (CL90) and a
Portuguese commercial rice husk ash. The rice husk ash was grounded and sieved in a
laboratory in order to study the influence of its particle size.
2.2
Rice husk ash
When burned under controlled conditions (temperature, time and air flow) the rice husk ash, can
be converted into an ash with a high percentage of amorphous silica (Metha, 1983). The
pozzolanic reactivity is affected by several factors such as the type of pozzolan or the surface
area. The latter is a relevant point to take into account in the pozzolanic reaction, since it
happens between the amorphous silica of ash and the lime (calcium hydroxide). In fact, the
reactivity will be greater with a larger surface area which, in general, is higher for smaller
particle sizes. Some authors studied several mortars with addition of ashes with different
particle sizes and they observed that smaller particle sizes leaded to higher values of
mechanical strength (Almeida, 2008; Agarwal, 2004). Nevertheless, other authors (Payá,
2000)concluded that ashes with smaller particle sizes did not maximize the pozzolanic reactivity
(Payá, 2000). In fact, according to other authors, the surface area of rice husk depends not only
on particle size distribution but also on its roughness (Metha, 1983).
2
The pozzolanic reactivity can be assessed by two methods: mechanical and chemical. The first
one considers that the addition of a pozzolan increases the mechanical strength of lime based
mortars with pozzolanic materials. The second one assesses the evolution of consumption’s
rate of Ca(OH)2 reacted with pozzolan or measures the conductivity of the saturated Ca(OH) 2
solution added with pozzolan. Latter studies referred two methods to the analysis of the
pozzolanic reactivity, however, it should be noticed that the referred methods, when used to
evaluate the pozzolanic reactivity of the same material, might lead to different classifications
(Velosa, 2006).
The rice husk ash studied throughout this work were provided by the Portuguese company
CINCÁS. By the visual observation of ash, it was possible to identify a difference in tone in the
recorded particles and also the presence of particles with different sizes. This occurs because
the process used by the manufacturer to burn rice husk does not allow monitoring the thermal
gradient and the air flow across the material. According to several authors, there is a correlation
between the color of ashes and its chemical composition. The light shade of gray reveals
evidence of a high percentage of silica concentration.
In order to reduce the use of the darker ashes and consequently minimize the use of ash with
high carbon content, the commercial rice hush ash (C) was sieved through the sieve opening
500 m for a 10 minutes period. At the end of this process it was obtained approximately 35%
of ash (CP) retained on the sieve opening of 500 m.
The sieved ash, CP, was then grounded in a Los Angeles mill to increase its fineness and
consequently the specific surface. The process was made in two distinct phases: one from the
minute zero to 45 minutes the milling was carried out using 6 balls of steel (≈400g/ball), and
another from 45 to 75 minutes using 10 balls. Comparative analysis of the ash before the milling
process (CP) and the obtained at the end (CPm75) shows that this process has proved to be
effective in reducing the size of the ash CP. In order to analyze the influence of fineness, by the
mechanical characterization of mortars, the rice husk ash sieved and grounded (CM) was used
with four different grading curves. Each curve was obtained by sieving the ash through the
sieves meshes 500 m, 250 m, 125 m and 75 m leading to the synthesis of CM500, CM250,
CM125 e CM75. Figure 1 and 2 shows the grading curves of commercial rice husk ash (C), ash
sieved (CP). Figure 1 and 2 shows the grading curves of commercial rice husk ash (C), ash
sieved (CP) and grounded and sieved through the sieves mentioned above.
3
90
90
30
20
1,000
0
0,500
CM75
20
10
0,250
CM125
30
0
0,125
CM250
40
10
Sieze size[mm]
CM500
50
1,000
40
CPm75
60
0,500
CP
50
70
0,250
C
60
80
0,125
70
0,063
0,075
80
Percentage passing [%]
100
0,063
0,075
Percentage passing [%]
100
Sieve size[mm]
Figure 1 – Grading curves of C and CP.
Figure 2 – Grading curves of CPm75,
CM500, CM250, CM125 and CM75.
2.3
Tests
To achieve the purpose of this research and taking into account the proposed on ASTM C59306, the mortars with ash addition were formulated with 180g of hydrated lime, 360g of rice husk
ash and 1480g of river sand (Ratio by weight 1:2:8 – lime:pozzolan:aggregate). The pure lime
mortar was formulated with a lime/aggregate ratio by weight of 1:8. The amount of water used in
the formulation of mortars was established in order to ensure a consistency of 165 ± 5 mm,
measured according to EN 1015:3.
For each mortar, six prismatic specimens (40x40x160mm) were prepared in accordance with
NP EN 196:1. Since the diffusion of carbon dioxide is slow in water, the carbonation reactions
are reduced in saturated environments. Hence, the specimens of mortars made with ash were
stored until their characterization under controlled conditions of 23±3ºC and 95±5 % RH. In this
way their hardening was mainly due to pozzolanic reactions. The specimens of lime mortar were
stored at 23±3ºC and 50±5 % RH. All the samples were demoulded 7 days after their
preparation.
All tests performed were based on procedures described in international specifications or in
several research works.
4
MECHANICAL AND PHYSICAL CHARACTERIZATION OF
3
MORTARS
Table 1 shows the tested mortars, the type of ash used in each one, the ratio water/ (blinder
mixture used in all mortars studied), values of consistence of fresh mortar, water retention, bulk
density and blending.
Table 1 – Fresh state characterization.
Mortar
Type of
ash
water/
(lime+ash)
Consistence
[mm]
Water
retention
[%]
Bulk
density
3
[kg/m ]
Blending
[%]
CAL
A500
A250
A125
A75
CM500
CM250
CM125
CM75
1,83
1,15
1,15
1,14
1,09
162
162
163
162
167
90,0
90,0
87,7
88,3
89,1
2080
1870
1890
1890
1930
2,6
1,5
1,3
2,1
2,1
When considering only the water / (mixed blinders) values and the values of the consistence of
lime based mortar with rice husk ash, it appears that, the reduction of the maximum particle size
of the ashes used is responsible for a lower amount of water required to obtain mortars with
2
1,16
1,8
1,15
water/(lime+ash)
1,6
1,14
1,4
1,13
1,2
1,12
1
0,8
1,11
0,6
1,10
0,4
1,09
0,2
water/(lime+ash)
similar consistency (Figure 3 and 4).
1,08
500 400 300
200 100
0
Maximum particle size of ash [μm]
0
Cal A500 A250 A125 A75
Maximum particle size of ash [μm]
Figure 3 – Water/(lime+ash) ratio for
Figure 4 – Relation between
obtaining a consistence of 165±5mm.
water/(lime+ash) ratio and maximum size of
ash.
5
The values of water retention for reference mortar are higher when compared with the values of
lime based mortar with ashes, Figure 5.
With the exception of A500, the presence of ash is responsible for a slight decrease in the water
retention capacity of mortars when compared with the reference mortar. However, for mortar
A250, A125 and A75, it shows that the value of the retention capacity of water has a slight
tendency to increase with the increasing of the ash particles fineness.
The obtained water retention values for the mortars are satisfactory, allowing a good behavior
of this type of mortar against the adverse weather conditions when applied on porous walls.
100
2100
Water retention [%]
Bulk density [kg/m3]
2050
90
80
70
2000
1950
1900
1850
1800
1750
1700
60
Cal
A500
A250
A125
Cal
A75
A500
A250
A125
A75
Mortar type
Mortar type
Figure 5 – Water retention.
Figure 6 – Bulk density.
From the Figure 6, it can also be observed that all mortars made with rice husk ash have lower
values of density, when compared to pure lime mortar. In relation to the lime mortar with the rice
husk ash, it is clear that the increasing the fineness of the ash is accompanied by an increase in
the bulk density of the different mortars. Whether this trend or the difference found between the
reference mortar and the other mortars can be justified by the occupation of empty spaces by
small particles of ash. It appears that the incorporation of fly ash in mortar causes a variation of
exudation, especially for the values corresponding for mortar A500 and A250, where there is a
reduction of exudation with the progressive decrease of the particle’s size.
6
2,25
2,00
Blending [%]
1,75
1,50
1,25
1,00
0,75
0,50
0,25
0,00
Cal
A500
A250
A125
A75
Mortar type
Figure 7 – Exudation.
Table 2 – Mechanical characterization of mortars.
Mortar
Compressive
strength [MPa]
CAL
A500
A250
A125
A75
14 days
0,3
2,7
3,4
3,6
3,5
Flexural strength
[MPa]
14 days
0,2
1
1,4
1,4
1,5
Compressive
strength [MPa]
28 days
0,5
3,2
3,5
3,8
5,5
Flexural strength
[MPa]
28 days
0,2
1,4
1,5
1,6
2,3
Considering the mechanical strength values of mortars made with ashes against the lime mortar
values, it can be concluded that the ash showed a considerable reactivity. In fact, this could be
also noticed because the mortars made with ash have been subjected to a moist curing. In this
way the hardening process and the values of mechanical strengths of these mortars are a
consequence of the formation of hydrated compounds, like calcium silicates, resulting from the
pozzolanic reactions.
Comparing the values of all the mechanical strength of mortars with 14 and 28 days old, it
appears that the values obtained at an age of 28 days are higher, which leads to the conclusion
that the ash of rice husk reveals a significant reactivity, especially A75 where the increase in the
mechanical strength is higher. The mechanical characterization points out the influence of the
7
fineness of the ash used, since there was an increase of mechanical strength with the maximum
size reduction of the ash used.
Flexural tensile strength [MPa]
Compressivel tensile strength
[MPa]
6
14
days
14 dias
28
days
28 dias
5
4
3
2
1
0
3
14
14 days
dias
28 days
dias
28
2
1
0
Cal
A500
A250
A125
A75
Cal
A500
Mortar type
Figure 8 – Specimen compressive tensile
th
A250
A125
A75
Mortar type
Figure 9 – Specimen flexural tensile
th
th
strength on the 14 and 28 days
th
strength on the 14 and 28 days
It can be observed that there is a growing trend in the propagation’s velocity of ultrasound for
the mortars with the addition of rice husk ash. This indicates that this test was sensitive to the
increase in pozzolanic reactivity manifested by a progressive increase of the fineness of the
ash.
Capillarity ater absorption
[kg/m2.s0,5]
Prismatic specimens ultrasonic
propagation velocity, 28 days
[m/s]
2.700
2.400
2.100
1.800
1.500
1.200
Cal
A500 A250 A125 A75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
Cal
A500
A250
A125
A75
0
Mortar type
200
400
600
800
1000
Duration [s1/2]
Figure 10 – Prismatic specimens ultrasonic
Figure 11 – Capillarity water absorption.
propagation velocity.
8
By analyzing the different absorption curves, is identified in each of the mortars the existence of
three sections with different rates of absorption, Figure 11. The first section is characterized by
an absorption rate a lot higher than the others, and from this it can be determined the capillarity
coefficient, mentioned above. (Rato, 2006)says that the initial velocity of this section depends
mainly on the pore size, being higher in mrtars with larger pores. The second section represents
the transition between the absorption and initial stabilization phase. Finally, the third section
corresponds to the stabilization phase and it is associated with a significantly reduced rate of
absorption, since the samples are already close to its saturation. It is based on this last phase
that determines the asymptotic value, which depends mainly on the open porosity of mortars.
Except for the reference mortar, CAL, the remaining mortars, particularly the A75, present
coefficient of water absorption values by capillarity above the recommended bibliography at the
mortars for plastering. (Veiga, 2003) states that these values should be between 0.13 and 0.20
2
kg/m .s
0, 5
. It should be noticed that the low value obtained in the mortar CAL can be associated
to the fact that it has only 28 days old, having a part of its hair structure filled with water.
2.600
Real and apparent mass [kg/m3]
40
Porosity [%]
30
20
10
0
Cal
A500
A250
A125
A75
Aparent
density
Massa Volúmica
Aparente
Massadensity
Volúmica Real
Real
2.400
2.200
2.000
1.800
1.600
1.400
1.200
1.000
Cal
A500 A250 A125
A75
Mortar type
Mortar type
Figure 12 – Open porosity.
Figure 13 – Real and apparent density.
9
It can be verified that the reference mortar has a lower value of open porosity than all the mortar
with the addition of rice husk ash, regardless of the particle size. Assessing only the lime based
mortar with the addition of rice husk ash, The Figure 12 makes explicit the trend of porosity with
the increasing fineness of the ash particles. A75 shows to be an exception.
It can also be observed that all mortars made with rice husk ash have higher values of porosity
and consequently lower values of density, than when compared with pure lime mortar. This
does not result from the addition of ash, but by the kind of environment in which they were
submitted during their setting. (Almeida, 2008) after submitting a mortar made with rice husk
ash to dry and saturated environments during their setting, he concludes that the latter
environment was responsible for higher values of porosity. The same trend was found by other
author (Faria-Rodrigues, 2004) in mortars made with different pozzolanic materials. The author
explained this trend by the fact that in the saturated environment the mortar looses water slowly
and when part of it is eliminated, the mortar is sufficiently hardened. Consequently the pores
may have a larger size and lead to higher values of open porosity.
26,0
24,0
22,0
20,0
18,0
16,0
14,0
12,0
10,0
8,0
6,0
4,0
2,0
0,0
0,7
0,6
0,5
0,4
Cal
0,3
A500
A250
0,2
A125
0,1
A75
0,0
0,0
Cal
A500
A250
A125
W[%]
Water absortion [Kg/m2]
0,8
A75
0
5,0 10,0 15,0 20,0 25,0 30,0 35,0
Time [s1/2]
5
10
15
20
25
Time [days]
Figure 14 – Karsten tube penetration test.
Figure 15 – Cinetic of dry process.
Associated to the higher values of porosity, higher values of coefficient of water absorption are
also observed.
It can be observed that the reference mortar has the higher initial capacity of absorption,
represented by the steep slope of its absorption curve. From the analysis of mortars with
addition of ashes, the A500 is the mortar that has the highest value of the initial capacity of
absorption.
10
The behavior of mortars with incorporation of ashes in Karsten tube penetration test is
consistent with results of open porosity test, since the water was easily penetrated in high
porosity mortars.
It is possible to conclude that the reference mortar shows a better performance in the drying
test.
4 CONCLUSION
This paper showed that the workability of mortars is reduced with the incorporation of lower
husk ash particle size. However, 10 minutes later after the mixing, the mortars showed greater
consistency. It could be considered that for a period of higher mixing, consistence sought would
be achieved with a smaller amount of water.
The used ashes, regardless of their finenesses, showed pozzolanic reactivity since the strength
values of mortars made with ash was significantly increase compared to the values of lime
mortar – the mechanical and flexural strength values of mortar A500 were ten times higher than
lime mortar values. Since the mortars with rice husk ash had higher porosity values compared
to the value of lime mortar, it can be concluded that the recorded increase is mostly due to the
formation of hydrated compounds, like calcium silicates, and not because they have a more
compact porous structure.
When only the mortars with rice husk ash were analyzed, it was found that the reduction of the
rice husk ash particle size was responsible for the mechanical strength values increase. The
latter values could be an indicator of a greater pozzolanic reactivity associated to an higher
specific surface.
The addition of rice husk ash in lime based mortar resulted in an increase of open porosity in
relation to the reference mortar. The fineness increase of the ash caused a downward trend in
the porosity of the mortars with incorporation of ashes.
Taking into account some of the recommendations for old buildings (Rosário Veiga, et al.,
2001), the addition of rice husk ash based on lime mortars should be performed in smaller
proportions as the formulation used (1:2:8 by weight – lime: rice husk ash: sand) led to mortars
with high values of mechanical strength.
This study allowed us to measure the great potential of using rice husk ash in lime mortars for
old masonry use, contributing thus to the development of sustainable practices in the field of
rehabilitation, through the incorporation of industrial by-products.
11
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