NOTICE: this is the author's version of a work that was accepted for publication in Construction and Building Materials. Changes
resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control
mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for
publication. A definitive version was subsequently published in Construction and Building Materials, Vol. 29, no. 4 April 2012, pp.
51-55: DOI: 10.1016/j.conbuildmat.2011.09.012
Properties of Cement Mortar Incorporating De-inking Waste Water
from Waste Paper Recycling
Shiqin Yan*, Kwesi Sagoe–Crentsil and Gretta Shapiro
CSIRO Materials Science and Engineering, PO Box 56, Highett, Victoria 3190,
Australia
ABSTRACT
This paper presents results of an investigation into the use of recycling paper mill
waste water to replace mixing water in cement mortar systems for manufacturing
concrete masonry products. The physical and mechanical properties of mortar
containing various amounts of de-inking waste water were studied. It was shown that
replacing mortar mixing water with waste water significantly improved the
workability and consistency of cement mortar mixtures. The flow of mortar mixtures
increased with the increase in waste water content up to 50 wt% and then levels off
when waste water content exceeds 50 wt%. The flow of mortar mix with 100% waste
water replacement at water/cement ratio of 0.50 was found to be equivalent to that of
the reference mortar mix with 100% potable water at water/cement ratio of 0.60,
indicating potential use of de-inking waste water as a water reducing admixture.
Compared to the reference mortar at water/cement ratio of 0.60, replacing 10 wt% and
100 wt% potable water respectively resulted in 19% and 60% reduction of
compressive strength of mortar samples. This also corresponded to 5% and 16% drop
in bulk density. Similarly, 10 wt% and 100 wt% waste water replacement induced
~3% and ~19% increase in drying shrinkage, respectively. Water absorption and
volume of permeable voids increased with increasing waste water dosage, albeit
marginally, suggesting overall potential for de-inking waste water in production of
light weight cementitious building and masonry elements.
Key words: de-inking waste water, mortar, compressive strength, drying shrinkage,
environmental management
*
Corresponding author. Tel: +61 3 9252 6294; fax: +61 3 9252 6244. E-mail address:
[email protected]
1. Introduction
De-inking waste water is the raw processing water prior to dewatering treatment in
waste paper recycling plants. It contains about 1 ~ 2 wt% solids with a pH value of 7
~ 8. Around 30,000 tonnes of de-inking waste water are generated annually in the
State of Victoria (Australia) and need to be processed for water recycling. This
process also produces 1,300 tonnes de-inking sludge (filter cake) disposed off in
landfills. The de-watering process and subsequent disposal of filter cake are not only
very costly but also presents severe environmental challenges [1-3]. Primarily due to
the need for reprocessing prior to disposal to the waterways, therefore developing
potential uses of de-inking waste water in manufacture operations, such as concrete
products, could deliver great social, economic and environmental benefit.
Many studies have been conducted recently on using paper sludge in construction
materials, as opposed to de-inking waste water [1-7]. Published work on waste water
usage has generally been directed to the use of washing waste water from pre-cast
concrete plants in concrete manufacturing [8-12]. Investigations of using washing
waste water from ready-mixed concrete plant as water source for concrete
manufacture have found that up to 85% of compressive strength can be retained by
replacing 100% water with washing waste water [8]. There is however very limited
research data on reusing de-inking waste water for pre-mix concrete applications. The
present study aims to explore the feasibility of reusing de-inking waste water from
recycling paper mill as mixing water replacement for manufacturing construction
products by evaluating the physical and mechanical properties as well as durability of
cementitious products incorporating de-inking waste water.
2. Materials and Experimental Program
De-inking waste water was sourced from a major waste treatment plant in Victoria,
Australia. Characterisation of the waste water was carried out. Moisture content (Loss
on Drying-LOD) and Loss on Ignition (LOI) were determined after heating waste
water samples at 110°C and 900°C respectively. Elemental analysis was determined
on the post-LOI ash by ICP after Microwave Decomposition of the material in a
mixture of nitric, hydrochloric and hydrofluoric acid.
2
A general purpose ordinary Portland cement (OPC) complying with AS 3972
(Australian Standard) and regular river sand were used in this work. The sand was
dried before use to ensure exact moisture levels in all mix formulations. Mortar
samples were prepared in accordance with AS 2350.12-2006 for standard mortar
sample fabrication. Specimens were prepared by replacing potable water with deinking waste water. The solid component of waste water was considered as sand
replacement.
Mixing was carried out in a 10L Hobart mixer by first placing potable water and deinking waste water in cement and mixing for about 30 seconds. River sand was then
added and mixing continued for another 60 seconds. The flow of the mixture was
measured in accordance with ASTM C 1437-07 before casting.
Mortar slurry was then cast into two types of steel moulds: 50 mm cubes for
compressive strength testing and 160 × 40 × 40 mm mould for drying shrinkage
measurement. The cast moulds were vibrated for 1 minute to achieve adequate
compaction. The samples were initially cured at ambient temperature under general
laboratory condition at 23°C and 50% RH (Relative Humidity) for 24 hrs. The
specimens were then demoulded and stored in a standard conditioned room at 23°C
and 100% RH for scheduled testing.
The following tests were conducted:
•
Compressive strength: The majority of samples for compressive strength testing
were prepared at the water to cement ratio of 0.6. However, samples with water to
cement ratio of 0.50 for 100% waste water replacement were also prepared to
compare the compressive strength under similar consistency. Compressive
strength tests were carried out at 1, 7, 28 and 91 days under saturated surface dry
(SSD) in accordance with AS 1012.9 -1997 at a loading rate of 20 MPa/min.
Compressive strength value under each condition constitutes the average results of
three to four mortar samples.
•
Drying shrinkage: Shrinkage measurements were performed at regular intervals
up to 91 days in accordance with a modified procedure of AS 1012.13–1992
3
(Determination of the Drying Shrinkage of Concrete for Samples Prepared in the
Field or in the Laboratory). Drying shrinkage value is the average results of three
mortar specimens.
•
ASTM C 642–06 (Standard Test Method for Density, Absorption, and Voids in
Hardened Concrete) was adopted for water absorption, bulk density and volume of
permeable voids tests. Each test result constitutes the average values of minimum
three mortar samples.
3. Results and Discussion
3.1. Characterisation of de-inking waste water
The pH of the waste water was determined to be about 7.8 ~ 8.0 at 21°C, measured
using a TPS pH Cube Meter. The chemical composition of the de-inking waste water
is shown in Table 1. The supplied de-inking waste water contained about 98.7 wt%
liquid, 0.76 wt% organic matter and 0.54 wt% ash. The ash composition differs from
most washing water sludge ashes in that it contains significants amount of Ti, Ba and
Cu [8, 9]. The organic matter in waste water contains mainly cellulose [4, 7], and
some surfactants, such as fatty acids and salts of sulphate lignins etc used in the deinking floatation process [13-15]. The surfactants appear to play a significant role
when de-inking waste water is used in the concrete mix. This is because these
surfactants (fatty acids and salts of sulphate lignins etc.) are well known air entraining
agents and could potentially induce entrained air in concrete [16]. Air entrainment is
known to improve workability and consistency of plastic concrete/mortar while
reducing its bleeding and segregation. It also improves the durability of the concrete
to deterioration caused by freeze/thaw exposure cycles. However, as a result of
entrained air, the concrete strength may decrease.
4
Table 1. Chemical composition of de-inking waste water
Component
Mass percentage
Moisture content
98.7
LOI (Loss on ignition) 0.76
Ash
0.54
Ash composition (after ignition at 900°C)
SiO2
16.70
Al2O3
16.53
CaO
22.46
TiO2
32.39
BaO
5.43
CuO
2.59
S2O3
1.17
Fe2O3
0.92
Na2O
0.33
3.2. Mortar flow behaviour with addition of de-inking waste water
It was observed that introducing de-inking waste water into mortar mix changed its
flow behaviour. Mortar samples were prepared by replacing potable water with waste
water where the solids component of the waste water was considered as sand
replacement. The sand-to-cement weight ratio was typically set at 3.
Fig. 1 shows variation of mortar flow with waste water content. Improvement on
mortar mix rheological properties using waste water is evident. The flow property
increases significantly with the increase of waste water content up to 50 wt%. The
flow nearly levels off when waste water content exceeds 50 wt%. As stated earlier,
this improvement is mainly attributed to the surfactants used in the de-inking process,
such as fatty acids and salts of sulphate lignins etc, well known as air entraining
admixtures [16].
The increase in workability of mortar mixes also suggests potential use of de-inking
waste water as a water reducing admixture, i.e. reducing water to cement ratio to
maintain similar mix consistency, while the physical and mechanical properties are
improved or at least retained. Fig. 1 also shows that flow of mortar mixture with
100% waste water at w/c of 0.50 is about 88%, which is equivalent to that of the
mortar mix with 100% potable water at w/c of 0.60, i.e. 89% of flow.
5
140
Flow (%)
130
120
110
w/c=0.6
w/c=0.5
100
90
80
0
20
40
60
80
100
Waste water content (%)
Fig. 1. Flow of hydraulic cement mortar vs. waste water content
3.3. Compressive strength
Results of compressive strength tests up to 91 days are shown in Fig. 2 and Fig.3.
Fig.2 shows compressive strength development at fixed w/c =0.6 for varying waste
water loadings while Fig.3 shows compressive strength results at varying w/c ratios.
Compressive strength decreases with increase in the amount of de-inking waste water
added. The compressive strength of mortar samples with 10 wt% and 100 wt% waste
water replacement for potable water is respectively 32.5 MPa and 16.3 MPa at 91
days, corresponding to 81% and 40% of the reference mortar strength (40.3 MPa).
As shown in Fig.3, the compressive strength is significantly reduced at 100% waste
water replacement compared to the reference mortar mix at w/c ratio of 0.60.
However, the compressive strength of mortar samples with w/c ratio of 0.50, which
have the equivalent consistency to reference mix at w/c ratio of 0.60, was about 34.0
MPa at 91 days, i.e. retaining 85% compressive strength of the reference mortar
samples. This trend demonstrates clear benefits of using de-inking waste water by
6
providing an effective way to reduce water to cement ratio while retaining
compressive strength of mortar samples.
As discussed in Section 3.2, the surfactants used in de-inking process produce
entrained air in the mix [16], leading to changes in porosity and pore size distribution
of the matrix and resulting in lower density of mortar block. This certainly induces
lower compressive strength. The observed lower bulk density and porous structure
generated by the addition of de-inking waste water in mortar mix is further discussed
in Section 3.5.
Compressive strength (MPa)
45
40
35
30
25
20
15
10
Reference
25 wt%
75 wt%
5
10 wt%
50 wt%
100 wt%
0
0
20
40
60
80
100
Mortar age (days)
Fig. 2. Compressive strength development with time at fixed w/c ratio of 0.6
7
Compressive strength (MPa)
45
40
35
30
25
20
15
10
Reference at w/c=0.60
100% waste water at w/c=0.60
100% waste water at w/c=0.50
5
0
0
20
40
60
80
100
Mortar age (days)
Fig. 3. Compressive strength development at varying water/cement ratios
3.4. Drying shrinkage
The results of drying shrinkage measurements of the mortar samples are shown in Fig.
4 and Fig. 5. Fig. 4 shows variation in drying shrinkage with time for varying deinking waste water loadings at a fixed w/c=0.6. Fig. 5 shows drying shrinkage
performance of systems incorporating de-inking waster water with w/c ratio of 0.5
and 0.6, compared to the reference system with w/c=0.6. The drying shrinkage of
mortar specimens slightly increases with increased waste water content. For w/c ratio
of 0.60, the drying shrinkage of mortar specimens with 10 wt% and 100 wt% waste
water replacement at 91 days corresponds to 964 and 1115 microstrains, which are
~3% and ~19% increase respectively, compared to the reference mortar sample (935
microstrains).
Reducing w/c ratio from 0.60 to 0.50 for 100% waste water replacement, as shown in
Fig. 5, the drying shrinkage at 91 days is around 1011 microstrains, which is only 8%
higher than that of the reference sample (935 microstrains) at the equivalent
8
consistency. The marginal increase in drying shrinkage indicates de-inking waste
water has the potential to be used in manufacturing concrete products.
The drying shrinkage of cementitious systems is mainly caused by loss of moisture in
cement paste and hence this could lead to debonding at the cement paste/aggregate
interfaces and development of intergranular cracks [17, 18]. Large drying shrinkage is
undesirable mainly due to the risk of inducing cracking in concrete elements. The
organic matter constituents and residual cellulose fibres in de-inking waste was
reported [2] to absorb additional water in mortar mix, which was released with time
and subsequently lost, resulting in increased drying shrinkage. However, due to low
solid components in de-inking waste water (1~2 wt% solids), the increase of drying
shrinkage for waste water-containing mortar samples was marginal.
0
10
20
30
40
50
60
70
80
90
100
0.000
% change in length
-0.020
Reference
50%
10%
75%
25%
100%
-0.040
-0.060
-0.080
-0.100
-0.120
Mortar age (days)
Fig. 4. Variation of drying shrinkage with time at a fixed w/c ratio of 0.6
9
0
10
20
30
40
50
60
70
80
90
100
0.000
% change in length
-0.020
Reference (w/c=0.6)
100% waste water at w/c=0.6
100% waste water at w/c=0.5
-0.040
-0.060
-0.080
-0.100
-0.120
Mortar age (days)
Fig. 5. Variation of drying shrinkage with time at varying water/cement ratios
3.5. Water absorption, bulk density and volume of permeable voids
Water absorption, bulk density and volume of permeable voids of the mortar samples
were measured and the results are shown in Fig. 6 through to Fig. 8. From these
figures, both water absorption and volume of permeable voids increase and bulk
density decreases with increased waste water loadings.
At w/c ratio of 0.60, replacing 10 wt% and 100 wt% potable water with waste water
delivered mortar densities of 2070 and 1840 kg/m3 respectively, equivalent to
medium density concrete production for wall panel applications. However, the
volume of permeable voids only increases slightly from 18.2% to 19.0%, as shown in
Fig. 8. This may be related to the presence of impermeable voids, which is not
reflected Fig. 8. It is well known that air voids introduced by air entraining admixture
are discrete and on average less than 0.05 mm in diameter, compared to entrapped air
voids, which are relatively large, typically 1 to 10 mm in diameter and may form
continuous channels, and hence increasing concrete permeability [19]. These
entrained air voids could be largely impermeable in the current water permeability
tests.
10
As shown in Fig. 6 to Fig. 8, reducing w/c ratio from 0.60 to 0.50 for 100% waste
water replacement, water absorption and volume of permeable voids of the mortar
samples significantly reduced, indicating free water available in the mortar matrix
remains the primary source of observed open pores. This is also associated with the
increase of its bulk density.
Fig. 9 shows back scattered SEM micrographs comparing fracture surface
morphology of reference sample and waste water-containing mortar samples at an
equivalent magnification of 100X. A relatively more porous structure is evident on
the sample containing 100% de-inking waste water, compared to the reference
sample, confirming trends observed for density and permeable voids measurements.
12
Absorption after immersion w/c=0.60
Absorption after boiling w/c=0.60
Absorption after immersion w/c=0.50
Absorption after boiling w/c=0.5
Water absortion (%)
11.5
11
10.5
10
9.5
9
0
20
40
60
80
100
Waste water content (%)
Fig. 6. Water absorption of mortar specimens after immersion and boiling
11
2200
Bulk density (kg/m 3)
w/c=0.60
100% waste water at w/c=0.50
2120
2040
1960
1880
1800
0
20
40
60
80
100
Waste water content (%)
Volume of permeable voids (%)
Fig. 7. Variation of bulk density of mortar specimens with waste water content
19
18.7
18.4
18.1
w/c=0.60
17.8
100% waste water at w/c=0.50
17.5
0
20
40
60
80
100
Waste water content (%)
Fig. 8. Volume of permeable voids vs. waste water content
12
(a)
(b)
Fig. 9. SEM micrographs comparing the fracture surface morphology of
reference sample (a) and 100% waste water-containing sample (b)
13
4. Conclusions
1. Replacing mortar mixing water with de-inking waste water significantly improves
rheological properties of the mortar mix. The flow of mortar mix with 100% waste
water replacement at water/cement ratio of 0.50 is equivalent to that of the mortar
mix with 100% potable water at water/cement ratio of 0.60, indicating potential
use of de-inking waste water as water reducing chemical admixture. This
enhanced effect is attributable to the surfactants used in de-inking process.
2. Replacement of 10 wt% and 100 wt% mortar mixing water with waste water
respectively induced mortar densities of 2070 and 1840 kg/m3, indicating deinking waste water has the potential for use as a foaming agent to produce
lightweight concrete or masonry block systems.
3. At water/cement ratio of 0.60, the compressive strength of mortar samples with
10wt% and 100wt% waste water is respectively 32.5 MPa and 16.3 MPa at 91
days, corresponding to 81% and 40% of the reference mortar. However, at
water/cement ratio of 0.50, the compressive strength of the mortar samples with
100 wt% waste water is 34 MPa, i.e. retaining about 85% of the reference strength
at the equivalent consistency.
4. At water/cement ratio of 0.60, measured drying shrinkage of mortar specimens
with 10 wt% and 100 wt% waste water replacement at 91 days corresponds to 964
and 1115 microstrains, which are ~3% and ~19% increase respectively, compared
to the reference mortar sample.
5. Water absorption and volume of permeable voids increase with increased amount
of waste water, while the bulk density decreases.
Acknowledgements
This work was conducted with the support of EPA Victoria (Australia) through the
Australian Industry Group.
14
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