VGB POWERTECH 10 (2020) - International Journal for Generation and Storage of Electricity and Heat

VGB PowerTech 10 l 2020
Wood fly ash as cement replacement
ing to 550 °C resulted in the CaO peak to
disappear in WA2, which is on the contrary
to [Lopinti et al. 2020], who saw beginning
calcination (CaCO 3 transformation to
CaO) already at 500 °C. The loss of CaO in
WA2 after heating reveals that Ca enters
into other phases.
The washing in water and acid resulted in
mass losses of 29 % and 37 % in WA1, respectively,
and 13 % and 22 % in WA2.
Washing in water and acid removed the
soluble salts KCl and K 2 SO 4 from the ashes
(Ta b l e 4 ). In the washed WA1, MgO and
Mg(OH) 2 were identified, whereas no crystalline
Mg was found before the washing,
where the concentrations probably were
too low for identification. When washing
out part of the mass (the soluble fraction)
the concentration of the non-soluble minerals
increases. In the water washed WA2
Ca(OH) 2 appeared in a low concentration,
and in WA1 the concentration of Ca(OH) 2
seemingly decreased. In the acid washed
WAs, Ca(OH) 2 was not identified.
3.2 The investigated WAs in mortar
3.2.1 Workability issues during casting
During the castings, it was observed that it
was difficult to vibrate the fresh mortar
well in the molds when WA was in the mixture,
and it was increasingly difficult when
increasing the replacement percentage of
cement from 5 % to 10 %. The low workability
resulted in the mortar not flowing
well during the vibration and the casted
mortar prisms did have larger visual pores
than the references. Other researchers
have reported similar finding, e.g. Udoeyo
et al. [2006] reported slump test results of
concrete containing varying percentages of
WA and was evident that WA concrete mixes
exhibited less workability than the reference.
3.2.2 Compressive strength (7 days) of
mortar
The screening of the effect from the WAs
and pretreated WAs on the ease of casting
and the 7 days compressive strength are in
Ta b l e 5 and F i g u r e 2 , respectively. The
general pattern is that the mortars with
WAs have lower compressive strength than
the reference mortar (F i g u r e 2 ). The
casting of some of the mortars with WA
was influenced by a lower workability (Ta -
b l e 2 ), and the hence heterogeneities in
the casted mortar prisms influenced the
compressive strength. Thus, evaluating the
compressive strength of the different mortar
mixes includes both different characteristics
for the WAs and treated WAs as well
as the influence from the differences in
workability. In order to evaluate the influence
from the pretreatments and changes
of phases, it is suggested to use plasticizer
to have the same workability in future
work, however, in this work, it was decided
to investigate the effect of a 1:1 cement replacement
with WA only.
Tab. 5. Evaluation of the casting process 0 = similar to reference, - = slightly drier mix than reference
but homogeneous prisms were obtained after vibration, -- = difficult to vibrate and
the mortar prisms had larger airvoids, --- = impossible to vibrate and the mortar bars had
many larger air voids and appeared very heterogeneous.
The mortars with WA and pretreated WA1s
all had lower compressive strength when
replacing 10 % cement than 5 %. This was
not the case for WA2 as received and hydrated
WA2, but for the use of WA2 after
the other pretreatments, as well. The casting
of mortar with 10 % replacement was
generally more difficult than casting mortars
with 5 % replacement, replacement,
however as this is not a focus area for this
study, this was not investigated further.
The compressive strength for the mortars
with WA1 and WA1-Dried (5 % replacement)
are very similar. The difference between
the two recipes was the use of asreceived
or dried WA1. The drying did not
result in new mineral phases (Ta b l e 4 ),
but it is likely that carbonation has taken
place. The differences were in addition to a
higher degree of carbonation also the content
of WA1 dry matter and water. The water
content of WA1 was 19 %, which means
that the 5 % cement (22.5 g) was replaced
with only 18.2 g WA1 dry matter and 2.3 g
water, and for the 5 % replacement, this
did not overall influence the compressive
strength. For the 10 % replacement on the
contrary, the compressive strength was
highest when using WA1 as received,
which is an indication of the slightly more
water and better workability (Ta b l e 5 ).
The changes in mineralogy (Ta b l e 4 )
needs to be considered when comparing
the compressive strength of mortars with
WA2 and WA2-hydrated. The compressive
WA1 – 5 % WA1 – 10 % WA2 – 5 % WA2 – 10 %
WA 0 - - 0
WA-dried 0 ---
WA-hydrated - 0
WA-550°C 0 --- 0 0
WA-WW - --- 0 --
WA-AW -- --- -- ---
(a)
Compressive strength in MPa
50
45
40
35
30
25
20
15
10
5
0
Ref
WA1
WA1- Dried
WA1-550
WA1-WW
WA1-AW
(b)
Compressive strength in MPa
50
45
40
35
30
25
20
15
10
5
0
5 % 10 % 5 % 10 %
Ref
WA2
WA2- hydrated
WA2-550
WA2-WW
WA2-AW
Fig. 2. The 7days compressive strength of mortar prisms with the investigated ashes as received or
pretreated. (a) WA1 and (b) WA-2.
strength is higher for the mortar with WA2
than with WA2-hydrated (5 % replacement).
It might indicate that the hydration
occurring in the mortar mix when using
the WA2 as received added to the strength,
but the significantly higher compressive
strength for the mortar with WA2-hydrated
than WA2 for 10 % replacement indicates
the opposite tendency, and no conclusion
can be drawn.
The compressive strength of the mortars
with 5 % cement replacement WA-heated
was high for both WAs (42 MPa and 41 MPa,
respectively), which was only slightly lower
than the compressive strength of the reference
(43 MPa) corresponding to a 2 %
decrease. The heating was performed to
diminish the content of organic material,
but from Ta b l e 4 it is also seen, that the
mineral phases changed, not at least the Ca
containing minerals. In common to the two
heated WAs is that CaO was not detected
by XRD, probably due to carbonation. During
the casting it was noted that the mortar
with WA1-heated (10 %) was impossible to
vibrate into homogeneous prisms, even
with higher frequency and longer duration
of the vibration. This low workability contributes
to almost halving the compressive
strength for mortar with 10 % relative to
5 % cement replacement. The same pattern
was not seen for heated WA2, where the
compressive strength was similar for to the
reference for the two percentages of replacement
(Ta b l e 5 ).
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