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

Wood fly ash as cement replacement VGB PowerTech 10 l 2020
The content of total alkalis was too high for
both ashes to comply with EN450-1, and
water washing of the ashes was performed
to investigate if removing the soluble salts
could increase the value of the WAs as cement
replacement. For WA1, the washing
did not change the compressive strength of
the mortar, in which WA1 or WA1-WW was
used, neither for 5 % nor 10 % replacement.
The mortar with 5 % cement replacement
with WA2-WW had the highest compressive
strength, which was almost the same as the
reference (42 MPa vs. 43 MPa). This mortar,
when fresh, had a good workability.
The workability of the fresh mortar with
10 % cement replacement with WA2-WW
also had good workability and the molding
was unproblematic, but here the compressive
strength did decrease, which thus can
be a result of the higher replacement.
Ca is present in too high concentrations in
both WAs in comparison to the requirements
in EN 450-1 (Ta b l e 3 ). Some Ca is
washed out in acid (Ta b l e 2 ). For the 5 %
cement replacement with WA-AWs the
compressive strength were similar to the
obtained with 5 % WAs as received. For
10 % cement replacement with WA-AW,
the compressive strength decreased both
compared to 5 % cement replacement with
WA-AW and with the 10 % WA replacements.
The casting of both mortar bars
with replacement percentages with WA2-
AW and WA1-AW 5 % were without problems,
whereas the casting of mortar WA1-
AW 10 % was almost impossible influencing
the very low compressive strength of
this mortar.
4 Conclusions
The quality of two different WA’s were investigated
for partial cement replacement
in concrete. The investigation includes the
WAs as received from the incinerator facility
and after pretreatments: hydration,
drying, heating to 550 °C, washing in water
and washing in acid. The WAs as received
did not comply with the composition from
EN 450-1 for fly ashes as mineral additions
in concrete. The content of CaO and total
alkalis were too high and the content of primary
oxides too low in both WAs. One of
the WAs did not comply with the upper
limit for MgO either. Both WAs did though
comply with the Cl and the P 2 O 5 content.
The investigated pretreatments changed
the mineralogy and/or the content of
chemical elements. The compressive
strength of all mortars with WAs and pretreated
WAs were less than that of the reference;
however, it was only a 2-4 % reduction
for some of the specimens - those with
heated WAs at 5 % cement replacement,
and with one of the water washed WAs.
The casting process was influenced negatively
due to a low workability of most of
the mortars with WAs and pre-treated
WAs, which again influenced the compressive
strength negatively. Thus, a 1:1 cement
replacement with the WAs is not advisable,
but use of plasticizer is recommended
to overcome this issue in future
work. The two investigated WAs differed in
composition, and one of the WAs was hydrated
just after incineration. This hydrated
WA met less of the limiting values in EN
450-1 compared to the non-hydrated ash.
In general, the non-hydrated ash gave better
results than the hydrated WA in regards
to the compressive strength of mortar with
low-level cement replacement.
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