Check nowIn this experiment, the CCSB was mainly made up of the volcanic ash-based composite gelling system composed of fly ash, silica fume, desulfurized gypsum, and carbide slag. Four different alkali activators were selected to analyze the overall performance of the CCSB based on the optimal ratio of raw materials.
Alkali-activated cementitious materials have a long history in the world. Purdon 7 first proposed the use of NaOH in stimulating blast-furnace slag and dissolving its silicon and aluminum structures to form hydration products. Glukhovsky 8 put forward the concept of "soil cement”, using calcium aluminosilicate and alkali metal solution to prepare a cement material, as a high strength, acid and alkali corrosion-resistant material. Alkali-metal cations play a structure-directing role in gel hardening and eventual crystal formation. 9 , 10 The addition of calcium compounds improves the mechanical properties of fly ash-based geopolymers. 11 , 12 The use of Ca(OH) 2 could increase the plastic shrinkage of the alkali-activated slag concrete and reduce the mesopore volume. 13 , 14 The compressive strength of fly ash and granular blast-furnace slag preparation mortar is significantly correlated with the specific alkali quality coefficient range. 15 , 16 The compressive strength of kaolin geopolymers reached the highest value when S/L and Na 2 SiO 3 /NaOH ratios were 1.00 and 0.32, respectively. 17 , 18 With the increase of silica modulus, the micromechanical properties of sodium silicate-activated slag showed a slight improvement as macroscale strength improved significantly. 19 , 20 The slag mortar made from Na 2 SiO 3 , NaOH, and Na 2 CO 3 showed that the mechanical properties were sensitive to the change in alkali-activator temperature. 21 , 22 The blast-furnace slag cement sample activated with a MgO–NaOH mixture and cured at 20 °C developed a strength of 54 MPa at 90 days. 23 The alkali-silica reaction of these samples was significantly lower than that of the ordinary Portland cement-based concrete and far below the ASTM-specified threshold. 24 The influence model of the alkali-slag concrete (ASC) air-void structure on the freeze–thaw resistance could be used to analyze and predict the freeze–thaw resistance of the ASC. 25 − 27
Composite calcium silicate boards (CCSBs) are a new kind of environment-friendly building material that gets widely used in building engineering such as ceilings, 4 , 5 floor coverings, and drying furnace walls. 6 By mixing and stirring raw materials containing calcium and silicon into a gelling system and then undergoing pressure forming, prenurturing, and autoclave curing, it obtains advantages such as high strength, impact resistance, thermal resistance, deformation resistance, and high-efficiency processing. At present, it is favored by many experts and scholars as a new type of building material with a long-term vision.
The rapid development of the national economy highly relies on the sufficient supply of energy and production materials. However, along with the massive supply of energy, huge amounts of fly ash, silicon powder, and carbide slag are also emitted into the atmosphere, aggravating water and air pollution. At the same time, the mass production of building materials will intensify the loss of natural, high-quality mineral resources such as quartz sand. 1 At present, powders such as fly ash are widely used in cement mortars and green building materials due to their properties. 2 , 3 However, in order to alleviate the tension of resource availability, more effort should be put into reducing the consumption of natural, high-quality mineral resources. Here, a novel method to utilize waste byproducts was introduced.
2. Results
and Discussion
2.1. Effect of the Single-Doped
Alkali Activator
on Flexural Performance of the CCSB
The position of the dotted
line in represents
the standard sample strength (11.53 MPa), where no alkali activator
is doped. The flexural strength of the CCSB is optimal when the NaOH
dosage is 6%. However, it is much lower than the standard sample strength
when the dosage reaches 12%. This trend indicates that NaOH can stimulate
the volcanic ash in the composite gelling system when the dosage is
low. However, it will lead to mechanical defects of the CCSB when
the dosage is too high.
Open in a separate windowThe content of CaO on
the flexural strength of the CCSB is apparent.
The flexural strength of the CCSB reaches a maximum when the content
of CaO is 3%. The flexural strength shows a sharp decline with the
increasing amount of CaO and the billet is even challenging to form
when the CaO content reaches 12%. The Ca(OH)2 content in
the system finally saturates and crystallizes out when CaO is highly
doped. The excess Ca(OH)2 crystal dissolves again and forms
many holes in the product when the low-strength board billet is steamed
and maintained in an autoclave;28 these
Ca(OH)2 crystals intersperse between hydrated bodies, which
result in insufficient strength.
It can be seen from that incorporating
a small amount of Na2CO3 into the cementitious
material can slightly increase the
strength of the CCSB. Because Na2CO3 is a strong
base weak acid salt, the OH– generated by hydrolysis
can provide an alkaline environment for raw material reactions. A
large amount of CO32– will consume Ca2+ in the system to form CaCO3 precipitates, which
can adversely affect the strength when the amount of Na2CO3 is continuously increased. The combination of Na+, active silicon, and aluminate ions contribute to the formation
of the monoclinic sodalite. This phenomenon subsequently causes a
decrease in the production of C–A–S–H. Therefore,
the incorporation of Na2CO3 will both promote
the pozzolanic effect and inhibit the production of the C–A–S–H
gel.29
Na2SiO3 hydrolyzes to form some OH– and active Si(OH)4 when it is dissolved. The active silicon
tetrahedral content in the composite system is continuously supplemented
with the continuous incorporation of Na2SiO3. Moreover, more hydration products are formed in the early stage
of the hydration reaction, which gives the board billet an early initial
strength. Therefore, the flexural strength of the CCSB is enhanced
as the amount of Na2SiO3 (Na/Si = 1.03 ±
0.03) is continuously increased, and the flexural strength reaches
a maximum at a dosage of 9%.
In summary, appropriate doping
of the abovementioned four alkali
activators individually can increase the mechanical strength of the
CCSB. Within the range of the current dosage of the alkali activator,
based on the increase in strength, the activating effect of the four
alkali activators can be ranked as CaO > NaOH> Na2SiO3 > Na2CO3.
2.2. Effect of Compounded Alkali Activators on
Flexural Performance of the CCSB
The flexural strength of
the CCSB after incorporating composite alkali activators is shown
in Table 1. The flexural
strengths of S1, S10, and S11 exceed 14 MPa, and the flexural strength
of S11 reaches a maximum of 14.72 MPa. When 3% NaOH and CaO are mixed
as a composite activator, the alkalinity in the gelation system and
the content of Ca2+ are increased. Therefore, the fly ash
particles can be activated efficiently during the preconservation
period. A lot of C–A–S–H gel is produced, thereby
increasing the compactness of the billet. However, it hinders the
pozzolanic effect of the raw material when the OH– concentration is too high. Therefore, the flexural strength of the
CCSB is lower than the strength of the standard sample S0 when NaOH
incorporation is increased to 6%.
Table 1
addition
(percentage of mass)/% samplesNaOHCaONa2CO3Na2SiO3flexural
strength/MPaS0000011.53S1330014.53S2630010.62S3303013.41S4603010.06S5606011.03S6300613.61S7600610.52S8033010.32S903609.61S10030614.25S11030914.72S12003613.56S13006611.87Open in a separate windowThe flexural strength of
the CCSB can reach 13.41 MPa when 3% NaOH
and 3% Na2CO3 is compositely added to it. However,
it rapidly drops to 10.06 MPa when the dosage of NaOH increases to
6%. The significant decrease in mechanical strength is caused by the
early precipitation or even dissolution of the aluminosilicate gel
due to excess NaOH. After increasing the content of Na2CO3 to 6%, the flexural strength rebounded to 11.03 MPa,
suggesting that the increase in CO32– concentration can effectively prevent the precipitation of the hydrated
gel. Test results also show that the incorporation of 6% NaOH and
Na2SiO3 is invalid for increasing the flexural
strength of the CCSB, and the composite of CaO and Na2CO3 even makes the flexural strength lower than the standard
sample S0 due to the hydrolysis limitation of Na2SiO3 in a strong alkaline environment. However, the combination
of CaO and Na2SiO3 benefits the mechanical properties
of the CCSB. The flexural strength of the CCSB reaches 14.72 MPa when
the CaO dosage is 3% and the Na2SiO3 dosage
is 9%. This is because the combination of these two alkalis provides
not only an appropriate alkaline environment but also the main substances
involved in the hydration reaction. The combination of 3% Na2CO3 and 6% Na2SiO3 can increase
the flexural strength to 13.56 MPa. However, with the increase of
the Na2CO3 dosage, excessive CO32– consumes Ca2+ in the system to form calcium
carbonate crystals, eventually reducing the strength of the product.
According to the flexural strength test results of the CCSB, the
compounded alkali activator to some extent further improved the flexural
strength of the CCSB compared to the single-doped alkali activator.
The rank of the strengthening effect by four alkali activators on
the strength of the CCSB is (CaO + Na2SiO3)
> (NaOH + CaO) > (Na2SiO3 + Na2CO3) > (NaOH + Na2CO3) > (Na2CO3 + CaO). The S11 CCSB (CaO + Na2SiO3) possesses the best strength. Therefore, the S11 CCSB is
used as a representative of the best alkali activator to perform various
performance tests.
2.3. Effect of the Alkali Activator
on the Freeze–Thaw
Performance of CCSB Samples
The S0 and S11 CCSBs were selected
as the test subject. The appearances of the two CCSBs before and after
the freeze–thaw cycle (FTC) are shown in , while the cut surface of the CCSB before
and after the FTC is shown in . Neither “rotten angle”, cracking, nor
delamination was found on the CCSBs, which have clean surfaces and
corners after 25 FTCs. Therefore, these two types of CCSBs meet the
requirements of the Chinese national standards. The flexural strength
of the boards before and after FTCs, the mass-loss rate, and the strength
loss rate are shown in Table 2.
Open in a separate window
Link to wuxingOpen in a separate windowTable 2
mass
of the CCSB (g)strength
of the CCSB (MPa)loss
rate (%)samplesBFaAFbBFAFmassstrengthS0265.65252.4311.8710.184.9814.22S11246.54235.4414.7213.304.219.65Open in a separate windowBoth the mass loss
and strength loss of the two CCSBs comply with
the specification in GB/T11969-2008. Moreover, the mass loss and strength
loss of the CCSBs are effectively reduced after incorporating an alkali
activator into the raw material. Colombo points out that the main
reason for the damage of the product during the FTC is due to the
9% volume expansion of water in the void hole during freezing.30 Free water in the gap of the product keeps moving
during the continuous growth of the ice crystals. When the moisture
in the capillary reaches the saturation point, pressure is generated
and causes mechanical defects at weak points of the product. This
promotes the continuous expansion of this mechanical defect and eventually
leads to the destruction of the product during the reciprocating FTC.
Therefore, the frost resistance of the product has an important relationship
with the internal void volume.
2.4. Effect
of the Alkali Activator on Pore Structure
Performance of CCSB Samples
a,b shows desorption/adsorption isotherm
curves of S0 and S11 CCSBs, respectively. The curves in both figures
are anti-S shape, which is closest to the type IV desorption/adsorption
isotherm of the adsorption isotherm by IUPAC.31 The adsorption curves show a slow increase in nitrogen adsorption
at P/P0 ≤ 0.4,
while microporous filling or N2 monolayer adsorption occurs
in the pores of the sample. The adsorption amount of the adsorption
curve starts to increase slowly when 0.4 ≤ P/P0 ≤ 0.8. The N2 multilayer
adsorption occurs and gradually transitions to capillary condensation
at this stage. The adsorption curve tends to be vertical when P/P0 reaches 1, but its adsorption
amount is still not saturated. N2 capillary condensation
occurs in voids with large pore size at this stage, which indicates
that a large number of mesopores exist in the sample. The desorption
curve running above the adsorption curve indicates that the denitrification
rate is higher than the nitrogen adsorption rate. This indicates that
no new crack or pore is formed inside the sample as P/P0 increases.32 Both desorption and adsorption isotherms of the two figures separate
at P/P0 = 0.4 and thus
form a hysteresis loop. The curves can describe characteristics of
the pore structure via the shapes of hysteresis and desorption/adsorption.
The almost same shapes of hysteresis loops shown in indicate that the shape, size,
and distribution range of the pores in the sample are similar. De
Boer33 divided the hysteresis loop into
four categories. However, the pore structure in the actual sample
is relatively complicated, and the hysteresis loop of the pores is
often a combination of multiple types. The desorption isotherm curve
is coincident, and no hysteresis loop is generated when the relative
pressure P/P0 ≤
0.4. Such a phenomenon indicates that the micropores in the sample
are mostly cylindrical, parallel plate, or ink bottle pores with one
end closed. The shape of the desorption isotherm curves in the high-pressure
zone is almost parallel. In addition, it could form a hysteresis loop
as the relative pressure rises close to the hysteresis loop H1. Besides, the amount of nitrogen adsorbed keeps increasing
with relative pressure ( ) and is still not saturated in the pressure saturation region,
which is close to the hysteresis loop H3. This phenomenon
indicates that cylinder-shaped pores opening at both ends and parallel
plate-slit pores are the main types of pores when the relative pressure
is over 0.4. Therefore, the hysteresis loop in the CCSB is a hybrid
of H1 and H3, and micropores and mesopores are
the majority inside the board.
Open in a separate windowAs shown in , the cumulative pore-size distribution of
two CCSBs was obtained
according to the BJH model processing data by the nitrogen adsorption
method. The total adsorption volume of nitrogen increases linearly
with the expansion of pore size as the pore size does not exceed 20
nm and subsequently stabilizes after exceeding 20 nm. This phenomenon
indicates that the size of the pores in two CCSBs is less than 20
nm. At the same time, the curve of S11 is beneath S0 for all pore
radii. This curve suggests that the pore-size distribution range and
the total adsorption pore volume of the CCSB doped with the composite
alkali activator are significantly reduced. Besides, the addition
of the alkali activator significantly improves the total pore volume
in the CCSB.
Open in a separate windowThe pore-size distribution of the CCSB is 1–93 nm
( ). The bimodal
distribution
of the pore-size distribution curve indicates that the pore size of
the CCSB accounts for a large proportion in the corresponding range.
The distribution of the pore size over 20 nm shows a downward trend,
indicating that there are fewer pores with a pore diameter greater
than 20 nm in the CCSB. Peaks of the S0 CCSB are at 1.926 and 4.869
nm. Two peak positions of the S11 CCSB, which is doped with the composite
activator in the raw material, are shifted left to 1.525 and 4.779
nm, respectively. This phenomenon indicates that the incorporation
of alkali activators can refine the average pore diameter in the CCSB.
In addition, the specific surface area of the CCSB without the alkali
activator is 67.48 m2·g–1 and reduces
to 47.05 m2·g–1 after incorporating
the alkali activator, which can be calculated by applying the BET
model. Therefore, the average pore size of the CCSB is refined under
the activation of the alkali activator, which converts some harmful
pores and less harmful pores into harmless pores. This reduces the
total pore volume and pore-size distribution range, and the overall
mechanical properties of the CCSB are improved. As can be seen from Table 2, the mass-loss rate
and the strength-loss rate of the S11 CCSB are lower than those of
the standard sample S0. It is known that the alkali activator contributes
to the refinement of the average pore size and the void volume inside
the sample, thereby improving the frost resistance of the CCSB.
Open in a separate window2.5. Effect of the Alkali Activator on the Physical
Performance of CCSB Samples
The test results are shown in Table 3. The physical properties
of the CCSBs, prepared using four industrial solid wastes, 3% CaO,
and 9% Na2SiO3, meet the requirements. The ball
drop impact resistance test verified that there is no crack on the
surface; no dripping on the reverse side of the board was observed,
which qualified the board to be impervious. The strength of the CCSB
can reach the class III standard in the DI.5 series.
Table 3
indexstandard requestresultthermal conductivity≤0.350.31density 1.45 g·cm–3dry shrinkage ratio<0.19%0.05%moisture content≤10%8%swelling ratio≤0.25%0.22%heat shrinkage ratio≤0.50%0.37%nonflammablelevel AqualifiedOpen in a separate window2.6. XRD Analysis
From the microscopic
point of view, the S11 CCSB has the best strength. Its X-ray diffraction
(XRD) pattern is shown in . The main mineral components consist of tobe mullite,
calcium carbonate, calcium hydrated calcium aluminate gel, and a small
amount of ettringite. By adding the alkali activator to the pozzolan-based
composite gelling system, silicon and calcium raw materials undergo
a complete hydration reaction and then produce calcium silicate crystals
composed of tobe mullite with good crystallinity. This can be proven
by the sharp diffraction peak seen in . The production of CaCO3 may
result from the carbonization of Ca(OH)2 crystals and the
hydrated gel by CO2 in the high-temperature curing process.
Open in a separate window2.7. Comparative
Analysis
Industrial cement,
river sand, or machine sand is commonly used as the main calcium and
silicon raw materials in the traditional CCSB preparation process.
Addition of solid waste raw materials in the cement slurry was widely
applied when preparing CCSB materials.34 Nowadays, much attention is focused on industrial transformation
and upgrading. River sand excavation is prohibited due to environmental
issues. Raw materials including sand and cement cannot meet the increasing
demands. Average prices of river sand and PO42.5 cement are 150–220
and 440–510 yuan·t–1 in China,35,36 respectively. Shortages are occurring in some areas, triggering
downstream soaring prices of products such as concrete. In this experiment,
fibers were blended into a composite gelling system composed of four
kinds of pure solid wastes. Ideally, the flexural strength of the
CCSB can reach a maximum of 14.72 MPa. At this point, a large volume
of solid waste can be consumed and the related environmental pollution
caused by cement production can be eliminated. This provides a new
idea for the manufacture and application of the CCSB. The raw materials
used in this experiment are cheap and easily accessible. The price
of carbide slag is very low (10–30 yuan·t–1) or free.37 Compared with the traditional
process, the cost of calcium and silicon raw materials can be reduced
by about 200 yuan·t–1.