X. HE et al.: EFFECT OF DRY-WET CIRCULATION ON THE MECHANICAL PROPERTIES AND PORE STRUCTURE ...
177–182
EFFECT OF DRY-WET CIRCULATION ON THE MECHANICAL
PROPERTIES AND PORE STRUCTURE OF AUTOCLAVED
AERATED CONCRETE
VPLIV SUHO-MOKRE CIRKULACIJE NA MEHANSKE
LASTNOSTI IN STRUKTURO POR V AVTOKLAVU
PREZRA^ENEGA BETONA
Xiao He, Jian Yin
*
, Jiewen Yang, Qiao Liang, Songyun Wu
School of Civil Engineering, Central South University of Forestry and Technology, no. 498 Shaoshan South Road, Tianxin District, Changsha
410004, Hunan, China
Prejem rokopisa – received: 2018-08-30; sprejem za objavo – accepted for publication: 2018-10-18
doi:10.17222/mit.2018.190
Dry-wet circulation is the main service environment for autoclaved aerated concrete. In this study, we looked at selected auto-
claved aerated concrete with dry-wet circulation times of (0, 30, 60, 150, 270 and 340) min. The tested mechanical properties
include the compressive strength and the splitting tensile strength. Scanning electron microscopy (SEM) and Image-Pro-Plus
(IPP) software were chosen to quantitatively characterize the microstructure. The results showed that as the dry-wet circulation
times increased, the compressive strength decreased after first increasing, and the splitting tensile strength reduced. In addition
to the pore structure, microcracks and internal acicular ettringite were produced. The pore size distribution was concentrated
from 1.0 mm to 1.5 mm, when dry-wet cycle times were 60 min or more.
Keywords: autoclaved aerated concrete, dry-wet circulation, mechanical properties, Image-Pro-Plus, pore structure
Suho-mokra cirkulacija je glavni naravni postopek za prezra~evanje betona v avtoklavu. Avtorji opisujejo prezra~evanje betona
v avtoklavu s suho-mokro 0, 30, 60, 150, 270 in 340-minutno cirkulacijo. Vzorcem betona so nato dolo~ili mehanske lastnosti in
sicer tla~no ter cepilno trdnost. Kvantitativno so okarakterizirali mikrostrukturo z vrsti~nim elektronskim mikroskopom (SEM)
in programsko opremo za obdelavo slike Image-Pro-Plus (IPP). Rezultati ka`ejo, da z nara{~ajo~im ~asom suho-mokre
cirkulacije tla~na trdnost po prvem dvigu v primerjavi z necirkuliranim betonom pade, cepila trdnost pa se zmanj{uje. V porozni
strukturi betona so nastale mikrorazpoke in interni igli~asti etringit (hidroksidni Ca-Al sulfatni mineral). Porazdelitev velikosti
por je bila koncentrirana med 1,0 in 1,5 mm po 60-minutnem ali {e dalj{em suho-mokrem ciklu.
Klju~ne besede: v avtoklavu prezra~en beton, suho-mokra cirkulacija, mehanske lastnosti, obdelava slike s programsko opremo
Image-Pro-Plus, struktura por
1 INTRODUCTION
Autoclaved aerated concrete has good performance
with respect to heat preservation and heat insulation, and
also saves clay resources. The excellent properties of
autoclaved aerated concrete depend on its unique pore
structure.
1–4
But in the actual service process, dry-wet
circulation causes deformation. Studies have shown that
one of the main factors was the dry-wet circulation.
5–7
In
addition, as the dry-wet circulation times increased, the
performance of the heat preservation and heat insulation
declined.
Autoclaved aerated concrete, as a wall material, is
often in the environment of rainfall infiltration and
drying evaporation. Studies showed that dry-wet circu-
lation speeded up the invasion rate of harmful media,
such as sulfate and chloride, which lead to destruction,
and also changed the moisture content for deformation.
Autoclaved aerated concrete with the characteristics of
dry shrinkage and wet expansion, for inflation happen
when the internal moisture content increased, on the
other hand, contraction was produced. Studies also
showed that the humidity inside the autoclaved aerated
concrete emerged to a graded distribution, and larger
internal stress occurred and when the stress exceeds a
certain limit, cracking of the autoclaved aerated concrete
happened, which results in its unique pore structure
being damaged, the mechanical properties declining, and
life of service being reduced.
8–10
This paper studies the relationship between the me-
chanical properties and the pore structure of the auto-
claved aerated concrete under dry-wet circulation.
Scanning electron microscopy (SEM) was used to
characterize the micro-pore structure. By employing
Image-Pro-Plus software to a binary image of the pore
structure, we obtained a quantitative characterization of
the pore structure, in order to calculate the pore diameter
distribution. A fractal dimension was used to character-
ize the pore size distribution, and the fractal dimension
had a quadratic correlation with the mechanical proper-
ties though the regression equation.
Materiali in tehnologije / Materials and technology 53 (2019) 2, 177–182 177
UDK 620.1:67.017:691.32:542.468 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(2)177(2019)
*Corresponding author e-mail:
csuyj700930@163.com

2 EXPERIMENTAL PART
2.1 Materials
Autoclaved aerated concrete was obtained from
Changle building materials co. LTD in Changsha, China.
Table 1 shows the properties of the autoclaved aerated
concrete.
Table 1: Main properties of autoclaved aerated concrete
Model
Bulk
density
Intensity of
average
Size
A3.5 B06 <625 >3.5 MPa (600 × 200 × 100) mm
2.2 Dry-wet circulation test methodologies
The methods of the specimens’ processing and
performance testing were referred to GB/T 11969-2008
"test methods of performance for autoclaved aerated con-
crete". The dry-wet circulation of a specimen prepared
with a sawing machine, chose a group based on the parts
from the above, middle and below, for along the direc-
tion of the gas, specifically included the above distance
30 mm from the top of the specimens, the block at the
center of the products, and the block a distance 30 mm
from the bottom. The size of the specimen was (100 ×
100 × 100) mm and a group of specimens was three
pieces.
The chosen specimen was dried with an electric drum
dryer to remain constant quality, then cooling to a
temperature of 20±5 °C and maintained for 20 min. Next
it was soaked in a water tank at a temperature of
20±5 °C, and the level of water exceeded the height of
specimen by 30 mm, held for 5 min, then removed, and
kept for 30 min at a temperature of 20±5 °C. Finally, in
the selected electric drum dryer, it was dried for7hata
temperature of 60±5 °C, then cooled for 20 min, later
soaked in the water at a temperature of 20±5 °C, for one
cycle. The selected times of the dry-wet circulation were
(0, 30, 60, 150, 270 and 340) min.
2.3 Mechanical test methodologies
The moisture in the specimen was maintained at 8 %
to 12 %. The specific methods were as follows: chose the
brush, evenly apply water on the specimen, then select
the PP bag packaging specimen for1dat20°C,usethe
water to achieved a uniform distribution, chose the uni-
versal mechanical testing machine for testing the com-
pressive strength and the splitting tensile strength. And
the size of specimen was (100 × 100 × 100) mm
3
.
2.4 Pore structure test methodologies
Scanning electron microscopy was chosen to observe
the micropore structure of the autoclaved aerated con-
crete, the size of test block was (10 × 10 × 10) mm
3
.
Image-Pro-Plus software was selected for a binary image
of the pore structure, so as to provide a calculated and
quantitative characterization of the pore diameter distri-
bution.
3 RESULTS AND DISCUSSION
3.1 Mechanical properties of autoclaved aerated con-
crete under dry-wet circulation
Figure 1 shows the compressive strength and splitt-
ing tensile strength of the autoclaved aerated concrete
under dry-wet circulation. When the dry-wet circulation
increased, the tendency of the compressive strength and
splitting tensile strength of autoclaved aerated concrete
was to decrease after first increasing. With dry-wet cir-
culation times of 30 min, the compressive strength was
greater than the specimen with no dry-wet circulation.
The compressive strength was 4.27 MPa, increased by
0.47 % compared to the no dry-wet circulation. As we
continued to increase the times of dry-wet cycle,
compressive strength showed a tendency to decline, such
as with times of the dry-wet cycle equal to (60, 150, 270
and 340) min, its compressive strength was (4.24, 4.23,
4.16 and 3.99) MPa. When the dry-wet circulation times
reached 340 min, the value of the compressive strength
was the lowest, and had declined by 6.1 % compared to
the no dry-wet specimen.
The splitting tensile strength showed the same ten-
dency as the compressive strength. When the dry-wet
circulation times went up to 30 min, the splitting tensile
strength reached a maximum of 0.568 MPa, compared to
the no dry-wet circulation, it increased by 0.53 %. And
with the dry-wet circulation times increased to (60, 150,
270 and 340) min, the value declined, respectively, to
(0.563, 0.562, 0.556, 0.541) MPa. Compared with the
specimen that had no dry-wet cycles, the splitting tensile
strength declined by 4.24 %, when the dry-wet circul-
ation times went to 340 min.
The reason for this behavior was that with an
increase in the times of dry-wet circulation, the moisture
was made to move in and out of the specimen more
X. HE et al.: EFFECT OF DRY-WET CIRCULATION ON THE MECHANICAL PROPERTIES AND PORE STRUCTURE ...
178 Materiali in tehnologije / Materials and technology 53 (2019) 2, 177–182
Figure 1: Effect of mechanical properties of autoclaved aerated con-
crete under dry-wet circulation

frequently. For the times of dry-wet circulation equal to
30 min, the influence of the hydration products on the
sand particles was negligible, and hence the mechanical
properties increased. As the dry-wet circulation times
increased, the hydration products reacted further with the
porous structure, thus resulting in a decrease in perform-
ance.
3.2 Microstructure of autoclaved aerated concrete un-
der dry-wet circulation
Figure 2 shows SEM images of the autoclaved
aerated concrete under dry-wet cycle times of 340 min,
and respectively magnified by 1000 and 1600 times.
According to Figure 2a the internal cracks of the
specimen appeared, with those microcracks expanded
into large cracks due to the external conditions, which
resulted in a degradation of the macroscopic perform-
ance, e.g., the strength and dynamic modulus of
elasticity. Also, the generation of cracks erosion of other
ions, caused further damage during the dry-wet circul-
ation of the specimen. Figure 2b shows acicular
ettringite, with the increased dry-wet circulation times,
and the ettringite began to grow inside the crack, which
results in the ettringite appearing in the pore structure.
3.3 Pore structure of autoclaved aerated concrete un-
der dry-wet circulations
3.3.1 Characterizing methods for the pore structure
By employing the technology of image analysis we
analyzed the pore structure. Figure 3 shows the methods.
First, smear the surface of autoclaved aerated concrete to
the color of black, next binarization of the image through
the soft of the Image-Pro-Plus, for the pore structure
more clearly, then identify the pore structure with the
X. HE et al.: EFFECT OF DRY-WET CIRCULATION ON THE MECHANICAL PROPERTIES AND PORE STRUCTURE ...
Materiali in tehnologije / Materials and technology 53 (2019) 2, 177–182 179
Figure 3: Schematic diagram of autoclaved aerated concrete pore structure analysis
Figure 2: Scanning electron microscopy images of the autoclaved
aerated concrete under dry-wet circulation: a) magnified 1600 times,
b) magnified 1000 times

Image-Pro-Plus and count it. Finally, obtain the para-
meter of the pore structure for the pore size.
3.3.2 Pore size distribution
Figure 4 shows the distribution of the pore size.
When the time of the dry-wet circulation was 0, the
distribution of the pore size concentrate was 0.1–0.5 mm,
and the ratio reached 48.25 %. With an increase of the
dry-wet circulation times to 30 min, the pore size
distribution was concentrate to 0.5–1.0 mm, the propor-
tion was 46.09 %, and with dry-wet cycles times in-
creased to (60, 150, 270 and 340) min, the aperture size
concentrate was 1.0–1.5 mm, the percentage achieved
was 35.14 %, 33.12 %, 34.23 % and 31.02 %, respect-
ively. For the specimen that had no dry-wet circulation,
the amount of pore size distribution that more than 1.5
mm is zero, and under dry-wet circulation, an aperture
size distribution of 1.5 mm appeared, and the dry-wet
circulation times reached to (60, 150, 270 and 340) min,
X. HE et al.: EFFECT OF DRY-WET CIRCULATION ON THE MECHANICAL PROPERTIES AND PORE STRUCTURE ...
180 Materiali in tehnologije / Materials and technology 53 (2019) 2, 177–182
Figure 5: Fractal dimension of size pore distribution, a) to f) for dry-wet circulation of (0, 30, 60, 150, 270 and 340) min
Figure 4: Pore size distribution of autoclaved aerated concrete under dry-wet circulation, a) to e) for dry-wet circulation times of (0, 30, 60, 150,
270 and 340) min

and the proportion, respectively, of 29.24 %, 39.70 %,
42.38 % and 39.70 %.
3.3.3 Fractal dimension of the pore size distribution
We used the method of box-counting dimension. The
specific method was as follows: chose a square with side
length of r to cover the pore, and statistically the number
of the square is N (r), by changing the side length of the
square, the statistical number corresponds to the square,
obtained a series of corresponding points, the slope is the
fractal dimension for D
c
. Equation (1) was used for the
fractal dimension.
D
Nr
r
c
d
d
=
(ln ( ))
(ln )
(1)
The fractal dimension of the pore size distribution
characterization for the uniformity of the pore size was
shown in Figure 5, when dry-wet circulation times of (0,
30, 60, 150, 270 and 340) min, the fractal dimension of
the pore size distribution was 1.41, 1.45, 1.51, 1.52, 1.56
and 1.64.
Figure 6 shows the relations between the fractal
dimension of the pore size distribution and the mecha-
nical properties. The research showed the relations
among the fractal dimension of the pore size distribution
with the compressive strength and the splitting tensile
strength. The results present a good quadratic correlation
of the fractal dimension and the mechanical properties
though the regression equation. The value of the fractal
dimension increased, the compressive strength and
splitting tensile strength tended to decrease. The result
implies that the pore size distribution has an influence on
the mechanical properties.
4 CONCLUSIONS
Dry-wet circulation is one of the most severe
environmental conditions. The relationship between the
mechanical properties and the pore structure of auto-
claved aerated concrete under dry-wet circulation is
discussed in this study. When the times of the dry-wet
circulation increased, the compressive strength decreased
after first increasing and the splitting tensile strength
declined. For dry-wet circulation times of 340 min, the
compressive strength and the splitting tensile strength
were 3.99 MPa and 0.541 MPa, and reduced by 6.1 %
and 4.24 %, compared to the no dry-wet circulation.
The microstructure of the autoclaved aerated concrete
under dry-wet circulation was obtained by SEM, and the
results showed that ettringite emerged. Less ettringite
may lead to better mechanical properties, but more can
break the pore structure, and reduce the properties. For a
quantitative characterization of the pore structure we
used a binary image and Image-Pro-Plus for a clear pore
structure. Then we calculated the pore diameter distribu-
tion, and compared the pore structure of the autoclaved
aerated concrete under a dry-wet cycle. We chose the
fractal dimension to characterize the pore size distribu-
tion and the value of the fractal dimension increase with
the time of dry-wet circulation increase. The pore size
distribution was found to affect the mechanical proper-
ties.
Acknowledgments
This research was supported by National Key R&D
Program of China (no. 2016YFC0700801-01).
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X. HE et al.: EFFECT OF DRY-WET CIRCULATION ON THE MECHANICAL PROPERTIES AND PORE STRUCTURE ...
182 Materiali in tehnologije / Materials and technology 53 (2019) 2, 177–182