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0Alexandria Engineering Journal (2014) 53, 119-130
Alexandria University Alexandria Engineering Journal
www.elsevier.com/locate/aej www.sciencedirect.com
ORIGINAL ARTICLE
Utilization of crushed clay brick in cellular concrete production
Ali A. Aliabdo, Abd-Elmoaty M. Abd-Elmoaty *, Hani H. Hassan
Structural Engineering Department, Faculty of Engineering, Alexandria University, Egypt
Received 17 August 2013; revised 9 October 2013; accepted 14 November 2013 Available online 8 December 2013
KEYWORDS
Masonry wastes; Recycled aggregates; Crushed clay bricks; Autoclave aerated concrete; Foamed concrete; Micro-structural analysis; TGA
Abstract The main objective of this research program is to study the effect of using crushed clay brick as an alternative aggregate in aerated concrete. Two series of mixtures were designed to investigate the physico-mechanical properties and micro-structural analysis of autoclave aerated concrete and foamed concrete, respectively. In each series, natural sand was replaced with crushed clay brick aggregate. In both series results showed a significant reduction in unit weight, thermal conductivity and sound attenuation coefficient while porosity has increased. Improvement on compressive strength of autoclave aerated concrete was observed at a percentage of 25% and 50% replacement, while in foamed concrete compressive strength gradually decreased by increasing crushed clay brick aggregate content. A comparatively uniform distribution of pore in case of foamed concrete with natural sand was observed by scanning electron microscope, while the pores were connected mostly and irregularly for mixes containing a percentage higher than 25% clay brick aggregate.
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1. Introduction
Crushed clay brick is not considered a recyclable material in the Middle East as opposed to recycled concrete aggregates which a few Arab countries began to produce for non-structural purposes. The first use of crushed brick with Portland cement was recorded in Germany (1860) for the manufacturing
* Corresponding author. Tel.: +20 1110595678.
E-mail address: abduo76@yahoo.com (A.-E. M. Abd-Elmoaty).
Peer review under responsibility of Faculty of Engineering, Alexandria
University.
of concrete products, but the first significant use of crushed brick as aggregates in new concrete has been recorded for reconstruction after the World War II [1]. A number of researches have been reported to evaluate the potential of using crushed clay bricks as an alternative aggregate. Most current researches use crushed clay brick as a coarse and/or fine aggregate in normal conventional concrete. Few researches reported that crushed brick powder, CBP, could be used as partial replacement of cement in concrete. Moriconi et al and Turanli et al [2,3] classified CBP as a pozzolanic material. Recycling crushed clay brick wastes needs more researches to make the maximum use of these wastes. Producing aerated concrete with crushed clay brick as an alternative aggregate will present solution for these recyclable wastes. The preparation of aerated concrete by incorporation of pozzolanic siliceous material received further attention because of the economical use of
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naturally occurring raw materials, waste material recycling and saved energy [4].
Nowadays aerated concrete is widely used because of its unique characteristics. It possesses high flowability, low self-weight, controlled low strength, excellent thermal insulation properties and fire resistance. Classification of aerated concrete based on the method of pore-formation can be summarized as air-entraining method (gas concrete), foaming method (foamed concrete) and combined method [4].
The first type of aerated concrete is autoclave aerated concrete (AAC) which is classified as a gas concrete produced by cement and lime as calcareous materials, and by quartz sand as the siliceous materials with traces of aluminum powder as a pore forming agent. After mixing these components with water, aluminum powder reacts with calcium hydroxide which liberates hydrogen gas and forms bubbles that lead to a porous structure concrete. Autoclave curing is a heat treatment which has been used to accelerate the strength development of concrete products. Because the hydration rate of cement increases with the temperature increase, the gain of strength can be speeded up by curing concrete in steam [5]. Several researchers have investigated the possibility of replacing the traditional raw materials of autoclave aerated concrete by industrial waste, such as coal bottom ash [5], natural zeolite [6], air-cooled slag [7], lead zinc tailings [8], iron ore tailings [9] and sand-phosphorus slag-lime [10].
The second type of aerated concrete is foamed concrete, FC, which has not involved chemical reactions. Introduction of pores is achieved through mechanical means either by preformed foaming (foaming agent mixed with a part of mixing water) or by mix foaming (foaming agent mixed with the mortar). A lot of studies have explored the use of alternative pozzo-lanic and/or siliceous materials in foamed concrete production. Fly ash and ground granulated blast furnace slag have been used in the range of 30-70% and 10-50%, respectively as cement replacement. Also, fly ash, lime, chalk, crushed concrete and recycled glass were used as alternative fine aggregates. All of these researches aimed to reduce the density of foamed concrete and/or to use waste recycled materials [11].
2. Research significance and scope
The major objective of this study is the utilization of clay brick aggregate and powder in producing cellular concrete. This study mainly focuses on exploring new suitable alternative siliceous materials for cellular concrete production. In this study, fine crushed clay brick aggregate is used as raw materials to prepare cellular concrete. It is considered as an alternative siliceous resource to reduce the consumption of sand and it is also expected to enhance mechanical properties for both autoclave aerated concrete and foamed concrete because of its recorded pozzolanic reactivity.
3. Experimental research program
3.1. Materials
Crushed clay brick wastes were manually crushed using a steel hammer, then screened and grouped to different sizes in accordance with ASTM C33 to comply sizes presented in Table 1. This grading was used for foamed concrete preparation.
Table 1 The properties of the used aggregates in foamed
concrete FC.
Properties Natural Recycled Limits
aggregate aggregate
Specific gravity 2.710 2.430 -
Fineness modulus 2.32 2.44 -
Absorption (%) 0.90 20.0 -
Particle size Percent passing <3/16"
distribution (mm)
4.75 (No. 4) 100 100 95-100
2.36 (No. 8) 100 100 80-100
1.18 (No. 16) 81 74 50-85
600 im (No. 30) 55 52 25-60
300 im (No. 50) 30 24 5-30
150 im (No. 100) 2 6 0-10
Additionally, the crushing process produces smaller aggregates. This dust-clay brick powder, CBP, was separated by 75 im mesh sieve. Clay brick powder was used as an alternate aggregate in autoclave aerated concrete mixture preparation.
Evaluation of the particles shape showed CBP grains to be a semi-oval shape and a semi-smooth surface. Fig. 1 shows the particle shape of clay brick powder grain shape.
Portland cement CEM I 42.5 N, lime with 83.9% CaO, natural sand and crushed clay brick with particle size up to 75 pm, aluminum powder with percentage of 94.2% of purity as a gas generating agent and potable water were used for producing autoclave aerated concrete, AAC. Physical, mechanical and chemical properties of cement and clay brick powder are given in Tables 2 and 3.
The used materials for foamed concrete, FC, were the same ordinary Portland cement CEM I 42.5 N used in AAC and natural siliceous sand with fineness modulus of 2.34. Clay brick aggregate was obtained by manual crushing as previously mentioned and Table 1 shows the physical properties and grading of either sand or fine crushed clay brick aggregate used in foamed concrete. The used doses of Type F chemical admixture for concrete mixes are 2.25% by weight of cement. Synthetic foaming agent has been used as a foaming agent during this section.
3.2. Mix proportions and sample preparation 3.2.1. Autoclave aerated concrete
Firstly, five different types of autoclave aerated concrete, AAC, were prepared by progressive incorporation of clay brick powder, CBP. Autoclave aerated concrete is always produced in a specialized factories. In these factories, the measure of the used materials is conducted by weight not by volume. Thus, in autoclave aerated concrete samples' preparation, the replacement by weight was respected as followed by earlier studies and practical applications. The replacement levels were 0%, 25%, 50%, 75% and 100%. Cement:fine aggregate:lime ratio (C:F:L) was chosen as 1:3:0.2 by weight. This ratio was chosen based on trials. The water to solid ratio was 0.6. Aluminum powder was added at 1.0% by weight of solid. Although previous researches used lower doses of aluminum powder, trial mixes in this research program showed that 1.0% aluminum
Table 2 Physical and mechanical properties of cement.
Test Test results Limits
Standard water required (w/c) 27% -
Initial setting (min) 157 P45a
Final setting (min) 381 6600a
Le chatelier expansion (mm) 4.0 610a
3 days mortar compressive strength (MPa) 21.5 P 18a
7 days mortar compressive strength (MPa) 27.5 P27a
Specific gravity 3.15
a In accordance with Egyptian code of practice number 203 issued 2009.
Table 3 Chemical composition of materials used.
Chemical composition% Cement Clay brick powder
Silicon dioxide (SiO2) 18.9 54.2
Iron oxide (Fe2O3) 3.1 7.6
Aluminum oxide (Al2O3) 5.1 15.4
Calcium oxide (CaO) 63.3 6.8
Magnesium oxide (MgO) 2.1 2.5
Sulfur trioxide (SO3) 3.2 1.1
Loss on ignition (LOI) 2.05 6.2
powder was the best dose for the desired density. Hydrogen gas generation depends on the fineness, source and purity of the used aluminum powder. The mixtures were mixed in a rotary mixer. After that, the mixtures were casted into cubic molds (70 \ 70 \ 70 mm). After casting; the specimens were subjected to two steps of curing cycles. At the first cycle, specimens were heated in furnace at 40 0C for 3 h to achieve the desired setting and volume stability and then the surfaces wirecut. At the second curing cycle, cubes were hydrothermally treated in an autoclave curing under 12 bar and 180:200 0C steam pressure for 18 h. Fig. 2 shows the specimens after the first cycle of curing.
3.2.2. Foamed concrete
The second studied cellular concrete is foamed concrete. Trial mixtures confirmed that high porosity and water absorption of the crushed clay brick affected badly on stability and workability of foamed concrete.
The pre-wetting of aggregates for 24 h was respected to avoid this problem. Fine aggregate cannot be fully soaked. Thus, the required water for absorption process was added
Figure 2 The specimens after the first cycle of curing For AAC.
to the aggregates and tightly covered with plastic sheets for 24 h before mixing.
Five different mixtures of foamed concrete, FC, were prepared by progressive incorporation of clay brick aggregate, CBA. The replacement by volume was considered for foamed concrete because it seems to be more appropriate for in-site applications, whereas foamed concrete is often used in-site applications.
The replacement levels were 0%, 25%, 50%, 75% and 100% by volume. Mixture proportion was designed according to ASTM C796-04.
Cement to sand ratio was 1:2. Water to solid ratio was 0.36. Super-plasticizer admixture Type F was chosen to reduce the water content which could enhance fresh and hardened properties. The determination of the best mixture proportion for foamed concrete is so sensitive. Thus, trial mixtures were conducted in order to choose the best fresh density. The control mixture was about 1100 kg/m3, with a variation of ±40 kg/ m3. This fresh density was chosen based on trials.
Pre-foaming method was applied in order to achieve the desired density. It comprises the production of a base mix and performing foam separately and then thoroughly blending foam into the base mix. Fig. 3 illustrates the used pre-foaming method for foamed concrete. The base mortar was prepared using 2/3 of the required total water. The rest third of water was used for foaming process. The foam was generated using foam generator and the average density of foam was 29.7 kg/ m3. The foaming agent to water dose was 5% by weight. Table 4 presents the mix proportions of foamed concrete.
Figure 3 Pre-foaming method diagram for foamed concrete. 3.3. Testing
For autoclave aerated concrete; compressive strength, porosity, sound attenuation coefficient, thermal conductivity, ther-mo-gravimetric analysis, X-ray diffraction analysis and micro-structural analysis were studied. For foamed concrete, compressive strength, splitting tensile strength, sound attenuation coefficient, thermal conductivity, porosity and microstructural analysis were studied.
Compressive strength, splitting tensile strength, ultrasonic pulse velocity and porosity were conducted according to the specifications given in Table 5.
The powder method of X-ray diffraction was adopted in the present study for the identification of the most probable phases of paste modified with clay brick powder. The X-ray tube voltage and current were fixed at 40 kV and 30 mA respectively.
Thermo-gravimetric analysis, TGA, was also conducted on autoclave aerated concrete samples. TGA uses heat to force reactions and physical changes in materials. It provides quantitative measurement of mass change in materials associated with thermal degradation.
Micro-structural evolutions of specimens were observed by scanning electron microscope, SEM, on gold-coated sections.
Thermal conductivity was measured by a simple technique for comparison only according to BS12664. In this method, two series of thermocouple were fixed on both sides of tested specimen as shown in Fig. 4. The thermal conductivity can be expressed as a function of DT where DTis the difference between hot side and cold side of specimen.
All specimens have the same dimensions = 150 \ 150 \ 20 mm. It was carefully wrapped with insulation in order to arrest any lateral heat transfer in the tested specimen. Dissipation of heat through the sides was not possible during the test. Each specimen was exposed to 5 min heating, then values of DT were measured for constant temperature T1 = 100 0C.
The porosity of cellular concrete was calculated using the difference between the whole volume of specimen and the powder volume of this specimen as given in Ref. [12].
The sound attenuation coefficient is determined by measuring the reduction in amplitude of an acoustic wave, which has travelled for a known distance through a material and is given by:-
a =(-20/X) log(Ax/Aa
Table 4 The mixture proportions of foamed concrete.
Mix no. Rep% Cement (kg) Water liter Aggregate SSD Admixture liter Density (kg/m3)
Sand (kg) CBA (kg) Theoretical Practical
1 0% 270 290 540 0 6.75 1106 1137
2 25% 270 290 405 121 6.75 1092 1061
3 50% 270 290 270 242 6.75 1078 1053
4 75% 270 290 135 363 6.75 1064 1027
5 100% 270 290 0 484 6.75 1050 1012
Table 5 The used tested specimens and age of testing.
Properties Dimensions of specimen Category Specifications and references Age of testing
Compressive strength 150 x 150 x 150 mm cube Foamed concrete BS 1881: Part 3 7, 28, 90 days
70 x 70 x 70 mm cube Autoclave aerated concrete ASTM 349-82 7 days
Splitting tensile strength Cylinder of 75 mm diameter Foamed concrete ASTM C 496 7, 28, 90 days
and 150 mm length
Ultrasonic pulse velocity 150 x 150 x 150 mm cube Foamed concrete BS 1881: Part 203 7, 28, 90 days
70 x 70 x 70 mm cube Autoclave aerated concrete 7 days
Porosity 70 x 70 x 70 mm cube Foamed concrete Ref. [12] 28 days
Autoclave aerated concrete 7 days
Thermal conductivity 150 x 150 x 20 mm Foamed concrete BS EN 12664:2001 28 days
Autoclave aerated concrete 7 days
Sound attenuation coefficient 150 x 150 x 150 mm cube Foamed concrete Ref. [13] 28 days
70 x 70 x 70 mm cube Autoclave aerated concrete 7 days
Figure 4 Thermocouple monitor and sample preparation.
where A0 is the initial amplitude of the wave and Ax is the amplitude after it has travelled for the distance X (in the presence of the tested specimen). The output wave amplitude (Ax) is the absolute peak voltage of the received signal, while the amplitude of the pulse entering the specimen was measured separately on a face-to-face configuration of the transducers [13] (see Fig. 5).
4. Test results of cellular concrete
0 25 50 75 100
%Repalcement ratio
Figure 6 Effect of clay brick powder content on autoclave aerated concrete compressive strength.
and 100% clay brick powder content compared to control mix, respectively. The pozzolanic characteristics of crushed brick powder may be the main cause of the compressive strength enhancement. This trend agrees with Kurama et al. [5].
4.1.2. Physical properties
4.1.2.1. Unit weight. The low unit weight of autoclave aerated concrete is considered one of the most unique characteristics. The oven-dry unit weight variation in autoclave aerated concrete specimens is graphically presented in Fig. 7. In this figure, the unit weight of autoclave aerated concrete decreases by
4.1. Autoclave aerated concrete test results 4.1.1. Compressive strength
Autoclave aerated concrete has lower mechanical properties compared with normal concrete. This paper focused on com-pressive strength property as an indication of the mechanical properties of the modified autoclave aerated concrete with crushed brick powder. The variations in the compressive strength of autoclave aerated concrete versus clay brick powder replacement ratio are presented in Fig. 6. Gradual com-pressive strength increase was obtained by increasing replacement ratio of clay brick powder up to 50%, whereas a significant reduction was observed at replacement level higher than 50%. The variations in the compressive strength are + 12.92, +20.22, -24.16 and -25.84% at 25%, 50%, 75%
25 50 75 % Recycled aggregate
Figure 7 Effect of clay brick powder content on autoclave aerated concrete unit weight.
(a) Experimental setup diagram. (b) The output amplitude
Figure 5 Sound attenuation experimental set up.
increasing the amount of clay brick powder. The reduction in dry unit weight as a result of using crushed brick powder as sand replacement is 8%, 10%, 18% and 23% for 25%, 50%, 75% and 100% clay brick powder content compared with control mix, respectively. This result could be attributed to the porous structure of clay brick aggregate compared to natural sand.
The measured ultrasonic pulse velocity values against clay brick powder substitution ratios are presented in Fig. 9. From this figure, it is obvious that the use of recycled aggregates decreases ultrasonic pulse velocity compared with control mix. The test results of ultrasonic pulse velocity confirm those of porosity where there is a direct relation between ultrasonic pulse velocity and porosity.
4.1.2.2. Porosity. The porosity of recycled aggregate itself, in addition to cracks created during the production process reflects directly on the global porosity of autoclave concrete. Fig. 8 shows the effect of clay brick powder content on concrete porosity. In this figure, the increase in clay brick powder content increases the autoclave aerated concrete porosity. The increase in porosity for varied aggregates contents is 7%, 10.6%, 10.9% and 12% for 25%, 50%, 75% and 100% sand replacement content compared with control mix, respectively.
4.1.2.3. Ultrasonic pulse velocity. The measuring of ultrasonic pulse velocity is a common technique employed for analyzing the porous structure of concrete to detect the internal defects (voids, cracks, etc.).
Solid materials transfer sound faster than porous materials do. The porous structure and relatively lower specific gravity of the clay brick powder than that of sand made ultrasonic pulse velocity values of the autoclave aerated concrete decreased by increasing of clay brick powder replacement ratio.
25 50 75 % Recycled aggregate
Figure 8 Effect of clay brick powder content on autoclave aerated concrete porosity.
4.1.2.4. Thermal conductivity. Fig. 10 shows the effect of aggregate replacement on values of AT for autoclave aerated concrete. In this figure, it is clear that the increase in the clay brick powder content increases autoclave aerated concrete AT as a result of lower unit weight of autoclave aerated concrete containing clay brick powder. The increase in AT for varied aggregates contents is 3.7%, 4.6%, 6.2%, 9% for 25%, 50%, 75% and 100% replacement level compared with control mix, respectively. Thus, it can be said that the incorporation of clay brick powder decreases the thermal conductivity of autoclave aerated concrete which lead to higher thermal resistivity of the new autoclave aerated concrete.
4.1.2.5. Sound attenuation coefficient. Sound attenuation coefficient values as a function of the substitution percentage of natural sand are graphically presented in Fig. 11. In this figure, it can be observed that, the sound attenuation coefficient of the autoclave aerated concrete decreases by increasing clay brick powder content. The decrease in sound attenuation coefficient for varied aggregate contents is 13%, 23%, 37%, 43% at 25%,
25 50 75
% Recycled aggregate
Figure 10 Effect of clay brick powder content on autoclave aerated concrete AT.
2300 a 2200
Cfl tj
U » I
2100 2000 1900 1800 1700 1600 1500
25 50 75
% Recycled aggregate
•S «
0.4 -|-
25 50 75
% Recycled aggregate
Figure 9 Effect of clay brick powder content on autoclave aerated concrete ultrasonic pulse velocity.
Figure 11 Effect of CBP content on autoclave aerated concrete attenuation coefficient.
50%, 75% and 100% replacement level compared to control mix, respectively.
Sound attenuation coefficient is significantly affected by unit weight. Exponential relationship is presented in Fig. 12. The concluded equation between unit weight kg/m3, W, and attenuation coefficient dB/mm, A, can be expressed as follows:
A = 0.023e°'°°3w (2)
4.1.3. XRD, TGA and micro-structural analysis 4.1.3.1. X-ray diffraction analysis. Although X-ray qualitative diffractometry does not provide any reliable quantitative information, it is considered a sensitive technique which gives acceptable information about the most probable phases. In all autoclave aerated concrete specimens diffraction peaks indicate the presence of quartz, calcite, albite and calcium silicate hydrate. In cement paste test specimen diffraction peaks indicate the presence of calcite, ettringite, quartz, calcium silicate hydrate and portlandite.
No harmful compounds were detected in all samples as shown in Figs. 13 and 14. Notably, Neither portlandite (calcium hydroxide) nor ettringite were detected in all autoclave aerated concrete specimen in contrast to cement paste. Port-landite may be completely consumed during the reactions at the first and the second curing cycles previously mentioned. A dehydration of ettringite may have occurred as a result of high temperature during the second cycle of curing [14].
o S 0.20
S 0.15 0.10
500 600 700 800
Unit weight (kg/m3)
Figure 12 Relationship between unit weight and attenuation coefficient.
Counts/S
80 70 60 50 40 30 20 10 0
Figure 14 X-ray diffraction patterns of cement paste of the used cement. C: calcite & E: Ettringite & Q: quartz & CSH: calcium silicate hydrate & P: Portlandite.
4.1.3.2. Thermo-gravimetric analysis. Fig. 15 shows the relation between the relative weight of specimen to the original weight of sample (w/w original) and applied temperature. According to this relation, the total weight loss at 1000 0C was 8.94%, 13.03%, and 12.61% for the specimens containing 0%, 50% and 100% clay brick powder, respectively. It can be observed that the incorporation of clay brick powder increases the weight loss of autoclave aerated concrete. It may be attributed to the higher loss on ignition of clay brick powder than sand.
Fig. 16 shows the relation between the relative weight of specimen to the original weight of sample (w/w original) and applied temperature for two samples, cement paste and autoclave aerated concrete containing 100% clay brick powder, in order to present a significant comparison between the behavior of both samples under high temperatures.
In this figure, the total weight loss at 1000 0C was 29% and 12.61% for cement paste and autoclave aerated concrete, respectively. It means that autoclave aerated concrete has lower weight loss under high temperatures than cement paste. Moreover, at zone one between 100 0C and 300 0C which is attributed to the dehydration of C-S-H and ettringite, a significant reduction is observed in case of cement paste. In contrast to autoclave aerated concrete no significant reduction was observed within this zone. This result confirms the aforementioned results of X-ray diffraction analysis which indicates
Position(2* Theta)
Postion(2* Theta)
Figure 13 X-ray diffraction patterns of autoclave aerated concrete contain different clay brick powder contents. C: calcite
- Control
-50% CBP
-100% CBP
Q 1.00
Q - , i A C+CSH - » A Q+CSH Q+A Q+A Q+C+A . . Q+A C C+A . 100%
Q 1 Q CBP al n 2 0.95
C+CSH A Q+A Q+A Q+A A Q+C+A C+A Q+A 50% or £
Q CBP f 0.90
Q C+CSH A A Q+C+A Q+A Q+A Q+A C+A Q 0%
0 80 160 240 320 400 480 560 640 720 800 880 960 Temperature oC
Figure 15 Thermo-gravimetric analysis curves for autoclave
& A: Albite & Q: quartz & CSH: calcium silicate hydrate phases. aerated concrete.
- Cement paste —A— Autoclave aerated concrete
0.7 +TTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTT"ri
0 80 160 240 320 400 480 560 640 720 800 880 960 Temperature °C
Figure 16 Comparison between autoclave aerated concrete and cement paste under TGA.
Figure 18 Micro-pore structure shape for 0% clay brick powder.
the absence of ettringite. In the third zone, ranging from 450 to 510 0C which was attributed to the dehydration of calcium hydroxide, sudden weight loss drops easily observed within the third zone in the cement paste. For autoclave aerated concrete, almost the weight is constant within zone between 450 and 510 0C which also confirms the results of X-ray analysis whereas no calcium hydroxide has been detected in case of autoclave aerated concrete while in cement paste portlandite is considered the most probable phase.
4.1.3.3. Micro-structural analysis. The material structure of autoclave aerated concrete is characterized by its solid micro-porous matrix and macro-pores. Fig. 17 presents an example of macro-pores in autoclave aerated concrete specimen. The macro-pores generated by hydrogen are shown in generally envisaged diameter of more than 60 pm. The macro-pore shape and size remained unchanged before and after autoclav-ing. The micro-pore distributions are very sensitive to the products formed by hydrothermal [4,5]. SEM micrographs presented in Figs. 18 and 19 illustrate that the samples with 50% clay brick powder possess more refined and denser microstructure than reference specimen.
The SEM of autoclave aerated concrete modified with 100% clay brick powder shows a modification of CSH to a
Figure 19 Micro-pore structure shape for 50% clay brick powder sample.
semi-crystalline structure as shown in Fig. 20. This behavior may be due to the reduction in silica content and increase in calcium oxide as reported by Mostafa [7].
4.2. Foamed concrete results
4.2.1. Mechanical properties
4.2.1.1. Compressive Strength. Fig. 21 shows the effect of using clay brick aggregate as sand replacement on compressive strength of foamed concrete. In this figure, it is clearly observed that the use of clay brick aggregate has very noticeable negative effect at the early age of 7 days. This negative effect decreases with the time increase. The enhancement of compres-sive strength with the time increase may be due to the pozzo-lanic effect of clay brick.
As an example, the reduction in concrete compressive strength after 90 days is 3%, 20%, 29% and 36% at 25%, 50%, 75% and 100% replacement level compared with control mix, respectively.
4.2.1.2. Splitting tensile strength. The results of splitting tensile strength are graphically presented in Fig. 22. In early ages, no significant variation was observed due to the weakness of the
Figure 17 Macro-pore structure shape at 100% clay brick powder.
25 50 75
%Repalcement ratio
Figure 21 Effect of clay brick aggregate content on foamed concrete compressive strength.
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
1 1 1 1
! 1 1 1
— ft = 0.205 fcu - 0.109 R= 0.90 "I 1 - m X ■
■ y/jk
1 Wr 1
■ 1 1
0.0 1.0 2.0 3.0
Compressive strength (Mpa)
Figure 23 Relationship between compressive strength and splitting tensile strength.
-7 days
-28 days
-90 days
£ 0.50
J= 0.40
n e 0.30
le 0.20
fl e 0.10
......-W i i Xrrr^SE^r^_________J__________
1-- 1 ^V/S^ 1
I fr .......... I ...............
i i 1 1
- -<>-- 1
1 —?-
1 1 1 1
25 50 75
%Repalcement ratio
Figure 22 Effect of clay brick aggregate content on foamed concrete tensile strength.
foamed mortar. Specimens that contain 25% and 50% clay brick aggregate have higher splitting tensile strength at 28, 90 day compared to the control mix. This enhancement may be due to the pozzolanic effect of clay brick powder, which enhances the mechanical properties. As an example, the change in concrete splitting tensile strength after 90 days is +10%, + 1%, 22% and 33% at 25%, 50%, 75% and 100% replacement level compared to control mix, respectively.
The relation between the compressive strength and splitting tensile strength for all mixtures is shown in Fig. 23.
The correlation between compressive strength and splitting tensile strength of foamed concrete using the best fitting can be expressed as follows:
ft = 0.205fc - 0.109
where fc is the compressive strength in MPa for values from 1 MPa to 3.2 MPa and ft is the splitting tensile strength in MPa. It is clear that the compressive strength of concrete is proportional to its splitting tensile strength; the higher the compressive strength, the higher the splitting tensile strength.
4.2.2. Physical properties
4.2.2.1. Porosity. The strength of concrete is influenced by the volume of all voids in the concrete (entrapped air, capillary pores, gel pores and entrained air) and other parameters. Baozhen and Erda [15] concluded that the compressive strength of foamed concrete decreases as the porosity increases.
The use of clay brick aggregate as an alternative aggregate decreases the unit weight of the foamed concrete due to the porous structure and relatively lower specific gravity of the clay brick aggregate than that of sand.
The porosity of clay brick aggregate itself reflects directly on the global porosity of foamed concrete. Fig. 24 shows the effect of clay brick aggregate on the foamed concrete porosity. Generally, in this figure, the increase in clay brick recycled aggregate increases the resulting porosity. For example the increase in porosity at 90 days was 5.9%, 2.5%, 8.2% and 9.0% for foamed concrete at 25%, 50%, 75% and 100% aggregates compared with control mix, respectively.
The correlation between the compressive strength and porosity is shown in Fig. 25. It indicates the importance of
80.0 70.0 60.0 -50.0 -40.0 -30.0 -20.0 -10.0
25 50 75
% Recycled aggregate
Figure 24 Effect of CBA content on foamed concrete porosity.
69 68 67 66 65 64 63 62
- P = -4.959 fcu + 79.16 - R = 0.91
1 1 ■
1 1 1 1 --1-1- -
1.6 2.0 2.4 2.8
Compressive strength (Mpa)
Figure 25 Relationship between compressive strength and porosity.
the media porosity on the mechanical performance. If a linear relationship is fitted to all data, the following equation will be obtained:
P = -4.96/c + 79.16
where fc is the compressive strength in MPa and P is the concrete porosity.
4.2.2.2. Ultrasonic pulse velocity. Due to the porous structure of crushed brick aggregate, It is obvious that the increase in recycled aggregate percentage in concrete decreases ultrasonic pulse velocity compared to concrete without recycled aggregates. Fig. 26 indicates that ultrasonic pulse velocity values of the foamed concrete decrease with the increase in clay brick aggregate replacement ratio.
For example the reduction in the ultrasonic pulse velocity at 28 days was 2.82%, 6.33%, 17.84% and 20.14% for the specimen containing 25%, 50%, 75% and 100% clay brick aggregate replacement compared to control mix, respectively.
4.2.2.3. Thermal conductivity. Foamed concrete has good thermal insulating properties. The thermal conductivity of foamed concrete of density 1000 kg/m3 is reported to be one-sixth the value of typical cement-sand mortar. Insulation is inversely proportional to density of concrete. A decrease in concrete dry density by 100 kg/m3 results in a reduction in thermal conductivity by 0.04 W/mK of lightweight aggregate foam concrete as reported by Baozhen and Edra [15]
2700 2600 2500 2400 2300 2200 2100 2000 1900 1800
—O— 7 days —■— 28 days —is— 90 days
i i i i
^^^ ' i
1 1 I ^
-----------1---------- ----------T------^^J
25 50 75
% Recycled aggregate
Figure 26 Effect of CBA content on foamed concrete ultrasonic pulse velocity.
The thermal conductivity also depends upon the pore structure of the lightweight concrete. The thermal conductivity results as a function of DT of foamed concrete specimens is shown in Fig. 27. The replacement of clay brick aggregate has significant affect on the thermal conductivity of the foamed concrete. As DT increases thermal conductivity decreases. For example the increase in DT values was 1%, 2%, 4% and 8% for the specimen containing 25%, 50%, 75% and 100% clay brick aggregate content compared with control mix, respectively.
4.2.2.4. Sound attenuation coefficient. Fig. 28 shows the effect of clay brick aggregate content on the attenuation coefficient
25 50 75
% Recycled aggregate
Figure 27 Effect of CBA content on foamed concrete DT.
25 50 75
% Recycled aggregate
Figure 28 Effect of CBA content on foamed concrete attenuation coefficient.
Figure 29 Pore distribution of control mix foamed concrete.
Figure 30 Pore distribution of 50% CBA foamed concrete.
of foamed concrete. Based upon the aforementioned results of autoclave foamed concrete, the sound attenuation coefficient of the cellular concrete decreases by increasing clay brick aggregate content. The decrease in sound attenuation coefficient is 6.3%, 18.2%, 39.5% and 36% for the specimen containing 25%, 50%, 75% and 100% clay brick aggregate content compared with control mix, respectively.
4.2.3. Micro-structural analysis
The strength of cellular concrete is influenced by the volume and the distribution of voids. Fracture surfaces of the specimens viewed through a scanning electronic microscope are presented in Figs. 29-31. A comparatively uniform distribution of pores easily observed in the case of foamed concrete with natural sand, while the pores are connected mostly and irregularly for mixes containing higher than 25% clay brick aggregate content. This indicates that high contents of clay brick aggregate cause clustering of bubbles to form irregular pores. This trend meets with Narayanan and Ramamurthy [16].
5. Conclusions
Based on the work undertaken here, the following conclusions may be drawn:-
5.1. Autoclave aerated concrete
• The pozzolanic characteristics of crushed brick powder may be the main cause of the compressive strength enhancement especially at 50% replacement ratio.
• Unit weight and thermal conductivity of autoclave aerated concrete decrease by increasing crushed clay brick content while porosity increase.
• The incorporation of clay brick powder has no effect on the qualitative hydration products and no harmful compounds were detected by X-ray diffraction analysis.
• Hydrogen gas generation reaction and curing by autoclave may cause the absence of portlandite in the most probable phases detected by X-ray diffraction analysis. whereas, these reactions need calcium hydroxide to complete.
• Ettringite dehydrates under high curing temperature of autoclave.
5.2. Foamed concrete
• Pre-wetting for crushed clay brick aggregate is recommended for foamed concrete mixes to enhance workability and volume stability.
• The incorporation of 25% clay brick aggregate almost has no significant effect on compressive strength, especially at prolonged curing.
• Increase crushed clay brick content increases the porosity as result of porous structure of recycled aggregate.
• Incorporation of clay brick aggregate enhances the splitting tensile strength of foamed concrete at 25% and 50% replacement percentage.
• Based on micro-structural analysis and bad volume stability of foamed concrete containing high contents of crushed clay brick. Clay brick aggregate content should not exceed 25% of total aggregate content.
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