analysis

J. Ind. Eng. Chem., Vol. 13, No. 7, (2007) 1103-1108

Analysis of Microstructure and Properties of Autoclaved Aerated

Concrete Wall Construction Materials

Yothin Ungkoon, Chadchart Sittipunt*, Pichai Namprakai, Wanvisa Jetipattaranat**,

Kyo-Seon Kim***, and Tawatchai Charinpanitkul****,†

Division of Energy Technology, School of Energy Environment and Materials, King Mongkut’s University of

Technology Thonburi, Bangmod, Rasburana, 126 Pracha U-thit Rd., Thungkru, Bangkok 10140, Thailand

*Department of Civil Engineering, Faculty of Engineering, Chulalongkorn University, Phayathai Rd., Phathumwan,

Bangkok 10330, Thailand

**Research Development Center, Superblock Public Co., LTD, 9/1 Moo 11, Singburi Pak Dong Rd., Bangrachan, Kai

Bangrachan, Singburi 16150, Thailand

***Department of Chemical Engineering, Faculty of Engineering, Kangwon National University, Chuncheon 200-701,

Korea

****Center of Excellence in Particle Technology, Faculty of Engineering, Chulalongkorn University, Phayathai Rd.,

Phathumwan, Bangkok 10330, Thailand

Received July 11, 2007; Accepted November 9, 2007

Abstract: Microstructure analyses in non-autoclaved aerated concrete (Non-AAC) and autoclaved aerated con-crete (AAC) were conducted using optical microscopic and scanning electron microscopic (SEM) methods, while their chemical analyses were also carried out using X-ray diffraction (XRD). Laboratory investigation in-cluding physiothermal properties and field tests using four lab-scale houses built with the Non-AAC and AAC blocks were also compared. It could be observed that the AAC exhibited properties superior to that of the Non-AAC. Interestingly, the AAC blocks could provide lower humidity accumulation than the Non-AAC blocks, resulting in possibility to prevent mold growth in such construction materials.

Keywords: autoclaved aerated concrete, microstructure, tobermorite

Introduction1)

At present, construction works, such as high-rise build-ings or offices and residential houses, in many countries including Thailand are growing very fast every year. Concrete has mainly been used as fundamental con-struction material for most of residential building be-cause of its outstanding mechanical properties, low cost and availability. However, structure and foundation of buildings tend to become larger due to an increase in their scale, leading to much more time-consumption and cost. In monsoon region including the Southeastern Asian countries, the ambient condition is hot and humid so that

†To whom all correspondence should be addressed.

(e-mail: [email protected])

accumulation of heat and moisture in building wall plays an important role in its maintenance and energy conservation. As a result, ventilating fans and air-condi-tioners have been employed to remove heat for providing comfortable environment for residents. Meanwhile, both the economic and energy crisis has stimulated awareness of energy conservation, resulting in a drastic increase in studies on construction material which incorporates en-ergy conservation. It should be noted that an approach which has been used in Civil Engineering works is the usage of construction materials as an insulator to prevent heat transfer from outsides of buildings. Khedari and coworkers reported that internal structure of concrete consisting of coconut and durian fibers mixed with ce-ment and sand at various proportions could exhibit light weight characteristics [1,2]. They revealed that low den-sity and thermal conductivity of blocks consisting of ad-

Yothin Ungkoon, Chadchart Sittipunt, Pichai Namprakai, Wanvisa Jetipattaranat, Kyo-Seon Kim, and Tawatchai Charinpanitkul1104

ditive fibers could help prevent heat transfer into building. However, because of their low compressive strength, they could only be recommended to use for non-load bearing concrete masonry units. John and coworkers con-ducted an assessment of degradation on a wall panel composite made of low alkaline, clinker free, activated slag cement reinforced with coconut fibers [3]. Though the low alkaline cement was not able to prevent decom-position and leaching of lignin contained in the fibers, the leaching of lignin exerted insignificant effect on the wall performances. Meanwhile, various studies on non-autoclaved and autoclaved light weight concrete which consisted of cement and some other additives, such as ash, zeolite or polystyrene foam were also con-ducted to elucidate their structures and engineering prop-erties [4-6]. For instance, effect of polymer cement modi-fiers on mechanical and physical properties of mortar us-ing waste concrete fine aggregate was investigated and found that porosity of the mortar could be increased by addition of higher contents of polyacrylic ester (PAE) modifier [7]. Such polymeric modifier could lead to en-hanced performance in hot water resistance and higher compressive strength but worse flexural strength [8]. There are two types of autoclaved aerated concrete (AAC) production methods which are chemical and me-chanical processes. In the chemical process, some metal-lic compounds would be added to react and generate tre-mendous amount of air bubbles in concrete texture while in mechanical process expansive foaming agent is nor-mally employed. In general, AAC could be prepared in a high pressure autoclave under conditions of temperature and pressure higher than 180 oC and 12 bar, respectively [5]. Approximately the porosity is 80 % of the volume of the processed cement, resulting in its very light weight. Additionally, AAC has excellent properties of acoustic insulation, fire resistance and allergy-free while it tends to suffer edge damage or breakage if it is subject to abra-sion or collision. In order to produce more promising AAC, various researches on its physical structure have been initiated using microscopic analyses and X-ray dif-fraction (XRD) to investigate their chemical and struc-tural characteristics [9-13]. To our knowledge, there are very few systematic inves-tigations on thermal and physical properties of AAC in Thailand. So far, only some comparisons of engineering and thermal properties of clay bricks, concrete blocks, AAC and Non-AAC have been reported [14,15]. Studies on thermal inertia of concrete blocks by Ropelewski and coworkers [16] and Ungkoon and coworkers [17] are in-volving with heat transfer by radiation to buildings made of AAC and Non-AAC. It was found that Non-AAC blocks with higher density are likely to be able to transfer heat faster than the lighter AAC and in turn lead to short-er delay time in heat transmission. Ungkoon and cow-

orkers has also conducted a study on moisture and ther-mal resistance of building walls made of AAC blocks and clay bricks [18]. Their experimental results showed that the wall made of AAC could resist moisture and re-duce heat transfer due to its inner porous structure. However, there is insufficient understanding of micro-structure which could affect on thermal properties of AAC. Therefore, this work sets its aim at examining the porous structure of both AAC and Non-AAC in order to elucidate the formation process of their microstructures using optical microscopy and scanning electron micro-scopy (SEM). Also chemical and structural analyses are conducted using X-ray diffraction (XRD) to Figure out the effect of their composition on their thermal properties. Comparison of thermal and mechanical proper-ties of AAC and Non-AAC has also been reported.

Materials and Method

Specimens of AAC were collected from a manufacturer which has been certified by Thailand Industrial Standard (TIS) 1505-2541. 120 pieces of AAC blocks with a uni-form size of 200 × 600 × 75 mm were randomly sampled for investigation. Meanwhile, the Non-AAC blocked of the same size and amount manufactured at the same fac-tory were sampled for comparison. Both AAC and Non- AAC specimens were subjected to tests as follows; 1) Thailand Industrial Standard (TIS) 1505-2541 and Din 4165-1986 for determining dry density, compressive strength, and flexural strength, 2) JIS A 2618 standard for examining thermal conductivity using Thermal Con- ductivity Tester (model 88 K-FOTOR, ANACON), 3) DSC analysis for determining specific heat capacity us-ing a differential scanning calorimeter (Model DSC 7), 4) microscopic analysis using scanning electron micro-scopy (SEM, Model JEOL JSM 5800 equipped with Energy Dispersive Spectroscope), and 5) chemical analy-sis using XRD (X-ray Diffractometer, model Bruker D8).

Results and Discussion

In general, it is known that tobermorite with ortho-rhombic structure could be stable with Ca/Si ratio of 0.8∼1.0 and temperature of ambient condition up to 150 oC. It generally coexists within tetrahedral silicate layers and octahedral calcium layers [17]. With hydrothermal reaction between SiO2 and Ca(OH)2 a system of Ca- lcium-Silicate-Hydrate (so-called C-S-H system) could be formed as slurry phase by following a two-step process. In the first step, calcium silicate hydrate gel (C-S-H gel) will be generated on the surface of SiO2 and then react with Ca(OH)2 to form well-organized crystal-

Analysis of Microstructure and Properties of Autoclaved Aerated Concrete Wall Construction Materials 1105

Figure 1. Model of Tobermorite structure consisting of proto-

nated silicate ions and water.

line [19]. In the second step, curing of crystalline product would be taken into account. There are two alternatives of curing process which could provide specimen of dif-ferent properties. The first curing process making use of air to cure specimens could create hydrogarnet crystalline of which the structure would strongly depend on curing time period. The other is an autoclave curing process in which high pressure and temperature would be applied to the specimens. At a pressure of 10∼12 bars and a tem-perature of 180∼190 oC, tobermorite could be formed in the specimens. As depicted in Figure 1, the tobermorite structure model consists of Ca2+ ions which are entrapped in the protonated silicate ions [9]. Therefore, the to-bermorite crystal is stable to carbonation reaction under the condition of ambient temperature and the pressure but it will be disintegrated at the temperature higher than 650 oC. From the SEM investigation on the surface morphology of the two typical samples of light weight AAC and Non-

AAC blocks, it could clearly be observed that the surface of Non-ACC sample consisted of large pores while the ACC sample exhibited smoother surface with much smaller porosity as shown in Figure 2. For further investigation on the porous structure of typi-cal samples of Non-AAC and AAC, SEM micrographs with higher magnification shown in Figure 3 are taken into account. In Figure 3(a) Non-AAC samples consist of particulate clusters of evenly distributed sizes, which sit-uate around craters generated by the existence of uncon-trollable air bubbles in curing process. It could further be observed that the crystallinity of the Non-AAC was re-sulted from random formation of gas bubbles, leading to formation of cavities with various depths. Also seen in Figure 3(a), some cracks observed on the Non-AAC sur-face would possibly lead to a decrease in the mechanical strength because they could become sources of fatigue growth. On the other hand, Figure 3(b) revealed the sam-ples of AAC exhibit smoother surface with some poros-ity and particulate of narrower size. It is notable that there are no cracks observed on the surface of denser AAC samples. This could be implied that under high pressure and temperature in autoclaving process, denser phase of C-S-H could be formed. For further investigation of their crystallinity, SEM mi-crographs with further higher magnification were employed. Figure 4(a) clearly reveals that Non-AAC sam-ples are comprised of particulate calcium silicate of un-evenly distributed morphologies. Primary particles exist-ing in the Non-AAC samples exhibit average size of 200∼400 nm. Sub-micron particulates gathering as agglom-erates with average size of 5 µm could form loose matrix surrounding by voidage, resulting in lower density and thermal conductivity of the Non-AAC samples. Howev- er, considering Figure 4(b), one could clearly observe

(a) (b)

Figure 2. SEM micrographs of non-AAC and AAC specimens.


(a) (b)

Figure 3. Comparison of surface morphology of Non-AAC and AAC specimens.

(a) (b)

Figure 4. Comparison of crystalline structure of Non-AAC and AAC specimens.

Figure 5. XRD patterns of (a) Non-AAC and (b) AAC speci-

mens (Q: quartz , T: tobermorite and C: calcite).

that finer needle-like crystalline structures exist thor-oughly the surface of AAC samples. The average size of

sub-micron needle crystal is ca. 400 nm with aspect ratio of 20∼30. Tremendous amount of finer porosity within the matrix of fine particulate would also result in much lower specific density of AAC samples. In order to con-firm the crystal structure of both Non-AAC and AAC samples, XRD analysis was also carried out. XRD pat-terns in Figure 5 reveals that Non-AAC samples mainly consist of calcite and quartz. Meanwhile, in AAC sample main product is crystalline in as much as tobermorite and quartz are present. With a high pressure of 12 bar and temperature of 180∼200 oC, calcite in the tested speci-mens could undergo phase transformation process to be-come tobermorite. Such experimental results are in a good agreement with those previous reports [9-11]. Difference in such crystal morphologies and phases in Non-AAC and AAC samples are supposed to exert sig-nificant effects on not only mechanical but also thermal characteristics of the samples. Accordingly, 250 pieces of Non-AAC and AAC samples were randomly collected

Analysis of Microstructure and Properties of Autoclaved Aerated Concrete Wall Construction Materials 1107

Table 1. Mechanical and Thermal Characteristic Test of Non-AAC and AAC Samples

Characteristics of sample testedNon‐AAC AAC

Min Max Avg. Min Max Avg.

Density (kg/m3) 733 750 741 573 577 575

Compressive strength (MPa.) 1.6 1.7 1.6 3.7 4.8 4.3

Flexural Strength (MPa.) 0.55 0.64 0.59 0.90 1.23 1.06

Thermal conductivity (W/mºC) 0.179 0.189 0.184 0.132 0.135 0.133

Heat capacity (J/kgK.) 1,303 1,595 1,449 1,193 1,256 1,224

for the statistical test of their mechanical and thermal characteristics. Table 1 summarizes the mechanical and thermal investigating results. It could be clearly observed that AAC samples exhibit specific density almost 20 per- cent lower than that of Non-AAC. Meanwhile, average compressive and flexural strength of AAC samples are 2.7 and 1.8 times of those of Non-AAC, respectively. These superior mechanical properties of AAC could be attributed to the present of higher crystallinity of to-bermorite phases in AAC samples [13,15]. On the other hand, it could also be seen that average thermal con-ductivity and heat capacity of AAC samples are sig-nificantly lower than those of Non-AAC. As could be confirmed by microscopic analyses, the existence of mi-cro-sized porosity in AAC samples would lead to higher insulating performance. Moreover, it has been reported that AAC could also exhibit lower drying shrinkage when compared with Non-AAC [9]. Finally, field tests using four lab-scale houses built with the Non-AAC and AAC blocks were also carried out. Details of field test facilities are available elsewhere [18]. It could be observed that walls built with the AAC blocks could exhibit insulating and humidity adsorbing perform-ance superior to that of a wall built with the Non-AAC blocks. Temperature in the houses built with the AAC blocks is significantly lower than that in the Non-AAC houses. Additionally, the AAC blocks could also provide lower humidity accumulation than the Non- AAC blocks, resulting in possibility to prevent mold growth in such construction materials. In summary, it could be confirmed experimentally that suitable treatment of aerated concrete specimens using autoclaving method could enhance phase transformation of calcite to tobermorite and control formation of micro-porosity in the cement matrix. The existence of crystal-line tobermorite phase and microporosity, which are jus-tified by microscopic and spectroscopic analyses, could enhance both mechanical and thermal properties of AAC specimens compared with those of Non-AAC. Based on these investigating results it is reasonable to recommend that AAC could be more suitable to apply as wall con-struction material which exerts superior mechanical strength with better insulating performance.

Conclusion

Under a condition of high pressure and temperature treatment, calcite in aerated cement specimens could un-dergo phase transformation process to become tobermor-ite, which is more stable than former phase. High pres-sure autoclaving could also control formation of evenly distributed micropores within the cement matrix. The ex-istence of tobermorite phase in AAC plays an important role in providing various outstanding properties. With autoclaved aerating process, AAC sample exhibits lower density than that of Non-AAC. Additionally, AAC could exhibit superior compressive strength and thermal in-sulating properties. With a series of experiments it could be confirmed AAC could be employed as a promising concrete building material.

Acknowledgments

The authors gratefully acknowledge the Superblock (public) Co., Ltd. for financial support to this research work. T. C. would gratefully acknowledge financial sup-port from Silver Jubilee Fund of Chulalongkorn University for CEPT. Support from the Ministry of Education and Human Resources Development (MOE) and the Ministry of Commerce, Industry and Energy (MOCIE) through the fostering project of the Industrial- Academic Cooperation Centered University is also acknowledged.

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