Introducing a Method to Determine Nonautoclaved Aerated Concrete Air content Based on Packing Theory Downloaded from ascelibrary.org by Ali Akbar Shirzadi Javid on 12/29/17. Copyright ASCE. For personal use only; all rights reserved. Mohsen Mohammadi 1; Ali Akbar Shirzadi Javid 2; and Mehdi Divandari 3 Abstract: This paper presents an experimental study on fresh and hardened properties of nonautoclaved aerated concrete (NAAC) mixtures. It also attempts to address a new method to determine the packing density and air percentage of NAAC mixtures. Different types of NAAC mixtures with various aluminum powder percentages and water-to-cement ratios were made. Results revealed that the new method (called the wet packing theory) is able to determine the percentage of air pores in the aerated concrete with high accuracy. Results also showed that there is an optimum aluminum percentage of approximately 0.0934 from the perspective of minislump diameter and compressive strength. With an increase in the aluminum percentage compared to the optimum percentage, minislump (workability and flow-ability) and compressive strength reduced. Scanning electron microscope (SEM) images of NAAC indicated that the air voids were shaped as an artificial porosity. The results showed that with an increase in the amount of aluminum powder, packing density reduced. It was observed that the maximum air percentage increased with an increase in the amount of aluminum percentage from 0.0934 to 0.1869. DOI: 10.1061/(ASCE)MT.19435533.0002180. © 2017 American Society of Civil Engineers. Author keywords: Nonautoclaved aerated concrete; Air percent; Packing theory; Aluminum powder. Introduction Aerated concrete refers to concrete having a high value of pores and air voids. These air bubbles are produced to reduce the density of the mixture and provide good thermal insulation. Aerated concrete was first presented in the 1930s and was referred to as foamed, cellular, or gas concrete (Neville 1981; Holt and Raivio 2005; Elhaddad 1993; Ng et al. 2012). Various uses of lightweight concrete are well known all over the world. It is proposed that the reduction of concrete’s self-weight is the best option to reduce the dead load of structure and eventually reduces the dimension of structural elements. Aerated concrete is classified into two categories: autoclaved aerated concrete (AAC) and nonautoclaved aerated concrete (NAAC). Most research studies have been carried out on the autoclaved category. The aerated concrete manufacturing process involves the creation of macroporosity in a micromortar matrix made of sand, lime, cement, and water in the presence of an expansive admixture (Cabrillac et al. 2006; Narayanan and Ramamurth 2000a). In this type of concrete, aluminum powder reacts with the lime (released by the hydration of the binder) and water (Holt and Raivio 2005; Wittman 1983; Xia et al. 2013; Mostafa 2005; Cenk et al. 2010). The air voids generated by this chemical reaction cause the fresh mortar to expand and lead to an increase in air pores (Narayanan and Romamurthy 2000b). 1 Research Assistant, School of Metallurgy and Materials Engineering, Iran Univ. of Science and Technology, P.O. Box 16765-163, Narmak, Tehran, Iran. E-mail: [email protected] 2 Assistant Professor, School of Civil Engineering, Iran Univ. of Science and Technology, P.O. Box 16765-163, Narmak, Tehran, Iran (corresponding author). E-mail: [email protected] 3 Associate Professor, School of Metallurgy and Materials Engineering, Iran Univ. of Science and Technology, P.O. Box 16765-163, Narmak, Tehran, Iran. E-mail: [email protected] Note. This manuscript was submitted on December 1, 2016; approved on August 31, 2017; published online on December 29, 2017. Discussion period open until May 29, 2018; separate discussions must be submitted for individual papers. This paper is part of the Journal of Materials in Civil Engineering, © ASCE, ISSN 0899-1561. © ASCE The aluminum reacts with calcium hydroxide or alkali, which releases hydrogen gas and forms air bubbles, as shown in Eq. (1) (PFU 1972) as follows: 3CaðOHÞ2 ðaqÞðsÞ þ 2AlðsÞ þ 6H2 OðlÞ → 3CaO · Al2 O3 .6H2 OðlÞ þ 3H2 ðgÞ ð1Þ Holt and Raivio (2005) confirmed that adjusting the reaction speed and duration of the aluminum reactions is important. Therefore, it is necessary to make sure that the pore formation ends up coinciding with the initiation of AAC hardening. Hence, the adjustment of aluminum powder and water content should control the macropores’ size and distribution (Boesten 2012). Literature classifies the air pore system of aerated concrete with the aim of studying the water transport theory as artificial air pores, intercluster pores, and interparticle pores (air pore diameters in aerated concrete are usually 0.1–1 mm). The first general review on aerated concrete was reported by Valore (1954a, b), Rudnai (1963), and Short and Kinniburgh (1963). Studies have shown that lightweight concrete has a great potential as a structural material. Furthermore, performance and durability are other appropriate features of this concrete (Narayanan and Ramamurthy 2000a; Shamsuddoha et al. 2011; Alexanderson 1979; Scheffler and Paolo 2005; Esmaily and Nuranian 2012; Schwartz et al. 1998; Schapovlov 1994). In addition, Panesar (2013) proved that an increase in air pore percentage results in the reduction of compressive strength and elastic modulus after 7 and 28 days. Recent research studies also showed that in order to create uniform pore distribution in aerated concrete, the aluminum powder size should be enlarged. It is evident from the results that the amount of aluminum particles characterizes the percentage of overlapped pores and its distribution (Liu et al. 2013). The basic issue in this study is to calculate the percentage of air in the AAC or NAAC. The usual method commonly used in ordinary concrete [ASTM C231 (ASTM 2014)] cannot accurately determine air content of more than 7%. Therefore, there is no study that focused on the air void content measurement in the fresh aerated concretes 04017312-1 J. Mater. Civ. Eng., 2018, 30(3): 04017312 J. Mater. Civ. Eng. Downloaded from ascelibrary.org by Ali Akbar Shirzadi Javid on 12/29/17. Copyright ASCE. For personal use only; all rights reserved. specifically. Moreover, limited research studies have been carried out on NAAC properties. In the present study, lightweight nonautoclaved aerated concrete was prepared. The amount of porosity in the concrete was changed by a variation of the aluminum powder percentage. Subsequently, the compressive strength of mixtures was measured and compared at time intervals of 3, 7, and 28 days. Likewise, in order to examine the workability of the prepared concretes, the amount of fresh concrete’s minislump was measured. Furthermore, a scanning electron microscope (SEM) was used to analyze the type of concrete pores. Last and foremost, a novel formulaic method was proposed to determine the air percentage of NAAC. Moreover, the effects of various aluminum powders on the fresh and hardened properties of NAAC were evaluated. Experimental Programs The powder is known as 1940 grade aluminum powder. As mentioned previously, aluminum powder for aerated concrete should be flake. Hence, according to the SEM image (Fig. 2), the shape of the aluminum powder used is flake. Moreover, the size of the used aluminum powder is in the range of 0.1–10 μm. Mixture Proportions The proportions of the NAAC mixtures are summarized in Table 3. As could be seen, six types of NAAC mixtures were prepared with a constant water-to-cement (W/C) ratio of 0.6 but with various percentages of aluminum powder (Mixtures A1–A6). In order to evaluate the effect of W/C ratios, Mixtures A7–A9 were sampled with a W/C ratio of 0.5, 0.8 and 1, respectively. Sand content, accelerator percentage, and limestone powder content were constant in all the mixtures. Materials In this study, a locally made ordinary portland Type II cement according to the requirements of ASTM C150 (ASTM 2003) and limestone powder as filler were used. River sand with a specific gravity of 2,550 kg=m3 was also used as aggregate. The chemical and grading properties of cement, sand, and limestone powder are presented in Table 1 and Fig. 1, respectively. An accelerator admixture was also used in order to decrease the setting time of mixtures. Physical properties of accelerator admixture are presented in Table 2. The reason for using an accelerator with the proportion of 8% of the cement weight was to omit the autoclave process. An industrial oily type of aluminum powder was used in this study. Table 2. Physical Properties of Accelerator Admixture Property Quantity or type Physical mode PH Density (g=cm3 ) Chloride (ppm) Powder 7–8 1.10 None Table 1. Chemical Properties of Cement and Mineral Admixtures (% Mass) Composition Sand Cement Limestone powder SiO2 Al2 O3 Fe2 O3 MgO CaO SO3 Loss on ignition (%) 66.7 9.8 5.1 2.37 8.8 0.16 — 20.74 4.90 3.50 1.20 62.95 3.00 1.56 2.80 0.35 0.50 1.80 51.22 1.24 42.06 Fig. 2. SEM image of used industrial oily aluminum powder Table 3. Mixture Proportions of NAAC Mixtures Limestone Water Al/C Accelerator/C Cement Sand powder (%) Mix (kg=m3 ) (kg=m3 ) (kg=m3 ) (kg=m3 ) W/C (%) Fig. 1. Particle size distribution of each solid material © ASCE A0 A1 A2 A3 A4 A5 A6 A7 A8 A9 648 648 648 648 648 648 648 648 648 648 04017312-2 J. Mater. Civ. Eng., 2018, 30(3): 04017312 821 821 821 821 821 821 821 821 821 821 389 389 389 389 389 389 389 324 518 648 77 77 77 77 77 77 77 77 77 77 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.5 0.8 1 0.0934 0.0466 0.0934 0.1869 0.2849 0.3731 0.4673 0.0934 0.0934 0.0934 0 8 8 8 8 8 8 8 8 8 J. Mater. Civ. Eng. Mixture Preparation The water, limestone powder, and accelerator admixture were poured into the mixer. The mixer was turned on and allowed to mix well for 2 min. Cement and sand were later added to the mixture and allowed to mix together for 4 min. Aluminum powder was mixed separately in 100–200 g of water and then added to the contents of the mixer. The mixing process was continued for 1–2 min. Downloaded from ascelibrary.org by Ali Akbar Shirzadi Javid on 12/29/17. Copyright ASCE. For personal use only; all rights reserved. Test Procedure Workability of NAAC Mixtures The workability of mixtures was tested with the minislump device. The apparatus for the minislump test of NAAC mixtures consists of a cone 60 mm high with a diameter of 70 mm at the top and 100 mm at the base. The cone was placed at the center of a plate and was filled with mixture. Immediately after filling, the cone was lifted, and the average diameter (in millimeters) of spread mixture over the table was measured. Fig. 3. Density and minislump results of NAAC mixtures Density and Volume Test The density of fresh NAAC mixtures was tested according to ASTM C188 (ASTM 2015). Volume change in the mixtures (due to the production of air bubbles in the mixtures) was tested at the interval of a few seconds. Compressive Strength Test The compressive strength of NAAC mixtures was determined according to BS EN1881 (BSI 2013b) after 7 and 28 days. Water Absorption Test Typically, this test was carried out according to standard procedure BS 1881-Part122 (BSI 2013a); however, the difference here is that because of the high porosity of the mixture, it is filled with water in a relatively short time. Hence, the test was performed at shorter intervals (1, 2, 4, and 8 min). Fig. 4. Density and compressive strength results of NAAC mixtures Measurement of Packing Density In order to evaluate the packing density of NAAC mixtures, a method called wet packing is applied based on literature presented by Wong and Kwan (2008) and Ghoddousi et al. (2014). The process of the measurements is described subsequently. Table 4. Workability, Density, and Compressive Strength Results of Fresh Mixtures Mix Density (kg=m3 ) Minislump (cm) 7-day compressive strength (MPa) 28-day compressive strength (MPa) A0 A1 A2 A3 A4 A5 A6 A7 A8 A9 1,020 1,477 1,298 1,170 1,140 1,085 1,027 1,500 1,550 1,600 30 28 30 31.5 29.5 24.5 23 22 40 47 1.5 8.5 6 2 1.1 0.8 0.5 7.6 4.5 3.4 2.5 11.6 8 2.9 1.7 1.3 0.9 10.1 6.5 4.8 © ASCE Fig. 5. Compressive strength and minislump results of NAAC mixtures 04017312-3 J. Mater. Civ. Eng., 2018, 30(3): 04017312 J. Mater. Civ. Eng. After making the NAAC mixture in the mixer, concrete is filled into a mold of size 100 × 100 × 100 mm. If the NAAC mixtures include various materials, the volume of the solid materials (V C ) in the mold could be calculated from Eq. (2) as follows: Downloaded from ascelibrary.org by Ali Akbar Shirzadi Javid on 12/29/17. Copyright ASCE. For personal use only; all rights reserved. VC ¼ M ρ w u w þ ρ g R g þ ρ s Rs þ ρ c Rc þ ρ l Rl þ ρ m Rm ð2Þ where M and V = mass and volume of NAAC, respectively; ρw = water density; uw = ratio of water volume to solid volume of granular material; ρs , ρc , ρl , and ρm = densities of sand, cement, limestone powder, and accelerating admixture as filler, respectively; and Rs , Rc , Rl , and Rm = volumetric ratios of sand, cement, limestone powder, and accelerating admixture, respectively, to total solid materials. Given that the amount of aluminum powder and accelerator admixture weight used in the mixtures are very low, they do not affect the packing density and can be ignored in the equation. Fig. 6. Effect of water-to-cement ratio on density and compressive strength Having obtained V C and V, packing density (ϕ) may be calculated by Eq. (3) as follows: ϕ¼ VC V ð3Þ SEM Test The microstructure of NAAC mixtures was observed using a SEM (EVO 18, Zeiss, Oberkochen, Germany). Results and Discussion Workability, Density, and Compressive Strength of the Fresh NAAC The workability, density, and compressive strength results of fresh mixtures are given in Table 4. For a better understanding of the parameters listed in Table 4, they are compared in Figs. 3–5. As is evident from Fig. 3, there is an optimal aluminum percentage of 0.093 from the perspective of minislump diameter. The results show that minislump and workability decrease with an increase in the amount of aluminum powder in comparison to optimum percentage. This could be explained by the fact that as long as pores are separated, they are spherical and act as bearings, but when there are many pores, they join together and act as a network, which leads to flow-ability decrement. However, the air percentage increases with an increase in the amount of aluminum powder, resulting in forming more pores and consequently reducing the density. Fig. 4 shows that the increase in aluminum percentage leads to a decrease in compressive strength of the NAAC mixture. This is in agreement with the results presented by Muthu Kumar and Ramamurthy (2015). It seems that the optimum percentage of aluminum can be considered as 0.0934. By increasing the amount of aluminum powder from 0.0934 to 0.1869, compressive strength reduced significantly. However, increasing the percentage of aluminum powder caused a density decrement, but with regard to compressive strength, the optimum percentage (0.0934) would Fig. 7. Artificial pores produced in the NAAC © ASCE 04017312-4 J. Mater. Civ. Eng., 2018, 30(3): 04017312 J. Mater. Civ. Eng. Downloaded from ascelibrary.org by Ali Akbar Shirzadi Javid on 12/29/17. Copyright ASCE. For personal use only; all rights reserved. Fig. 8. SEM images of NAAC mixtures be suitable. Fig. 5 also clearly shows that there is an optimal percentage (0.0934) from the viewpoint of suitable compressive strength and workability. The effect of the water-to-cement ratio on fresh and hardened properties of NAAC mixtures is illustrated in Fig. 6. It is obvious that with an increase in water-to-cement ratio, compressive strength reduced, but from the perspective of both compressive strength and density, an optimal W/C ratio (0.6) can be considered. SEM Results The porosity in aerated concrete is divided into three categories: artificial, cluster, and particle porosity. Comparing the SEM images of the three categories with images obtained in this study, it was clearly found in Fig. 7 that the porosity formed in this study is an artificial porosity. When aluminum powder is very high (A6 mixture), detection of the number, type, and diameter of pores is very difficult (Fig. 8) because when the pore number increases, pores change to pore networks. This is in agreement with the results reported by Narayanan and Ramamurthy (2000a). Water Absorption of Mixtures Volumetric water absorption of NAAC mixtures at 1, 2, 4, and 8 min are shown in Fig. 9. As can be seen, an increase in the amount of aluminum powder leads to more absorbed water. The reason is that by increasing the aluminum powder, the size and relative value of pores increase. Packing Theory for Calculation of Air Percentage Table 5 presents the measured packing density of A1 and A2 mixtures at five intervals. The results show a reduction in packing density with time. This might be attributed to the air bubbles produced © ASCE Fig. 9. Water absorption of NAAC mixtures at different times during the reaction of NAAC constitutions. The results also show that with an increase in the amount of aluminum powder from 0.0934 to 0.1869 (A1 to A2), packing density reduces. This is due to a higher air percentage in the A2 mixture. Figs. 10 and 11 show the results of volume change ratio and density with time for A1 and A2 mixtures. It can be seen that by increasing air bubbles over time, the volume increases and density reduces. As mentioned in the Introduction, there is no study that focused on the air void content measurement in fresh aerated concretes specifically. Fig. 12 shows the measured air percentage of NAAC mixtures A1 and A2 over time based on packing theory. It is observed that with an increase in aluminum powder (mixture A2), the maximum aerated air percentage is higher than mixture A1 containing 04017312-5 J. Mater. Civ. Eng., 2018, 30(3): 04017312 J. Mater. Civ. Eng. Table 5. Measured Packing Density of A1 and A2 Mixtures Downloaded from ascelibrary.org by Ali Akbar Shirzadi Javid on 12/29/17. Copyright ASCE. For personal use only; all rights reserved. Time (s) Mix 0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 A1 A2 0.549 0.602 0.536 0.548 0.536 0.521 0.536 0.497 0.524 0.462 0.524 0.437 0.513 0.420 0.506 0.404 0.502 0.399 0.482 0.399 0.474 0.399 0.472 0.399 0.467 0.399 0.463 0.399 0.458 0.399 0.454 0.399 Fig. 10. Results of volume change ratio and density with time for the A1 mixture Fig. 12. Measured air percent of NAAC mixtures A1 and A2 with time based on packing theory Fig. 11. Results of volume change ratio and density with time for the A2 mixture Fig. 13. Relation between air percent of NAAC mixture with volume change less aluminum powder (21% for A2 and 16% for A1). To determine the air percentage of NAAC having a volume change on site or in a laboratory, the following equation is obtained from Fig. 13: Conclusions y ¼ 0.2484x þ 7.1436 ð4Þ where y = air percentage produced in the NAAC mixture; and x = percentage of final mixture volume change compared to the initial volume. © ASCE The following conclusions can be drawn from the results of this research: • There is an optimal percentage of aluminum (0.0934) from the viewpoint of minislump diameter and compressive strength. After the optimum point, the increase in percentage of aluminum results in minislump (workability and flow-ability) and compressive strength decrement. 04017312-6 J. Mater. Civ. Eng., 2018, 30(3): 04017312 J. Mater. Civ. Eng. Downloaded from ascelibrary.org by Ali Akbar Shirzadi Javid on 12/29/17. Copyright ASCE. For personal use only; all rights reserved. • Comparing the SEM images of three categories with images obtained in this study, it is clearly found that the porosity formed in mixtures is an artificial one. • With an increase in aluminum content, the percentage of water absorbed increases. The reason is that by increasing the aluminum powder, the size and relative value of pores increase. • The results show that an increase in the amount of aluminum powder reduces packing density in comparison to the mixture with lower aluminum percentage. • Increasing the water-to-cement ratio reduces the compressive strength, but from the viewpoint of density, an optimal waterto-cement ratio (0.6) can be considered. • This study proposes a new method of determining air percentage of aerated concrete based on packing theory. It observes that the air percentage of aerated concrete increases with an increase in aluminum powder. Notation The following symbols are used in this paper: M = mass; Rc = volumetric ratio of cement to total solid materials; Rl = volumetric ratio of limestone to total solid materials; Rm = volumetric ratio of accelerating admixture to total solid materials; Rs = volumetric ratio of sand to total solid materials; uw = ratio of water volume to solid volume of granular material; V = volume; V C = volume of solid materials; ρc = density of cement; ρl = density of limestone; ρm = density of accelerator admixture; ρs = density of sand; ρw = density of water; and ϕ = packing density. References Alexanderson, J. (1979). “Relations between structure and mechanical properties of autoclaved aerated concrete.” Cem. Concr. Compos., 9(4), 507–514. ASTM. (2003). “Standard specification for portland cement.” ASTM C150, West Conshohocken, PA. ASTM. (2014). “Standard test method for air content of freshly mixed concrete by the pressure method.” ASTM C231, West Conshohocken, PA. ASTM. 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