This chapter describes the different properties and the durability of autoclaved aerated concrete (AAC) masonry blocks and contains the following sections: the types of lightweight concrete, history, utilization, manufacturing, mechanism of AAC, physical properties, mechanical properties, microstructure, characterizations, thermal conductivity and durability of AAC. The types of lightweight concrete section describes different types of lightweight concrete and the different production process used in making lightweight concrete such as aerated concrete and foam concrete, paying particular attention to AAC. The utilization of AAC as masonry blocks is also mentioned. The mechanism involved in the production of AAC in relation to the pore-forming method used and the hydration of AAC is described in the mechanism section. The physical properties section describes the bulk density of AAC and the relation to the air voids. The mechanical properties section describes the compressive strength and flexural strength of the AAC, the relationships with the physical properties and the hydration products. The microstructure section describes the pore size formed and the morphology of the AAC microstructure characterized through the use of scanning electron microscopy (SEM) while the other characterization section involved the use of X-ray diffraction and thermogravimetric analysis techniques. Thermal conductivity is also discussed in the chapter. The durability section of the chapter describes the freeze-thaw resistance of the AAC.

Lightweight concrete can be classed according to its unit weight or density, which normally ranges from 320 to 1920 kg/m³, according to the ACI Committee 213 Guide for Structural Lightweight Aggregate Concrete (ACI 213, 2001). There are three different lightweight concrete type divisions in terms of strength range, which are low-density concretes (0.7–2.0 MPa), moderate-strength concretes (7–14 MPa) and structural concretes (17–63 MPa). The density of these concretes is in the range of 300–800 kg/m³, 800–1350 kg/m³ and 1350–≈1920 kg/m³ respectively. The use of lightweight concrete has been used since the early 1900s in the United States, and lightweight concrete has been used in multistory buildings, long span bridges, offshore platforms and large structures (Mindess, Young, & Darwin, 2003). A number of advantages in using low-density lightweight concrete in construction are due to its low density, low thermal conductivity, low shrinkage and high heat resistance, in addition to reduction in dead load, lower haulage cost and faster building rate (Wongkeo, Thongsanitgarn, Pimraksa, & Chaipanich, 2012).

The first commercial production of AAC was in 1923 in Sweden. Since then, it has now been used in more than 40 countries in Europe, America, Australia, the Middle East and the Far East. The modern uses of AAC in the United States began in 1990 for residential and commercial buildings (Masonry magazine, 2008; Kočí, Maděra, & Černý, 2012; Radhi, 2011). AAC masonry blocks can be made as precast building blocks and are used in residential construction, hospitals, office buildings and university accommodations (Hess, Kincl, Amasay, & Wolfe, 2010.). AAC masonry blocks have many advantages in comparison to conventional concrete: lighter weight (typically weigh one-sixth to one-third of conventional concrete), lower building costs and provides thermal and acoustic insulation (Hendry, 2001; Hess et al., 2010; Klingner, 2008). The thermal insulation property of AAC would make buildings more energy efficient as it has been reported in a case study in the United Arab Emirates where the use of AAC was found to reduce energy consumption in the residential sector by about 7% (Radhi, 2011). Due to the high porous structure with 60–70% air, AAC has the ability to dampen the mechanical vibration energy giving its excellent sound insulation, therefore the use of AAC would be very suitable for places like schools, hotels and apartments (Technology Brief, 2010). The design and specifications of AAC masonry blocks is covered by the Masonry Standards Joint Committee code and by ASTM C1386. The standard size of an AAC masonry block is larger compared to the concrete masonry unit (CMU) block; the length of an AAC masonry block is 610 mm compared to the 410 mm length of a CMU block (Hess et al., 2010). Although it is larger in size, it is lighter, so the large size of an AAC block is comparable to a medium-sized CMU block in terms of weight (Hess et al., 2010). A standard block is 50–375 mm thick, 200 mm in height and 610 mm in length (Klingner, 2008). A standard AAC block is shown in Figure 9.1. Overall, due to these desired properties of AAC, especially its lightweight, thermal and acoustic properties, AAC masonry block is an ideal construction material for walls in many residential and office buildings.

AAC is generally made from quartz-rich sand, lime, cement and aluminum powder (Kurama, Topcu, & Karakurt, 2009). Gypsum and fly ash can also be used in the production of AAC (Klingner, 2008). Aluminum powder is used as an air-entraining (aerating) or pore-forming agent and is the most common agent used (Narayanan & Ramamurthy, 2000a). It is based on the reaction of aluminum with the soluble alkalis in the cement to form the small bubbles of hydrogen as in Eqn (8.1) (Mindess et al., 2003). This causes the material to rise in the mold and after curing for 45 min, the products are cut into the required unit size by wires (Technology Brief, 2010).

The bulk density of bottom ash (BA) cement AAC specimens has been reported by Wongkeo et al. (2012) by comparing the addition of aluminum powder. The bulk density of autoclaved lightweight concrete without BA replacement decreased up to 23.47% when aluminum (Al) was added. Al addition was therefore found to have a significant effect on the bulk density of autoclaved concrete. Moreover, the bulk density of bottom ash AAC compared to Portland cement AAC increased from 2% to 7% when BA was used to replace Portland cement at 10% and 30% respectively. The increase in the bulk density of bottom ash AAC was due to the formation of the tobermorite phase when BA is used.

The microstructure of AAC consists of macropores and micropores and is influenced by the method of pore formation (Narayanan & Ramamurthy, 2000a). The pore sizes, those are initially formed from the reaction of aluminum with the cement alkalis, are somewhat larger in size typically in the range of 0.1–1.0 mm (Ioannou, Hamilton, & Hall, 2008; Mindess et al., 2003). The pore sizes created by the chemically reacted agent such as aluminum or the foaming methods are therefore large enough to be visible and easily seen. The formation of macropores is reported to be formed from the aeration process while the micropores appear in the wall of the macropores (Alexanderson, 1979). Petrov & Schlegel (1994) summarized the macropore and micropore size as being greater than 60 μm for the former and 50 nm or less for the latter (Alexanderson, 1979). Macropore sizes of 50–500 μm (0.05–0.5 mm) formed during aeration have also been reported (Alexanderson, 1979).

The thermal conductivity of AAC concrete has a direct relationship with its physical properties. The thermal conductivity of bottom ash cement AAC was reported to increase slightly with bottom ash used as supplementary cementitious material in concrete (Figure 9.3), because the BA content increased the overall unit weight or bulk density of the concrete (Wongkeo & Chaipanich, 2010; Wongkeo et al., 2012). Pore structure of the lightweight aggregates, density of concrete and the cement matrix has an effect on the thermal conductivity of concrete (Corinaldesi et al., 2011; Topcu & Uygunoglu, 2007). Thus, the thermal conductivity of BLWC tended to increase due to the decrease in the volume of permeable voids with increasing BA content (Wongkeo & Chaipanich, 2010). Albayrak et al. (2007) reported that the compressive strength and the thermal conductivity of AAC concrete are reduced with the decrease in bulk density. Albayrak et al. (2007) found the thermal conductivity of AAC in the range of 1.1–5.0 MPa and the density range of 270–500 kg/m³, showing an increase in the strength with density. Several other researchers (Blanco, Garcia, Mateos, & Ayala, 2000; Demirboga, 2003, 2007; Demirboga & Gul, 2003) also reported that the decrease in thermal conductivity is related to the reduction in the concrete density. Jerman, Keppert, Vyborny, & Cerny (2013) showed that the thermal conductivity depends on the density of AAC as well as the moisture content (Figure 9.4), where the increase in moisture content and density leads to an increase in the thermal conductivity. The thermal conductivity is therefore known to increase with the increase in the density of both normal and AAC concrete. The thermal conductivity of AAC can be as low as 0.08 W/m K at 25°C. However, the compressive strength as a consequence is very low (2.05 MPa) (Jerman et al., 2013). When AAC with bottom ash is used as supplementary cementitious material, the thermal conductivity is in the range of 0.58–0.61 W/m K (Wongkeo et al., 2012). On the other hand, when bottom ash is used as aggregate, the thermal conductivity of the AAC concrete is reported to be in between 0.220–0.361 W/m K and was found to reduce with increasing BA content used as sand replacement (Kurama et al., 2009).

Since the AAC has voids, which are large due to the formation of the initial reaction for aeration, it is expected to be frost resistant (Mindess et al., 2003). However, the degree of saturation is important for freeze-thaw reactions (Roulet, 1983), as aerated concrete is susceptible to liquid and gas penetration due to its high porosity that may cause damage to the concrete (Narayanan & Ramamurthy, 2000a; RILEM, 1993). The maximum degree of saturation is reported to be in the range of 20–40%. With a higher degree of saturation, the concrete would be become brittle and crack completely (Roulet, 1983).

While the focus of this chapter is generally on the properties and durability of AAC, which is a review of past and recent research works on the subject, the future trends involving AAC would largely depend on the application of the AAC and the specification required by the construction industry, as well as those set by the standards. Research will be heavily focused on new science and technology leading to innovative AAC products, taking into account the specification requirement, application, economical and environmental aspects. As such, new technology and new materials should be used in producing AAC. For example, a different and more suitable source of silica could be used, and if it is environmentally friendly and is also economical, the better. That is probably the reason why the research on pozzolanic materials have been so successful and have attracted many interests. New cost-effective production should be looked at also, not least the new materials such as nano-size or new waste and by-products that can benefit the mechanical and durability properties of AAC.

Narayanan & Ramamurthy (2000a) have reviewed the works on AAC, which described a number of properties of AAC. Wongkeo and Chaipanich (2010) and Wongkeo et al. (2012) reported the works on the use of pozzolans as binary and ternary blended cements and the results on the mechanical properties of AAC. Concrete by Mindess et al. (2003) is also a useful book to read to give an overall idea of lightweight concrete in general and the classifications of lightweight concrete. Published works by Meller et al. (2009), Taylor (1997) and Richardson (2008) are other useful reading sources on the hydration of calcium silicate and the hydration products at different temperatures and pressure.