Cellular Ceramics / p2

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fly ash is added. Cellular concretes containing fly ash are generally more prone to freeze/thaw due to their higher permeability to water [6, 7].

The recipes used by the various manufactures of cellular masonry are proprietary, and references to cellular concrete in the open literature are limited. However, patent literature on this subject is abundant, mostly concentrated in the Russian patent office. The microstructure of cellular concrete is unimpressive. The C–S–H that forms is noncrystalline with a microstructure containing porosity and an occasional large crystal of Ca(OH)2. SEM images of C–S–H in a cellular concrete cured at low temperature are given in Figs. 3 and 4. For comparison, the microstructure of a Class F fly ash AAC sample is shown in Figs. 7 and 8.

2.9.6.2

Autoclaved Aerated Concrete (AAC)

When mixed, autoclave-cured cements are normally much more fluid than cellular concretes. This is somewhat misleading, however, because they also contain CaO, which is slaked soon after mixing and thickens the mixture after 5–6 min. Metal powders used to foam AAC formulations are normally aluminum-based. The aluminum powder is ball-milled, and particle size is critical in achieving a specified bubble size. Envision the following: A micrometer-sized powder of thin aluminum flakes is introduced into an AAC slurry at the very last minute of mixing to provide the necessary hydrogen bubbles to make the slurry take on a highly cellular character. The flakes must be thin rather than round to provide a fine edge where the gas bubbles originate. The making of a particular size of bubble is necessary to minimize the merging of bubbles into very large ones that will move upward and be lost to the atmosphere. The viscosity of the mixture must be such that it begins to thicken during gas evolution and thus captures the bubbles before they migrate upward in the mix.

AAC has environmental advantages. Because it is tobermorite-based it does not need as much lime-containing ingredients to produce as does cellular cement paste. In addition it consists of 80 % void space and only 20 % solids, so 4 m3 of AAC can be produced from 1 m3 of starting material [11]. Final curing occurs in a steamheated autoclave, so emissions of CO2, CO, and NOx during curing are minimal when compared to high firing of ceramics in a kiln. Furthermore, waste materials such as Class F fly ash and ground granulated blast-furnace slag, as well as trimmings from the green block during sample sizing and condensate from the boiler, can be recycled into new AAC mixtures [11, 23–26].

Typically the ingredients are mixed and the slurry is poured and allowed to harden for 45–60 min, at which point the material has enough green strength to be demolded and cut to size with wire saws. The slurry normally contains ball-milled quartz flour (10–20 mm), Class F fly ash, or micrometer-sized copper mine tailings (the three silica sources used in the USA at present), Portland cement, vertical-kiln- fired lime that is slow to slake, recycled trimmings, and additional water if necessary. The silica sources are mixed with water maintained at about 60 C in constantly stirred thousand-gallon open tanks equipped with overhead stirrers. Waste material

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is similarly slurried in another tank. It consists of the waste material trimmed from the green cakes prior to autoclaving. The mixer is charged with known quantities of raw and recycled materials, to which are added the Portland cement and then the lime in a series of timed steps. Typical formulations are given in Tab. 1. Mixing is continuous and quite strong, forming a vortex in the middle of the mixer. Some manufactures add gypsum to their mix at this point, followed closely by a preslurried dose of aluminum powder. After pouring the mixture begins to develop the alkalinity needed to for the aluminum powder react. Ideally this occurs after the lime has slaked and the mixture has thickened slightly, making bubble retention easier. The mix can expand twoor threefold. Gas bubbles and capillary pores occupy as much as 80 % of the void space in the mixture. Densities can range from about 200 to 1000 kg m–3, and associated strengths increase with density. As density increases so does the amount of matrix material present; in this case the amount and kinds of aluminum added control the outcome. One hour after being poured into a massive preheated (38–40 C) steel mold coated with an oily release agent, the AAC has evolved into a cake. In some cases the mold is not attached to the base and is slightly tapered to facilitate its upward removal by using a lift. When casting reinforced panels in molds that are up to 6 m or more in length, the mold has a removable side and the beam is tipped out of the mold and manipulated so that the direction of rise is horizontal prior to cutting with piano-wire saws. Autoclaves are heated with superheated steam. Commercial autoclaves are manufactured by a variety of companies. Essentially they are large steel cylinders 40–50 m or more in length and nearly 3 m in diameter. Ideally they have doors at each end that allows the manufacturer to roll the green cake into one end of the autoclave while removing the cured material from the other. The AAC is cast on small steel carts mounted on railway tracks. An autoclave can take many cycles of green cakes. Because a heating cycle takes about 24 h for heat/cure/cool, cakes tend to accumulate. Some will sit for many hours while the last made enter the autoclave. Like a ceramic (and depending on whether or not the piece is reinforced with wire cage, the cake is preheated for up to 6–8 h as the temperature and pressure are raised to 180 C. Curing lasts for 8 (without metal reinforcement) to 12 h (with metal reinforcement), and cooling takes 6–8 h, too. The manufacturer can adjust the density of the AAC by adding or subtracting Al flake, which in turn affects the amount of air included in the sample.

The final AAC material consists of approximately 80 vol % void space and 20 vol % crystalline tobermorite [27]. Unlike Portland cement cured at room temperature, AAC consists of the crystalline calcium silicate hydrate mineral known as 1.1 nm tobermorite (Ca5Si6O17·5 H2O) [18, 28]. Although it is suggested that this mineral forms a poorly crystalline analogue at room temperature called C–S–H, this has never been proven [1].

Three classes of AAC based on compressive strength are commonly manufactured: low-, medium-, and high-strength. Table 2 summarizes the general characteristics of these classes of AAC [11]. As is the case for low-temperature cellular concrete, the density of AAC is also directly related to strength. Rilem [11] has published a plot of AAC strength versus dry density, reproduced here as Fig. 13. Other densityrelated properties that similarly increase with declining strength are thermal and acoustic insulating value [11].

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Table 2 Classification of AAC according to characteristic compressive strength [11]

Figure 13 Relationship of compressive strength to dry density. For any two equivalent density (dry) AAC and low temperature cellular concrete samples the expected strength of the autoclaved sample should always be higher [11].

AAC has mechanical properties that depend on its water content. After autoclave curing, AAC contains about 25–35 % (as much as 45 % in very low density materials) water by weight of the dry material [11]. Strength at this point is at its lowest. Some manufacturers test their samples at this point and base their formulations on meeting wet strength and the requirements of their class. Others test their samples when they are dried to about 10 wt % moisture. Strength increases as water is lost by the sample. The average water content of AAC after 1–3 years of service reaches an “equilibrium” value of about 3.5 wt %. Ninety percent of AAC samples in use have less than 5 wt % moisture content. Figure 14 depicts the relative compressive strength of an AAC sample as it dries [11]. The insulating value increases with the degree of drying [29–32] of AAC, and thus it is proposed that the degree of soundproofing will also increase as a sample dries.

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Fig. 14 Moisture content versus relative strength [11].

Strength and gas permeability are extremely sensitive to direction of rise [29]. Bubbles tend to rise somewhat, and the upper portion of the AAC is commonly slightly less dense than the rest of the sample. For this reason testing is always done perpendicular to direction of rise. Direct tensile strength is 15–35 % of compressive strength, and modulus of rupture (MOR) of AAC is approximated by Eq. (8), where ƒct is compressive strength (in MPa) [11]:

The shear strength is approximately 20–30 % of the compressive strength [11]. The use of fracture mechanics as a predictive tool for cellular concrete, AAC, and

concrete products in general is still in its infancy, although attempts have been made with a variety of models [11] with a fair degree of success and controversy. In terms of linear elastic fracture mechanics, the toughness of AAC can be compared and contrasted by measuring the critical strain energy release rate GC (in N m–1) and critical stress intensity factor or fracture toughness KIC (in MN m–3/2) [30–32]. These parameters can be equated to each other for “plane-stress conditions” by Eq. (9)

where E is the elastic modulus [11]. Representative fracture toughness data versus density are given in Fig. 15.

Nonlinear fracture mechanics have also been used to calculate toughness (fictitious crack model), but in this instance curve fitting (strain softening) and area measurement under the entire force–crack mouth opening displacement diagram are used to derive values [11, 33–35] which do in fact provide additional insight for welldefined ceramic systems, but for cement-based cellular concrete systems, the results should be viewed with some caution when comparing samples from different sources prepared in different ways, because of the uncertainty introduced by

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Fig. 15 Fracture toughness as a function of density. The normal block produced in the USA has a density of 400–600 kg m–3.

Its fracture toughness is approximately 0.05–0.08 MN m–3/2. Adding zeolites increases this value slightly [11].

multiple cracking [36]. On the other hand, if all measurements are carried out on the same samples as a function of a single variable such as AAC rising direction, then the results of nonlinear fracture mechanics are informative, and in this case illustrated that the brittle character of the samples does vary as a function of the rising direction of the AAC cake [35].

Toughness can be increased by judicious use of fiber reinforcement. Fibers can be introduced during mixing [36] or can be grown in situ during hydrothermal curing [37]. Asbestos-reinforced cement is a perfect example of the benefit derived from such a process. In a post-asbestos world, however, fiber reinforcement has never been the same. Glass, steel, cellulose, and plastic fibers have been introduced into all mannerer of cementitious material with varying degrees of success [36]. There is an ongoing effort to find a mineral replacement that can be used in lieu of asbestos. For example, Low and Beaudoin [38] used fibrous wollastonite to reinforce C–S–H pastes, and Grutzeck has grown zeolites in Class F fly ash AAC by augmenting the mix with significant amounts of sodium hydroxide [37]. To date just a slight increase has been documented (unpublished data).

AAC has an interlocking microstructure. Although it is able to float, it is not impermeable. Water will enter the structure, but relatively slowly [39–41]. The microstructure shown in Figs. 7 and 8 contains openings that are in the range of 0.03 mm in size, which compares to the pore size in the “green” block of close to 6 mm [28]. The coefficient of water permeability K ranges from 10–12 to 10–13 m2, while gas permeability for a dry AAC sample is approximately 10–14 m2 [11]. A typical absorption/desorption isotherm for water in AAC is given in Fig. 16. Note that

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the plot is extremely nonlinear. Its shape is reminiscent of those of clays [42] and could be related to the inclusion of liquid water in interlayers at very high humidities.

Fig. 16 Typical absorption and desorption isotherm of AAC [11].

Due to the sensitivity of AAC to water, the material is always coated with stucco or some type of waterproofing before it is exposed to water or high humidity [6, 7, 11]. Durability is not a problem if the AAC is continually frozen (arctic climate) or air-dried (arid climate), but it is an issue in temperate climates where conditions cycle many times during a season. Unexpectedly, the resistance of uncoated AAC to freeze/thaw and wet/dry cycling is not impressive compared to air-entrained ordinary Portland cement (OPC) concrete, even though it contains 50 vol % air bubbles.

2.9.7

Durability of Cellular Concrete

Although bubbles and voids in AAC are not interconnected, bubbles and pockets of evaporable water are normally isolated from one another by a rim of hydration products. The thickness of these rims and their relation to each other and how they grow together to form the matrix ultimately determines the permeability of the cellular concrete. As the density of AAC is decreased (air content is increased) the thickness of the matrix wall decreases and thus resistance to water ingress also decreases. In all cases water will slowly permeate the material with time, but it will do so more quickly in low-density material [43, 44]. Since water can freeze in bubbles filled with

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water and expand and cause cracks or shrink during drying, uncoated AAC is not considered durable when exposed to freeze/thaw and wet/dry cycling as occur in temperate climates. Cellular concretes exposed to environmental wet/dry and freeze/thaw cycles must be coated to prevent deterioration [11, 43, 44]. Unfortunately this sensitivity will increase the cost of AAC materials used in large-volume applications such as sound walls along highways and thus deter its use.

Moisture transport in AAC is not yet fully understood. Modeling studies have been carried out and models proposed [11, 39–41]. Interestingly, freeze/thaw resistance is related to bubble size and the water content of the air bubbles. A conventional AAC sample undergoing 100 freeze/thaw cycles lost 12 wt % of surface material, while a companion sample with very small bubbles lost 3 wt % [6]. Hasegawa [41] and Senbu and Kamada [43] both reported that freezing of water in matrix pores caused little damage. What did cause damage, however, was the presence of water in the large pores (ca. 40 % filled). When these froze the AAC sample began to spall. Compared to a conventional concrete that does not contain bubbles, freezing that occurs in the paste pores normally causes damage by expansion when the water freezes. However, if it is air-entrained (surfactant is included that provides 3–5 % air bubbles in the mix), the freezing water will expand into the air bubbles and damage will be averted. In AAC the damage is similarly averted if water content is low. But once the air bubbles become partially filled, then freeze/thaw damage will occur [43].

Higher porosity leads to an overall increase in permeability and thus an increase in susceptibility to the effects of freezing and thawing. Permeability and freeze/thaw damage increases as the density of the composite decreases. It is known that the addition of a pozzolanic material such as Class F fly ash and /or a reactive silica source to a Portland cement paste will increase long-term performance. The smaller particles usually fit between the hydrating cement grains and help the hydration products to “bridge the water-filled gap” between cement particles in the paste. These additives also provide a more tortuous path, since they react with the excess lime to produce more C–S–H. Reactivity of these additives (pozzolana) at ambient temperatures is limited in the short term; therefore, they often reduce early strength, but then enhance it over the long term. Finally, the degree of reactivity is generally correlated with temperature. Cellular concrete cast and cured at ambient temperatures matures over a period of weeks to months. Pozzolanic additives are usually considered relatively inert. Autoclaved concrete cured above 100 C matures in a day. In this instance the reactivity of the additives is comparable to that of the cement itself. Coating these materials with waterproofing materials and/or paints or stucco prevents ingress of water and thus makes them more durable [41, 44]. Many companies worldwide deal in such coating materials.