Cellular Ceramics / p2

193

2.9

Cellular Concrete

Michael W. Grutzeck

2.9.1

Introduction

Unlike conventional high-fired cellular ceramics discussed in earlier chapters, cellular concrete is one of the many ceramiclike materials that can be produced via a comparatively low temperature, chemically driven hydration reaction. Air-dried cellular concrete consists of two phases: a gaseous phase (gas bubbles, ca. 60 vol %) and void spaces/ micropores (ca. 20 vol %) that were formally occupied by evaporable water, and a surrounding matrix phase consisting predominantly of calcium silicate hydrate formed by a dissolution/precipitation reaction that occurs during the reaction of Portland cement with water. The water has three functions. First it makes it physically possible to mix, aerate, and place the cellular material. Second, it acts as a solvent for the microme- ter-sized anhydrous cement particles present in Portland cement. Third, it acts as a reagent that combines with the dissolved species in solution to form the insoluble hydrates that comprise the matrix material in cellular concrete. The hydrates that form, in descending order of importance are: calcium silicate hydrate, smaller amounts of calcium aluminate hydrates, and calcium hydroxide [1, 2]. Much like sintering causes a green ceramic body to develop its final properties, the root cause for hardening of cellular concretes is a series of complex hydration reactions.

The water consumed during hydration that becomes part of or closely associated with a newly formed hydrated phase is considered to be nonevaporable, because it occupies a lattice position or is chemically bound to a surface. Unlike excess water that remains in interstitial spaces and voids, water of hydration is normally not lost during drying at 105 C [3]. Thus, total porosity of cellular concrete is defined as the volume of space occupied by the gas phase and evaporable water divided by the total volume of the sample [4].

In the strictest sense, cellular concrete is not a concrete. By definition, concrete contains (by volume) approximately one part Portland cement, two parts fine aggregate (sand-sized limestone or quartz), three parts course aggregate (centimeter-sized limestone or quartz), and enough water to make the combination workable during mixing and placing [5]. For this reason “cellular concrete” is somewhat of a misnomer. Cellular concrete is little more than a hardened Portland cement slurry that has been aerated prior to setting to give it a homogeneous void or cell structure con-

Cellular Ceramics: Structure, Manufacturing, Properties and Applications.

Michael Scheffler, Paolo Colombo (Eds.)

Copyright 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 3-527-31320-6

194 Part 2 Manufacturing

taining 50–80 vol % or more of air bubbles, void spaces, and capillary porosity [6, 7]. (A slurry is a mixture of Portland cement and water. For sake of completeness, a mortar is a slurry that has been mixed with sand, and a concrete is a slurry that contains both sand and larger sized aggregate.)

Cellular concrete will undergo a process of setting and hardening much like its namesake. If the reaction is carried out below the boiling point of water, the calcium silicate hydrates that form are tens of nanometers in size; they have short-range order as observed by magic angle spinning nuclear magnetic resonance (MAS NMR), but only poorly organized long-range order [8]. It is suggested that the predominant calcium silicate hydrate that forms has a layerlike structure similar to certain clays (phyllosilicates) and is thus able to host interlayer water molecules [1–3]. These characteristics give the calcium silicate hydrates the distinction of being X-ray amorphous and are purportedly the cause of their gel-like properties. If cured above 100 C in a pressurized vessel under saturated steam pressure, the calcium silicate hydrates that form during mixing and molding will begin to develop crystallinity, which makes them easier to analyze and understand in the thermodynamic sense, that is, their solubilities and phase equilibria are relatively straightforward [9].

The phases that form are determined by the bulk composition and final curing conditions of the mixtures, and each of the phases that form has a given set of characteristics. Some hydrates have desirable properties while others do not. For example, it has been repeatedly observed that a conventional cement paste cured significantly above 100 C will develop calcium silicate hydrate phases that are unsound [1,2]. It is for this reason that autoclave-cured products are commonly augmented with 50 wt % or more of a silica source. This causes enough of a change in bulk composition that subsequent autoclave curing will lead to the production of a stronger phase [1, 2, 9]. Although phase relations at low temperatures are still uncertain, at this point in time enough data exist to allow one to piece things together and offer hypothetical versions of phase diagrams for the low-temperature phases occurring in the system CaO–SiO2–H2O [1,8]. Phase compatibility and phase properties are important design tools that can be used to formulate cellular cements for different applications.

2.9.2

Types of Cellular Concrete

Curing temperature has the single greatest effect on the outcome of a given hydration reaction. If we adopt an arbitrary dividing line (e.g., ca. 100 C) we can divide cellular concretes into two groups: one that consists of cast-in-place fill materials and precast masonry products that are cured at ambient or slightly elevated temperatures, and another that consists of precast masonry products and reinforced slabs and panels that are molded, precured to develop green strength, and then autoclaved at temperatures significantly higher than 100 C in a pressurized, steam-heated environment. These two types of cellular concretes share a common name and have a similar appearance (see Fig. 1), but their chemistries are radically different.

2.9 Cellular Concrete 195

Fig. 1 Cellular characters of the two cellular concretes are nearly the same. The one on the left is a cellular concrete made at ambient or slightly higher temperature (taken from A.J. Voton’s web site, http://www.ajvoton.com/foam-concrete.html). The one on the right is a piece of aerated autoclaved concrete (AAC) made at 180 C by Hebel GmbH.

2.9.2.1

Low Temperature Cured Cellular Concrete

Low temperature cured cellular concrete consists of gas bubbles and voids enclosed by a matrix that is, for the most part, identical to that associated with conventional hydrated cement products. In fact one can visualize a cellular concrete as being a mixture of a conventional Portland cement slurry or mortar mixed in a stationary mixer or ready-mix truck that has been blended with up to 80 vol % of air bubbles, voids, and micropores. Both foamed and unfoamed materials have the same phase composition; e.g. the calcium silicate hydrate in the cured products has a Ca/Si molar ratio close to 1.7. Because Portland cement is the main ingredient and other additives such as sand or gravel are essentially inert when cured at low temperatures, the calcium silicate hydrates that form in cellular concrete provide the same strength and impermeability to the matrix as would be expected to occur in a solid hydrated paste or mortar. The inclusion of a gas phase reduces the density of the material, making it lightweight, self-insulating, and soundproof, but at the same time the gas phase makes the cellular concrete less strong, more permeable to water vapor, and ultimately less durable than its more massive counterparts. Although not widely accepted at present in the USA it is anticipated that cellular concrete of all types will increase in popularity as its positive attributes become more widely known.

There are two categories of lightweight low-strength cellular concrete: those that are cast in place and those that are precast and cured in a mold or formwork with/ without reinforcing (here collectively referred to as masonry). Wet densities of foamed slurries and mortar usually range from 100 to 900 kg m–3 for cement slurries and up to 1600 kg m–3 for mortars containing sand or fly ash. ASTM has two standards that are instructive in that they provide some background on cellular concrete as well as a description of appropriate test methods for making and testing hardened cellular concrete. ASTM C 796-97 (Standard Test Method for Foaming Agents for Use in Producing Cellular Concrete Using Preformed Foam) provides the reader

196 Part 2 Manufacturing

with a composition and a means of producing foam and test samples; it is to be used in conjunction with ASTM C 869-91 (Standard Specification for Foaming Agents Used in Making Preformed Foam for Cellular Concrete). The former standard provides recipes for a Type I and a Type III Portland cement slurry that are designed to have wet densities within the 600–700 kg m–3 range when used with different preformed foaming agents, while the latter standard contains a table that lists the physical requirements of the test specimens themselves. Specifications for the mandated 0.58 water/cement ratio Type I and 0.64 water/cement ratio Type III cement slurries are as follows: compressive strength (1.4 MPa), tensile splitting strength (0.17 MPa), maximum water absorption (25 vol%), and maximum loss of air during pumping (4.5 vol %). Other variables that are determined are oven dry density and air content.

Cast-in-place cellular concrete is commonly used for geotechnical applications as a replacement for unstable soils, for example, nonslumping backfill for highway construction, especially around bridge approaches and bridge piers; backfilling of tunnels; and insulation of buried steam pipes and utility-containing culverts that may have to be reexcavated in the future [10]. When used as backfill, it is considerably more stable than clay-rich soil, which tends to creep with time, causing slabs cast on top of such soils to buckle and crack. When used as fill around steep bridge embankments it will not flow as would a clay rich soil. For example, densities of one set of foamed slurries studied by Cellular Concrete LLC ranged from 320 to 960 kg m–3, and their associated strength ranged from 3.5 to 65 kg cm–2. Production of a lightweight fill material is relatively simple. A surfactant/air entrainer or a preformed foam is added to a concrete mix in a ready-mix truck, allowed to mix to generate foam and/or distribute preformed foam evenly, and then poured into place where it sets and cures [10].

Lightweight masonry includes materials that are cast in place in formwork or precast in a factory and cured with low-pressure steam. Materials in this category include UL fire and FM rated roof decks, UL fire-rated soundproofing floor fill, leveling fill, cellular acoustic paneling, and impact/energy adsorbing block or panel [10]. Masonry formed by casting a slurry or mortar in formwork is commonly used in floor and roof systems worldwide. These serve as a foundation for the application of the final wearing course (e.g., compound roof, wood, or tile floor). Masonry products formed in a factory and then used elsewhere are less common in the USA. Although cellular concrete has great potential it tends to be less popular than equivalent Plaster of Paris based paneling or lightweight concrete made with lightweight aggregate (e.g., expanded perlite), but this trend may be changing. Cellular concrete is much more durable in wet places and thus better suited for bathroom and basement applications. In addition, cellular concrete masonry is able to adsorb energy caused by impacts of various types and adsorb traffic noise along highways when used as a sound wall. The cellular character and low strength of the material provides controlled energy adsorption as the cellular structure collapses. These masonry materials can be used for crash abatement at the bottom of hills or at the ends of airport runways as a means of stopping out of control vehicles and aircraft. Sound walls are another area where cellular concrete may find increased use once sensitivity to wet/ dry and freeze/thaw damage of uncoated material are minimized.

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2.9.2.2

Autoclave-Cured Cellular Concrete

The second type of cellular concrete is called autoclaved aerated concrete (AAC) because it is cured in a pressurized vessel of some type in the presence of free water at temperatures well above the boiling point of water. The most common curing chamber available to accomplish this is a steam-heated autoclave that is operated along the liquid/vapor curve of water at 180–190 C. The development of final properties of reinforced precast AAC units such as a slab or a beam used for floors, walls, or roofs, or of a masonry product such as a block or a panel that is later assembled with thin-set mortar and used for exterior and interior walls or panels normally occurs in two steps: during precure and during final cure in an autoclave.

There are two main types of AAC used worldwide: those based on quartz flour and those based on Class F fly ash. Both are durable once coated with a waterproofing agent and both have the same physical and mechanical properties, but one is gray and the other white. In both cases, approximately 60 wt % or more of the siliceous material is blended with smaller amounts of Portland cement and lime and enough hot water to make a thin slurry. During the last minute of mixing, microme- ter-sized aluminum metal flake is added to the mixture. The aluminum reacts with the caustic in the cement to form millimeter-sized hydrogen bubbles, which in turn cause the slurry to double or even treble in size. Slaking of the lime in the mixture and hydration of the cement causes the “cake” to thicken and harden in 45–60 min, that is, to develop green strength. The cake is strong enough at this point to be demolded and cut to size with wire saws. The blocks are then autoclaved at 180–190 C and saturated steam pressure [11]. Although not very common in the USA, AAC is a very popular construction material used worldwide [12, 13].

RILEM has published a set of recommended test methods much like ASTM has done for cellular concrete [11]. There are a total of 27 such recommendations which cover the entire gamut of characteristics discussed below. ASTM is currently developing similar test procedures for AAC as part of its Committee C 27.60. To date, two have been accepted and published: ASTM C 1386-98 (Standard Specification for Precast Autoclaved Aerated Concrete (PAAC) Wall Construction Units) and ASTM C1452-00 (Standard Specification for Reinforced Autoclaved Aerated Concrete Elements). Work in progress includes a standard for measuring the modulus of elasticity of AAC.

Even though cellular concrete does not normally contain fine/coarse aggregate, the choice to call these aerated materials “concretes” was intentional. By associating them by name with conventional concrete it was implied that the aerated cellular cement pastes that comprise cellular concrete would have the same performance, albeit at a lower level, and the same durability as conventional concrete. Second, the use of concrete as a descriptor made these materials more acceptable to engineers, who are keenly aware of the fact that cement pastes shrink and crack and do not perform well in traditional engineering applications [5].

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2.9.3

Per-Capita Consumption

If the per-capita consumption of cellular concrete masonry in all of its forms were ranked in descending order by geographical location, consumption in the USA would appear near the bottom of the list [12,13]. The regions appearing near the top would be Europe and Asia. One can only speculate as to the cause of this; more than likely it is a combination of economic and cultural factors. Cellular concrete products are currently more expensive in the USA than conventional concrete block; U.S. families tend to prefer wood framed houses over stucco-finished concrete block houses; and U.S. home owners tend to move every seven years or so and thus they consider price more so than long-term durability when choosing a residence.

Nevertheless, AAC is gaining increasing acceptance as a construction material for public and commercial buildings in the USA, because AAC is nearly maintenance free and thus more cost effective in the long term. Currently there are four U.S. producers and one Mexican distributor (Contec) based in Texas supplying the industry with AAC block and panel that is being used to construct factories, elementary schools, college dormitories, libraries, office buildings, hotels, and other commercial buildings. The history surrounding its introduction in the U.S. market is both interesting and telling; growing pains are clearly evident. Two successful German companies (Hebel and Ytong) built plants in the Southern part of the United States and for a period of ten years or so tried to establish an AAC presence in the U.S. They were not successful. There was little consumer acceptance. They subsequently sold their plants to U.S. companies who have continued to operate them. Ytong has become Aercon Florida LLC and Hebel has become Babb International in Georgia. ACCO Aerated Concrete Systems in Florida and E-Crete in Arizona were built by the current owners. At the same time EPRI constructed a mobile AAC plant that was used to manufacture on-site AAC block from the fly ash of ten coal burning power plants throughout the USA. They published three volumes containing analyses of the various fly ashes used, typical recipes, and a tabulation of the physical and mechanical properties of the AAC produced at each plant [14]. Open houses were held and samples were distributed, but very little interest was actually generated. In contrast, Mexico has the distinction of having the oldest continuously operating AAC plant in the Americas. Buildings made with their AAC have withstood countless earthquakes. Acceptance in Mexico is much higher than in the USA. Perhaps cellular concrete has been adopted by the Mexicans, Europeans, and Asians because it is both lightweight and self-insulating and is well suited as a replacement for solid stone and or cement block used in the stucco-finished housing common in these areas.

Whatever the cause may be, in general terms, the situation on a worldwide basis is very different from that in the USA. As acceptance by the commercial sector increases and the U.S. population is exposed to cellular concrete products and begins to adopt them, prices of cellular concrete masonry materials will come down and cellular concretes will be able to compete with wood in price. Given the current state of affairs for masonry, it is safe to say that the most commonly used cellular concretes in the USA at present are the cast-in-place lightweight geotechnical fill materials and controlled-strength fill materials used for construction purposes [6, 7].

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2.9.4

Overview of Cellular Concrete

Cellular concrete is a composite material. The hydrous solid dictates its performance and properties; the gas phase dictates its specific gravity and ease of handling. The properties of the matrix phase can be manipulated almost infinitely by varying bulk composition, type of starting materials, water/cement ratio, and precuring and final curing conditions. The gas phase can be introduced in a number of ways during the mixing and precuring stage. Although the gas phase affects final properties of the composite, it is essentially considered to be inert filler.

2.9.4.1

The Gas Phase

The gas phase present in cellular concrete is introduced by using chemical airentraining agents, pre-formed foams, and/or micrometer-sized aluminum flake. These substances are added to a Portland cement slurry during mixing to produce a stable array of gas bubbles in the slurry. The mechanisms of chemical air-entraining agents [15] are well known and are similar to what is discussed in earlier chapters of this book. Preformed foams are prepared by mixing a surfactant with water in a pressurized container and then adding it to the concrete. Such foams have densities in the 40–70 kg m–3 range [10]. Both foaming agents are normally used to foam cellular concretes that are cured at ambient temperatures or at temperatures well below 100 C. The resulting cellular pastes and slurries can be cast in place, or can be precast and cured in the factory.

Aluminum flake reacts with the caustic solution that evolves during the hydration reaction to form hydrogen gas bubbles. The formation of portlandite Ca(OH)2, which drives the reaction, is due to the fact that the calcium silicate and calcium aluminate hydrates that form during the hydration of Portland cement, that is, calcium silicate hydrate and the various calcium aluminate hydrates, contain less calcium that the starting materials. Because the reaction is a through-solution reaction, the excess calcium and hydroxyl ions remain in solution until concentrations significantly exceed the 20 mmol L–1 considered to be the solubility of Ca(OH)2 in water, that is, the solution becomes supersaturated with respect to portlandite, which begins to precipitate as a separate phase. At this point the pH is approximately 12, which is caustic enough to make the reaction of aluminum proceed. Aluminum flake is not normally used to foam cast- in-place cellular concrete cured below 100 C. Its use is restricted to applications that require production of foam after mixing and placing. For example, if one wishes to produce a reinforced masonry product containing reinforcing rebar or wire cages one has no choice but to introduce a thin slurry into the mold and allow it to rise and engulf the reinforcement. The process makes it possible to reinforce the product without introducing large air pockets along metal/cement contacts.

As one begins to experiment with the formulation of the extremely fluid AAC mixtures used by the industry, one soon arrives at the conclusion that the ability to produce a stable and homogeneous cellular structure with aluminum flake is actu-