Structure and properties of aerated concrete: a review

Abstract

Aerated concrete is relatively homogeneous when compared to normal concrete, as it does not contain coarse aggregate phase, yet shows vast variation in its properties. The properties of aerated concrete depend on its microstructure (void-paste system) and composition, which are influenced by the type of binder used, methods of pore-formation and curing. Although aerated concrete was initially envisaged as a good insulation material, there has been renewed interest in its structural characteristics in view of its lighter weight, savings in material and potential for large scale utilisation of wastes like pulverised fuel ash. The focus of this paper is to classify the investigations on the properties of aerated concrete in terms of physical (microstructure, density), chemical, mechanical (compressive and tensile strengths, modulus of elasticity, drying shrinkage) and functional (thermal insulation, moisture transport, durability, fire resistance and acoustic insulation) characteristics. 

1. Introduction

Aerated concrete is either a cement or lime mortar, classified as lightweight concrete, in which air-voids are entrapped in the mortar matrix by means of a suitable aerating agent. Broadly speaking aerated concrete falls into the group of cellular concrete (microporite being the other). The prominent advantage of aerated concrete is its lightweight, which economises the design of supporting structures including the foundation and walls of lower floors. It provides a high degree of thermal insulation and considerable savings in material due to the porous structure. By appropriate method of production, aerated concrete with a wide range of densities (300 - 1800 kg/m3) can be obtained thereby offering flexibility in manufacturing products for specific applications (structural, partition and insulation grades). There have been several investigations on the properties of aerated concrete in the past. The first comprehensive review on aerated concrete was presented by Valore and detailed treatment by Rudnai and Short and Kinniburgh. Although aerated concrete was initially envisaged as an insulation material, there has been renewed interest on its structural characteristics in view of its lighter weight, savings in material and potential for large scale utilisation of wastes like pullverised fuel ash. Hence, it was felt essential to compile and review the available literature on aerated concrete. This review aims to classify the studies on aerated concrete related to its material structure and properties.


2. Classification of aerated concrete

2.1 Based on the method of pore-formation

Air-entraining method (gas concrete): Gas-forming chemicals are mixed into lime or cement mortar during the liquid or plastic stage, resulting in a mass of increased volume and when the gas escapes, leaves a porous structure. Aluminium powder, hydrogen peroxide/bleaching powder and calcium carbide liberate hydrogen, oxygen and acetypene, respectively. Among these, aluminium powder is the most commonly used aerating agent. Efficiency of aluminium powder process is influenced by its fineness, purity and alkalinity of cement, along with the means taken to prevent the escape of gas before hardening of mortar, In the case of Portland cements with low alkalinity, addition of sodium hydroxide or lime supplement the alkali required.

Foaming method (foamed concrete): This is reported as the most economical and controllable pore-forming process as there are no chemical reactions involved. Introduction of pores is achieved through mechanical means either by pre-formed foaming (foaming agent mixed with a part of mixing water) or mix foaming (foaming agent mixed with the mortar). The various foaming agents used are detergents, resin soap, glue resins, saponin, hydropysed proteins such as keratin etc.

Combined pore-forming method: Production of cellular concrete by combining foaming and air-entraining methods has also been adopted using aluminium powder and glue resin.

2.2. Based on the type of binder

Aerated concrete is classified into cement or lime based depending on the binder used. Attempts have also been made to use pozzolanic materials such as pulverised fuel ash or slate waste as partial replacements to the binder or sand.

2.3. Based on the method of curing

Aerated concrete can be non-autoclaved (BAAC) or autoclaved (AAC) based on the method of curing. The compressive strength, drying shrinkage, absorption properties etc. directly depend on the method and duration of curing. The strength development is rather slow for moist-cured products. Autoclaving initiates reaction between lime and silica/alumina bearing ingredients. The hydrothermal reactions involved in autoclaving have been explained. There exists wide variation in the pressure and duration suggested by various authors (duration - 8 - 16 h, and pressure - 4 - 16 MPa). The other variables of significance are the age and condition of the mix at the start of the curing cycle and rates of change of temperature and pressure. Autoclaving is reported to reduce the drying shrinkage significantly and is essential if aerated concrete products are required within acceptable levels of strength and shrinkage.