Properties of sustainable concrete containing fly ash, slag and recycled concrete aggregate
Introduction
Minimising the environmental impact and energy- and CO2-intensity of concrete used for construction is increasingly important as resources decline and the impact of greenhouse emissions becomes more evident. Thus, it is logical to use life cycle and sustainable engineering approaches to concrete mix design. This requires several elements: maximizing concrete durability, conservation of materials, use of waste and supplementary cementing materials, and recycling of concrete. Waste and supplementary cementing materials such as fly ash, blast furnace slag, silica fume, rice husk ash and metakaolin can be used as partial replacements for portland cement. These materials can improve concrete durability, reduce the risk of thermal cracking in mass concrete and are less energy- and CO2-intensive than cement. Use of aggregate obtained from crushed concrete is an example of recycling and conservation of raw materials.
Examples of developments in sustainable philosophies for concrete have been published [1], [2], [3], [4], [5], [6]. Much research has been performed on the use of high volumes of fly ash and other supplementary cementitious materials to produce more sustainable and durable concrete [7], [8], [9], [10]. Recycled concrete is becoming of increasing interest for use as aggregate in structural concrete and recent research has examined its performance [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22].
The objective of the research described in this paper was to assess the properties of concrete that combine both supplementary cementitious materials and recycled concrete aggregate. The application of particular interest was wind turbine foundations since these use large quantities of concrete. A target 28 day compressive strength of 40 MPa was sought, in addition to durability in sulphate- and chloride-bearing environments to which onshore and offshore concrete foundations may be exposed. The mixes contained relatively high amounts of fly ash, blast furnace slag and combinations of both. The mixes were tested for basic mechanical and hydraulic properties in addition to durability in marine environments.
Section snippets
Materials and concrete mix design
Concrete mixes containing either natural or recycled concrete aggregates were studied and compared. Five different mix designs were investigated for the concrete with natural aggregate. The first of these was a control mix and did not contain any fly ash or ground granulated blast furnace slag. Two of the mixes contained 50% replacement of cement with either fly ash or slag and one contained 70% slag. The final mix used 25% fly ash and 25% slag as cement replacement. The cement used was ASTM C
Workability
Other than the trial concrete mixes containing dry RCA, all mixes exhibited good workability and were readily consolidated under laboratory conditions. Fly ash gave the most significant improvement in workability and required a shorter vibration time. This was reflected in the slump measurements presented in Table 2. Workability was similar for the mixes with different sized RCA and the slump values were 65–85 mm as shown in Table 4. In addition, the workability of mixes with natural and
Conclusions
The combined effects of partial cement replacement and use of recycled concrete aggregate to improve the sustainability of new concrete was investigated. The results indicated that concrete mixes containing 50% replacement of cement with blast furnace slag gave the best results in terms of mechanical properties and durability when either natural or recycled concrete aggregate was used. Recycled concrete aggregate was not significantly detrimental to strength, particularly when the concrete also
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