**3.2 Reducing the amount of cement in the concrete mix**

An alternative to minimise the harmful emissions generated by concrete is to directly reduce the consumption of cement in the mix. For that purpose, supplementary cementitious materials have been developed to replace part of the required cement [31].

**Figure 1.** *Material life-cycle [30].*

These alternative materials include the reuse of waste such as fly ash, which can replace 15–30% of cement [32–36] and silica fume which replace cement by 5–25% [37, 38]. There are other residues as well that have been used, such as ground granulated blast furnace slag [39], metakaolin [40, 41], sewage sludge ash [42, 43], rice husk ash [44], which can replace up to 20% of cement.

Alkaline activated cements can be used to replace more than 50% of conventional cement [45–48]. This material is a product with cementitious properties generated from the reaction of a powdery material with an aluminosilicic nature and an alkaline agent [49]. Alkaline cements are mainly characterised by low hydration heats, high mechanical performance, good durability against different chemical attacks, often using industrial waste as the only raw material and not requiring high energy consumption compared to the Portland cement manufacturing process [50].

Even a "cementless concrete" can be produced by implementing a sub-group of alkaline-activated materials called geopolymers [50]. In this regard, the main author of this chapter has the experience of design and making geopolymer material with Chilean aggregates in the Microlab TU Delft. The results were optimal, obtaining without problems the design strength.

However, the implementation of the technology of actively alkaline cements and geopolymers requires great care in their production. In addition, the polymerisation reaction is very sensitive to temperature and requires the curing of the geopolymer concrete to be at an elevated temperature under a strictly controlled temperature regime [51, 52]. These aspects coupled with the lack of standardisation of the material make the practical implementation of this technology limited to particular engineering solutions such as the construction of prefabricated structures like sewer pipes and marine members.

#### **3.3 Implementation of optimised designs**

Another approach to reduce the environmental impact of concrete is to implement sustainable design features. For instance, optimised structures requiring fewer concrete material to satisfy the in-service demands or optimised structures requiring less conditioning energy during their use phase. Optimisation has been applied in the design of hydraulic structures as dams [16–18]. Actually, Deepika and Suribabu used differential evolution algorithm to find the best optimal shape of a gravity dam reducing 20% the demand of concrete [53].

Similarly, Hashemian proposed a cambered curve beam which requires 20% less concrete than the conventional prismatic beam [54].

Other possibility of optimization is to take better advantage of intrinsic properties of the concrete material in the design of structures. An of example of is the concrete thermal inertia, which is the characteristic of taking a long time to warm up, but, once warm, taking a long time to cool down, and vice versa. This property incorporated in building design can result in structures that maintain a comfortable internal temperature by means of a better interaction with the environment [55, 56]. As a result, a reduction of the conditioning energy over the lifetime of the building is achieved.

#### **3.4 Improving the concrete durability**

From a sustainable and technical perspective, the greater the durability of the concrete material, the greater the benefits. Certainly, it is not the same to have a concrete material, for the same application, enduring 15 or 60 years. If the structure has good durability, then it will require less significant interventions and the service life can be extended. In this regard, the ICRI Committee 160 mentions that

#### *Different Approaches to Develop More Sustainable Concrete Alternatives DOI: http://dx.doi.org/10.5772/intechopen.100194*

the most effective sustainability strategy for concrete and masonry structures is to avoid the need of repairing [57].

A particular case is reinforced concrete, where it is very important to avoid or reduce the entrance of atmospheric agents that can corrode the reinforcement [58–60]. This can prevent advanced deterioration and damage, which repair works, require substantial resources. For example, in the UK the annual cost of repairing reinforced concrete structures near coastal areas is £755,000,000 [61]. Similarly, between 1991 and 2001, the cost of corrosion repair of reinforced concrete structures in the United States was \$276 million, representing 3.1% of the gross domestic product [62].

In order to prevent corrosion deterioration, it is essential to reduce the formation and propagation of micro-cracking generated by the concrete shrinkage phenomena at early age [60, 63, 64]. As alternatives to limit the micro-cracking and thus increase the concrete durability, Jonkers et al. developed a promising self-healing concrete technology based on the application of mineral produced by bacteria included in the mix [23]. This method allows to repair from inside the material without requiring external agents to activate the process.

Another widely evaluated alternative is the incorporation of fibres in the concrete, which can reduce the number, size and propagation of microcracks [21, 22]. In this case, compared to man-made fibres, natural fibres are less expensive, locally available (in some cases as industrial waste), renewable, lightweight, biodegradable and less energy intensive to produce [65–67].

#### **3.5 Reducing the amount of natural aggregates in the concrete mix**

The concrete industry generates a high demand for natural aggregates, due to the massive use of concrete, and the fact that approximately 70% of the volume of this material is composed by aggregates [68]. Uribe mentions that aggregate extraction processes generate loss of vegetation cover and soil [10]. Consequently, the landscape and the habitat of the existing fauna in the area are altered. Moreover, soil fertility and air quality are reduced due to emissions of particulate matter. Therefore, the life quality of the people living near the extraction sites is affected as well.

Another direct impact relates to construction costs. Indeed, as aggregates are a finite resource, they can become scarce, which can increase the cost of acquiring and transporting materials from more distant locations.

Globally, up to 17.5 Gt of aggregates have been consumed annually for concrete manufacturing [69]. In the United States, two billion tonnes of aggregates are produced annually [70]. Similarly, it is estimated that in Chile, aggregate extraction amounts to 7 million 500 thousand cubic metres produced annually, of which five million correspond to gravel and sand [71].

Therefore, in order to preserve natural resources and, at the same time, contributing to solve waste disposal problems, it has been evaluated to replace natural aggregates by different types of waste such as recycled crushed concrete, crushed bricks, recycled glass, rubber, ceramics, marble waste, textile effluent sludge [19], waste foundry sand [20], steel slag [72], copper slag [73], blast furnace slag, ferrochrome slag, class F fly ash, palm oil clinker [68] and various types of plastic waste [74].
