Life cycle analysis (LCA) for the incorporation of rice husk ash in mortar coating

Abstract

Considering that the civil construction segment is responsible for a significant amount of the environmental impact, once it consumes a large amount of natural resources and energy, the incorporation of waste in the process brings sustainable benefits for the companies as well as for society. In addition, the reduction of the useful life of mortar coating related to the lack of adherence to the substrate provokes a large generation of waste, increasing the environmental liability of the respective industry. This study aimed to determine the technical and environmental viability of the incorporation of rice husk ash (RHA) waste in mortar coatings, in order to reduce the consumption of natural resources and improve the conditions of adherence of the cementitious matrix. For this evaluation, mechanical tests were carried out to test the adherence to the substrate, besides a survey of all the processes related to mortar coating production, from natural resources extraction till the application of the product and its consequent impact. Finally it was used the life cycle analysis (LCA) tool to evaluate the environmental issue. The results showed an increase of 100% of bond strength for the mortars with 5% addition of RHA, and it was identified, among the processes surveyed, a smaller number of significant impacts in relation to the mortar with no addition. It is noticed that, for the conditions of this study, the mortars with RHA addition present better technical and environmental performance compared to the usual mortars.

Introduction

The growing concern about the production and use of materials and their environmental aspects has promoted the development of researches focused on the civil construction segment. It is also observed that this segment is responsible for a significant amount of environmental impacts, due to the large consumption of natural resources and energy.

The most efficient way to make the construction industry sustainable is through the incorporation of waste from other industries in their materials (Mehta, 2001). In this way, the waste recycling by the civil construction industry has been consolidating itself as an important practice for sustainability, be it reducing the environmental impact generated by the segment, be it reducing costs. However, it is noticed in the related works, a larger concern about evaluating the technological results of the applications, without studying the necessary environmental parameters that must be considered to viable the studied recycling waste.

The use of mortar coating, a product which is largely used by the segment, causes significant environmental impacts. Starting by its composition, formed by non-renewable raw-materials (cement, lime and sand), and, secondly, by the reduction of its useful life due to the lack of adherence to the substrate, which increases the civil construction environmental liability. Many researches demonstrate that the incorporation of waste containing silica such as rice husk ash (RHA) on pastes and mortars promotes a better performance of the product. On the other hand, it is observed a lack in studying issues related to the possible impacts that RHA recycling may generate.

Within this context, this work aimed at applying the life cycle analysis (LCA) methodology to qualitatively compare the environmental performance of mortar coating composed by cement, lime and sand, replacing part of the volume of cement for rice husk ashes.

The NBR ISO 14001 (ABNT, 2004) defines environmental aspect as the element from the organization's activities, products or services that can interact with the environment. The concept of environmental impact described by the norms refers to any modification of the environment, adverse or beneficial, which results, partially or totally, from the environmental aspects of the organization.

According to Höjer et al. (2008), due to the world changes and problems related to the environmental aspects and impacts generated by society, there is a need to look for efficient and powerful tools for understanding the economical and technological evolution, as well as the environmental systems evolution, in order to have a larger interaction between society and the organization planning.

The importance of identifying and evaluating the environmental aspects, according to Zobel et al. (2002), is related to:

• Being required by the environmental management system;

• Finding the significant environmental aspects;

• Tools to implement environmental adjustments in the system;

• Tools to promote continuous improvement.

The concern related to the environmental impacts generated by society has induced the development of new tools and methods that aim to help the understanding with control and the reduction of those impacts. There is, then, a number of different tools used in the analysis of the environmental impacts, which have been developed by various authors. Höjer et al. (2008) classify those tools in:

• Procedural tools: evaluation of the environmental impact, evaluation of environmental strategies and environmental management systems—focused on procedures and connections for decision making;

• Analytical tools: analysis of life cycle, material flow analysis—focused on the technical aspects of the analysis;

• Tools that include economical aspects: analysis of cost-benefit, life cycle cost, inputs and outputs analysis.

In the same way, for the estimate of the significant aspects and impacts, there is a series of methodologies already studied. According to Zobel and e Burman (2004) and Zobel et al. (2002), these are the main criteria used:

• The scale of the impact;

• The severity of the impact;

• The probability of occurrence;

• The permanence of the impact;

• The current or potential legal/regulatory exposition;

• The difficulty to change the impact;

• The relation with the parts involved.

Zobel et al. (2002) propose the use of the LCA method, according to ISO 14040 (Environmental Management: LCA—Principles and Structure) and ISO/WD 14042 (Environmental Management: LCA—Impact Analysis) and the documentation format according to ISO 14048, as a way to determine the significant environmental aspects. According to those authors, this proposition was taken for being a method with scientific rigor and with the possibility of being repeated. As a conclusion, the method has proved to be appropriate, although initially it represented an increase of work. However, it is expected to be reduced later, because the information such as definition of limits and description of process unities do not need to be updated frequently, once they do not change from 1 year to another.

The LCA is a procedure to evaluate the impact of the resources, of the environment and the health of a system (production process, product or service). In this case, the system is the process net necessary to provide a specific service (Jǿrgensen et al., 2004). This tool can be used to obtain a better understanding of the whole system used to produce a specific product and, consequently, improve it (Chehebe, 1997).

The LCA, standardized by ISO 14.040 (ABNT, 2001), is an instrument of evaluation of the environmental impact associated to a product or process that comprehend phases that go from the extraction from nature of the elementary raw-materials that belong to the productive system to the final product disposal after being used. From this point of view, the LCA consists of a supporting technique for environmental management and for sustainable development.

The methodology of LCA application comprehends 4 main phases: definition of the objective; definition of the scope; analysis of the inventory and evaluation of the impact. The definition of the objective describes the aim of the LCA use and the decisions used as support. The definition of the scope defines the physical characteristics and the geographical frontiers of the system to be analyzed, the functional unity, the amount of services (variable) that supply the system, being those variables organized in different scenarios. The analysis of the inventory registers the data concerning the variables involved, allocations among the variables, considering the environmental relevance. The analysis of impact is the interpretation of the results obtained in the analysis of the inventory, considering the satisfactory data for the decision making (Jǿrgensen et al., 2004).

The LCA applied in the civil construction can be used as environmental diagnosis and prognosis of the various areas, being more easily applied and used in the production of civil construction materials (Carvalho, 2002). It is known that the selection of products for the civil construction that cause as less impact as possible on nature is a way to reduce the damages caused to the natural environment during the extraction of raw-materials. However, identifying products economically and environmentally viable is not an easy task (Lippiatt, 1998).

Mastella et al. (2001) emphasize that there are not better materials but processes that present differentiated aspects regarding particular requirements. The choice for one specific material will depend on specific needs of each plant or each company.

The concern regarding the production and use of materials and their environmental aspects is in line with the concept of sustainable development which involves, among other issues, those related to the civil construction segment. So, according to Druszcz (2002), the use of constructive systems and less impacting materials must be encouraged. The pursuit of a sustainable development demands a reduction of waste from the process, as well as the arousal of new administrative strategies that minimize the environmental impacts (Güereca et al., 2006).

In this context, many researches use the LCA tool to verify the environmental impacts consequent from the use of a specific waste. Lee and Park (2005) used the LCA tool to quantify the environmental credits of the recycling blast furnace slag. The results demonstrated a larger amount of environmental credits when the waste was used as raw-material for the manufacturing of Portland cement and slag cement.

Gäbel and Tillman (2005) analyzed the life cycle for 9 processes of different cement manufacturing. The results of the analysis demonstrated that the emissions of the process can be reduced from 30 to 80% when waste is incorporated and alternative fuel is used.

Navia et al. (2006) studied, through ACV, the use of volcanic soil contaminated by heavy metals as raw-material for the production of cement. Two scenarios were compared: Scenario 1 which corresponds to the process of production of the existing cement, and Scenario 2 which represents the production of cement using contaminated volcanic soil. The comparative results were favorable to Scenario 2, especially considering the ecotoxicity category, mainly for avoiding the emissions of volcanic soil when disposed in landfill. The authors concluded that the use of alternative raw-materials (in this case, contaminated volcanic soil), which are the waste from another process, makes possible the development of cement production in a more sustainable way, making the process slightly cheaper.

The civil construction industry responds for a significant amount of the impacts generated by human activities on the environment, because its inputs are big consumers of natural resources and energy. One way to minimize the impacts generated by this segment is through the incorporation of industrial waste for the development of materials and constructive processes, making possible also the reduction in the costs of construction products.

It has been demonstrated that the use of recycled materials is environmentally preferred, once the environmental impacts associated to the process of recycled materials are smaller than those associated to the extraction and processing of raw-material (Lee and Park, 2005).

A large number of researches point towards the use of waste in the civil construction industry. The development and use of cement containing mineral admixture, is growing rapidly (Chindaprasirt and Rukzon, 2008). The mineral additions in cement in the form of pozzolanic material have been used with the aim of improving the mechanical strength and durability of mortar, associated to cost savings and the reduction of environmental impacts.

In this context, the rice husk ash appears as a pozzolanic material due to its high amount of SiO₂. In Brazil, most of the works that study the rice husk ash in civil construction show that large part of those ashes present pozzolanic characteristics, originated from their amorphous chemical structure and from their high concentration of silica.

The cement paste containing silica, in regard to the Portland cement paste with no additions, tends to exhibit a denser microstructure, less heterogeneous and more compact, and with smaller proportion of voids developed. The reaction of silica with calcium hydroxide formed in the cement hydration generates a resistant compound of hydrated calcium silicate. This compound tends to fulfill the big capillary voids, with excellent characteristics of adherence, different and even better than the hydrated calcium silicate obtained with the hydration of Portland cement (Silva and Libório, 2003).

In this way, when introducing materials in mortars, such as rice husk ash (high amount of silica), it will be possible to have a more effective link for the improvement of the paste versus aggregate, or paste versus substrate, interface zone (Silva, 2004).

Regarding the technological performance, it becomes evident that the use of rice husk ash improves the properties of mortar coating adherence. However, it is noticed a lack of researches that consider the developing product life cycle. Due to what exposed above, it is reinforced that this work considers in its analysis both the technological and the environmental aspects, relevant to the application of rice husk ash in mortars.