Editorial

Sustainable management of waste and recycled materials in construction

Due to limited economic resources, which impede access to specific advanced technologies, many developing countries are still facing the challenge of reducing human exposure to heavy metals, which is primarily associated with the consumption of water contaminated through the discharge of poorly treated wastewater.

In wastewater treatment technology, adsorption is sometime preferred to other approaches because of its high efficiency, easy handling, availability of different substrates and cost effectiveness. Moreover, increasing emphasis has recently been given to the use of low-cost adsorbents (generally solid wastes) for the treatment of polluted water, with a resulting double benefit for the environment.

In this paper, the use of red mud and pyrolusite has been investigated for the removal of As and Mn from drinking water. Adsorption equilibrium data have been examined through the application of constant temperature models (isotherms), while batch and dynamic tests have been used to clarify the effects of pH, initial metal ion concentration and temperature on the adsorption performance, aiming at identifying the best conditions for the treatment.

The combined use of the two adsorbents allows exploiting their properties synergistically, maximizing efficacy and sustainability without affecting process design and costs. In particular, ‘clean’ water (i.e. water with heavy metals contents below law limits) has been obtained even after the passage of a volume of solution higher than 40 bed volumes, and considering initial unrealistically high concentrations for the metals.

The sorption of As^V on red mud (bauxite residue), produced in the ALCOA-San Cibrao factory (Spain), was assessed in view of its potential use as sorptive liner of landfills for the attenuation of As-rich leachates. The operating parameters evaluated, using batch-type procedures, comprised the effects of time, solution pH, As^V concentration (sorption isotherm) and presence of phosphate on the As^V sorption. The results showed that the red mud efficiently sorbed As^V. The sorption was fast, with a major fraction of initial As^V being removed in a few minutes or hours of contact, depending on As^V concentration. The kinetic process was well described by the pseudo-second order equation, which points to chemisorption is involved, whereas surface (film) diffusion chiefly governs the rate of As^V sorption for the red mud system. Sorption of As^V was strongly pH-dependent. Maximum removal (>98%) was observed at slightly acidic pH (pH_max = 5.5–6), while As^V sorption considerably decreased at both highly acidic and alkaline pH. The percentages of sorbed As^V decreased with the increasing solution As^V concentration, and the As^V sorption capacity (up to 43.5 mmol/kg) of the red mud was higher (∼4 -fold) at pH ∼6 than at pH ∼9.2 (natural pH of the red mud). The presence of P at equimolar or 1:10 As/P molar ratios reduced As^V sorption by ∼20% and 30%, respectively. Simulations of As^V migration taking into account the effects of dispersion and diffusion through an hypothetical red mud liner, using the sorption parameters and the geotechnical–hydraulic conductivity characteristics of the RM, predicted a deeper migration of As^V in the liner at pH∼9.2 than at pH∼6 and a minimum thickness of ∼90 cm and ∼20 cm, respectively, for a RM liner to decrease the solution As^V concentration from highly toxic 1 mM to a safe <0.133 μM (<10 μg/L) level, after a 35-years period.

This paper studies the potential environmental impact of recycled coarse aggregate (RCA) for concrete production in China. According to the cradle-to-cradle theory, a closed-loop life cycle assessment (LCA) on recycled aggregate concrete (RAC) utilization in China with entire local life cycle inventory (LCI) is performed, regarding the environmental influence of cement content, aggregate production, transportation and waste landfilling. Special attention is paid on the primary resource and energy conservation, as well as climate protection induced by RAC applications. Environmental impact between natural aggregate concrete (NAC) and RAC are also compared. It is shown that cement proportion and transportation are the top two contributors for carbon dioxide (CO₂) emissions and energy consumption for both NAC and RAC. Sensitivity analysis also proves that long delivery distances for natural coarse aggregate (NCA) leave a possible opportunity for lowering environmental impact of RAC in China.

Even though construction and demolition waste (CDW) is the bulkiest waste stream, its estimation and composition in specific regions still faces major difficulties. Therefore new methods are required especially when it comes to make predictions limited to small areas, such as counties. This paper proposes one such method, which makes use of data collected from real demolition works and statistical information on the geographical area under study. Based on a correlation analysis between the demolition waste estimates and indicators such as population density, buildings ageing index, buildings density and land occupation type, relationships are established that can be used to determine demolition waste outputs in a given area. The derived models are presented and explained. This methodology is independent from the specific region with which it is exemplified (the Lisbon Metropolitan Area) and can therefore be applied to any region of the world, from the country to the county level. Generation of demolition waste data at the county level is the basis of the design of a systemic model for CDW management in a region. Future developments proposed include a mixed-integer linear programming formulation of such recycling network.

The construction industry provides extensive impacts to the environment with population increases further driving these pressures. In an attempt to mitigate these impacts, the industry as a whole has promoted and developed numerous green building rating systems (GBRS). These GBRS assign scores to buildings based on a variety of assessment criteria to determine a structures environmental impact. Previous research has identified shortfalls in certain areas related to these GBRS. In order to gain further insight into these systems an industry survey was conducted to establish an alternate perspective. The outcomes of this survey were consistent with that of previous research, identifying the requirement for an increase in the Embodied Energy (EE) consideration of structures in GBRS. The average estimation of the contribution of EE to a structures life cycle consumption was 28.4%. The survey findings also identified a number of other key areas through which changes in the consideration of sustainable development could be improved. The use of these findings and their comparison to previous research outcomes would assist in the development of supplementary mechanisms to improve the assessment of the environmental performance of structures.

The building industry has widespread social, economic and environmental impacts. Considering the materials used, such impacts depend on the production of concrete, since it is the most consumed material and its properties are associated with the consumption of Portland cement, which represents a significant part of CO₂ emissions from this sector. This project studied the utility of recycled tire rubber for lightweight concrete with added metakaolin, with the dual purpose of reducing cement consumption while achieving satisfactory strength. Metakaolin was produced in the laboratory, assessing the minimum temperature for formation from kaolin. The lightweight concrete (in which rubber was substituted for sand) was evaluated using compressive strength, calorimetry and thermal conductivity tests of mortar. The results showed that the metakaolin produced at 800 °C exhibits more efficient silica fume replacement and that producing mortar with 40% rubber yields a compressive strength of 20 MPa. Achieving this strength in lightweight concrete permits the production of materials with low cement consumption, 486 kg/m³ for mortar (22.9 kg of cement per m³ per MPa) and 260 kg/m³ for concrete (13 kg of cement per m³ per MPa). The concrete-rubber results showed the best thermal conductivity indices, thus permitting the design of building systems with improved energy efficiency, which reduces their operating costs.