The effective use of construction and demolition waste (CDW) and its application in reusable structural elements can simultaneously reduce waste dumping and decrease the need to use primary resources, both of which are important environmental aspects to be considered in responsible sustainability management. This represents an important contribution to the solution of one of the core objectives of the 2030 UN Agenda on Sustainable Development—Goal 12: Ensure sustainable consumption and production patterns [1
]. This goal is focused on economic growth based on efficient resource use and low environmental degradation while improving the well-being of people. This can be done by a shift towards more sustainable resource consumption and improved production processes.
Currently, this is a very real problem, not only from a local/regional perspective, but especially from a global point of view. The material footprint per capita of developing countries almost doubled in the last eight years, representing a significant and needed improvement in material standards of living [1
]. Most of this increase is connected with the rising consumption of nonmetallic minerals due to growth in infrastructure and construction. This also includes the environmental impact of concrete structures, which is still growing. Thus, concrete plays an important role in this process and represents a promising challenge for the future.
The replacement of natural aggregate (NA) by recycled aggregate (RA) from construction and demolition waste reduces consumption of primary recourses. However, its utilization mostly negatively affects the properties of concrete. For this reason, the utilization of recycled construction and demolition waste is mostly limited to the use of recycled concrete waste for constructing base layers in road structures or partial replacement of aggregates for concrete. Mixed recycled aggregate (MRA) with a high content of waste masonry has not found satisfactory utilization yet. There are concerns over the quality of recycled masonry as a technological material due to the complexity of the demolition and recycling process. Furthermore, this secondary raw material cannot be used as an aggregate for concrete according to Czech standards [2
]. However, the selective demolition and two-phase recycling process leads to higher-quality recycled masonry aggregate (RMA) without the unwanted impurities which negatively influence its properties. Despite the high quality of RMA, its use as an aggregate for recycled aggregate concrete (RAC) negatively influences mechanical properties and durability. However, RMA has a positive impact on thermal properties.
Many studies of the properties of RMA and its effect on the mechanical properties of RAC have been published [3
]. It has been recognized that RMA differs from NA mainly in terms of water absorption, density, and resistances, for instance, resistance to wear, abrasion resistance, or freeze–thaw resistance [24
]. The range of water absorption of coarse RMA has been established from 10% to 19%, which is up to 25 times higher than natural gravel [5
], and fine RMA from 12% to 15%, which is more than 10 times higher than natural sand [16
]. The dry density of coarse RMA ranges between 1800 and 2700 kg/m3
and fine RMA between 2000 and 2500 kg/m3
, which is generally lower than natural gravel and sand [5
The physical, mechanical, thermal, and durability properties of concrete are influenced by the physical, mechanical, thermal, and durability properties of aggregates. The higher water absorption of RMA affects the workability of fresh concrete [30
]. For this reason, it is necessary to determine the additional water needed and to add it to the concrete mixture during mixing [16
] or to soak RMA in water for 24 h before mixing [22
] to achieve workability that is similar to a conventional concrete mixture. Workability and the effective water/cement (w/c) ratio influence the compressive strength of concrete [26
]. The effective w/c ratio is the total amount of water which reacts with cement divided by the total amount of cement. For recycled aggregate concrete, this depends on the water absorption capacity of the recycled aggregate [31
]. The compressive strength of recycled aggregate concrete decreases with an increasing NA replacement rate [16
]. No significant decline of compressive strength has been found for recycled aggregate concrete with 15% coarse RMA. On the contrary, the compressive strength of concrete with full replacement of coarse NA by RMA decreases up to 35% [22
]. The partial replacement of natural sand by fine RMA has had no significant impact on the compressive strength of recycled aggregate concrete. The reason could be the silica and alumina contents in crushed bricks, which could lead to pozzolanic reactions [30
The thermal conductivity of concrete depends on the type of aggregate [32
], its thermal conductivity, as well as the density and porosity of the aggregate [33
]. Furthermore, the thermal conductivity of concrete is also strongly influenced by the w/c ratio [34
] and cement content [35
]. It has been established that lower aggregate thermal conductivity causes lower concrete thermal conductivity. This aspect could positively influence the usability of concrete with RMA due to its lower thermal conductivity caused by the low thermal conductivity of RMA, which ranges between 0.60 and 0.78 W/(m·K) [33
]. The thermal conductivity of NA depends on its mineralogical characteristics, composition, and degree of crystallization. The heat conduction of NA with a crystalline structure is higher than amorphous and vitreous NA of the same composition [36
In order to decrease the thermal conductivity of concrete, various kinds of thermal insulation materials have been added to self-insulating concrete. For instance, expanded polystyrene (EPS) beads, vermiculite, and glazed hollow beads have been used as insulating materials added to concrete. The utilization of thermal insulation materials as partial replacement of sand in concrete mixtures diminishes the mechanical properties, density, and thermal conductivity of concrete. Moreover, the decrease of these properties depends on the type of additional material [37
]. The density of self-insulating concrete with recycled EPS ranges from 1070 to 1250 kg/m3
, with the thermal conductivity between 0.34 and 0.5 W/(m.K) and compressive strength between 7.74 and 15.55 MPa at 28 days (see Table 1
]. In another study [38
], two types of lightweight concrete were manufactured. One concrete mixture with a fresh density of 400 kg/m3
was produced with the strength of 3.0 MPa and thermal conductivity 0.09 W/(m.K), and another concrete mixture with a fresh density of 800 kg/m3
was produced with the strength of 13.0 MPa and thermal conductivity 0.25 W/(m.K). Polystyrene foamed concretes of densities ranging from 150 to 1200 kg/m3
with an EPS volume between 0% and 82% were compared with foamed concrete of 800 kg/m3
density without EPS [39
]. The results of this study indicated a significant decline of compressive strength and a reduction of thermal conductivity caused by the increased EPS content. The concrete mixture containing 45% EPS had compressive strength of about 0.85 MPa and thermal conductivity of 0.16 W/(m·K), while the concrete mixture with 82% EPS had compressive strength of 0.08 MPa and thermal conductivity of 0.08 W/(m·K). Recycled EPS can be also used as a partial replacement of aggregate in self-insulating concrete for structural utilization [40
Research on recycled materials as a partial or full replacement of NA in structural applications such as concrete blocks, paving blocks or floor blocks has already been published. The main reason for replacement of aggregate, which is the major component in concrete blocks, is the primary sources savings [50
]. There are many recycled waste materials which is possible to use as partial or full replacement of aggregate in concrete blocks such as recycled concrete waste [51
], crushed brick waste [11
], glass waste [67
], crump rubber waste [71
], ceramic and tile industry waste [74
], marble waste [75
], plastic waste [77
] and concrete slurry waste [78
]. Moreover, due to its unique characteristics, the recycled materials could positively influence some properties of concrete blocks such as thermal conductivity, thermal resistance [65
] or mechanical properties [67
The use of RMA as a partial or full replacement of aggregate in structural concrete was examined for manufacturing precast prestressed beams [11
], paving with precast concrete [58
], and paving blocks or hollow tiles [59
]. It was found that the most affected property of concrete was the modulus of elasticity, while compressive and tensile strengths were maintained at acceptable values for the full replacement of NA. The maximal acceptable replacement rate of RMA was found to be up to 35% for concrete with RMA in precast prestressed joists of building floors. Recycled aggregates from CDW containing more than 50% of waste concrete, more than 20% of waste clay bricks, and around 20% of cement or mortar stone were also used as a partial replacement of natural aggregates for preparing concrete masonry blocks suitable for indoor applications [60
]. In this research, full blocks of 95% RA and hollow blocks of 75% RA were manufactured and tested.
This paper presents the environmentally based optimization of a concrete mixture containing recycled materials for mortarless masonry wall structures. Due to the low thermal conductivity of RMA and EPS, their utilization could have great potential for manufacturing concrete blocks for mortarless masonry walls of low-rise buildings, despite the decline of strength. From technical and/or economic viewpoints, the principle of mortarless masonry permits easy wall deconstruction for the most effective reuse of structural elements after their end of life.
2. Materials and Methods
In total, 10 concrete mixtures were prepared and tested in order to verify the properties of concrete made using recycled masonry aggregate. One of them was a reference mixture with a natural aggregate only, and other mixtures contained recycled masonry aggregate and recycled expanded polystyrene in various ratios as a partial or full replacement of natural aggregate.
2.1. Recycled Aggregate
This research used one type of NA, two types of RMA, and one type of recycled EPS. Both types of RMA originated from construction and demolition waste and were delivered in fractions of 0–8 and 8–16 mm by a Czech recycling center (see Figure 1
). For utilization as a substitute for fine-grained aggregate, in mixtures with an aggregate of fraction 0–16 mm in various replacement ratios, the fractions 0–4 and 4–8 mm were separated from the aggregate of fraction 0–8 mm in the laboratory. For mixtures containing a 0–8 mm fraction, RMA 0–8 mm was used without any laboratory treatment. Physical properties of RMA, especially water absorption, differ from NA. Therefore, the physical properties (see Table 2
) are presented to show the differences in the materials used for the preparation of the concrete mixtures.
Selected properties of RMA were tested according to valid Czech standards. The properties most influencing the recipe design were tested. The basic physical properties of RMA are shown in Table 2
, the granulometry is shown in graphs in Figure 2
, and the composition of RMA is listed in Table 3
2.2. Recycled Aggregate Concrete Mixtures
Ten concrete mixtures with the same exposition class XF1, effective w/c ratio 0.5, and amount of cement CEM I 42.5 R 320 kg/m3
were prepared for laboratory measurements. One mixture of conventional concrete of strength class C30/37 only with NA of fraction 0–16 mm was manufactured as a reference to compare with the other mixtures in which NA was replaced in various ratios by RMA (five mixtures) and recycled EPS in addition (four mixtures). Two mixtures of RMA concrete (RMAC E and RMAC EPS D) only with RMA of fraction 0–8 mm was manufactured (see Table 4
). The sample fragments of each material and the composition of RMA concrete mixtures are shown in Table 5
and Figure 3
The physical, mechanical, deformation, and thermal properties were tested according to valid Czech standards. Samples of dimensions 100 × 100 × 400 mm, 150 × 150 × 150 mm, and 100 × 100 × 100 mm were used for testing.
2.3. Evaluation Methodology
Samples were stored and cured in a stable laboratory environment during solidification and maturation, and after 28 days, the following properties were determined by laboratory tests: physical (density and capillary absorption), mechanical (compressive strength and tensile strength), deformation (static modulus of elasticity in compression), and thermal (volume heat capacity and thermal conductivity).
The mechanical properties, such as compressive strength, flexural strength, and static modulus of elasticity, were examined according to European and Czech standards. Water absorption capacity by immersion was tested on cubic specimens 100 × 100 × 100 mm. Specimens were treated by water, and after stabilization of weight, dried in an oven at a temperature of 105 ± 2 °C until stabilization of weight. The saturated surface-dried density and dry density were measured on these samples. Capillary water absorption was determined by measuring the rate of water absorption by capillaries. The ends of fractured prismatic specimens of 100 × 100 × approx. 150 mm, which were tested after the tensile strength test, were immersed in water up to a maximum height of 5 mm for 72 h or until their weight stabilized. The amount of water absorbed at different time intervals was measured by periodically weighing the surface-dried sample. Weighing intervals were 5 min, then 15 min for the first hour, then every hour for the first 6 h, and finally, every 12 h.
Measurement of thermal properties was done by the portable hand-held system ISOMET 2114 (Applied Precision Ltd., Bratislava, Slovakia) for measurement of the heat transfer properties of the materials. This applies a dynamic measurement method, which enables reducing the measurement time in comparison with steady-state measurement methods. It is equipped with a surface probe for measuring solid and hard materials. A flat surface of at least 60 mm diameter is satisfactory for the probe. Demand for the accuracy of the surface flatness increases as the thermal conductivity value of the tested material increases. The expected minimal thickness of the evaluated materials ranged from 20 to 40 mm depending on their diffusivity (conductivity).
Measured quantities and measurement ranges:
Thermal conductivity λ (W/(m·K)): 0.04–6.00;
Thermal diffusivity a (m2/s);
Volume heat capacity cρ (J/(m3·K)): 4.0 × 104 to 3.0 × 106;
Temperature T (°C): −15 to +50.
The thermal properties were tested on three cube samples of concrete of dimensions 100 × 100 × 100 mm for each mixture (see Table 3
). The samples were tested under constant laboratory conditions. The temperature was 23 ± 3 °C.