Thermal Performance of Concrete with Recycled Concrete Powder as Partial Cement Replacement and Recycled CDW Aggregate

: This novel study was triggered by a lack in the international literature of the simultaneous use of ground recycled concrete (GRC) as a cement replacement and mixed recycled aggregate as part of the granular skeleton in recycled concrete. It explores the thermal behaviour of concrete mixes bearing 10 wt% or 25 wt% GRC as a cement replacement and 25 wt% or 50 wt% mixed recycled aggregate (MRA) sourced from construction and demolition waste (CDW). The experimental programme conducted assessed concrete’s dry density, open porosity, electrical and thermal conductivity and speciﬁc heat capacity. The ﬁndings showed that the use of 10% and 25% GRC, in conjunction with 50% MRA, reduced thermal conductivity by 7.9% to 11.8% and raised speciﬁc heat capacity by 6.0% to 9.1% relative to concrete with 100% natural aggregate (NA). A cross-property analysis revealed that improved thermal performance was linearly related to lower density and higher porosity. The results also support the conclusion that these new recycled aggregate concrete mixes are more energy-e ﬃ cient construction materials than conventional concrete.


Introduction
Concrete production is responsible for serious environmental impacts worldwide. These impacts can be divided into two main problems: use of natural aggregates, and Portland cement production, which leads to an average consumption of 125 kW/h of electricity and results in an emission to the atmosphere of 800 kg of CO 2 . Furthermore, the latter also leads to the consumption of a vast number of natural resources [1,2].
In addition to these two problems, the construction industry is also faced with the challenge of adequately disposing the large amount of waste that the industry itself generates. It is recalled that construction and demolition waste (CDW) represents more than 30% of solid waste generated among all economic sectors [3].
It is essential to find, on the one hand, solutions for the use of these CDW, in addition to their use as recycled aggregates (RA), and, on the other hand, sustainable binders that can replace cement. Hence, a hypothesis to solve these environmental problems is the use, in concrete, of binders formed by these CDW.
Despite this variation, several other investigations [13][14][15][16][17] also point to decreases in thermal conductivity higher than 20% in mixes with the replacement of natural aggregates with RA from concrete or glass.
This work aims at analysing the thermal behaviour of concrete with recycled concrete powder, as partial cement replacement and RA from CDW, in order to fill the complete lack of information in the literature on this subject. This study started with the collection of CDW from a recycling plant, to be used as RA, and with the production of concrete powder, to be used instead of cement as a binder. In order to carry out this investigation, concrete was produced with RA from CDW (0% and 50%) and recycled cement with recycled concrete powder (0%, 10% and 25%) mixed with Portland cement. The analysis of the thermal behaviour of concrete was carried out through thermal conductivity and specific heat capacity tests, as well as by confronting these properties with the air content, the compressive strength, the open porosity and the electrical resistivity of these mixes. The joint analysis of these concrete properties and the physical and chemical properties of recycled concrete powder allowed a detailed analysis and understanding of the thermal performance of these mixes. The main innovation of this investigation is the analysis of the thermal behaviour of concrete with recycled concrete powder as partial cement replacement, which, as mentioned above, is totally innovative. Furthermore, the present study also evaluates the thermal behaviour of concrete that simultaneously contains recycled cement and recycled aggregates from CDW.

Binders
The three binders used, all European standard EN 197-1-compliant (Table 1), were: a type 1 42.5 R (CEM I 42.5 R) ordinary Portland cement (OPC); a blend of 90% OPC and 10% ground recycled concrete (GRC), labelled RB 10 ; and a blend of 75% OPC and 25% GRC, labelled RB 25 . The GRC was obtained by crushing and grinding (to a maximum size of 147 µm) laboratory-prepared concrete specimens, batched as per the Faury method [18]. The composition and fresh and hardened 28-day characteristics of that concrete mix are given in Table 2. Further to the X-ray fluorescence (XRF)-determined chemical composition of the GRC and OPC shown in Figure 1a, the major oxides were SiO 2 (46.10 wt%) and CaO (40.0 wt%). The figure also presents graphically the minimum and maximum ground concrete replacement ratios used in earlier studies [20][21][22][23][24][25][26] on pastes, mortars and concrete.
The large, irregularly shaped particles visible in the micrograph in Figure 1b were identified as aggregate with bound mortar, and the small clusters with a rough surface as the paste. GRC's density was 2.54 g/cm 3 , a value lower than 3.11 g/cm 3 observed in OPC.
The large, irregularly shaped particles visible in the micrograph in Figure 1b were identified as aggregate with bound mortar, and the small clusters with a rough surface as the paste. GRC's density was 2.54 g/cm 3 , a value lower than 3.11 g/cm 3 observed in OPC.

Aggregates
The crushed limestone coarse natural aggregate used was graded in two particle sizes, 4 mm to 12 mm (NG-M) and 12 mm to 22 mm (NG-G). The fines consisted of two siliceous river sands, graded to 0 mm to 2 mm (NS-F) and 2 mm to 4 mm (NS-C).
The mixed recycled aggregates (MRA) supplied by a CDW recycling plant in Lisbon, Portugal, were sieved and classified at the laboratory, where sizes <4 mm and >22 mm were rejected. The composition of the coarse (4 mm to 22 mm) MRA aggregate used, complying with European standard EN 933-11 [27], is given in Table 3. On the grounds of those data and Spain's structural concrete code (EHE-08) [28], the MRA is designated a mixed recycled aggregate for its Rcu (Rcu = R + Ru) content was ≤95 wt% and its Rb content >5 wt%. It would also be classified under that category in the Agrela et al. [29] proposal, with Rcu >70 wt% and Rb <20 wt%. The physical and mechanical properties of the aggregates given in Table 4 show that the MRA was less dense and less abrasion-resistant than the NA (NC-M and NC-G). This was attributed to the higher water absorption of the bound mortar and fired clay materials comprising the MRA. The Los

Aggregates
The crushed limestone coarse natural aggregate used was graded in two particle sizes, 4 mm to 12 mm (NG-M) and 12 mm to 22 mm (NG-G). The fines consisted of two siliceous river sands, graded to 0 mm to 2 mm (NS-F) and 2 mm to 4 mm (NS-C).
The mixed recycled aggregates (MRA) supplied by a CDW recycling plant in Lisbon, Portugal, were sieved and classified at the laboratory, where sizes <4 mm and >22 mm were rejected. The composition of the coarse (4 mm to 22 mm) MRA aggregate used, complying with European standard EN 933-11 [27], is given in Table 3. On the grounds of those data and Spain's structural concrete code (EHE-08) [28], the MRA is designated a mixed recycled aggregate for its R cu (R cu = R + Ru) content was ≤95 wt% and its Rb content >5 wt%. It would also be classified under that category in the Agrela et al. [29] proposal, with R cu >70 wt% and Rb <20 wt%. The physical and mechanical properties of the aggregates given in Table 4 show that the MRA was less dense and less abrasion-resistant than the NA (NC-M and NC-G). This was attributed to the higher water absorption of the bound mortar and fired clay materials comprising the MRA. The Los Angeles coefficient (LA), a measure of abrasion fragility, was higher in the MRA (LA = 46 wt%) than in the NA (LA = 26 wt% to 28 wt%), due to the greater hardness of NA relative to the MRA components such as concrete, brick and tile. The MRA also exhibited a higher flakiness index (FA), 20 wt%, than that of the NA, 13 wt%, likewise attributable to its components, particularly brick and tile, which are flakier than NA.  Table 5 lists the fresh state and 28-day properties of the concrete mixes analysed and the standards followed to determine them. All the mixes were batched with the same volume to ensure non-interference by parameters irrelevant to the study. The specimens were prepared and cured as per European standard EN 12390-2 [34]. After moulding, performed with particular care to minimise pouring and consolidation times, the specimens were wrapped in plastic and stored for 24 h. The demoulded specimens were subsequently stored in a humidity chamber at 20 ± 2 • C and 95 ± 5% humidity until tested.

Testing and methodology
Thermal testing was conducted on an Applied Precision Ltd. ISOMET 2114 analyser. Two 100 × 100 × 500 mm prismatic specimens of each mix cured 28 days in a dry chamber (20 ± 2 • C, 50% RH) were analysed in these tests, taking at least three readings per sample.

Concrete Design
The six concrete mixes batched for the tests were divided into two groups: mixes containing 100% NA and GRC as OPC replacement at ratios of 0% (labelled NAC), 10% (labelled N10/0) or 25% GRC (labelled N25/0); and mixes with 50% MRA and GRC as an OPC replacement at ratios of 0% (labelled R0/50), 10% GRC (labelled R10/50) or 25% (labelled R25/50). The mixes with 100% NA were designed to study the effect of GRC on conventional concrete prepared with NA, and those with 50% to determine the joint effect of the two recycled materials. No plasticizers were used in any of the mixes.
All the mixes were batched as set out in European standard EN 206-1 [19] to durability class X2 and strength class C25/30, although using 300 kg/m 3 of binder (OPC + GRC), slightly more than the standard minimum of 280 kg/m 3 for that exposure class. All were also prepared for a target workability of S2, as defined in EN-206-1 [19], equivalent to a 70 ± 20 mm slump [32].
The particle size distribution of the (fine + coarse) aggregate, irrespective of whether it was natural or recycled, fit the theoretical curve defined by Faury [18] for a maximum size of 22 mm. The batching for the six materials used in the study is given in Table 6. The total amount of water used in each mix was defined as the effective water plus the water needed to offset the amount absorbed by aggregate when soaked for 10 min (approximately the mixing time) [42]. All the aggregates were used moist, subtracting the water required from the total in the mix.

Fresh Properties
The fresh-state properties of the mixes are given in Table 7, according to which they all fall in the same workability class, S2 (70 ± 20 mm), defined in European standard EN 206-1 [19].  Figure 2 shows that fresh-state density (R 2 > 0.848) and air content (R 2 > 0.779) varied linearly with the w/b eff ratio. Both the rise in air content and the decline in density with rising w/b eff were induced by the use of GRC and MRA, which are respectively less dense than OPC and NA. That behaviour was consistent with the findings reported by Cantero et al. [10] in a study of concrete mixes prepared with both 25% ground recycled CDW (RC-CDW) as a cement replacement and 50% MRA. The authors found a linear rise in fresh-state air content (R 2 > 0.842) and a linear decline in density (R 2 > 0.921) as a result of using the two recycled materials.
The air content in all the mixes studied here was lower than the 4.5 vol% recommended by ACI [43] for concrete mixes prepared with a maximum aggregate size of 22.4 mm.

Mechanical Properties
The 28-day mean compressive (f cm ), mean splitting tensile (f st ), relative compressive (∆f cm ) and relative splitting tensile (∆f st ) strengths are plotted in Figure 3. In terms of compressive strength, four of the recycled material mixes (R10/0, R25/0, R0/50 and R10/50), which exhibited f cm > 25 MPa, would be apt for structural applications in building construction. A value under 25 MPa was found only in the mix with 25% GRC and 50% MRA (R25/50), whose use would be restricted to non-structural purposes.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 18 The 28-day mean compressive (fcm), mean splitting tensile (fst), relative compressive (∆fcm) and relative splitting tensile (∆fst) strengths are plotted in Figure 3. In terms of compressive strength, four of the recycled material mixes (R10/0, R25/0, R0/50 and R10/50), which exhibited fcm > 25 MPa, would be apt for structural applications in building construction. A value under 25 MPa was found only in the mix with 25% GRC and 50% MRA (R25/50), whose use would be restricted to non-structural purposes.   Figure 3 also shows that, irrespective of the GRC replacement ratio and the presence or absence of MRA, the experimental materials had lower compressive and splitting tensile strengths than those of the reference mix. Letelier et al. [44], studying concrete mixes with 10% or 15% ground brick and 30% RCA, found the incorporation of ground brick at both replacement ratios induced declines in 28day compressive and flexural strength in the mixes, whether made with 100% NA or 30% RCA.

Per cent (%)
Relative  The 28-day mean compressive (fcm), mean splitting tensile (fst), relative compressive (∆fcm) and relative splitting tensile (∆fst) strengths are plotted in Figure 3. In terms of compressive strength, four of the recycled material mixes (R10/0, R25/0, R0/50 and R10/50), which exhibited fcm > 25 MPa, would be apt for structural applications in building construction. A value under 25 MPa was found only in the mix with 25% GRC and 50% MRA (R25/50), whose use would be restricted to non-structural purposes.   Figure 3 also shows that, irrespective of the GRC replacement ratio and the presence or absence of MRA, the experimental materials had lower compressive and splitting tensile strengths than those of the reference mix. Letelier et al. [44], studying concrete mixes with 10% or 15% ground brick and 30% RCA, found the incorporation of ground brick at both replacement ratios induced declines in 28day compressive and flexural strength in the mixes, whether made with 100% NA or 30% RCA.    3 also shows that, irrespective of the GRC replacement ratio and the presence or absence of MRA, the experimental materials had lower compressive and splitting tensile strengths than those of the reference mix. Letelier et al. [44], studying concrete mixes with 10% or 15% ground brick and 30% RCA, found the incorporation of ground brick at both replacement ratios induced declines in 28-day compressive and flexural strength in the mixes, whether made with 100% NA or 30% RCA.
Adding 10% GRC to the NA mix (R10/0) lowered compressive strength by 18.7% and tensile strength by 12.9%, whilst at a replacement ratio of 25% (R25/0) the decline was 38.1% in compressive and 19.4% in tensile strength relative to NAC, due to the lower reactivity of GRC relative to cement and the smaller amount of the latter in the mixes. In a study of the effect of replacing OPC with 15%, 30% or 45% ground crushed concrete, Kim [22] attributed the declines in compressive and flexural strength observed to the lower reactivity of the addition.
With the combined use of GRC and MRA, compressive strength was 33.4% and splitting tensile strength was 17.5% lower in R10/50 than in NAC, whilst in R25/50 compressive strength was 57.2% and splitting tensile strength was 48.2% lower. Those findings were the result of two factors: low GRC reactivity [22] and the lesser density and hardness of MRA than NA [45][46][47]. The declines recorded were steeper than the up to 20% reported for compressive strength and up to 24% for splitting tensile strength in a study [10] of concrete mixes with ground fired clay-based materials recycled from CDW, which were more reactive than the present GRC, and 50% of a harder MRA (LA < 36 wt%) than the one used here (LA = 46 wt%). Table 8 lists the 28-day dry density (ρ dry ), open porosity (P o ) and electrical conductivity (EC) values for all the mixes, as well as the effects of using GRC, MRA and the two jointly.

Dry Density
Analysing the data from Table 8, dry density (ρ dry ) was lower in all the mixes containing GRC and MRA than in the NAC reference. Figure 4a shows that replacing OPC with GRC lowered ρ dry linearly (R 2 ≥ 0.998) in the mixes with NA and MRA due to the lower density of the recycled materials (see Section 2.1.1), which also yielded less hydration product (low reactivity) [5,6]. Dry density was 2.8% lower in N10/0 and 5.8% lower in N25/0 than in NAC and 2.3% lower in R10/50 and 5.3% lower in R25/50 than in R0/50. The vertical distance between the two trend lines in Figure 4a, labelled the 'MRA effect', measures the effect of replacing 50% NA with MRA. The mean 8.0% decline in ρ dry in the MRA mixes (R0/50, R10/50 and R25/50) relative to the NA mixes (NAC, N10/0 and N25/0) was attributable to the lower mean dry density of MRA (ρ dry = 2069 kg/m 3 ) relative to that of NA (ρ dry = 2591 kg/m 3 ). The joint use of GRC+MRA, in turn, induced declines in dry density relative to NAC of 10.4% in R10/50 and 13.1% in R25/50, findings consistent with earlier observations in concrete made with recycled (coarse and fine) aggregate consisting of crushed waste clay brick [48].

Open Porosity
The open porosity (P o ) data in Table 8 show greater pore volume in all the mixes containing GRC and MRA than in the reference NAC. This was attributed to two factors: the larger w/b eff ratio in the new cementitious matrices associated with the greater water demand in the new binders (RB 10 and RB 25 ); and higher water absorption MRA (9.1%) than NA (1.3%) (higher open porosity) primarily due to the presence of bound mortar and fired clay-based material in MRA [49]. Those results are consistent with earlier studies on concrete batched with up to 100% MRA [50], up to 100% RCA [51] and concrete with up to 100% RCA fine and 100% RCA coarse aggregate [52].
The use of GRC raised P o by 5.0% in N10/0 and 13.2% in N25/0 relative to NAC and by 4.6% in R10/50 and 11.2% in R25/50 relative to R0/50, an indication that the joint use of CDW as additions and aggregates had no adverse effect on open porosity. As Figure 4b shows, a direct linear relationship (R 2 ≥ 0.997) was observed between open porosity and GRC replacement ratio in all the mixes, whether containing NA or MRA, where it was added. The vertical distance between the two trend lines depicts the impact of replacing 50% NA with MRA, which induced a maximum rise in the mean P o of up to 25% in the MRA (R0/50, R10/50, R25/50) relative to the NA mixes (NAC, N10/0, N25/0).
All the P o values lay within the 12.5% to 17% range reported in the literature for concrete made with 50% MRA [52][53][54] and concrete made jointly with 30% ground granulated blast furnace slag and 50% RCA [55].
with recycled (coarse and fine) aggregate consisting of crushed waste clay brick [48].

Open Porosity
The open porosity (Po) data in Table 8 show greater pore volume in all the mixes containing GRC and MRA than in the reference NAC. This was attributed to two factors: the larger w/beff ratio in the new cementitious matrices associated with the greater water demand in the new binders (RB10 and RB25); and higher water absorption MRA (9.1%) than NA (1.3%) (higher open porosity) primarily due to the presence of bound mortar and fired clay-based material in MRA [49]. Those results are consistent with earlier studies on concrete batched with up to 100% MRA [50], up to 100% RCA [51] and concrete with up to 100% RCA fine and 100% RCA coarse aggregate [52].
The use of GRC raised Po by 5.0% in N10/0 and 13.2% in N25/0 relative to NAC and by 4.6% in R10/50 and 11.2% in R25/50 relative to R0/50, an indication that the joint use of CDW as additions and aggregates had no adverse effect on open porosity. As Figure 4b shows, a direct linear relationship (R 2 ≥ 0.997) was observed between open porosity and GRC replacement ratio in all the mixes, whether containing NA or MRA, where it was added. The vertical distance between the two trend lines depicts the impact of replacing 50% NA with MRA, which induced a maximum rise in the mean Po of up to 25% in the MRA (R0/50, R10/50, R25/50) relative to the NA mixes (NAC, N10/0, N25/0).
All the Po values lay within the 12.5% to 17% range reported in the literature for concrete made with 50% MRA [52][53][54] and concrete made jointly with 30% ground granulated blast furnace slag and 50% RCA [55].

Electrical Conductivity
The electrical conductivity (EC) values in Table 8 show that the parameter rose with the inclusion of GRC in both the NA and MRA mixes. That observation is related to the greater ion mobility resulting from a more inter-connected saturated pore structure induced by the addition, which in turn raises electrical conductivity [56]. The graph in Figure 5 shows a direct linear relationship (R 2 ≥ 0.978) between EC and Po or the volume of permeable pores in the NA and MRA mixes. The use of GRC raised EC linearly (R 2 ≥ 0.980) (Figure 4c)) by 7.1% in N10/0 and 16.8% in N25/0 relative to NAC and by 3.0% in R10/50 and 10.9% in R25/50 relative to R0/50. Using 50% MRA raised EC by a mean

Electrical Conductivity
The electrical conductivity (EC) values in Table 8 show that the parameter rose with the inclusion of GRC in both the NA and MRA mixes. That observation is related to the greater ion mobility resulting from a more inter-connected saturated pore structure induced by the addition, which in turn raises electrical conductivity [56]. The graph in Figure 5 shows a direct linear relationship (R 2 ≥ 0.978) between EC and P o or the volume of permeable pores in the NA and MRA mixes. The use of GRC raised EC linearly (R 2 ≥ 0.980) (Figure 4c)) by 7.1% in N10/0 and 16.8% in N25/0 relative to NAC and by 3.0% in R10/50 and 10.9% in R25/50 relative to R0/50. Using 50% MRA raised EC by a mean of 7.2% relative to the mixes with 100% NA.
All the increases in EC observed lie within the 7% to 24% range reported in the literature for concrete mixes with 25% or 50% coarse fired clay aggregate [53] and mixes with 25% or 50% RCA fines [57].

Electrical Conductivity
The electrical conductivity (EC) values in Table 8 show that the parameter rose with the inclusion of GRC in both the NA and MRA mixes. That observation is related to the greater ion mobility resulting from a more inter-connected saturated pore structure induced by the addition, which in turn raises electrical conductivity [56]. The graph in Figure 5 shows a direct linear relationship (R 2 ≥ 0.978) between EC and Po or the volume of permeable pores in the NA and MRA mixes. The use of GRC raised EC linearly (R 2 ≥ 0.980) (Figure 4c)) by 7.1% in N10/0 and 16.8% in N25/0 relative to NAC and by 3.0% in R10/50 and 10.9% in R25/50 relative to R0/50. Using 50% MRA raised EC by a mean of 7.2% relative to the mixes with 100% NA.
All the increases in EC observed lie within the 7% to 24% range reported in the literature for concrete mixes with 25% or 50% coarse fired clay aggregate [53] and mixes with 25% or 50% RCA fines [57].

Thermal Properties
Twenty-eight-day thermal conductivity (λ) and specific heat capacity (cp) are given for all the mixes in Table 9, which also lists the effects of including GRC, MRA and the two jointly.

Thermal Properties
Twenty-eight-day thermal conductivity (λ) and specific heat capacity (c p ) are given for all the mixes in Table 9, which also lists the effects of including GRC, MRA and the two jointly.  Figure 6 shows that mixes N10/0 and N25/0 exhibited lower thermal conductivity (λ) than NAC and mixes R10/50 and R25/50 lower λ than R0/50, inferring that the recycled material concrete would afford more effective thermal insulation than conventional materials. The use of GRC lowered λ by 2.5% in N10/0 and 6.9% in N25/0 relative to NAC and by 3.1% in R10/50 and 7.2% in R25/50 relative to R0/50. Those values lay in the 2.8% to 6.8% range observed for concrete made with up to 25% RA to replace NA fines [9] and concrete prepared with 10% ground granulated blast furnace slag as an OPC substitute [58].

Thermal Conductivity
The inverse linear relationship (R 2 ≥ 0.997) between λ and GRC in NA and MRA mixes is plotted in Figure 6a. The mean decline of 5.3% in λ (determined as the vertical distance between the two trend lines: the MRA effect) attributed to the use of 50% MRA was very close to the 5.5% decline reported for concrete with 50% RA of a type similar to the RA used in this study [12]. The joint use of GRC and MRA lowered thermal conductivity by 7.9% in R10/50 and by 11.8% in R25/50 relative to the reference NAC.
All the recycled material mixes exhibited λ in the 2.01 W/m·K to 1.60 W/m·K range found for concrete with up to 50% RA fines and up to 50% RA coarse recycled CDW aggregate [12], as well as within the range recommended by Eurocode 2 [59]: 1.95 W/m·K to 1.33 W/m·K. Figure 6 shows that mixes N10/0 and N25/0 exhibited lower thermal conductivity (λ) than NAC and mixes R10/50 and R25/50 lower λ than R0/50, inferring that the recycled material concrete would afford more effective thermal insulation than conventional materials. The use of GRC lowered λ by 2.5% in N10/0 and 6.9% in N25/0 relative to NAC and by 3.1% in R10/50 and 7.2% in R25/50 relative to R0/50. Those values lay in the 2.8% to 6.8% range observed for concrete made with up to 25% RA to replace NA fines [9] and concrete prepared with 10% ground granulated blast furnace slag as an OPC substitute [58].
The inverse linear relationship (R 2 ≥ 0.997) between λ and GRC in NA and MRA mixes is plotted in Figure 6a. The mean decline of 5.3% in λ (determined as the vertical distance between the two trend lines: the MRA effect) attributed to the use of 50% MRA was very close to the 5.5% decline reported for concrete with 50% RA of a type similar to the RA used in this study [12]. The joint use of GRC and MRA lowered thermal conductivity by 7.9% in R10/50 and by 11.8% in R25/50 relative to the reference NAC.
All the recycled material mixes exhibited λ in the 2.01 W/m·K to 1.60 W/m·K range found for concrete with up to 50% RA fines and up to 50% RA coarse recycled CDW aggregate [12], as well as within the range recommended by Eurocode 2 [59]: 1.95 W/m·K to 1.33 W/m·K. .

Specific Heat Capacity
Specific heat capacity (c p in J/kgK) is calculated as c p = c ρ dry , where c is volumetric heat capacity in J/m 3 K and ρ dry is concrete dry density in kg/m 3 . Further to the data in Table 9, the use of GRC raised c ρ by 1.4% in N10/0 and 4.1% in N25/0 relative to NAC and by 1.8% in R10/50 and 4.8% in R25/50 relative to R0/50, an indication that the new mixes would be less affected by abrupt temperature changes than NAC [60]. Figure 6b plots the direct linear relationship (R 2 ≥ 0.950) between c ρ and GRC in NA and MRA mixes and shows that the use, in addition, of 50% MRA (MRA effect) raised c ρ by a mean 4.5% in the MRA mixes relative to the NA mixes, a value similar to the mean 5.3% decline observed for λ. The joint use of GRC and MRA induced a rise in c ρ of 6.0% in R10/50 and 9.1% in R25/50 relative to the reference NAC. Those increases with the use of GRC and MRA infer greater thermal inertia in the new recycled material mixes than in the reference concrete and, consequently, greater thermal stability when exposed to changing outdoor temperatures.
All the c ρ values lay within the normal range (0.790 kJ/m 3 ·K to 0.960 kJ/m 3 ·K) for concrete used in residential building construction [13].

Thermal Properties and Air Content
Thermal conductivity and specific heat capacity are plotted against fresh-state air content in Figure 7. The linear relationships denote a good correlation between these properties with determination coefficients R 2 of over 0.84, a value higher than R 2 = 0.57 reported by Bravo et al. [12] for a study on the thermal behaviour of concrete in which NA was replaced with 10% to 100% (coarse and fine) recycled aggregate from several CDW management plants.
Thermal conductivity and specific heat capacity are plotted against fresh-state air content in Figure 7. The linear relationships denote a good correlation between these properties with determination coefficients R 2 of over 0.84, a value higher than R 2 = 0.57 reported by Bravo et al. [12] for a study on the thermal behaviour of concrete in which NA was replaced with 10% to 100% (coarse and fine) recycled aggregate from several CDW management plants.

Thermal Properties and Compressive Strength
Analysing Figure 8, both thermal conductivity and specific heat capacity were linearly related to compressive strength with determination coefficients R 2 above 0.92, the former inversely and the latter directly, i.e., the lower the thermal conductivity or the higher the specific heat capacity, the

Thermal Properties and Compressive Strength
Analysing Figure 8, both thermal conductivity and specific heat capacity were linearly related to compressive strength with determination coefficients R 2 above 0.92, the former inversely and the latter directly, i.e., the lower the thermal conductivity or the higher the specific heat capacity, the higher the compressive strength. Those findings were consistent with the results reported by Bravo et al. [12] and Pavlu et al. [61] for concrete with up to 100% RMA fine and coarse aggregate.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 18 higher the compressive strength. Those findings were consistent with the results reported by Bravo et al. [12] and Pavlu et al. [61] for concrete with up to 100% RMA fine and coarse aggregate. Conversely to compressive strength, open porosity was inversely related to thermal conductivity and directly to specific heat capacity (Figure 9) with determination coefficients R 2 for the respective linear relationships above 0.823. According to these data, the larger the pore volume, the more effective the thermal insulation (lower conductivity) is and the greater the thermal stability

Thermal Properties and Open Porosity
Conversely to compressive strength, open porosity was inversely related to thermal conductivity and directly to specific heat capacity ( Figure 9) with determination coefficients R 2 for the respective linear relationships above 0.823. According to these data, the larger the pore volume, the more effective the thermal insulation (lower conductivity) is and the greater the thermal stability (higher specific heat capacity) when exposed to abrupt outdoor temperature changes. Earlier studies also related lower concrete thermal conductivity to higher pore volumes [41,44,45,47,48].

Thermal Properties and Open Porosity
Conversely to compressive strength, open porosity was inversely related to thermal conductivity and directly to specific heat capacity ( Figure 9) with determination coefficients R 2 for the respective linear relationships above 0.823. According to these data, the larger the pore volume, the more effective the thermal insulation (lower conductivity) is and the greater the thermal stability (higher specific heat capacity) when exposed to abrupt outdoor temperature changes. Earlier studies also related lower concrete thermal conductivity to higher pore volumes [41,44,45,47,48].   The thermal conductivity of the solid matrix (λs) in concrete mixes is given in Table 10. This parameter separates the effect of the volume of open pores from the effect of adding both recycled materials (GRC and MRA) to the mixes. It exhibited higher values than overall thermal conductivity (Table 9). With GRC, λ s was 1.8% lower in N10/0 and 5.2% lower in N25/0 than in NAC and, in the mixes with MRA, 2.4% lower in R10/50 and 5.4% lower in R25/50 than in R0/50. The combined use of GRC and MRA lowered the mean λ s value by 1.7% in (R0/50, R10/50 and R25/50) relative to the reference NAC. In other words, the use of 25% GRC had a greater impact than the use of 50% MRA.

Thermal Properties and Electrical Conductivity
As Figure 10 shows, electrical conductivity, like open porosity, was inversely related to thermal conductivity and directly to specific heat capacity with determination coefficients R 2 for linearity above 0.830. The explanation for these findings lies in the greater electrical conductivity of the recycled materials mixes than the reference mix, resulting from the greater volume of saturated pores in the former (see item 3.3.3). Earlier authors also mentioned a rise in electrical conductivity (reported as a decline in electrical resistivity) with the higher porosity attributed to using MRA coarse aggregate [50], RCA coarse aggregate [62], RCA fines [57] or RCA coarse and fine aggregate jointly [63].
denotes the type of concrete mix.

Thermal Properties and Electrical Conductivity
As Figure 10 shows, electrical conductivity, like open porosity, was inversely related to thermal conductivity and directly to specific heat capacity with determination coefficients R 2 for linearity above 0.830. The explanation for these findings lies in the greater electrical conductivity of the recycled materials mixes than the reference mix, resulting from the greater volume of saturated pores in the former (see item 3.3.3). Earlier authors also mentioned a rise in electrical conductivity (reported as a decline in electrical resistivity) with the higher porosity attributed to using MRA coarse aggregate [50], RCA coarse aggregate [62], RCA fines [57] or RCA coarse and fine aggregate jointly [63].

Conclusions
The conclusions that may be drawn from this study are set out below: • The use of GRC to replace 25% of OPC and MRA to replace 50% of NA lowered concrete freshstate density by 6.2% and raised its air content by 30%. All the recycled material concrete mixes

Conclusions
The conclusions that may be drawn from this study are set out below: • The use of GRC to replace 25% of OPC and MRA to replace 50% of NA lowered concrete fresh-state density by 6.2% and raised its air content by 30%. All the recycled material concrete mixes studied had an air content under 4.5%, the upper limit recommended by the ACI committee for structural concrete; • Porosity rose by 39.7% in the mixes with 25% GRC and 50% MRA and by 13.2% in the mixes with 25% GRC and 100% NA relative to the reference mix made with 100% OPC and 100% NA; • The decline in dry density and the rise in electrical conductivity were associated with the incorporation of GRC in mixes with 100% NA as well as those with 50% MRA due to the higher porosity in the recycled materials; • The use of 25% GRC in conjunction with 50% MRA reduced thermal conductivity by 11.8% and raised specific heat capacity by 9.1%, whilst the values for 25% GRC with 100% NA were a 6.9% reduction in thermal conductivity and a 4.1% rise in specific heat capacity, both relative to concrete with 100% natural aggregate (NA); • Due to their greater porosity, the new recycled materials concrete may provide better thermal insulation and greater thermal inertia than conventional concrete; • Cross-referencing concrete properties showed that, whereas using GRC and MRA as replacement materials had an adverse effect on concrete's compressive strength, it improved its thermal properties; • Mixes with 25% GRC and 100% NA and those with 10% GRC and 50% MRA, with compressive strength values of >25 MPa, are not only apt for use in building construction, but afford greater energy efficiency than conventional concrete. Funding: This research was funded by Spanish Ministry of Science, Innovation and Universities mobility grant ETS18/00313, awarded in connection with pre-doctoral university professor scholarship 16/02693, under which Blas Cantero conducted research at the Lisbon University Instituto Superior Técnico construction laboratory. European Regional Development Fund (ERDF) funding was also received for an Interreg-POCTEP research grant (0008_ECO2CIR_4_E, 'Project for cross-border cooperation to introduce the eco-and circular economy by reducing waste generation and enhancing waste recycling, management and valorisation in Central Spain, Extremadura and Alentejo) and, in conjunction with the Government of Extremadura, under grant GR 18,122 awarded to the MATERIA research group.