Reusing Construction and Demolition Waste to Prepare Alkali-Activated Cement

Large amounts of waste are derived not only from construction processes, but also the demolition of existing buildings. Such waste occupies large volumes in landfills, which makes its final disposal difficult and expensive. Reusing this waste type is generally limited to being employed as filler material or recycled aggregate in concrete, which limits its valorisation. The present work proposes reusing construction and demolition waste to manufacture alkali-activated cement to improve its sustainability and recovery. Construction and demolition waste (C&DW) from a demolition waste collection plant in Valencia (Spain) was physically and chemically characterised. This residue contained large fractions of concrete, mortar, bricks, and other ceramic materials. X-ray fluorescence (XRF) analysis showed that its chemical composition was mainly CaO, SiO2 and Al2O3. X-ray diffraction (XRD) analysis revealed that it presented some crystalline products, and quartz (SiO2) and calcite (CaCO3) were the main components. Blends of C&DW and blast furnace slag (BFS) were alkali-activated with mixtures of sodium hydroxide and sodium silicate. The corresponding pastes were characterised by techniques such as thermogravimetry and scanning electron microscopy (SEM). The alkali-activated mortars were prepared, and the resulting mortars’ compressive strength was determined, which was as high as 58 MPa with the 50% C&DW-50% BFS mixture. This work concluded that it is possible to make new sustainable binders by the alkali activation of C&DW-BFS without using Portland cement.


Introduction
The European Commission Waste Framework Directive provides basic waste managementrelated definitions. To comply with the objectives of this Directive, European Union Member States, by 2020, must use 70% non-hazardous waste from construction and demolition works [1]. Construction and demolition waste (C&DW) is assumed to comprise at least 30% of all total solid waste worldwide [2]. Depending on the construction type, several construction material waste types can be found in C&DW compositions: mortar, concrete, wood, bricks, ceramic sanitaryware, steel, etc.
In 2018, China produced 2360 million tonnes of C&DW. The EU generated a significant amount the same year: for example, Germany alone contributed 225 million tonnes [3]. China is the largest producer of C&DW, followed by India and the United States. To date, the main use of C&DW is to replace natural aggregates to produce mortar and concrete. Nevertheless, large-scale C&DW reuse towards a circular economy system needs to be enhanced [4]. ent precursors: natural volcanic pozzolan with ground granulated BFS, FA, C&DW and red clay brick waste. The only mixture that was considered as low-strength structural block was the mixture with C&DW because the block did not meet the water absorption criterion.
Two marine sediments and different C&DW fractions were studied as precursors to prepare pastes activated with KOH and SS [14]. Sediments substituted fractions of 10% and 30% tiles, bricks and concrete. The concrete paste proved to be the worst sample because this fraction had a small quantity of SiO 2 and Al 2 O 3 . One of the studied sediments enhanced the compressive strength of all the C&DW fractions. The same conclusions were reached in another paper published by the same author, which demonstrated that concrete waste always yielded compressive strengths below 15 MPa versus the tile and brick waste, which, respectively, obtained 49.5 MPa and 57.8 MPa [15].
The present study aims to assess the potential of using real C&DW from a wastesorting and recovery plant in Valencia (east Spain). Systematic waste characterisation was performed, and preliminary studies associated with its reactivity were carried out. The compressive strength and microstructural properties of the alkali-activated cements based on C&DW/BFS activated at two different SiO 2 /Na 2 O molar ratios were assessed for two different curing conditions. The use of BFS as part of the precursor is due to the fact that it is a well-known material in the literature of alkaline activation and that it has amply demonstrated its good behaviour both mechanically and in terms of durability [16].
The main objective of the investigation is to valorise C&DW residue in the construction industry. Its recovery would contribute to the fact that a waste produced by the same industry could be used again as a raw material, contributing to the development of the circular economy precursors: natural volcanic pozzolan with ground granulated BFS, FA, C&DW and red clay brick waste. The only mixture that was considered as low-strength structural block was th mixture with C&DW because the block did not meet the water absorption criterion.

C&DW Conditioning and Characterisation
Two marine sediments and different C&DW fractions were studied as precursors t prepare pastes activated with KOH and SS [14]. Sediments substituted fractions of 10% and 30% tiles, bricks and concrete. The concrete paste proved to be the worst sample be cause this fraction had a small quantity of SiO2 and Al2O3. One of the studied sediment enhanced the compressive strength of all the C&DW fractions. The same conclusions wer reached in another paper published by the same author, which demonstrated that con crete waste always yielded compressive strengths below 15 MPa versus the tile and brick waste, which, respectively, obtained 49.5 MPa and 57.8 MPa [15].
The present study aims to assess the potential of using real C&DW from a waste sorting and recovery plant in Valencia (east Spain). Systematic waste characterisation wa performed, and preliminary studies associated with its reactivity were carried out. Th compressive strength and microstructural properties of the alkali-activated cements based on C&DW/BFS activated at two different SiO2/Na2O molar ratios were assessed for tw different curing conditions. The use of BFS as part of the precursor is due to the fact tha it is a well-known material in the literature of alkaline activation and that it has amply demonstrated its good behaviour both mechanically and in terms of durability [16].
The main objective of the investigation is to valorise C&DW residue in the construc tion industry. Its recovery would contribute to the fact that a waste produced by the sam industry could be used again as a raw material, contributing to the development of th circular economy   Figure 2a shows the as-received C&DW. Figure 2b illustrates the two fractions ob tained in step 4 before manual classification.  Two C&DW fractions were obtained by sieving in step 4: fine and coarse fractions (see Figure 2b). In this study, the coarse fraction was selected to prepare alkali-activated cements because the finest fraction contained more impurities (plastic, wood chips, paper, gypsum). As this waste was of no value, it was subsequently taken to a landfill. The only valuable waste obtained in the plant was in step 7, which was later used as gravel.

C&DW Conditioning and Characterisation
About 500 kg of coarse C&DW was collected from the recovery plant and characterised for this study. C&DW was placed inside a jab crusher (Retsch BB20 model) to produce particles of size < 2 mm. This material was milled in a ball mill for 15 minutes (200 g of C&DW with 96 alumina balls in Gabrielli Mill 2 equipment) to obtain a similar mean diameter particle to the other binding materials used in this research. Different instrumental techniques were applied to characterise C&DW: X-ray fluorescence (Philips Magix Pro spectrometer); X-ray diffraction in a Brucker AXS D8 Advance of Billerica (XRD spectra were taken from 10° to 70° 2θ, at 20 mA and 40 kV, in an angle step of 0.02°); a laser dispersion granulometric analysis using Malvern Instruments Mastersize 2000 (measurements in aqueous medium); field emission scanning electron microscopy (Zeiss FESEM ULTRA 55); thermogravimetric analysis by Mettler Toledo TGA 850 (using alumina crucibles, and temperature range from 35 °C to 1000 °C at a heating rate of 20 °C/min in an air atmosphere). To analyse XRD pattern, DRXWin&Creafit 2.0 software was used. The samples analysed by microscopy were coated with carbon to make them conductive and to be observed in the equipment.

Other Materials
Calcium hydroxide (Ca(OH)2, 96% purity, supplied by Panreac Química SLU) was used to study the pozzolanic reactivity of C&DW following the evaluation method proposed by Tashima et al. [17].
BFS was supplied by Cementval SL (Puerto de Sagunto, Valencia, Spain). It was dried at 100 °C and dry-milled in a ball mill for 30 minutes (450 g of BFS with 98 alumina balls in Gabrielli Mill 2 equipment). The mean particle diameter obtained after milling was 26.09 µm. The main BFS compounds in the mass were: CaO (40.35%), SiO2 (30.04%) and Al2O3 (10.6%).
Siliceous sand, with a fineness modulus of 4.1 and a humidity percentage < 0.1%, was used to prepare the mortars. The alkaline solutions were composed of a mixture of commercial reagents: the mixture contained NaOH pellets (98% purity, Panreac Quimica SLU), Na2SiO3 (28% SiO2, 8% Na2O, 64% H2O, supplied by Merck S.L.U) and tap water. Two C&DW fractions were obtained by sieving in step 4: fine and coarse fractions (see Figure 2b). In this study, the coarse fraction was selected to prepare alkali-activated cements because the finest fraction contained more impurities (plastic, wood chips, paper, gypsum). As this waste was of no value, it was subsequently taken to a landfill. The only valuable waste obtained in the plant was in step 7, which was later used as gravel.
About 500 kg of coarse C&DW was collected from the recovery plant and characterised for this study. C&DW was placed inside a jab crusher (Retsch BB20 model) to produce particles of size < 2 mm. This material was milled in a ball mill for 15 min (200 g of C&DW with 96 alumina balls in Gabrielli Mill 2 equipment) to obtain a similar mean diameter particle to the other binding materials used in this research. Different instrumental techniques were applied to characterise C&DW: X-ray fluorescence (Philips Magix Pro spectrometer); X-ray diffraction in a Brucker AXS D8 Advance of Billerica (XRD spectra were taken from 10 • to 70 • 2θ, at 20 mA and 40 kV, in an angle step of 0.02 • ); a laser dispersion granulometric analysis using Malvern Instruments Mastersize 2000 (measurements in aqueous medium); field emission scanning electron microscopy (Zeiss FESEM ULTRA 55); thermogravimetric analysis by Mettler Toledo TGA 850 (using alumina crucibles, and temperature range from 35 • C to 1000 • C at a heating rate of 20 • C/min in an air atmosphere). To analyse XRD pattern, DRXWin&Creafit 2.0 software was used. The samples analysed by microscopy were coated with carbon to make them conductive and to be observed in the equipment.

Other Materials
Calcium hydroxide (Ca(OH) 2 , 96% purity, supplied by Panreac Química SLU) was used to study the pozzolanic reactivity of C&DW following the evaluation method proposed by Tashima et al. [17].
BFS was supplied by Cementval SL (Puerto de Sagunto, Valencia, Spain). It was dried at 100 • C and dry-milled in a ball mill for 30 min (450 g of BFS with 98 alumina balls in Gabrielli Mill 2 equipment). The mean particle diameter obtained after milling was 26.09 µm. The main BFS compounds in the mass were: CaO (40.35%), SiO 2 (30.04%) and Al 2 O 3 (10.6%).
Siliceous sand, with a fineness modulus of 4.1 and a humidity percentage < 0.1%, was used to prepare the mortars. The alkaline solutions were composed of a mixture of commercial reagents: the mixture contained NaOH pellets (98% purity, Panreac Quimica SLU), Na 2 SiO 3 (28% SiO 2 , 8% Na 2 O, 64% H 2 O, supplied by Merck S.L.U) and tap water.

Experimental Procedure
A preliminary study of the C&DW reactivity was performed to assess its potential use as a precursor in alkali-activated systems. First of all, the reactivity of C&DW with Ca(OH) 2 (CH) was evaluated following the electrical conductivity method proposed by Tashima et al. [17]. The pH and electrical conductivity (using Micro PH2001 and Micro CM2201 supplied by Crison, respectively) for the C&DW and CH mixtures were monitored for the different CH:C&DW proportions (0.5:9.5; 1.0:9.0; 1.5:8.5; 2.0:8.0; 2.5:7.5 and 3.0:7.0) at 60 • C for 7 days. The suspension was prepared with 50 mL of deionised water in a 100 mL plastic screw-cap Erlenmeyer flask and the total amount of solid (C&DW plus CH) was set at 1 g. This flask was tightly sealed and placed inside a thermal bath (JULABO-SW22 equipment) with continuous shaking at 60 • C. Next, the corresponding amount of CH was added in each case to completely dissolve it, or until liquid saturation. At this point the electrical conductivity values were measured to obtain a control of the saturated suspension with CH, and C&DW was added. Electrical conductivity was measured every 24 h for 7 testing days.
An alkali-activated paste based on C&DW was prepared using 9 mol·kg −1 of sodium at the SiO 2 /Na 2 O molar ratio of 1.21 and a water/binder ratio of 0.45. This paste was cured for 7 days at 65 • C, and the thermogravimetric analysis and XRD pattern of paste were assessed.
Mortars were prepared using five different C&DW/BFS proportions (100/0, 80/20, 70/30, 60/40, 50/50 per mass) as a binder (precursor). For the activating solution, two different Na + concentrations (7 and 9 mol·kg −1 ) were tested, for which two distinct SiO 2 /Na 2 O molar ratios (1.21 and 1.56) were assessed. The water/binder ratio and the siliceous sand/binder ratio were set at 0.45 and 3, respectively. Mechanical strength was measured on 40 mm × 40 mm × 160 mm specimens according to Standard UNE-EN 196-1 [18]. Compressive strength was measured after 3 and 7 curing days at 65 • C, and also after 28 and 90 curing days at room temperature. The selected alkali-activated pastes cured at 65 • C for 7 days (without sand) using similar mix proportions as the mortars were studied by FESEM. Figure 3 shows an outline of the studies carried out in the paper.

Experimental Procedure
A preliminary study of the C&DW reactivity was performed to assess its potential use as a precursor in alkali-activated systems. First of all, the reactivity of C&DW with Ca(OH)2 (CH) was evaluated following the electrical conductivity method proposed by Tashima et al. [17]. The pH and electrical conductivity (using Micro PH2001 and Micro CM2201 supplied by Crison, respectively) for the C&DW and CH mixtures were monitored for the different CH:C&DW proportions (0.5:9.5; 1.0:9.0; 1.5:8.5; 2.0:8.0; 2.5:7.5 and 3.0:7.0) at 60 °C for 7 days. The suspension was prepared with 50 mL of deionised water in a 100 mL plastic screw-cap Erlenmeyer flask and the total amount of solid (C&DW plus CH) was set at 1 g. This flask was tightly sealed and placed inside a thermal bath (JULABO-SW22 equipment) with continuous shaking at 60 °C. Next, the corresponding amount of CH was added in each case to completely dissolve it, or until liquid saturation. At this point the electrical conductivity values were measured to obtain a control of the saturated suspension with CH, and C&DW was added. Electrical conductivity was measured every 24 h for 7 testing days.
An alkali-activated paste based on C&DW was prepared using 9 mol.kg −1 of sodium at the SiO2/Na2O molar ratio of 1.21 and a water/binder ratio of 0.45. This paste was cured for 7 days at 65 °C, and the thermogravimetric analysis and XRD pattern of paste were assessed.
Mortars were prepared using five different C&DW/BFS proportions (100/0, 80/20, 70/30, 60/40, 50/50 per mass) as a binder (precursor). For the activating solution, two different Na + concentrations (7 and 9 mol.kg −1 ) were tested, for which two distinct SiO2/Na2O molar ratios (1.21 and 1.56) were assessed. The water/binder ratio and the siliceous sand/binder ratio were set at 0.45 and 3, respectively. Mechanical strength was measured on 40 mm × 40 mm × 160 mm specimens according to Standard UNE-EN 196-1 [18]. Compressive strength was measured after 3 and 7 curing days at 65 °C, and also after 28 and 90 curing days at room temperature. The selected alkali-activated pastes cured at 65 °C for 7 days (without sand) using similar mix proportions as the mortars were studied by FESEM. Figure 3 shows an outline of the studies carried out in the paper.  Figure 4 depicts the percentages of the construction material types found in C&DW. Low percentages of paper, plastic and wall paints were found (about 0.24%), whereas ceramic materials such as sanitaryware and ceramic floors appeared at 14.9%. The highest proportions were composed of mortar and concrete (approximately 39.8%) and tiles and bricks (about 25.4%). More than 65% C&DW was based on mortar, concrete, tiles and bricks. Petreous materials comprised mainly limestone materials, which are extremely abundant in the area where the plant is located.  Figure 4 depicts the percentages of the construction material types found in C&DW. Low percentages of paper, plastic and wall paints were found (about 0.24%), whereas ceramic materials such as sanitaryware and ceramic floors appeared at 14.9%. The highest proportions were composed of mortar and concrete (approximately 39.8%) and tiles and bricks (about 25.4%). More than 65% C&DW was based on mortar, concrete, tiles and bricks. Petreous materials comprised mainly limestone materials, which are extremely abundant in the area where the plant is located.  After the milling process, C&DW presented a wide granulometric distribution (Figure 5) and a mean particle diameter of 19.24 µm. The particle size distribution parameters for C&DW appeared, such as d (0.1), d (0.5) and d (0.9), which were 0.83 µm, 6.79 µm and 58.40 µm, respectively.  Table 1 shows the chemical composition of C&DW analysed by XRF. The main oxides present in waste were CaO (23.41%) and SiO2 (38.41%). These percentages were similar not only the C&DW investigated by Mucsi et al. [19] with CaO (21.1%) and SiO2 (38.7%), but also to the waste used by Robayo-Salazar et al. [12] with CaO (21.2%) and SiO2 (47.6%). Regarding the composition of the BFS, this residue had a lower percentage in CaO and slightly higher percentage in SiO2.   After the milling process, C&DW presented a wide granulometric distribution ( Figure 5) and a mean particle diameter of 19.24 µm. The particle size distribution parameters for C&DW appeared, such as d (0.1), d (0.5) and d (0.9), which were 0.83 µm, 6.79 µm and 58.40 µm, respectively.  After the milling process, C&DW presented a wide granulometric distribution (Figure 5) and a mean particle diameter of 19.24 µm. The particle size distribution parameters for C&DW appeared, such as d (0.1), d (0.5) and d (0.9), which were 0.83 µm, 6.79 µm and 58.40 µm, respectively.  Table 1 shows the chemical composition of C&DW analysed by XRF. The main oxides present in waste were CaO (23.41%) and SiO2 (38.41%). These percentages were similar not only the C&DW investigated by Mucsi et al. [19] with CaO (21.1%) and SiO2 (38.7%), but also to the waste used by Robayo-Salazar et al. [12] with CaO (21.2%) and SiO2 (47.6%). Regarding the composition of the BFS, this residue had a lower percentage in CaO and slightly higher percentage in SiO2.   Table 1 shows the chemical composition of C&DW analysed by XRF. The main oxides present in waste were CaO (23.41%) and SiO 2 (38.41%). These percentages were similar not only the C&DW investigated by Mucsi et al. [19] with CaO (21.1%) and SiO 2 (38.7%), but also to the waste used by Robayo-Salazar et al. [12] with CaO (21.2%) and SiO 2 (47.6%). Regarding the composition of the BFS, this residue had a lower percentage in CaO and slightly higher percentage in SiO 2 . The XRD pattern of C&DW appears in Figure 6. The main peaks corresponded to the presence of quartz (Q, SiO 2 , PDFcard 210816) and calcite (C, CaCO 3 , PDFcard 050583). Gypsum was identified as the secondary peak (G, CaSO 4 ·2H 2 O, PDFcard 210816). These minerals derived from the composition of mortars and concretes (Portland cement and aggregates). Some peaks derived from the ceramic compounds present in C&DW, such as albite (A, NaAlSi 3 O 8 , PDFcard 090466) and muscovite mica (M, MgAlSi 4 O 10 (OH) 2 , PDFcard 210993). The presence of these crystalline compounds was similar to the C&DW studied by other authors [19,20]. The XRD pattern of C&DW appears in Figure 6. The main peaks corresponded to the presence of quartz (Q, SiO2, PDFcard 210816) and calcite (C, CaCO3, PDFcard 050583). Gypsum was identified as the secondary peak (G, CaSO4.2H2O, PDFcard 210816). These minerals derived from the composition of mortars and concretes (Portland cement and aggregates). Some peaks derived from the ceramic compounds present in C&DW, such as albite (A, NaAlSi3O8, PDFcard 090466) and muscovite mica (M, MgAlSi4O10(OH)2, PDFcard 210993). The presence of these crystalline compounds was similar to the C&DW studied by other authors [19,20]. The derivative thermogravimetric curve (DTG) of C&DW is represented in Figure 7. The main peak occurred at around 850 °C, which was attributed to loss of CO2 from CaCO3. The other peak was around 150 °C and was attributed to the dehydration of gypsum and the hydrated compounds that derived from the hydration process of the Portland cement present in the mortar and concrete fractions. Total mass loss was 13.75% and the mass loss that derived from carbonates was 8.07%. Intensity (arbiratry units) The derivative thermogravimetric curve (DTG) of C&DW is represented in Figure 7. The main peak occurred at around 850 • C, which was attributed to loss of CO 2 from CaCO 3 . The other peak was around 150 • C and was attributed to the dehydration of gypsum and the hydrated compounds that derived from the hydration process of the Portland cement present in the mortar and concrete fractions. Total mass loss was 13.75% and the mass loss that derived from carbonates was 8.07%.  Figure 8 shows the FESEM images of the milled C&DW. The micrographs of this material displayed well-graded irregular-shaped particle distribution. Many particles were not porous.

C&DW Reactivity as Pozzolanic Material
Tashima et al. [17] proposed using a parameter denominated loss of electrical conductivity (LC%) to evaluate the unsaturation criterion of CH/pozzolan suspensions. A value higher than 30% for LC% would indicate that the aqueous solution was unsaturated

C&DW Reactivity as Pozzolanic Material
Tashima et al. [17] proposed using a parameter denominated loss of electrical conductivity (LC%) to evaluate the unsaturation criterion of CH/pozzolan suspensions. A value higher than 30% for LC% would indicate that the aqueous solution was unsaturated in relation to CH and, therefore, pozzolan would have consumed the lime-forming cementitious products. As seen in Figure 9, only the suspensions with a small quantity of CH (0.5:9.5, 1.0:9.0 and 1.5:8.5) achieved the unsaturation setup for the CH:C&DW system. in relation to CH and, therefore, pozzolan would have consumed the lime-forming cementitious products. As seen in Figure 9, only the suspensions with a small quantity of CH (0.5:9.5, 1.0:9.0 and 1.5:8.5) achieved the unsaturation setup for the CH:C&DW system. According to the classification proposed by Tashima et al. [17], the pozzolanic reactivity shown by C&DW was low. Frías et al. [21] used six different wastes generated during concrete waste crushing (particles of <5 mm). They assessed pozzolanic reactivity with an accelerated method in a pure pozzolan/CH system and concluded that these waste types exhibited medium-low fixation capacity. These results were consistent with the present research. Low reactivity was due to the high proportion in mortar, concrete and stones for C&DW, which are components with no pozzolanic material. Pozzolanic reactivity is attributed to the presence of ceramics and bricks.

C&DW Reactivity as Precursor Material in Alkaline Activated Pastes
A 100% C&DW paste was activated using sodium hydroxide and sodium silicate (9 According to the classification proposed by Tashima et al. [17], the pozzolanic reactivity shown by C&DW was low. Frías et al. [21] used six different wastes generated during concrete waste crushing (particles of <5 mm). They assessed pozzolanic reactivity with an accelerated method in a pure pozzolan/CH system and concluded that these waste types exhibited medium-low fixation capacity. These results were consistent with the present research. Low reactivity was due to the high proportion in mortar, concrete and stones for C&DW, which are components with no pozzolanic material. Pozzolanic reactivity is attributed to the presence of ceramics and bricks.

C&DW Reactivity as Precursor Material in Alkaline Activated Pastes
A 100% C&DW paste was activated using sodium hydroxide and sodium silicate (9 mol·kg −1 of sodium, an SiO 2 /Na 2 O molar ratio of 1.21, and a water/binder ratio of 0.45). The paste was cured for 7 days at 65 • C to study the products that formed during the activation reaction. Figure 10 represents the TGA and DTG curves for this paste obtained within the heat interval 35 • C and 1000 • C. There are three significant zones which can be related to the decomposition of reaction products. In zone 1, mass loss can be attributed to the decomposition of CSH, CASH, NASH, and C(N)ASH that derive from either the alkaline activation reaction or unreacted waste. Similar products have been observed by Escalante et al. for active limestone with sodium hydroxide and waterglass [22]. The peak observed in Figure 7, centred at 850 • C, unfolded in two zones (zones 2 and 3). The peak in zone 3 appeared at the same temperature as that observed in the original waste (850 • C). A peak centred at 550 • C in zone 2. This phenomenon demonstrated that calcium carbonate reacts and produces calcium carboaluminate hydrate products. The formation of carboaluminates has been observed in binary and ternary pastes with cement, ground BFS, limestone filler and ground oyster shells [23]. The authors of those research works established that calcium carbonate was consumed to form calcium hemi-and monocarboaluminate phases. Other authors have established the interval between 300 • C and 650 • C for carboaluminates decomposition [24] or within 800-850 • C [25]. Payá et al. [26] observed carbonate decomposition at low temperature when calcium carbonate was activated. The presence of products derived from the alkali activation reaction was corroborated by the XRD analysis. Figure 11 represents the activated paste and the formation of zeolitic phases, specifically the formation of zeolite Pt (Z, Na5.7Al5.7Si10.3O32.12H2O, PDFcard 340524). The phases of the original material simultaneously appeared, such as albite, quartz or calcite.  The presence of products derived from the alkali activation reaction was corroborated by the XRD analysis. Figure 11 represents the activated paste and the formation of zeolitic phases, specifically the formation of zeolite Pt (Z, Na 5.7 Al 5.7 Si 10.3 O 32 ·12H 2 O, PDFcard 340524). The phases of the original material simultaneously appeared, such as albite, quartz or calcite. The presence of products derived from the alkali activation reaction was corroborated by the XRD analysis. Figure 11 represents the activated paste and the formation of zeolitic phases, specifically the formation of zeolite Pt (Z, Na5.7Al5.7Si10.3O32.12H2O, PDFcard 340524). The phases of the original material simultaneously appeared, such as albite, quartz or calcite. Figure 11. XRD pattern of the alkali-activated paste of C&DW cured for 7 days at 65 °C.

Mechanical Strength
Although it was demonstrated that C&DW can be alkali-activated, the preliminary studies on the mortars demonstrated that the compressive strength of the mortar with only this waste type was very low. It was thought to use BFS as a precursor material along Intensity (arbitrary units) Figure 11. XRD pattern of the alkali-activated paste of C&DW cured for 7 days at 65 • C.

Mechanical Strength
Although it was demonstrated that C&DW can be alkali-activated, the preliminary studies on the mortars demonstrated that the compressive strength of the mortar with only this waste type was very low. It was thought to use BFS as a precursor material along with C&DW. The use of BFS with other less reactive materials such as ceramic sanitaryware had been explored in previous research obtaining good results. In this research it was corroborated that the resistance of mortars with 100% BFS obtain compressive strength values higher than 90 MPa for 7 days of curing at 65 • C, and 80 MPa at 28 days of curing at room temperature. The author used a Na + concentrations of 7.5 mol·kg −1 and an SiO 2 /Na 2 O molar ratios of 1.94 [27].
In the present experiment, the two selected SiO 2 /Na 2 O ratios (1.21 and 1.56) were chosen based on previous research [28], which employed hydrated cement. It was carbonated by two methods before being used as a precursor in an alkali-activated system. The prepared alkali-activated mortars contained this carbonated waste. The obtained compressive strength was between 10 MPa and 20 MPa at room temperature (28 curing days) depending on the carbonation conditions and the employed SiO 2 /Na 2 O ratio.
The compressive strength for the C&DW/BFS mortars with different C&DW percentages cured at 65 • C are depicted in Figure 12. The compressive strength was 7.4 MPa after 3 curing days for the mortars activated at the 1.56 SiO 2 /Na 2 O molar ratio (Figure 12a) containing 100% C&DW. By replacing only 20% C&DW with BFS, a 201% increment in compressive strength was achieved. A linear increase in compressive strength took place with the rise in BFS and yielded up to 58.2 MPa after 7 curing days at 65 • C for the 50% sample. Figure 12b presents the compressive strength of the mortars activated at the 1.21 SiO 2 /Na 2 O molar ratio. The maximum compressive strength (52.2 MPa) was yielded for the mortars containing 50% BFS after 7 curing days. A similar trend was generally observed for this series compared with the 1.56 SiO 2 /Na 2 O molar ratio. The main difference was related to the compressive strength of the 100% C&DW mortar. At the 1.21 SiO 2 /Na 2 O molar ratio, the 100% C&DW mortar yielded 18.0 MPa after 3 curing days, which was 2.5-fold higher than for the 1.56 SiO 2 /Na 2 O molar ratio. When the mixture contained 30% BFS or more, compressive strengths were slightly higher for the ratio with the smallest Na 2 O quantity. These results agreed with those published by Heikal et al. [29], who established that the compressive strength of BFS increases by alkali contents, but further increases can lower compressive strength. The increasing Na 2 O content probably resulted in changes in the products' structure.
compressive strength was between 10 MPa and 20 MPa at room temperature (28 curing days) depending on the carbonation conditions and the employed SiO2/Na2O ratio.
The compressive strength for the C&DW/BFS mortars with different C&DW percentages cured at 65 °C are depicted in Figure 12. The compressive strength was 7.4 MPa after 3 curing days for the mortars activated at the 1.56 SiO2/Na2O molar ratio (Figure 12a) containing 100% C&DW. By replacing only 20% C&DW with BFS, a 201% increment in compressive strength was achieved. A linear increase in compressive strength took place with the rise in BFS and yielded up to 58.2 MPa after 7 curing days at 65 °C for the 50% sample.  Figure 12b presents the compressive strength of the mortars activated at the 1.21 SiO2/Na2O molar ratio. The maximum compressive strength (52.2 MPa) was yielded for the mortars containing 50% BFS after 7 curing days. A similar trend was generally observed for this series compared with the 1.56 SiO2/Na2O molar ratio. The main difference was related to the compressive strength of the 100% C&DW mortar. At the 1.21 SiO2/Na2O molar ratio, the 100% C&DW mortar yielded 18.0 MPa after 3 curing days, which was 2.5fold higher than for the 1.56 SiO2/Na2O molar ratio. When the mixture contained 30% BFS or more, compressive strengths were slightly higher for the ratio with the smallest Na2O quantity. These results agreed with those published by Heikal et al. [29], who established that the compressive strength of BFS increases by alkali contents, but further increases can lower compressive strength. The increasing Na2O content probably resulted in changes in the products' structure.
When the alkali-activated mortars were cured at room temperature ( Figure 13), compressive strength slightly lowered, mainly for the 100% C&DW mortar compared with the results obtained at 65 °C. In this case, the mortar with the 1.21 SiO2/Na2O molar ratio yielded 10.3 MPa after 90 days, whereas compressive strength was 18.0 MPa after 3 curing days for the same mortar at 65 °C. This behaviour was observed only for the 1.21 SiO2/Na2O molar ratio. The mortars containing 70% C&DW presented very interesting When the alkali-activated mortars were cured at room temperature ( Figure 13), compressive strength slightly lowered, mainly for the 100% C&DW mortar compared with the results obtained at 65 • C. In this case, the mortar with the 1.21 SiO 2 /Na 2 O molar ratio yielded 10.3 MPa after 90 days, whereas compressive strength was 18.0 MPa after 3 curing days for the same mortar at 65 • C. This behaviour was observed only for the 1.21 SiO 2 /Na 2 O molar ratio. The mortars containing 70% C&DW presented very interesting mechanical behaviour, which yielded about 40 MPa after 90 curing days. The 50% C&DW mortars yielded more than 50 MPa compression. The results obtained by adding C&DW, compared with a mortar with 100% BFS in other investigations, were lower, although the concentrations of the activators were not exactly the same. The contribution of C&DW to the alkaline activating reaction was not very high. Nevertheless, no deleterious effect was observed in the mechanical strength that developed when C&DW was added to the mortar. Thus, depending on the mortar application, up to 80% C&DW can be added to mortar to yield about 20 MPa depending on the SiO2/Na2O molar ratio. This fact can be considered a new sustainable way towards C&DW valorisation.
The improvement of mechanical resistance when using other precursors together with C&DW has already been studied by other authors such as Vasquez et al. [11] where the use of 10% MK or 30% OPC improved mechanical resistance by 85% and 33%, respectively, compared with a system with 100% C&DW. The results obtained by adding C&DW, compared with a mortar with 100% BFS in other investigations, were lower, although the concentrations of the activators were not exactly the same. The contribution of C&DW to the alkaline activating reaction was not very high. Nevertheless, no deleterious effect was observed in the mechanical strength that developed when C&DW was added to the mortar. Thus, depending on the mortar application, up to 80% C&DW can be added to mortar to yield about 20 MPa depending on the SiO 2 /Na 2 O molar ratio. This fact can be considered a new sustainable way towards C&DW valorisation.
The improvement of mechanical resistance when using other precursors together with C&DW has already been studied by other authors such as Vasquez et al. [11] where the use of 10% MK or 30% OPC improved mechanical resistance by 85% and 33%, respectively, compared with a system with 100% C&DW.

FESEM Studies
The activated pastes at the 1.21 SiO 2 /Na 2 O molar ratio and with different proportions of C&DW/BFS were studied by FESEM. Figure 14 depicts the micrographs for the pastes cured for 7 days at 65 • C.
other investigations, were lower, although the concentrations of the activators were not exactly the same. The contribution of C&DW to the alkaline activating reaction was not very high. Nevertheless, no deleterious effect was observed in the mechanical strength that developed when C&DW was added to the mortar. Thus, depending on the mortar application, up to 80% C&DW can be added to mortar to yield about 20 MPa depending on the SiO2/Na2O molar ratio. This fact can be considered a new sustainable way towards C&DW valorisation.
The improvement of mechanical resistance when using other precursors together with C&DW has already been studied by other authors such as Vasquez et al. [11] where the use of 10% MK or 30% OPC improved mechanical resistance by 85% and 33%, respectively, compared with a system with 100% C&DW.

FESEM Studies
The activated pastes at the 1.21 SiO2/Na2O molar ratio and with different proportions of C&DW/BFS were studied by FESEM. Figure 14 depicts the micrographs for the pastes cured for 7 days at 65 °C. The pastes activated with only C&DW (Figure 14a,b) presented a type of gel as the chemical reaction product. Gel morphology was similar to that previously reported during the activation of this waste type [20]. Villaquirán-Caicedo and Mejía de Gutiérrez [20] activated C&DW mixtures, which corresponded to zeolitic phases. The pastes activated with only C&DW (Figure 14a,b) presented a type of gel as the chemical reaction product. Gel morphology was similar to that previously reported during the activation of this waste type [20]. Villaquirán-Caicedo and Mejía de Gutiérrez [20] activated C&DW mixtures, which corresponded to zeolitic phases.
For the BFS-containing pastes (Figure 14c-f), the morphology of the products was more compact than those in the paste with only C&DW. The micrographs reveal particles of unreacted BFS and the principal product of the reaction was C(N)ASH gel. The formation of C(N)ASH gel in the pastes with BFS was reported by Moraes et al. when they activated BFS with sugar cane straw ash [30]. Yang et al. also indicated the presence of this gel while performing an EDS analysis on alkaline-activated BFS and coal gangue samples [31]. In pastes with BFS, the presence of zeolitic phases has not been observed.

Conclusions
The main conclusions drawn from the present research work are: The studied C&DW contained considerable amounts of compounds CaO, SiO 2 , and Al 2 O 3 to a lesser extent, which are very important for the alkaline activation process The analysis of the electrical conductivity of the CH/C&DW suspensions showed that C&DW lacks important pozzolanic properties The compressive strength results of the alkali-activated C&DW mortars using a mixture of NaOH and sodium silicate as an activator demonstrated the possibility of valorising this waste in this way, with up to 18 MPa after 3 days of curing at 65 • C. These resistances are perfectly valid for non-structural applications where resistance requirements are less important.
When BFS was added as part of the precursor, improvements in mechanical strength occurred, with values above 55 MPa after 7 days of curing at 65 • C. These improvements in mechanical strengths by adding BFS would allow mixed BFS/C&DW systems to be used in structural applications.
In view of the obtained results, one might think that these systems can be used as prefabricated elements of different characteristics. Depending on the necessary mechanical strength, the percentage of BFS to be used would vary. It also opens the possibility of valorisation of C&DW in binary mixtures with other precursors such as fly ash or the fluid catalytic cracking residue.