Three binders were used: A blast furnace slag (BFS), a Coal fly ash (FA) and a CEM I 52.5R cement (OPC). Table 1 gives the chemical composition determined by XRF on PHILIPS PW-1004 join to loss on ignition (22) and free lime of OPC (23). Vitreous content of BFS was found through McMaster method as modified by Hooton and Emery (24), while vitreous content of FA was determined with a selective attack with 1% HF (25).

Particle sizes below which 10%, 50% and 90% of the distribution fell, as well as the Blaine fineness values for each sample (26) are listed in Table 2.

Three types of fine aggregates were used in this study: one was siliceous (S) and two aggregates (R and H) from recycled construction and demolition waste (C&DW). H aggregate come from concrete waste, while R aggregate, has also ceramic particles. Their chemical composition is listed in Table 3. Solubles SO₃ (27) and Cl^- total content (28) in these recycled aggregates were also determined.

The mineralogical composition of the fines, determined by XRD on Philips PW1710 X-ray diffractometer, and the respective quantitative Rietveld analysis are given in Table 4. Further to the data in Table 3, aggregate S was siliceous, with over 98% quartz in its composition. Aggregate R comprised mostly calcite and dolomite, although smaller percentages of basanite, quartz and muscovite were also identified. Aggregate H consisted essentially of dolomite (around 80% of the total) and calcite (16%), with quartz and muscovite as minority crystalline phases.

Given the dolomitic nature of these aggregates, it would be logical to think that they could be reactive to alkali-aggregate reaction. These recycled aggregates come originally from dolomitic aggregates of the area of Granada (Spain) that have been analyzed in various studies, resulting in non-reactive aggregates (29, 30).

Physical characteristics (31) of the three types of aggregate used to prepare the mortars are summarised in Table 5. Their particle size distribution through differencial sieving (32) is plotted in Figure 1. Figure 2 shows the visual aspect of each particle size group, after differential sieving, where it is possible to observe, due to its more reddish color, that aggregate R contains ceramic particles.

Portland cement (OPC) and alkali-activated slag (AAS) and fly ash (AAFA) mortars were prepared at an aggregate/binder ratio of 2:1. Five mortars were prepared with each binder: one with 100% siliceous sand (S), one with 100% aggregate R, one with 100% aggregate H and the other two with 80% S and 20% recycled aggregate R or H. OPC and AAFA mortars were mixed and prepared according to UNE-EN 196-1 (33). However, according to previous works (34) (35), longer mixing time (20 minutes) were used for AAS mortars to avoid fast setting and changes in rheology due to, mainly, primary a type of C-S-H gel formation (36). Liquid/Solid (L/S) ratios were determined for each mortar, based on slump test results. The liquid/solid ratios for the mortars were determined further to Spanish standard (37) to ensure the same slump and therefore, same consistency in all the materials.

The OPC mortars were water-hydrated while AAS mortars were activated with a waterglass solution (Wg) (Na₂O·nSiO₂·mH₂O + NaOH) with SiO₂/Na₂O ratio of 1.2 and 4% Na₂O by mass of slag mass and, finally AAFA mortars were activated with a 10 M solution of NaOH containing 15% Wg.

OPC and AAS mortars were cured at 99 % RH and 20±2 °C for 24 h (OPC mortars were submerged in water, until the test age). AAFA were cured in an oven at 85 °C and 99% relative humidity for the first 20 hours. They were subsequently removed from the moulds and stored in a climatic chamber in the same conditions as the AAS mortar specimens. Table 6 lists the OPC, AAS and AAFA mortars prepared.

• Mechanical strength and porosity of AAMs

Mechanical strengths of all mortars prepared were found for the 2, 7 and 28-day mortar specimens (4x4x16 cm) in a Ibertest press. Total porosity and pore size distribution were tested in the 7 day specimens with 20% replacement on a Micromeritics 9320 Porosimeter.

• Shrinkage behaviour of AAS mortars

AAS mortar shrinkage was studied on prismatic specimens measuring 2.5x2.5x28.7 cm using a Mitutoyo Absolute C112mxb retractometer. These specimens were cured as described above for the first 24 hours and subsequently stored in climatic chambers at 22±2 °C and at either 99 or 45% RH. The tests were conducted until 120-160 days.

• Fire resistance of AAFA mortars

Three-centimetre cubic specimens were prepared for the AAFA fire resistance tests. After curing for 28 days in the conditions described above, they were kept in a kiln at 50 °C for 24 h to eliminate any residual moisture. They were subsequently heated to 200, 400, 600, 800 and 1000 °C, ramping at a rate of 6 °C/min and holding the temperature constant for 1 h at each plateau. Specimen weight and compressive strength were determined.

Figure 3 gives the liquid/solid ratios used for the mortars based on slump test results.

Figure 3 shows that the replacement of S aggregate by R or H recycled aggregates increased liquid demand for all mortars, and this increment is higher as the replacement percentage raises. This effect has been previously reported by other authors (38). Corinaldesi and Moriconi (39), studying the amount of water needed to prepare OPC mortars with different types of recycled aggregate, observed that at 100% replacement, the l/s ratio climbed by 19 to 52%. Conversely, other authors who maintained the same l/s ratio observed that mortar slump values declined more steeply when more recycled aggregate was used. This effect depends on the nature of the recycled aggregate, its water absorption and shape, particularly in clay-based aggregate rate (38, 40).

The l/s ratio obtained for each type of OPC mortars is lower that found for corresponding AAS and AAFA mortars. This effect in mortars with aggregate/binder ratio of 2:1, was previously described by Alonso et al (36).

In OPC mortars, the replacement of aggregate S with 100 % R or H increased the l/s ratio by 51 and 39 %, respectively. In AAS mortars this increase was found to be around 32 and 16% for 100% R and H replacement respectively. In AAFA mortars, 100% replacement of S aggregate result in highest increase of liquid to solid ratio (76 and 64% for R and H aggregates respectively).

The replacement of aggregate S with 20% R or H called for raising the l/s ratio in OPC and AAFA by 7 and 25%, respectively. The l/s ratio increased sigthly when 80/20 recycled C&DW aggregate replacement was added to AAS mortars, however (see Figure 3), even though the liquid required in these mortars was greater than in the OPC and AAFA materials. This finding may be associated with the nature and properties of the activating solution. However, at 20% replacement, no differences were observed between mortars with aggregates R and H, despite the higher water absorption (Table 5) and greater fineness (Figure 1) in the former.

Moreover, the increase in liquid demand is the highest for mortars with 100% R aggregate due to its higher clay conten, higher finesses and therefore its higher water absorption (see Figure 1, Tables 3 and 4). Nevertheless, no relevant differences are observed between 80/20 R and 80/20H mortars.

The flexural and compressive strength for OPC, AAS and AAFA mortars is shown in Figures 4a-f. Table 7 shows 7 days’ porosity and pore size distribution for 100% S and 80/20% replacement mortars. This Figure 4 shows that, generally speaking, in all the mortars tested (OPC, AAS and AAFA) and at all ages, replacing siliceous aggregate S with 20 % or 100% of recycled aggregate induced a decline in mechanical strengths.

Strength grows in all the OPC mortars (Figures 4a and 4b) with test time. Total replacement (100%) of aggregate S declined flexural and compressive strength around 70% for R aggregate and around 60% for H aggregate. These effects are attributables to the higher water demand in the mortars with 100% recycled aggregate, which would concur with earlier reports (10) (13). Mortars with 100% of R aggregate show the lower mechanical performance, due to its clay nature and higher water demand.

A number of authors have reported that the replacement of natural with recycled aggregate in OPC concrete induced a downturn of 5 to 76% in mechanical strength for replacement ratios of 100% (14) (40). Such a wide variation would be due to the host of factors that affect strength in concrete bearing recycled aggregate, including l/s ratio; nature, particle size distribution and shape of the aggregate; number of reuses; replacement ratio; and type of aggregate replaced. In mortars in which sand was replaced, behaviour was also found to vary widely, particularly when the finest fraction was involved (38).

However, partial (20%) replacement of aggregate S lowered 7 and 28-day compressive strength by only around 15%. Note that the percentage decline in mechanical strength was lower than the aggregate replacement rate applied (20% of S and 15% decline). Total porosity climbed in the OPC mortars with the replacement of 20% S by R and H (see Table 7); while the rise was greater with aggregate H than R, the l/s ratio was the same in both. The especially steep rise observed in air pores (>10 μm) and macropores (10-0.05 μm) with 20% replacements may explain the decline in compressive strength in the mortars prepared with aggregates R and H.

As in the OPC mortars, flexural and compressive strength in AAS mortars (Figures 4c and 4d) was observed to rise with test time. Moreover, almost all the 7 and 28 day strength values were higher in all the AAS mortars than in the respective OPC materials, a finding likewise consistent with prior studies (41) (42) (43). As observed in OPC mortars, total replacement at 28-days curing, induces a decline in mechanical strength around 60-80%, and again, AAS100R mortars present worst performance. Seven-day mechanical strength declined by 15 and 5% when the standardised aggregate was partially replaced with 20% R and H, respectively. In the 28 day materials, the decline evened out at around 13% for both types of aggregate. Again as in OPC mortars, the slide in compressive strength was less steep than the aggregate replacement ratio, affording proof of the viability of using recycled aggregate in these AAS mortars. Since the liquid/solid ratio was not observed to change, however, these declines may be associated with alterations in mortar microstructure and compactness. In the AAS mortars, despite the flat l/s ratio, total porosity was observed to rise, with increases in the percentages of macropores (10-0.05 μm) and mesopores (0.05-0.01 μm) in the mortars containing aggregates R and H. The mortar bearing 20 % aggregate H exhibited the highest porosity.

Figures 4e and 4f show mechanical performance in mortars AAFA. Strength was much lower in these mortars than in the other two at all the ages studied (44). In the 28 day specimens, 100% replacement of siliceous aggregate by R or H aggregates induces a steep decline in compressive strength (72% and 56% for R and H respectively). For 7 day specimens (20% replacement) compressive strength dipped by 15-18% with both types of recycled aggregate. The 28 day values (20% replacement) were comparable or even slightly greater than in the mortars with standardised aggregate. Lastly, despite the 25% higher liquid demand in the AAFA mortars with 20 % replacement, porosity values rose only slightly or not at all, so mechanical strength are not so dramatically affected.

The graph in Figure 5 plots the 7 day values for the three variables studied (compressive strength, l/s ratio and porosity) in the OPC, AAS and AAFA mortars with 20% replacement. As the graph shows, since the starting materials (OPC, S and FA) differed widely, the resulting mortars occupied different regions in space.

The results of the shrinkage tests conducted at 99 and 45% RH on AAS mortars containing only siliceous (S) or recycled aggregate (R or H) or S with 20% aggregate R or H are shown in Figure 6.

Note that when the test was conducted at 99% RH, the AAS mortar specimens exhibited similar behavior for S and H aggregate, with slightly higher shrinkage than observed in prior studies (45). However, higher shrinkage values were found for mortars containing R aggregate, as expected. The drying shrinkage values (test conducted at 45% RH) were higher than found for autogenous shrinkage studies (45). The poorest performance was observed for mortar AAS100R, with total shrinkage of 0.75%, 35% higher than mortars AAS100S. AAS100H and AAS80:20S/H mortars presented shrinkage values 20% higher than AAS100S. The inclusion of recycled aggregates induced an increase in drying shrinkage, and this increase is higher as the replacement rises. These results are in aggrement with previous studies made with OPC concrete (46). However, recent works with AAS systems (47)showed contradictory results, indicating the need of a more in deep study on the effect of recycled aggregate in these alkaline systems.

Regarding mortars with 20% of recycled aggregate, higher shrinkage was found for AAS80:20S/H which was found to the most porous AAS mortar (see Table 7). Further to these findings, the inclusion of 100% of construction and demolition waste aggregate increased drying shrinkage.

Lastly, Table 8 gives the percentage of weight loss in AAFA mortars after exposure to high temperatures in the durability test. The photographs in Figure 7 depict the mortar specimens after testing at 200-1000 °C. Specimens from AAFA100R ans AAFA100H were more damaged. The AAFA100S specimens were in best condition, with fewest cracks and least weight loss. Their colour also darkened more than in the other two materials. The AAFA100H and AAFA80:20S/H specimens were in the poorest condition, with lowest density and the highest detachment rate, due to their highest weigth loss (Table 8).

Such greater loss in mortars with recycled aggregates was due in all likelihood to the calcareous nature of the replacement aggregates. At temperatures of over 600 °C these aggregates lost a substantial portion of their mass due to thermal decomposition.

Mortar with 100% of aggregattes R or H exhibited very poor behaviour for fire test. However, AAFA100S and AAFA mortars with partial replacement of S aggregate tested for durability exhibited similar compressive strength patterns (Figure 8). At 50 °C, strength was 16% lower in the mortars bearing aggregates R and H than in mortar AAFA100S. Strength values rose in all three mortars at 200 °C, due to higher activation of fly ash, and then declined until reaching 600 °C to subsequently climb through the end of the test.

This increase in mechanical strength is due to changes in pore size distribution with temperature (48). At 200 °C strength climbed by 52% in mortar AAFA100S but only by 25% in the mortars with 20% aggregate replacement. This increase in mechanical strength is due to an increase in FA reaction/activation and formation of N-A-S-H gel up to 200°C. Other authors have observed this effect until higher temperatures (49). However, at 600 °C, strength dropped in mortar AAFA100S to values comparable to 50 °C. In the recycled aggregate, however, strength was 40% lower than at 50 °C. According to literature (48) this decline in mechanical strength is due to an increase in porosity, and it is also related with the nature of the zeolites and reacctions product formed. Lastly, at 1000 °C mortars AAFA100S and AAFA80:20S/R reverted to the 200 °C values, whereas the mortar with 20% aggregate H barely attained its initial strength. At these high temperatures the liquid come into vapour which is released from the material, generating pressure into pore walls and producing the interconection of these pores. However the reaction products can melt, filling these pores and giving stability to specimens (48).