Commercial CAC, Secar 51 (containing 51 wt% alumina) from Imerys (France) and GGBFS from Ecocem (Belgium) were utilised in this work. The physical properties including measured densities and d₉₀, d₅₀, and d₁₀ values for the precursor materials are shown in Table 1, and the particle distribution curves are illustrated in Figure 1. The densities were measured using a Micromeritics AccuPyc II 1340 pycnometer and the particle size distributions using a Malvern Mastersizer 3000 instrument.

The chemical compositions of both CAC and GGBFS, represented as wt% of oxides, determined from X-ray fluorescence are presented in Table 2.

The X-ray diffraction (XRD) patterns of the raw materials are illustrated in Figure 2. The GGBFS pattern contains no distinct crystalline features and is determined to be predominantly amorphous, exhibiting a very broad peak centred at ∼ 29° 2θ. The crystalline phases identified in CAC are monocalcium aluminate (CaOAl₂O₃ or CA, PDF#04-013-0779), gehlenite (2CaOAl₂O₃·SiO₂ or C₂AS, PDF#01-073-2041), and monocalcium dialuminate (CaO·2Al₂O₃ or CA₂, PDF#00-007-0082).

Alkali-activated pastes were prepared by mixing GGBFS with 10 wt% CAC. The activator solution was prepared by mixing NaOH pellets (99% purity) with water, and subsequently adding sodium silicate solution (supplied by PQ corporation with a starting modulus of 2.07 and concentration of 44.1%) on cooling of the aqueous NaOH solution. Two activator concentrations were formulated by varying the modulus (molar ratio of SiO₂/Na₂O) from 0.56 to 0.91 and the alkali dose from 4 to 4.7 (expressed as Na₂O wt% relative to the precursors, GGBFS and CAC) for activator “a” and activator “b”, respectively. Solutions of NaOH were also prepared with a target concentration of 4 M (activator “c”). Table 3 shows the mix proportions and the respective sample nomenclature used henceforth. The water-to-binder ratio of 0.38 was maintained the same for all mixes.

The heat flow profiles of the alkali-activated and hybrid samples are shown in Figure 3A. An initial exothermic peak, designated as the pre-induction peak, appears immediately for all samples representing the wetting and dissolution of precursor materials. When GGBFS is activated with a sodium silicate solution of modulus of 0.56 (0C-a) the dissolution peak is followed by a dormant period which ends after ∼8 h with the beginning of the acceleration period. In contrast, GGBFS activated with 4M NaOH (0C-c) exhibits a very short dormant period with a subsequent acceleration period commencing after 1.5 h. The dormant period dependence on the activator solution is consistent with previous studies (Haha et al., 2011; Zuo and Ye, 2020). The large second exothermic peak is centred at ∼15 and ∼7 h for 0C-a and 0C-c, respectively, with the acceleration/deceleration stages concluded within the initial 24 h for both mixtures. This exotherm is associated with the precipitation, nucleation, and growth of reaction products, such as C-A-S-H type gels during polycondensation (Brough and Atkinson, 2002).

Hybrid samples activated with sodium silicate and sodium hydroxide, 10C-a and 10C-b, exhibit a markedly different response. Regardless of the activator dosage, the dormant period is prolonged up to ∼40 h. The 10C-a sample shows several overlapping heat evolution peaks between 35-50 h, resolving into a small narrow peak at ∼55 h. The appearance of these secondary exothermic peaks may indicate initial gel formation, i.e., progression of the polycondensation reaction. The subsequent induction-like period spans from 55 to 70 h, after which a visibly suppressed second exothermic peak is apparent with a maximum at ∼85 h. The induction period in the 10C-b sample also exceeds 35 h, followed by two more distinct exothermic peaks: a sharp narrow peak at ∼50 h and a broader peak at ∼ 80 h. As seen in the case of 10C-a, the slope of the last peak diminishes after 150 h. The exact designation of individual heat evolution peaks to describe the progress of several concurrent and competing reactions sequences in this multi-component system is difficult, however, it is clear from the data that both the 10C-a and 10C-b systems with the addition of CAC experience a serious retarding effect on the formation kinetics associated with polycondensation, similarly to what is observed for alkali-activated CAC (Pastor et al., 2009).

The 10C-c system also exhibits a prolonged dormant period compared with 0C-c. Only after ∼17 h does the acceleration stage begin and reaction products such as primary C-A-S-H gels form. The acceleration/deceleration period is seen to conclude within 50 h. Figure 3B shows the cumulative heat profiles of all samples, confirming the appearance of two distinct trends, with 10C-a and 10C-b showing a strong delay in the initial heat evolution, whilst 0C-a, 0C-c, and 10C-c present conventional heat profiles.

Table 4 presents mini-slump and setting time test results for all samples. The water/binder mass ratio used in the formulation of the binders was kept constant in order to compare the effects of individual binder components on workability. The addition of CAC results in a decrease in slump flow even though the CAC used in this work has a slightly larger particle size than the GGBFS (see Table 1), and literature on the effect of CAC additions on paste flow reports CAC particles having a low tendency to form agglomerates, and thus high mobility in aqueous solutions (Vafaei and Allahverdi, 2016). The reason behind the reduction in slump flow is due to the high early reactivity of CAC and the prompt formation of aluminate hydrates from the onset of reaction in the alkaline environment. Workability is found to be dependent on the nature of the activator solution, as clearly seen from the comparison between NaOH-activated and sodium silicate-activated pastes (Dai et al., 2022). The activator type is a significant factor affecting workability; in particular, the presence of dissolved silicates results in a direct increase in flow by acting as a plasticiser for alkali-activated mixtures (Collins and Sanjayan, 1999). The activator modulus and alkali content can be seen to reduce fresh-state flow, when comparing 10C-a and 10C-b.

Sample 0C-a starts to set after 3.33 h and reaches final setting after 4.67 h. These results show that the common issue of quick setting of alkali-activated cement has been avoided, giving setting times similar to those of traditional PC binders, confirming the suitability of the activator selected for the control mix 0C-a (Sasaki et al., 2019; Hertwig and Holschemacher, 2021). The second control formulation, 0C-c, prepared with 4M NaOH has a slightly longer setting time, albeit still less than 6 h to final setting.

Addition of CAC results in a delay in setting times in all cases. Setting times over 40 h were recorded for solutions “a” and “b”, made with sodium silicate solutions with different modulus, used in conjunction with CAC. When 4M NaOH is used as activator, type “c”, the setting times are still prolonged compared to 0C-c, however set within a reasonable timeframe (<13 h). These results are consistent with kinetics curves seen in Figure 3. The extent aluminate hydrate formation, which causes a reduction in flow, is not significant enough to trigger the setting process. This may be due to the low mass fraction of CAC in the mixes, which limits the extent of early hydrate formation.

Overall, CAC additions significantly compromise the fresh properties of hybrid binders, leading to decreased workability and delayed setting time of resulting binders.

Figure 4A shows the XRD diffraction patterns for control specimens 0C-a after 1, 2, 7, and 28 days. The pattern shows distinct broad humps at ∼7°, ∼29°, and ∼35° 2θ indicative of amorphous C-A-S-H gel formation (Tajuelo Rodriguez et al., 2017). Minor peaks are due to the formation of a hydrotalcite-group phase (HT, approximately Mg₄Al₂(CO₃) (OH)₁₂ 3H₂O, PDF#01-089-0460) which is has been observed extensively in alkali-activated slag systems (Wang and Scrivener, 1995; Myers et al., 2017; Walkley et al., 2017). The formation of crystalline hydrotalcite involves the dissolution of the Mg²⁺ component present within the GGBFS, and can result in enhanced properties of the binder, such as resistance to carbonation and chloride ingress (Kayali et al., 2012; Bernal et al., 2014). Calcite (CaCO₃, PDF# 01-086-0174) is formed as the main carbonation product, appearing as a sharp peak overlaid on the broad hump at ∼29° 2θ. The phase evolution between 1 day and 28 days is minimal. The humps attributed to C-A-S-H are noted to become narrower over time as a result of increased short range order and improved gel development.

The XRD patterns of 10C-a are presented in Figure 4B. After 1 day, the peaks at ∼30° and ∼32° 2θ are identified as unreacted CAC phases: calcium monoaluminate (CA), as well as gehlenite (G) which is generally characterised by low reactivity and expected to be stable within the system at ambient temperature during the first stages of reaction (Puerta-Falla et al., 2016). Both CA and G are the major constituents of CAC (Ideker, 2019). Further peaks are referenced to calcium monocarboaluminate (Mc, C₄A C ¯ H₁₁ PDF#00-041-0219), an AFm-like phase likely to be formed from the carbonation of CA hydration products, and hydrogarnet (C₃AH₆ PDF# 00-024-0217). Meta-stable hexagonal aluminate hydrates (CAH₁₀ and C₂AH₈), compounds that conventionally form in the early hydration of CAC, are not observed, suggesting either that in alkaline environments the formation of cubic aluminate hydrate is favoured from the onset of reaction, or that the conversion of the metastable aluminates to hydrogarnet has very fast kinetics (Pastor et al., 2009; Fernández-Jiménez et al., 2011; Reig et al., 2016; Vafaei and Allahverdi, 2016). The formation of AH₃ along with cubic aluminate hydrates is not excluded as a possibility, however this phase is not easily detected in its disordered forms. Both Mc and C₃AH₆ are attributed to the reaction of CA only, whilst the main reaction products of GGBFS alkali-activation (C-A-S-H and HT) are not yet observed. This indicates that CAC promptly dissolves and reacts, whilst the dissolution of GGBFS is inhibited.

The quick precipitation of reaction products (including C₃AH₆) on the surface of GGBFS grains might be the barrier that affects the dissolution and diffusion of the ions into solution for polycondensation reaction (Sun and Vollpracht, 2018) causing a delay in C-A-S-H formation, extended duration of the dormant period in the calorimetry data (Figure 3A) and retarded setting times (Table 4). Calcite is formed as carbonation product after 2 days, whilst the intensity of C₃AH₆ is progressively reduced during the following days of curing until it becomes undetectable after 7 days. Early C-A-S-H formation is slow to occur due to the inhibited dissolution of GGBFS. Increased C-A-S-H formation after 7 days is accompanied by the appearance of strätlingite (C₂ASH₈, PDF# 01-085-8414). Strätlingite crystals are hexagonal AFm-structured plates incorporating Si, and therefore can form at relatively early ages if there exists a sufficient source of soluble silica promoted by sodium salts in a moderately alkaline environment (Ding et al., 1995). Strätlingite formation is also reported to be favoured when high contents of aluminium are present (Lothenbach and Gruskovnjak, 2007; Myers and Lothenbach, 2015), and it is possible that the Al provided by the redissolution of the C₃AH₆ is contributing to this process. The stability and coexistence of strätlingite with C-A-S-H can be confirmed after 28 days (Kwan et al., 1995; Okoronkwo and Glasser, 2016).

The XRD patterns of 10C-b in Figure 4C include the formation of vaterite (CaCO₃ PDF# 01-072-0506) along with C₃AH₆, while Mc formation is not clearly defined until 28 days (Figure 4C). C₃AH₆ is present in the XRD data until 7 days as C-A-S-H forms slowly, similar to the 10C-a sample, and no peaks referenced to strätlingite are present. The lack of strätlingite is explained by its unfavourable formation at very high pH, in agreement with previous studies (Fernández-Jiménez et al., 2011) whereby strätlingite did not form from CAC in highly alkaline environments. It is hypothesised that as strätlingite requires a readily available source of soluble silica to form, there is insufficient dissolution of silica in the 10C-a sample after 2 days to allow for significant formation of this phase to be detected. All three samples containing CAC show a defined C-A-S-H peak at ∼29° 2θ that narrows over time, indicating an increase in short range ordering due to a higher degree of polymerisation of the gel phase (Maddalena et al., 2019).

The XRD pattern of 0C-c in Figure 4D contains peaks indexed to hydrotalcite, calcite, C-S-H, and also tetracalcium aluminate hydrate (C₄AH₁₃) as secondary AFm phase with major peaks overlapping hydrotalcite peaks. The formation of AFm-like secondary products alongside HT has been identified in NaOH-activated GGBFS (Bonk et al., 2003; Escalante-García et al., 2003; Gong and White, 2016; Gijbels et al., 2020). The evolution of phases with time is very limited, similar to the 0C-a control sample, meaning that the primary phase assemblage and structures formed since the initial stages of reaction remain stable.

Figure 4E shows the XRD patterns for 10C-c. Similar to the 0C-c samples, formation of hydrotalcite, calcite, C-S-H, and a secondary AFm phase is noted. Unreacted CA inclusions are observed from a secondary peak at ∼30° 2θ (which disappears after 1 day) and unreacted gehlenite which remains stable throughout alkali-activation. The identified aragonite peak (CaCO₃, PDF# 00-041-1475) at ∼28° represents a secondary carbonation product along with calcite. C₃AH₆ is formed from the onset of reaction, as seen for 10C-a and 10C-b, however remains stable even after 28 days. It is noted that C-A-S-H forms initially, unlike for samples activated with sodium silicate, consistent with the calorimetry data as discussed above which show much less severe inhibition of the dissolution of GGBFS by CAC addition in systems activated by NaOH.

Figure 5 shows the FTIR results obtained for alkali-activated and hybrid samples after 2 and 28 days. The band at 1650 cm⁻¹ is due to H-OH bond bending, indicative of weakly bonded water molecules trapped on the surface or in pores within the samples (Uchino et al., 1991; Heah et al., 2012). The bands at ∼1490–1410 cm⁻¹ are related to O-CO bond stretching and corresponding asymmetric bending at ∼900–820 cm⁻¹ (Trezza and Lavat, 2001; Lee and van Deventer, 2002; Reig et al., 2002; Hidalgo et al., 2009). All samples show modes relating to carbonation products. The major absorption band between 1200–900 cm⁻¹ is associated with asymmetric Si-O-T (T: tetrahedral Si or Al) bond stretching and is significant in understanding the formation of reaction products (García-Lodeiro et al., 2008; Vafaei and Allahverdi, 2016). It is noted for all binders that after alkali-activation, the Si-O-T band shifts to lower wavenumbers and sharpens. The band is the combination of Si-O-Si asymmetric stretching vibrations with varying Al substitution, resulting in a change in bond stretching contributions. The remaining symmetric Si-O-T bond stretching ∼700 cm⁻¹ and Si-O-T bond bending ∼520–420 cm⁻¹ bands are also considered to include contributions from substituted Al.

In general, the major absorption band ∼950 cm⁻¹ in alkali-activated specimens becomes more narrow and tends to shift to lower wavenumbers after 28 days compared with 2 days (Walkley et al., 2016). This is reported in the literature to be due to increased incorporation of Al, and the subsequent contribution from Al-O-Si linkages, within which the Al-O bonds are weaker than Si-O (Król et al., 2018). The integrated intensity of the band is also noted to shift to lower wavenumbers due to Al perturbations in local Si-O bonding (Allahverdi and Skvara, 2001). This is reflected in both 10C-a and 10C-b samples, where the major bands shift from 976 to 984 cm⁻¹ (respectively) to 954 and 957 cm⁻¹ after 28 days, and the shape of the band markedly changes from broad to sharp. The contribution of Si-O symmetric stretching vibrations appears to be larger for samples 0C-c and 10C-c activated with NaOH. Due to the contributions of LDH phases these samples show three distinct bands in the range ∼510–420 cm⁻¹, which are attributed to the Al-O bonds of HT, C-A-S-H, and C₄AH₁₃ (Horgnies et al., 2013; Gijbels et al., 2020). The bands at ∼ 896 and ∼590 cm^-1 for samples with CAC addition can be attributed to hydrogarnet C₃AH₆. (Horgnies et al., 2013).

²⁹Si MAS NMR spectra, deconvolutions, and simulated spectra for the 2 and 28 days cured samples activated with sodium silicate and NaOH are shown in Figure 7 and Figure 8, respectively. The raw GGBFS spectrum is simulated with one peak centred at isotropic chemical shift, δ_iso, = −75 ppm (Figure 6A) and is linearly scaled and included in the simulation of the data for all alkali-activated samples to account for remnant unreacted GGBFS, with the assumption that this lineshape does not change throughout the reaction. The simulated spectrum for GGBFS in this work is consistent with previous studies (Tänzer et al., 2014). Any Si content within CAC is related to the presence of gehlenite, as seen in the XRD data of the precursors (Figure 2). The deconvoluted spectrum in Figure 6B shows Q⁰ and Q¹ sites (δ _iso = −68, −71, −74, and −78 ppm) as the main Si environments (Stebbins and Xue, 2014; Allu et al., 2017).

Table 5 gives the deconvolution data for all ²⁹Si MAS NMR spectra and quantification of Qⁿ environments identified in all samples after 2 and 28 days of curing.

Resonances at δ _iso = −68 and −71 ppm are attributed to Q⁰ and Q¹ units of unreacted GGBFS and CAC (Figure 6). Both precursors have terminal and isolated Si groups present (Florian et al., 2012). The fraction of GGBFS is reduced with increasing curing times as can be seen from the comparison between the 2-day samples (Figures 7A, C, E, 8A, C) with the 28-day samples (Figures 7B, D, F, 8B, D). Signals at δ _iso = −74 and −78 ppm are attributed to the presence of two different types of Q¹ sites in C-S-H or C-A-S-H type gels, which are the main reaction products in all the investigated systems. The assignment of Q¹ sites in C-A-S-H gel is challenging as each individual unit may be bonded to Si or Al and have Ca²⁺, Na⁺, or H⁺ as balancing cations in various ratios that when combined cause variation in Si chemical shifts. Q¹ is simulated with two peaks in the region between −70 and −80 ppm which are named Q¹(I) and Q¹(II) without assigning a specific chemical environment to each one, as a consequence of the complexity of such interactions (Bernal et al., 2013).

Network modifying cations can also affect the chemical environment of Qⁿ (mAl) species, increasing the chemical shift, especially for strongly polarising cations such as calcium (Pena et al., 2008), resulting in additional overlap of the individual Qⁿ (mAl) environments. The presence of distorted Al tetrahedra, Al(V), and Al(VI) species in octahedral configurations further complicates peak assignment in the ²⁹Si MAS NMR spectra.

Resonances at δ _iso = −81.5, −84, and −85.5 ppm are attributed to Q²(1Al) and Q² units indicating the formation of an Al-substituted C-S-H (C-A-S-H) gel with a tobermorite-type structure. These resonances are found in all samples (Figures 7A–F, 8A–D). The signals at δ _iso = −89 and −93 ppm are attributed to highly cross-linked Q³(1Al) and Q⁴(3Al) environments which overlap within the spectra (Myers et al., 2013). These are found in the control sample 0C-a activated with sodium silicate (Figure 7B), whilst not detected in the 0C-c sample activated using NaOH (Figure 8B). This formation of greater cross-linked C-A-S-H type gels when using sodium silicate has been observed in previous studies (Puertas et al., 2011).

The ²⁷Al MAS NMR spectra for both precursors and alkali-activated materials are shown in Figure 9. The GGBFS spectrum exhibits one broad resonance spanning between δ _obs = 80 and 40 ppm, centred at δ _obs = ∼63 ppm, and which is assigned to a distribution of tetrahedral Al environments (Al_T). There is not a single well-defined Al environment distinguishable within the spectrum due to the amorphous structure of the GGBFS precursor (Bernal et al., 2013; Walkley et al., 2016) (Figure 9C). The main characteristic of the CAC spectrum is an intense broad signal with centre at δ _obs = 81 ppm, attributed to the presence of Al_T (Cong and Kirkpatrick, 1993; Pena et al., 2008; Hughes et al., 2019). The asymmetry of this peak is due to the convolution of signals assigned to two different CaAl₂O₄ components (Pena et al., 2008; Hughes et al., 2019). A minor signal between δ _obs = 10 and 13 ppm is associated with the presence of octahedral aluminium (Al_O) in CAH₁₀, that may appear as a consequence of material weathering (Arbi et al., 2013).

The ²⁷Al NMR spectra for 2-day samples in Figure 9A show a similar broad signal spanning from δ _obs = 80 to 50 ppm assigned to Al in a slightly distorted tetrahedral environment. The sharp peak that appears at δ _obs = ∼75 ppm in the 0C-a 2-day spectrum is not present in either the 10C-a or 10C-b 2-day spectra. This peak is attributed to Al in a well-defined Al_T coordination and incorporated in bridging tetrahedra, in which Al_T sites are bonded to Q²(1Al) sites (q²(2Si)) (Richardson et al., 1993; Sun et al., 2006). This resonance becomes more intense and narrower for all samples with increasing curing time, particularly in the 10C-b 28-day spectrum, typical of a well-structured C-A-S-H type gel.

The resonance at δ _obs = 64 ppm in the 10C-a 28-day spectrum is attributed to strätlingite formation which is identified from the XRD data (Figure 4B). The narrow resonance at δ _obs = ∼11 ppm present in all sample spectra is assigned to Al_O, found in AFm-like phases. The intensity of this band for the 10C-a 28-day sample is greater than the other samples due to the convolution of responses from both phases and strätlingite (Kwan et al., 1995; Dai et al., 2014). This resonance is also attributed to C₃AH_6, which is initially present in both 10C-a and 10C-b samples. The shoulder band at δ _obs = ∼7 ppm in the 0Ca 28-day spectrum may indicate the presence of hydrotalcite-like phase as major AFm phase as identified in the XRD data in Figure 4A (Andersen et al., 2003; Bonk et al., 2003).

The ²⁷Al NMR spectra for the mixtures activated with NaOH solution, 0C-c and 10C-c, (Figure 9B) exhibit two signals related to Al_T and Al_O environments. For both samples, intense peaks are found at δ _obs = 11 and 12 ppm. Hydrotalcite, AFm-like phases, and katoite-type phases as C₃AH₆ (for 10C-c) are associated with this peak, identified from XRD data in Figures 4D, E. The Al_T signal in the 0C-c spectrum evolves with increasing curing time, becoming sharper and narrower, with the main resonance found at δ _obs = ∼75 ppm attributed to q²(2Si) environment. The shoulder identified in the 10C-c spectrum at ∼63 ppm can be attributed to q³(3Si) due to additional Al incorporation into the gel (Wang and Scrivener, 2003; Arbi et al., 2013; Walkley and Provis, 2019). This may be attributed to formation of strätlingite as seen for 10C-a 28-day, however strätlingite is not identified from the XRD data of these samples.

Figure 10 shows backscattered electron (BSE) images of all alkali-activated and hybrid samples after 2 and 28 days of curing.

The initial formation of C-A-S-H gel (grey region) in the 0C-a 2-day sample (Figure 10A) is seen surrounding partially unreacted GGBFS particles (brighter particles), with apparent porosity identified by the darker regions. Comparison with the sample after 28 days (Figure 10D) reveals that the 0C-a specimen progressively densifies, generating a more compact binder that appears homogenous, albeit with the persistent inclusion of remnant unreacted GGBFS particles.

The 10C-a and 10C-b 2-day image (Figures 10B, C) shows limited matrix gel formation; numerous unreacted small and large GGBFS particles are present with reduced interparticle spacing, resulting in a highly porous binder. Large CAC particles are also present, for which two distinctive areas of different greyscale levels are identified as CA and gehlenite (Figure 11). The change in microstructure of both samples after 28 days (Figures 10E, F) is similar to the 0C-a sample with increased densification and gel phase formation. Most of the small GGBFS particles are dissolved.

Samples 0C-c and 10C-c after 2 days (Figures 10G, H) also show prominent unreacted GGBFS particles, and are highly heterogeneous composed by distinctive regions with a jagged appearance. The progression of C-A-S-H gel formation is visibly improved after 28 days (Figures 10I, J). The result is similar to the gel formed in sample 0C-a, however the gel formed in 0C-c appears more porous with respect to 10C-c.

Changes in the Ca/Si and Na/Si atomic ratios of the binders cured for 2 and 28 days were determined by SEM-EDS point analysis and are shown in Figures 12A, B.

Each sample after 2 days displays a range of values for the ratio Ca/Si between ∼1 and 2, similar to the precursors. The ratio Na/Si appears to be more elevated for sample 10C-c (>2) and sample 0C-c (∼1), where NaOH is used as an activator. Other samples have values ranging between 0.1–0.5. Increasing the curing time to 28 days, Figure 12B, the Ca/Si ratio slightly decreases for all samples, this is due to the further incorporation of Si in C-A-S-H type gels as the polycondensation reaction proceeds as a result of the slower dissolution of Si with respect to Ca from the precursor materials (Walkley et al., 2017). All data points appear to become more tightly clustered as the curing time increases, meaning that the gel type developed has a sim ilar chemical composition for all samples.

The 7 and 28 days compressive strength results for alkali-activated and hybrid samples are displayed in Figure 13. The 10C-a and 10C-b specimens after 7 days exhibit a slightly inferior strength (∼35 MPa) with respect to the control specimen 0C-a (∼44 MPa). As seen in the previous sections [analysis of kinetics (Figure 3), FTIR (Figure 5), XRD data (Figure 4), and SEM (Figures 10B, C)], the gel formation reaction during the first few days of curing is partially inhibited by the competitive and concurrent formation of other phases, such as hydrogarnet, AFm-like phases, and strätlingite. These compounds when formed at later stages do not result in diminished mechanical integrity, however their initial formation may prevent the complete dissolution and subsequent reorganisation of slag particles during polycondensation. The increase in compressive strength seen after 28 days, from 35 to 61 MPa for 10C-a and from 37 to 60 MPa for 10C-b, is considerable. This suggests that whilst formation of additional gel is gradual due to the initially retarded dissolution of aluminium and calcium sources, it is accompanied by significant AFm-like phases and strätlingite phase formation, which are known to promote the densification of the matrix (Midgley and Bhaskara Rao, 1978; Gosselin et al., 2010; Puerta-Falla et al., 2016; Yang et al., 2019), resulting in a more compact gel structure (Fernández-Jiménez et al., 2008; Arbi et al., 2013; Vafaei and Allahverdi, 2016). Both of these formulations surpass the 28-day strength developed by the control formulation 0C-a, which is 58 MPa.

When using NaOH as an activator the compressive strength developed by the control formulation 0C-c is inferior to that of 0C-a, reaching 22 and 33 MPa after 7 and 28 days, respectively; these values are consistent with those found in the literature (Aydin and Baradan, 2014; Sasui et al., 2019; Gijbels et al., 2020; Hakem Aziz et al., 2020). In this case the addition of CAC is beneficial as the increase of strength is substantial, surpassing 42 MPa and 52 MPa after 7 and 28 days. This provides evidence of good incorporation of Al into the binding phases.