Effect of CaO on the Autogenous Shrinkage of Alkali-Activated Slag Mortar

.e autogenous shrinkage of alkali-activated slag (AAS) is significantly higher than that of ordinary Portland cement (OPC). .e higher risk of concrete cracking due to autogenous shrinkage is a critical drawback to wider use of this promising alternative binder..e effect of CaO content on the autogenous shrinkage of AASmortar was investigated..e autogenous shrinkage of AAS mortars was determined by comparator. .e pore structure of the pastes was determined by mercury intrusion porosimetry. .e hydration products of the pastes were determined by Fourier transform-infrared, thermogravimetric analysis, X-ray diffraction, and Si solid-state magic-angle spinning nuclearmagnetic resonance..e results show that the amount of portlandite increases as CaO content increases. CaO in the paste causes the partial replacement of C-S-H(I) (low stiffness) by C-S-H(II) (high stiffness). .e hydration reaction of AAS is inhibited by the addition of CaO. .e increase of polymerization degree of C-(A-)S-H and rearrangement of C-S-H(I) during hydration are inhibited by the addition of CaO, and micropores closure is also inhibited. .erefore, the autogenous shrinkage of AAS mortar decreases with the increase of CaO content.


Introduction
Ground granulated blast furnace slag (GGBFS) is a byproduct of iron smelting. Alkali-activated GGBFS is a replacement for Portland cement in concrete. Alkali-activated slag (AAS) concrete has low embodied energy, a low CO 2 footprint, and equivalent or better strengths than ordinary Portland cement (OPC) concrete. AAS is also more resistant to fire, chloride-induced corrosion, and chemical (e.g., acid and sulfate) reactions than OPC [1]. However, a significant problem in using AAS is its volumetric instability. AAS concretes are prone to significant shrinkage due to selfdesiccation and have a high risk of cracking [2]. ere have been various studies over the past decade aimed at determining the influence of formula on autogenous shrinkage of AAS base material.
Allahverdi et al. [3] studied the effect of conditioning AAS mortar with liquid sodium silicate (LSS) on autogenous shrinkage. Autogenous shrinkage increased as the quantity of LSS increased. ey concluded that the quantity of the activator greatly influenced the mechanical properties of the mortar, its porosity, and the degree of hydration, which are factors that determine autogenous shrinkage. Neto et al. [4] also studied the effect of adding LSS on the autogenous shrinkage of AAS mortar and reached the same conclusion.
Cartwright et al. [2] studied the effects of conditioning AAS mortar with NaOH and varying the LSS modulus on autogenous shrinkage. Autogenous shrinkage increased as quantities of NaOH increased and as the LSS modulus increased. ey suggested that greater saturation, finer pore structure, lower elastic stiffness, and greater chemical shrinkage led to greater autogenous shrinkage. ey also suggested that other mechanisms, such as creep and Gibbs-Bangham shrinkage, further contribute to greater autogenous shrinkage of AAS mortar and must be considered in future research.
Lee et al. [5] studied the effects of the proportion of slag, the quantity of sodium silicate powder, and the water/binder ratio on the autogenous shrinkage of alkali-activated fly ashslag (AFS) mortar. Autogenous shrinkage increased as the slag or sodium silicate content increased. For the same Na 2 O/H 2 O and SiO 2 /H 2 O ratios of alkali activators, the autogenous shrinkage of the AFS mortar increased as the water/binder ratio decreased. ey attributed the greater autogenous shrinkage to greater capillary stress resulting from higher mesopore volume.
Neto et al. [6], Allahverdi et al. [3], and Lee et al. [5] all attributed the larger autogenous shrinkage of AAS or AFS mortar to its finer structure. Cartwright et al. [2] also suggested that creep is a factor in increased autogenous shrinkage of AAS mortar.
CaO can alter the initial Ca/Si ratio of AAS, which may lead to the change of the species and proportions of hydration products and modification of the structure of C-(A-) S-H. e expansion of CaO can also counter the autogenous shrinkage. To our knowledge, the effect of CaO on the autogenous shrinkage of AAS mortar, which was activated by NaOH, has not been reported. In this study, we investigated the effect of CaO (at proportions of 3%, 6%, 9%, and 12%) on the autogenous shrinkage of AAS mortar, using NaOH (Na 2 O% � 4% by mass of slag) as an activator. Table 1, and its main chemical composition is shown in Table 2. NaOH powder was used as an alkaline activator. River sand was used as a fine aggregate. Properties of the fine aggregate are given in Table 3, and gradation of the fine aggregate (residue on each sieve) is given in Table 4.

Mixture Proportions.
e compositions of AAS cement are provided in Table 5 and are denoted with specific codes. e labels m Na , m Ca , and m s denote the NaOH content (by mass of Na 2 O), the mass of CaO, and the mass of slag. e numbers 0, 3, 6, 9, and 12 denote the CaO content (% mass). e water/binder (GGBFS + activator) mass ratios of AAS mortars and pastes were 0.4. e sand/binder mass ratio of AAS mortars was 2 : 1.

Testing Methods.
ree mortar cuboids measuring 25 × 25 × 280 mm were cast for each group. Autogenous shrinkage tests were performed at ages 1, 2, 3, 4, 5, 6,7,9,11,14,17,21,28,35,42,49,56,63,70,77,84,91,98,112,125,140,154,168,182,196,210,224,238, and 252 days. Length variances were determined in accordance with ASTM C490 [7]. All cuboids were wrapped with an inner layer of polyethylene film and an outer layer of aluminum foil and sealed with aluminum tape. e cuboids used for the autogenous shrinkage tests were demoulded at 24 h and immediately stored in a curing room at a constant temperature of 20 ± 0.3°C. e paste specimens for the microscopic tests were prepared and cured using the same method. e pore structure of the AAS pastes was measured using mercury intrusion porosimetry (MIP) in accordance with ASTM D4284-07 on a PoreMaster-60 (Quantachrome Corp.).
Fourier transform infrared (FTIR) spectra were collected using a Nicolet iS50 FTIR spectrometer in transmittance mode from wavenumbers 4000 to 400 cm −1 using a standard KBr technique. ermogravimetry and differential scanning calorimetry (TG-DSC) curves were obtained using a NETSZCH STA449C thermobalance with a heating ratio of 10°C/min in a nitrogen atmosphere.
X-ray diffraction (XRD) data were collected using a PANalytical X'Pert PRO diffractometer in a conventional Bragg-Brentano θ-2θ configuration.
e CuKα X-ray (λ �1.5418Å) was generated using 40 mA and 40 kV. A Kβ nickel filter was used to remove the β diffraction spectra. e AAS samples were scanned continuously between 5°and 75°2 θ. e d (002) values were calculated by the Bragg equation [8]: where d is the crystalline interplanar distance, θ is the grazing angle, n is the diffraction series, and λ is the X-ray wavelength. e 29 Si MAS NMR spectra were recorded on a Bruker AVANCE III 500 spectrometer. e 29 Si chemical shift was referenced relative to tetramethylsilane, Si(CH 3 ) 4 . e 29 Si MAS NMR spectra were deconvoluted using a Gaussian-Lorentzian function. e mean chain length of C-(A-) S-H (MCL) can be calculated by (2) [9]. e Al/Si ratio of C-(A-)S-H can be calculated by (3) [9]. e hydration degree of slag (α) of the slag can be calculated by (4) [10]. denotes the sum of relative integral intensity of Q 0 and Q 1 slag . e specimens used for MIP, FTIR, XRD, TG-DSC, and 29 Si MAS NMR were vacuum-dried to a constant weight before being used. Figure 1 shows that the autogenous shrinkage of AAS mortar decreases with the increase of CaO content. e ordering of 252 d autogenous  Figure 2 and Table 6. e total porosity and average pore diameter are shown in Table 6. e total porosities of C12 at 28 d and 56 d were 6.45% and 5.28% less than those of C0. e average pore diameters of C12 at 28 d and 56 d were 29.62% and 25.63% less than those of C0.

Autogenous Shrinkage.
ese results indicate that the increase in CaO content results in the pore structure becoming finer in the AAS paste. Figure 2 also shows that the threshold pore diameter of the first peak (around 7-9 nm) for C0 increases from 7.07 nm (28 d) to 8.24 nm (56 d). is indicates that some micropore closure occurs as hydration continues. However, the threshold pore diameter of the first peak (around 7-9 nm) for C12 increases slightly from 8.70 nm (28 d) to 8.73 nm (56 d). Figure 3 shows the FTIR spectra of the AAS pastes. e narrow bands at 3652-3641 cm −1 on the spectra are characteristic of the stretching vibrations generated by the O-H bonds in portlandite (Ca(OH) 2 ) [11]. It can be inferred that, due to the high initial Ca/Si ratio in C12, a certain amount of portlandite precipitated with the C- is indicates the increased polymerization of C-(A-)S-H [12]. e band for C12 also moves from 949 cm −1 (28 d) to 950 cm −1 (56 d). It indicates that the polymerization of C-(A-)S-H in C12 is less than that of C0, and the increased polymerization of C-(A-)S-H in C12 is less than that of C0 over 28-56 d. No Q 3 units were detected by this technique (i.e., at 1200 cm −1 [13]) during this study.  Figure 4 shows the TG, DSC, and DTG curves of the hardened C0 and C12 pastes for 56 d curing. e presence of C-(A-)S-H in C0 and C12 is shown by the peaks below 200°C on the DSC and DTG curves. e weight loss on the TG curves in this temperature range are 8.37% (C0) and 7.41% (C12) and are related to C-(A-)S-H, Aft, and Afm [14].      Figure 4(a)) and 378.6°C (C12, Figure 4(b)) [15]. e weight losses for the second broad peak on the TG curve are 3.94% (C0, Figure 4(a)) and 3.38% (C12, Figure 4(b)). ese results indicate a reduction of the hydrotalcite phase as CaO content increases. e portlandite (Ca(OH) 2 ) peak occurs at 451.7°C on the DSC and DTG curves for C12 (Figure 4(b)), but there is no distinct peak in the temperature range 400°C-500°C on the DSC and DTG curves for C0 (Figure 4(a)). ese results show that portlandite content increases with the increase of CaO content.    Figure 5 shows the XRD patterns of C0 and C12 at 28 d and 56 d. e hydration products in both C0 and C12 are C-S-H (I), hydrotalcite phases, hydrogarnet, ettringite, and portlandite, which is consistent with existing research [16]. Calcite was also detected in AAS due to slight carbonation during mixing or sample preparation. ere is a considerable increase in peak intensity of portlandite and C-S-H (II) on the patterns for C12 at 28 d and 56 d. is   Figure 6 shows the deconvolution of 29 Si MAS NMR patterns for C0 and C12 at 28 d and 56 d. e ranges of chemical shifts corresponding to Q n and Q n (1Al) units in aluminosilicate were used for fitting the NMR spectrum of AAS [17]. It can be seen that raw slag consists mainly of Q 0 and Q 1 slag , while C-(A-)S-H gel consists primarily of Q 1 , Q 2 (1Al), and Q 2 . is indicates that there are no cross-link structures in the specimens. e signal corresponding to Q 2 (1Al) indicates that some Si in the [SiO 4 ] was substituted by Al. Table 8 (Figure 2 and Table 6). e threshold pore diameter of the first peak (around 7-9 nm) for C0 moves significantly from 28 d to 56 d (7.07 nm to 8.24 nm) (Figure 2). e threshold pore diameter of the first peak (around 7-9 nm) for C12 moves slightly from 28 d to 56 d (8.70 nm to 8.73 nm) (Figure 2).

29 Si MAS NMR.
ese results indicate that micropore closure, which is one cause of autogenous shrinkage, is reduced by the increased CaO content.

Hydration Products and Hydration Degree.
Analysis of the results of FTIR (Figure 3), TG-DSC-DTG (Figure 4), and XRD ( Figure 5) shows that the amount of portlandite increases when the CaO content increases from 0% to 12%. e initial Ca/Si ratio of the AAS paste increases as CaO content increases. When the Ca/Si ratio of C-(A-)S-H is equilibrated with the pore solution, which is saturated with Ca 2+ , the Ca 2+ in the pore solution cannot be consumed to increase the Ca/Si ratio of C-(A-)S-H. Portlandite precipitates out and fills the capillary pores. e autogenous shrinkage of AAS is also counterbalanced by the volumetric expansion of calcium oxide's hydration. e characteristic peak of C-S-H (II) is seen in the XRD spectrum for C12 ( Figure 5). It is known that Ca/Si ratio of C-S-H (I) is 0.8-1.5, whereas the Ca/Si ratio of C-S-H (II) is approximately 1.8 in Portland cement [5].
is indicates that the Ca/Si ratio of C-(A-)S-H increases when the CaO content increases from 0 to 12% due to the increase in the initial Ca/Si ratio of the AAS paste. In addition, NaOH-activated slag (low Ca/Si ratio) was reported to be less stiff than OPC (high Ca/Si ratio) in the existing research [2].
Analysis of the results of TG-DSC-DTG (Figure 4), XRD ( Figure 5), and 29 Si MAS NMR ( Figure 6 and Table 8) shows that α at 28 d and 56 d decreases when the CaO content increases from 0 to 12%. In other words, the addition of CaO inhibits the hydration reaction. us the quantity of C-(A-) S-H, in which deformation mainly occurs, decreases as CaO content increases.

MCL and Mean Basal Distances of C-(A-)S-H.
Analysis of the results of 29 Si MAS NMR ( Figure 6 and Table 8) shows that Si polymerization occurs between adjacent C-(A-)S-H during hydration. e results of FTIR ( Figure 3) agree with those of 29 Si MAS NMR. ey indicate that MCL in AAS decreases as CaO content increases and that polymerization degree increases as curing time increases but it is inhibited by the addition of CaO. e addition of CaO initially increases the Ca/Si ratio of the AAS paste. is leads to an increase in the Ca/Si ratio of C-(A-)S-H. e MCLs decrease with the increase of Ca/Si ratio of C-(A-)S-H [11].
Deprotonation of ≡SiOH in C-(A-)S-H occurs in an alkaline environment as shown in Ca 2+ and Na + in the solution compete to compensate the negative charge of ≡Si − O − . Ca 2+ preferentially compensates the negative charge over Na + [18]. us adsorption of Na + decreases as Ca 2+ concentration increases in the pore solution [19]. e increased polymerization of C-(A-)S-H during hydration (in (6)), which increases the density of C-(A-)S-H [20], is inhibited by the addition of CaO until the Ca/Si ratio of C-(A-)S-H reaches the maximum as it equilibrates with the Ca 2+ saturated solution.
e Al/Si ratios of C-(A-)S-H in C12 are lower than those in C0 (Table 8). is indicates that the Al/Si ratios of C-(A-)S-H in AAS decrease when CaO is added. e reduction in MCL indicates a decrease in the proportion of bridge [SiO 4 ] tetrahedra (Q 2b ). According to the result of the ab initio algorithm [21], Al is easier to substitute Si in Q 2b than that in Q 2p . erefore, Al has a lower probability of substituting the Si of [SiO 4 ] tetrahedra in C-(A-)S-H when   CaO is added.
e substitution of Al 3+ for Si 4+ creates negative charge that is compensated by the uptake of Na + ions or Ca 2+ ions in the gel [11]. erefore, the amount of adsorbed Na + decreases as the Al/Si ratios of C-(A-)S-H decrease.
Analysis of the results of XRD (Table 7) shows that the mean basal distance of C-S-H (I) decreases with the age, which is caused by the rearrangement of C-S-H (I) (Figure 7) [22]. e percentage reduction of d (002) from 28 d to 56 d decreases with the addition of CaO. It indicates that the addition of CaO inhibits the rearrangement of C-S-H (I).
e addition of CaO decreases the mean basal distance of C-S-H (I), which decreases the interlayer friction coefficients [23,24], while the decrease in the quantity of adsorbed Na + increases the interlayer friction coefficients [23,24]. e results show that this latter process is dominant. In addition, the adsorption of Na + results in a more irregular stacking of C-(A-)S-H [19] and then creates a larger space for shrinkage because of the differences in ionic radii.

Conclusions
is paper presents the effect of CaO on the autogenous shrinkage of alkali-activated slag (AAS) mortar, using NaOH (Na 2 O% � 4% by mass of slag) as an activator. Based on the experimental results, the following conclusions are drawn: (1) C-S-H (I) (low stiffness) is partially replaced by C-S-H (II) (high stiffness) because of the addition of 12% CaO. In addition, the amount of portlandite increases as CaO content increases. ese can lead to higher stiffness of AAS paste. (2) e addition of CaO inhibits hydration. e hydration degrees of slag (α) in AAS with 12% CaO at 28 d and 56 d are 8.64% and 9.66% lower than those without CaO, respectively. In addition, the micropore closure, which is one cause of autogenous shrinkage, is inhibited by the addition of 12% CaO. with the increase of CaO content. e 252 d autogenous shrinkage of AAS mortar activated by NaOH (Na 2 O% � 4% by mass of slag) is reduced by 50.53% due to the addition of 12% CaO.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.