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A ternary optimization of alkali-activated cement mortars incorporating glass powder, slag and calcium aluminate cement
Construction and Building Materials 240 (2020) 117983
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
A ternary optimization of alkali-activated cement mortars incorporating glass powder, slag and calcium aluminate cement Pingping He, Binyu Zhang, Jian-Xin Lu, Chi Sun Poon ⇑ Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
h i g h l i g h t s The optimal ratios of GP, GGBS and CAC in AAC mortars using GC as aggregate were obtained. The effect of ternary binders on the mechanical property and durability was investigated. The performance prediction equations of the ternary composite mortars were obtained.
a r t i c l e
i n f o
Article history: Received 6 November 2019 Received in revised form 27 December 2019 Accepted 28 December 2019
Keywords: Waste glass Alkali-activated cement Alkali-aggregate reaction Calcium aluminate cement Drying shrinkage
a b s t r a c t This paper introduces the use of a simplex-centroid design method, together with a ternary contour diagram, to evaluate the effect of blending different proportions of soda lime glass powder (GP), ground granulated blast furnace slag (GGBS) and calcium aluminate cement (CAC) on the compressive strength, alkali-silica reaction (ASR) expansion, drying shrinkage and high temperature resistance of alkaliactivated cement (AAC) mortars prepared with the use of crushed glass cullet (GC) as aggregates. The relationship between performance of the geopolymer and precursor composition was established. It was found the ASR expansion increased with the increase of GP content and the addition of CAC as the replacement of GGBS could significantly reduce the ASR expansion. The drying shrinkage increased with the increase of GGBS contents. The residual strength coefficient of the mortars after high temperature exposure increased with the increase of CAC content and could be higher than 100%. And the strength coefficient increased with the increase of GP content resulted from its partial melting at the high temperature. It was found that the GP content should be in the range of 77–90%, the GGBS content should be lower than 5% and the CAC content should be higher than 10% to achieve acceptable ASR expansion and drying shrinkage. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction A wide range of studies on alkali-activated cement (AAC), an attractive alternative to Portland cements, has been carried out during the past several decades [1,2]. This cement showed high strength, good resistance to chemical attack and high temperature [3,4]. It was found that not all the AAC had a low carbon footprint [5] and only one part geopolymers showed a lower carbon footprint compared to Portland cement [6,7]. Therefore, some researchers tried to use waste-based activators to decrease the carbon footprint [8–10]. Ground granulated blast furnace slag (GGBS) is a common material used to fabricate alkali-activated slag cement [11]. However, it also has some drawbacks such as rapid setting, high ⇑ Corresponding author. E-mail address: [email protected] (C.S. Poon). https://doi.org/10.1016/j.conbuildmat.2019.117983 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
shrinkage, excessive expansion due to the alkali-aggregate reactions and prone to efflorescence [12–14]. A large quantity of waste glass is being generated in municipalities such as Hong Kong and they are mostly disposed of at landfills due to the lack of a glass manufacturing industry, which induces severe resources and environmental problems. Many researchers tried to recycle waste glass in AAC for use as a construction material. Glass powder (GP) could be used as a partial substitute of GGBS to retard setting time of the alkali-activated slag cement due to the lower alkali reactivity of GP compared to that of GGBS. It can also reduce the drying shrinkage and alkali-silica reaction (ASR) expansion of alkali-activated slag cement [15]. Moreover, some researchers used GP as the replacement of fly ash to improve the performance of AAC mortar as the GP can prevent the transition of the amorphous gel into crystallised zeolites [16,17]. Glass cullet (GC) can be included in AAC mortar as a partial replacement of sand to maximize the recycling of waste glass, but this would
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P. He et al. / Construction and Building Materials 240 (2020) 117983
induce the risk of ASR expansion. In our previous studies, calcium aluminate cement (CAC) was added and successfully mitigated the ASR expansion of AAC mortars prepared with GC [18–20]. The Simplex centroid design method, introduced by Scheffe in 1958 [21], uses only seven batches of experiments to correlate the compositions of ternary composites with different performance. This design method is commonly applied in optimization of industrial product formulations [22,23]. Douglas et al. [24] and Wang et al. [25] successfully used this method to predict the compressive strength of mortars prepared with cement, slag and fly ash. Furthermore, some researchers used this method to study the relationship between materials and properties of cements such as hydration properties [26], ASR expansion [27], chloride-ion permeability [28], drying shrinkage [29], etc. It was found the experimental and predicted results correlated very well [27]. However, limited studies were carried out using this method to investigate the effect of composites of precursors on the properties of AAC materials. But Li et al. [30] proposed an optimal ratio of coarse to fine aggregate of alkali-activated slag-based concrete and Sun et al. [31] optimized the aggregate gradation of alkali-activated metakaolin mortars using this method. The objective of this study is to use the Simplex centroid design method to obtain the optimal ratio of GP, GGBS and CAC in alkaliactivated slag mortars using GC as aggregate. It was reported that the alkali-activated binders had much higher shrinkage compared to the cement-based mortars due to a small volume of water remaining as interstitial water [32,33]. Besides, the high silica phase content in GC could react with alkaline in the pore solution and induce high ASR expansion. Therefore, the most important drawback limiting the use of GC-blended AAC mortars are the high ASR expansion and high drying shrinkage compared to that of the GC-Portland cement composites. The AAC binder compositions affect the characteristics of the products, the pore size distribution and the pore solution, etc. So the effects of the ternary binder on the drying shrinkage, and ASR expansion were discussed in this study. The compressive strength, high temperature resistance of the AAC mortar were also investigated. With the aid of the factorial design method, the optimal binder proportion for AAC mortars with low ASR expansion and drying shrinkage, high resistance to elevated temperature was obtained. The microscopic analysis was presented in another study to investigate the effect of different ratios of precursors on the properties of the AAC mortars. It should be noted that this optimal binder proportion was obtained based on the modulus of the activators and amount and grading of the aggregate used in this study. But influence of the type and dosage of activators, and the dosage and grading of aggregates were not investigated in this study as it was difficult to report all the parameters in a single paper. The optimal proportion may be changed when different modulus or grading of aggregate were used. But this method on assessing the binder was still efficient [27].
2. Experimental program 2.1. Materials The following materials were used in this work: GGBS and CAC, supplied by a commercial source; GP, obtained by milling recycled GC collected from a local glass recycler; a commercially sourced water glass, composed of 28.3% SiO2, 8.6% Na2O and 58.4% H2O; AR grade NaOH. The chemical compositions of GGBS, GP and CAC are shown in Table 1. The particle size distributions of these three powders are shown in Fig. 1. The gradation of the fine aggregate, GC, is shown in Table 2.
2.2. Sample preparation The simplex-centroid design method [24,25] was used to obtain the optimized composition or composition range in GGBS-GP-CAC ternary AAC mortars. When the mixture is composed of three components x1 , x2 and x3 , then
Y ¼ b1 x1 þ b2 x2 þ b3 x3 þ b12 x1 x2 þ b13 x1 x3 þ b23 x2 x3 þ b123 x1 x2 x3
ð1Þ
where Y = response; bi = parameter to be estimated; and x1 ,x2 ,x3 = percentages of GP, GGBS and CAC. The sum of x1 , x2 , x3 were 1. Based on our previous study [34], the proportion range of GP used in this study were 75%-100% in order to maximize the use of waste glass. The proportion ranges of CAC were 0–25% as it was found the AAC mortar containing more than 25% of CAC had a very low compressive strength [19]. The GGBS content was controlled to be lower than 25% so that the setting time was not too short. The seven-batches of designed compositions are listed in Table 3 and plotted in a ternary diagram as shown in Fig. 2. Due to the low reactivity of GP, the mortar containing 100% of GP could not be demolded after 1 day of ambient curing and the ASR expansion and drying shrinkage data could not be obtained. Therefore, another batch of mortar comprising 90% of GP (S5G90C5) was added in the study. The mass ratio of activator/powder, GC/powder, water/ powder and modulus of activator (SiO2/Na2O) were 0.164, 2.7, 0.4 and 1.5, respectively based on our previous study [34]. The powders included GGBS, GP, and CAC. The fresh mixture after mixing by a mechanical mixer was cast into the 40 mm*40 mm*40 mm plastic molds for compressive strength test and the 25 mm*25 mm*285 mm steel molds for drying shrinkage and ASR expansion tests. Then they were cured in laboratory conditions (25 °C and 75% RH) for 1 day before demolding. 2.3. Mix design and experimental methods The compressive strength of cubic specimens was evaluated using a 300 kN capacity compression testing machine with the fixing loading rate of 0.6 MPa/s. After ambient curing for 28 days, some specimens were dried in an oven at 105◦C for 24 h and then placed into an electric furnace which was heated up at a rate of 2.5◦C/min to 800◦C. The maximum temperature was kept for 2 h. After natural cooling to room temperature, the residual compressive strength was tested to evaluate the high temperature resistance of AAC mortars. The drying shrinkage of mortars was evaluated according to the method described in BS ISO 1920--Part 8 [35]. The 25 mm*25 mm*285 mm samples were immersed in a water bath at 25 °C for 48 h and the length was regarded as the initial length. After that, they were moved to a chamber with a temperature of 27 ± 2◦C and a relative humidity of 50%. The length changes at 14 days and 28 days were recorded using a calibrated dial gauge. The measurement of ASR expansion of the mortar bars was conducted following the ASTM C1260 [36]. The 25 mm* 25 mm*285 mm samples were firstly immersed in a water bath at 80 °C for 24 h, followed by placing them in a 1 M NaOH solution contained in a plastic container. The whole setup was then placed in an 80 °C water bath. The length changes of the specimens after 14 days and 28 days of alkaline immersion were recorded. The pore size distributions of the samples were determined using a Poresizer 9320 mercury porosimeter with a maximum mercury intrusion pressure of 210 MPa. The assumed contact angel and a mercury surface tension were 120° and 0.483 N/m, respectively.
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P. He et al. / Construction and Building Materials 240 (2020) 117983 Table 1 Chemical compositions of GGBS, GP and CAC (% by mass).
GGBS GP CAC
3.5
SiO2
CaO
AlO3
Na2O
Fe2O3
K2O
SO4
MgO
35.14 73.29 2.65
37.79 12.14 38.76
13.24 1.10 57.03
– 10.54 –
0.33 0.30 0.65
0.85 0.84 0.21
3.25 0.24 0.32
7.85 1.25 0.24
GGBS GP CAC
3.0
Volume (%)
2.5
2.0
1.5
1.0
0.5
0.0 0.01
0.1
1
10
100
1000
Diameter (mm) Fig. 1. Particle size distributions of GGBS, GP and CAC. Fig. 2. Composition design for GGBS-GP-CAC ternary AAC composites. Table 2 Particle grading of GC. Sieve size
Percentage (%)
2.36 mm-5 mm 1.18 mm-2.36 mm 0.6 mm-1.18 mm <0.6 mm
42.4 22.2 21.4 14.1
obtained due to the severe deformation resulted from the high GP content after the high temperature exposure. 3.1. Compressive strength The corresponding coefficients can be obtained using Eq. (1) and the compressive strength values in Table 4. The compressive strength prediction equations of the ternary composite mortars at 28 days can be described by Eq. (2):
Table 3 Mix proportions of the ternary AAC mortars. Mixtures Notation
GGBS
GP
CAC
S0G100C0 S25G75C0 S0G75C25 S12G87C0 S12G75C12 S0G87C12 S8G83C8 S5G90C5
0 0.25 0 0.125 0.125 0 0.083 0.05
1 0.75 0.75 0.875 0.75 0.875 0.834 0.9
0 0 0.25 0 0.125 0.125 0.083 0.05
The Vickers micro hardness test was carried out using HVX-1000A micro hardness tester to evaluate the mechanical characteristics of the sample before and after subjecting to the high temperature exposure. A flat surface of the sample was obtained by polishing. The test load was 10 g with a 15 s contact time.
3. Results and discussion The compressive strength, ASR expansion, drying shrinkage and residual strength coefficient after exposure to high temperature of AAC mortars are summarized in Table 4. The effect of ternary binders on the properties of mortars are discussed in the following sections. The residual strength coefficient of S5G90C5 was not
Y S28 ¼ 6:48x1 þ 193:91x2 56:91x3 149:23x1 x2 þ 114:5x1 x3 6746:04x2 x3 þ 9535:615x1 x2 x3
ð2Þ
The compressive strength contours of AAC mortars at 28 days in the ternary diagram are shown in Fig. 3(a). It can be seen that the compressive strength was increased with the increase of both GGBS and CAC contents. This was due to the higher alkali reactivity of GGBS and CAC compared to that of GP. And it seems that the effect of GGBS was more obvious than that of CAC. For example, the compressive strength of the mortar containing 15% GGBS was about 16 MPa, while the value was 12 MPa for that containing the same content of CAC. This might be related to the difference of the alkali reaction products formed with these two systems. According to our previous study, the product of the alkaliactivated GP/GGBS was C-N-A-S-H gel. The product in alkaliactivated GP/CAC was a zeolite phase and C-N-A-S-H gel [18]. The increase of zeolite content decreased the compressive strength of the AAC structure due to the lower strength of the zeolite compared to that of AAC gel [37]. Therefore, the alkali-activated GP/GGBS mortar had a higher compressive strength compared to the alkali-activated GP/CAC mortar when the GP content was the same. Besides, the contour suggested that the compressive strength firstly increased and then decreased with the increase of CAC content when the GP content was fixed at a certain value between 75% and 100%. This was in agreement with our previous
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P. He et al. / Construction and Building Materials 240 (2020) 117983
Table 4 Compressive strength, ASR expansion, drying shrinkage and residual strength coefficient after exposure to high temperature of AAC mortars. No.
Compressive strength (MPa)
S0G100C0 S25G75C0 S0G75C25 S12G87C0 S12G75C12 S0G87C12 S8G83C8 S5G90C5
ASR expansion (micro strain)
Drying shrinkage (micro strain)
Residual strength coefficient (%)
28d
90d
14d
28d
14d
28d
6.48 25.36 12.10 13.59 25.07 11.08 22.69 –
29.59 21.29 17.98 19.19 35.74 16.09 20.83 –
– 1776 1017 3719 389 871 682 2521
– 3216 2632 8404 700 1860 1216 5617
– 1262 1092 2611 2321 1152 1232 735
– 1206 1213 2651 2475 1350 1329 835
– 202 455 31.1 206 405 52 –
Note: Residual strength coefficient was calculated as the ratio between the residual compressive strength of the specimens after high temperature exposure at 800 °C for 2 h to the compressive strength of the specimens after 28 days of air curing.
0.00
2.000 4.000
0.25
6.000 8.000 10.00
0.05
0.20
12.00 14.00 16.00
0.10
18.00
0.15
CA
22.00
BS GG
C
20.00
0.15
24.00
0.10
26.00
12.00
0.20
0.05
10.00
0.25
0.00
0.75
0.80
0.85
0.90
0.95
1.00
GP Compressive strength at 28d
(a) 0.00
0.05
0.20
0.10
0.15
C CA
BS GG
22.00 0.15
0.10
20.00 0.20
0.05
0.85
0.90
0.95
492:48x1 x3 þ 4955:377x2 x3 5232:88x1 x2 x3
ð3Þ
Fig. 3(b) shows the compressive strength contours of AAC mortars at 90 days in the ternary diagram. It can be seen that the 90 days compressive strength of the AAC mortars was higher than that at 28 days. When the GP content was fixed at a certain value between 75% and 100%, the compressive strength of AAC mortar firstly increased and then decreased with the increase of CAC content. Unlike the compressive strength development at 28 days, the strength was firstly decreased and then increased with the increase of GP content when the GGBS content was a fixed value of lower than 10%. A similar strength development trend was also observed for the AAC mortars prepared with a fixed CAC content of lower than10%. These results were attributed to a late development of the alkali activity of glass powder [38]. 3.2. ASR expansion The ASR expansion prediction equations of the ternary composite mortars at 14 days and 28 days can be described by Eqs. (4) and (5):
Y A14 ¼ 1:2 104 x1 þ 3:8 105 x2 þ 2:4 105 x3 5:5105 x1 x2 3:7 105 x1 x3 3:2 106 x2 x3 þ 4:2 106 x1 x2 x3
ð4Þ
1:1105 x1 x2 5:1 105 x1 x3 þ 1:4 106 x2 x3 2:0 0.00
0.80
Y S90 ¼ 29:59x1 þ 296:39x2 þ 352:51x3 400x1 x2
Y A28 ¼ 1:7 104 x1 þ 4:2 104 x2 þ 3:4 105 x3
22.00
0.25 0.75
2.000 4.000 6.000 8.000 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00 26.00 28.00 30.00 32.00 34.00 36.00
0.25
to the formation of zeolite and katoite which had much lower strength compared to geopolymer gel [19]. Moreover, the compressive strength could be higher than 24 MPa when the GP content was lower than 85%. The compressive strength prediction equations of the ternary composite mortars at 90 days can be described by Eq. (3):
1.00
GP Compressive strength at 90d
(b) Fig. 3. Compressive strength (MPa) contours of ternary composite AAC mortars at (a) 28 days (b) 90 days.
study that the high aluminum content in CAC had a positive effect on the development of compressive strength. However, further increase of CAC content decreased the compressive strength due
106 x1 x2 x3
ð5Þ
Fig. 4 shows the contour lines of ASR expansion of the mortars after immersion in the alkali environment for 14 days and 28 days. The area with expansion values higher than 1000 micro strain is regarded as a potentially dangerous composition range as the expansion could lead to severe cracking and damage to the mortar. Some samples showed high ASR expansion after alkaline immersion for 14 days due to the high active silica content in GC compared to that in conventional aggregates such as river sand. When the CAC or GGBS content was fixed, the ASR expansion increased with the increase of GP content at 14 days. This means
P. He et al. / Construction and Building Materials 240 (2020) 117983
0.00
0.000
0.25
500.0 1000
0.05
When the CAC content was higher than 3.75%, the expansion was lower than 1000 micro strain at 14 days. The expansion contour of this ternary composite AAC mortars at 28 days was similar to that at 14 days.
1500
0.20
1000
5
2000
3.3. Drying shrinkage
2500
0.10
3000
0.15
3500
CA
C
BS GG
0.15
4000 1.240E+04
0.10
0.20
0.05
0.25
0.00
0.75
0.80
0.85
0.90
0.95
1.00
GP Expansion in alkali at 14 days
(a)
0.00
0.05
0.20
0.10
C 0.15
BS GG
CA
0.15 1000
1000
2000
0.10
0.20
0.25 0.75
0.000 500.0 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 8500 9000
0.25
0.05
1000 2000 0.80
Y D14 ¼ 1:5 103 x1 þ 1:2 105 x2 þ 3:3 4x3 1:7105 x1 x2 0.00
0.85
0.90
0.95
It was reported that the pore size distribution and the content of calcium had a critical influence on the magnitude of drying shrinkage. The drying shrinkage increased with the increase of mesopore (1.25–25 nm) proportion [14] and the Ca/Si ratio in the raw materials when the Ca/Si ratio was lower than 0.75 assuming that the raw materials were fully soluble [32]. Besides, the drying shrinkage decreased with the increase of compressive strength [29]. The pore size distributions of S12G87C0, S12G75C12, and S0G87C12 are shown in Fig. 5. Even though the porosities of these three samples were similar, the mesopore proportions were quite different. Table 5 summarizes the mesopore proportion, compressive strength, Ca/Si ratio in the raw materials and drying shrinkage of these three samples. It can be seen that when using CAC or GGBS to replace GP, the mesopore proportion increased. That might be related to the higher reactivity of CAC or GGBS compared to that of GP refining the pore matrix. When using CAC to replace GGBS, the mesopore proportion decreased, which might be due to the different characteristics of AAC gel. The sample S0G87C12 had the lowest mesopore proportion and low Ca/Si ratio, therefore it had the lowest drying shrinkage compared to S12G87C0 and S12G75C12. S12G75C12 had the highest mesopore proportion, but the high compressive strength could lead to the higher resistance to drying shrinkage. So the drying shrinkage of S12G75C12 was slightly lower than that of S12G87C0. The drying shrinkage prediction equations of the ternary composite mortars at 14 days and 28 days can be described by Eqs. (6) and (7):
1.00
GP Expansion in alkali at 28 days
(b)
5:6 104 x1 x3 1:6 106 x2 x3 þ 2:0 106 x1 x2 x3 Y D28 ¼ 1:9 103 x1 þ 1:3 105 x2 þ 7:0 104 x3 1:9105 x1 x2 1:1 105 x1 x3 2:0 106 x2 x3 þ 2:6 106 x1 x2 x3
Fig. 4. ASR expansion (micro strain) contours of ternary composite AAC mortars at a) 14 days and b) 28 days.
GP had a negative effect on the ASR expansion when the GP content was higher than 75% [15], which might be due to the higher alkali concentration in the pore solution resulted from the low reactivity of GP compared to that of CAC or GGBS. Besides, the alkalis released from the soda lime glass would also contribute to ASR gel formation. CAC could significantly suppress ASR expansion. For example, the ASR expansion was between 3000 and 3500 micro strain when using 15% of GGBS as the replacement of GP. While this value was lower than 500 micro strain for the AAC mortar using the same content of CAC. According to our previous study [18], the aluminum in CAC was incorporated in the gel matrix, which needed Na to balance the negative charge. Besides, the formation of zeolite absorbed more Na from the pore solution. Therefore, the alkali concentration of the alkali-activated CAC/GP mortars had a lower ASR expansion compared to that of the GGBS/GP mortars. As a result, the ASR expansion decreased with the increase of CAC content when the GP content was a fixed value between 75% and 100%. The AAC mortar containing 25% of GGBS and 75% of GP had an expansion higher than 1500 micro strain.
ð6Þ
Fig. 5. The cumulative porosity of S12G87C0, S12G75C12, and S0G87C12.
ð7Þ
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P. He et al. / Construction and Building Materials 240 (2020) 117983
Table 5 Compressive strength, mesopores proportion and drying shrinkage of S12G87C0, S12G75C12, and S0G87C12. No.
GP (%)
GGBS (%)
CAC (%)
28 days Compressive strength (MPa)
90 days Compressive strength (MPa)
Mesopores (%)
Ca/Si
Drying shrinkage (micro strain)
S12G87C0 S12G75C12 S0G87C12
87.5 75 87.5
12.5 12.5 0
0 12.5 12.5
13.59 25.07 11.08
19.19 35.74 16.09
2.30 6.19 1.74
0.176 0.246 0.189
2651 2475 861
0.00
200.0 400.0
0.25
600.0 800.0 1000
0.05
0.20
1200 1400
C CA
0.15
2000 2200
0.10
0.20
0.05
1000
0.75
1800
2400
0.15
0.25
1600
BS GG
0.10
0.80
0.00 0.85
0.90
0.95
1.00
GP Dry shrinkage at 14 days
when the GP content increased. When the GGBS content was lower than 15%, the drying shrinkage firstly decreased and then increased with the increase of GP content. The mesopore proportion, Ca/Si ratio and the compressive strength decreased with the increase of GP content. These three factors had opposite effects on the drying shrinkage. Therefore, the drying shrinkage firstly decreased due to the decrease of the mesopore proportion and Ca/Si ratio, and then increased due to the decrease of the compressive strength. When the CAC or GP content was fixed, the drying shrinkage of mortars increased with the increase of GGBS content, which might be related to the increase of mesopore proportion. It was reported that the AAC mortar prepared with a lower water absorption aggregate showed lower drying shrinkage [32]. The water absorption of glass is negligible. Therefore, the drying shrinkage of AAC mortars prepared with glass cullet could be lower than that prepared with conventional aggregates, like river sand. However, the drying shrinkage of most samples in this study was higher than 1000 micro strain after 14 days. The drying shrinkage could be controlled to be lower than 1000 micro strain at 14 days when the GGBS content was lower than 7.5%.
(a) 3.4. High temperature resistance 0.00
0.25
0.05
0.20
1200 1400
0.15
CA
BS GG
C
0.10
200.0 400.0 600.0 800.0 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800
0.15
0.10
1400 0.20
1200
1000
0.05
0.25 0.75
The MIP test results are illustrated in Fig. 7 and Fig. 8. The pore size distribution and micro hardness results are summarized in Table 6. Even though, the micro hardness of the matrix increased after the high temperature exposure, the porosity of S12G87C0 was increased significantly and the size of the major pores was larger than 1 lm. Therefore, the compressive strength decreased dramatically after the high temperature exposure. The reason for the increase of total porosity might be related to the decomposition of the reaction products in the AAC mortar incorporating GP and GGBS (C-N-A-S-H gel) during the high temperature exposure. After 2 h of high temperature exposure, the total porosity of S12G75C12
0.00 0.80
0.85
0.90
0.95
1.00
GP
(b) Fig. 6. Drying shrinkage (micro strain) contours of ternary composite AAC mortars at a) 14 days and b) 28 days.
The contour plots of drying shrinkage of mortars at 14 days and 28 days are presented in Fig. 6. The drying shrinkage of the mortars at 28 days was slightly higher than that at 14 days because the drying shrinkage developed fast at early age and almost reached to a constant value after about 14 days [39]. When the GGBS content was a fixed value of higher than 15%, the drying shrinkage of mortars increased with the increase of GP content. As mentioned above, the compressive strength decreased when the GP content increased. This might explain the increase of drying shrinkage
Fig. 7. Cumulative pore volume of S12G87C0, S12G75C12 and S0G87C12 before and after high temperature exposure.
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P. He et al. / Construction and Building Materials 240 (2020) 117983
0.00
0.05
0.20
100.0
0.15
CA
BS GG
C
0.10
0.000 50.00 100.0 150.0 200.0 250.0 300.0 350.0 400.0 450.0 500.0 550.0 600.0
0.25
0.15
0.10
0.20
0.05
0.25
0.00
0.75 Fig. 8. Pore size distribution curves of S12G87C0, S12G75C12 and S0G87C12 before and after high temperature exposure.
was decreased and the micro hardness of matrix increased, which might result in the increase of compressive strength after high temperature exposure. The decrease of total porosity might be related to the melting of glass filling the pores in the paste. When comparing S12G87C0 and S12G75C12, it can be found that the AAC mortars incorporating CAC showed much lower porosity after the high temperature exposure. This might be related to the different characteristics of the products, which needs further research. The total porosity of S0G87C12 was significantly decreased and the micro hardness of matrix was increased after the high temperature exposure, which might result in the dramatic increase of compressive strength [40]. S0G87C12 showed a higher residual strength coefficient compared to S12G75C12, which might be related to the higher micro hardness of the matrix after the high temperature exposure. Besides, S0G87C12 had a higher proportion of smaller pores than S12G75C12, which might be because more GP in S0G87C12 was melted which refined the pores. Fig. 9 shows the residual compressive strength of 28 day cured AAC mortars. The residual strength coefficient prediction equation of the AAC mortars could not be obtained due to the missing data in Table 4 as the data of the mortars incorporating a very high percentage of GP could not be obtained due to the severe deformation of the mortars after the high temperature exposure. It was found that the compressive strength of the AAC mortars using other aggregates, like crushed limestone, decreased after high temperature exposure [41]. However, the residual strength of some samples in this study could be higher than that the original strength before the high temperature exposure. The reason for the increase of the compressive strength might be related to the partial melting of the glass (including GP and GC) at the elevated temperature that induced the formation of a stronger bonding between the aggregates and the paste [42]. When the GP content or GGBS content
0.80
0.85
0.90
0.95
1.00
GP Fig. 9. Residual strength coefficient (%) of AAC mortars after exposed to 800 °C for 2 h.
was fixed, the residual strength coefficient increased with the increase of CAC content as the total porosity decreased or the proportion of smaller pores increased when using CAC to replace GP or GGBS as shown in Table 6. The effect of CAC on improving the high temperature resistance might also be related to the dehydration of the CAC hydration products during the high temperature exposure accommodating the expansion of the mortar samples, which will be reported in our another work using differential thermal analysis. When the CAC content was a fixed value of higher than 10%, the residual strength coefficient was higher than 100% and increased with the increase of GP content. The increase of residual strength coefficient might be related to the refinement of the pores in the matrix and the increase of the micro hardness of the matrix after the melting of GP. The residual strength coefficient decreased with the increase of GGBS content, which might be related to the decomposition of products (AAC gel) leaving large pores as mentioned above. 3.5. Optimizing AAC mortar mixture design Based on the above results, the critical values required for the AAC mortars to meet their functions are drawn in Fig. 10. The overlapped area (the grey zone) in Fig. 10 is regarded as the optimal AAC mortar composition which would produce a AAC mortar with low ASR expansion and good drying shrinkage resistance. The optimal composition is an AAC mortar prepared with a GP content in the range of 77% 90%, a GGBS content of lower than 5% and a CAC content of higher than 10%. The 28-day compressive strength of the AAC mortar could reach 16 MPa which can be used as partition wall blocks and the residual strength coefficient after exposure to 800° for 2 h was higher than 500% (gain in strength).
Table 6 Pore size distribution, residual strength coefficient of S12G87C0, S12G75C12, and S0G87C12. No.
S12G87C0 S12G75C12 S0G87C12
Pore size distribution before high temperature exposure (%)
Pore size distribution after high temperature exposure (%)
Micro hardness HV of matrix
porosity
<50 nm
50– 1000 nm
>1000 nm
porosity
<50 nm
50– 1000 nm
>1000 nm
Before high temperature
After high temperature
14.35 15.17 18.04
4.38 10.0 9.49
2.39 1.42 3.36
7.58 3.74 5.19
34.52 6.48 14.00
1.11 1.60 4.77
1.00 2.37 4.86
32.41 2.52 4.38
63.00 57.43 74.99
737.21 384.14 673.74
Residual strength coefficient (%)
31.1 206 405
8
P. He et al. / Construction and Building Materials 240 (2020) 117983
CRediT authorship contribution statement Pingping He: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Data curation, Writing original draft, Visualization. Binyu Zhang: Resources. Jian-Xin Lu: Writing - review & editing. Chi Sun Poon: Writing - review & editing, Supervision, Project administration, Funding acquisition.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement Fig. 10. Optimal binder composition for AAC mortars.
4. Conclusions The main conclusions derived from this work are: (1) The AAC mortars incorporating a higher GP content showed a lower compressive strength at 28 days. When the GP content was higher than 92.5%, the strength was lower than 10 MPa. The compressive strength of AAC mortars firstly increased and then decreased with the increased content of CAC at 28 days and 90 days. (2) When the GP content was higher than 75%, the ASR expansion increased with the increase of GP content. The expansion of the mortars prepared with 85% GP and 15% GGBS could reach 3000 micro strain after 14 days of alkaline immersion. The addition of CAC as the replacement of GGBS could significantly decrease ASR expansion. The expansion could be controlled to lower than 1000 micro strain if the CAC content was higher than 3.75%. (3) When the GGBS content was fixed at a value of higher than 15%, the drying shrinkage increased with the increase of GP contents. When the GGBS content was lower than 15%, the drying shrinkage firstly decreased and then increased with the increase of GP contents. The drying shrinkage of mortars increased with the increase of GGBS contents when the GP or CAC content was fixed. However, the drying shrinkage was lower than 1000 micro strain after 14 days if the GGBS content was lower than 7.5%. (4) The residual strength coefficient after high temperature exposure increased with the increase of CAC content. The residual strength could be higher than that before the high temperature exposure if the CAC content was higher than 10%. When the CAC content was fixed, the residual strength coefficient increased with the increase of GP content. And the residual strength coefficient decreased with the increase of GGBS content. (5) To obtain AAC mortars with acceptable ASR expansion and drying shrinkage, and to maximize the use of waste glass, the GP content should be between 77% and 90%, the GGBS content should be lower than 5% and the CAC content should be higher than 10%. The compressive strength of the mortar in this range could reach 16 MPa, which can be used as partition wall blocks. The residual strength coefficient in this range was higher than 500%.
The authors wish to acknowledge the financial support of the Environment and Conservation Fund.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
A ternary optimization of alkali-activated cement mortars incorporating glass powder, slag and calcium aluminate cement Pingping He, Binyu Zhang, Jian-Xin Lu, Chi Sun Poon ⇑ Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
h i g h l i g h t s The optimal ratios of GP, GGBS and CAC in AAC mortars using GC as aggregate were obtained. The effect of ternary binders on the mechanical property and durability was investigated. The performance prediction equations of the ternary composite mortars were obtained.
a r t i c l e
i n f o
Article history: Received 6 November 2019 Received in revised form 27 December 2019 Accepted 28 December 2019
Keywords: Waste glass Alkali-activated cement Alkali-aggregate reaction Calcium aluminate cement Drying shrinkage
a b s t r a c t This paper introduces the use of a simplex-centroid design method, together with a ternary contour diagram, to evaluate the effect of blending different proportions of soda lime glass powder (GP), ground granulated blast furnace slag (GGBS) and calcium aluminate cement (CAC) on the compressive strength, alkali-silica reaction (ASR) expansion, drying shrinkage and high temperature resistance of alkaliactivated cement (AAC) mortars prepared with the use of crushed glass cullet (GC) as aggregates. The relationship between performance of the geopolymer and precursor composition was established. It was found the ASR expansion increased with the increase of GP content and the addition of CAC as the replacement of GGBS could significantly reduce the ASR expansion. The drying shrinkage increased with the increase of GGBS contents. The residual strength coefficient of the mortars after high temperature exposure increased with the increase of CAC content and could be higher than 100%. And the strength coefficient increased with the increase of GP content resulted from its partial melting at the high temperature. It was found that the GP content should be in the range of 77–90%, the GGBS content should be lower than 5% and the CAC content should be higher than 10% to achieve acceptable ASR expansion and drying shrinkage. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction A wide range of studies on alkali-activated cement (AAC), an attractive alternative to Portland cements, has been carried out during the past several decades [1,2]. This cement showed high strength, good resistance to chemical attack and high temperature [3,4]. It was found that not all the AAC had a low carbon footprint [5] and only one part geopolymers showed a lower carbon footprint compared to Portland cement [6,7]. Therefore, some researchers tried to use waste-based activators to decrease the carbon footprint [8–10]. Ground granulated blast furnace slag (GGBS) is a common material used to fabricate alkali-activated slag cement [11]. However, it also has some drawbacks such as rapid setting, high ⇑ Corresponding author. E-mail address: [email protected] (C.S. Poon). https://doi.org/10.1016/j.conbuildmat.2019.117983 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
shrinkage, excessive expansion due to the alkali-aggregate reactions and prone to efflorescence [12–14]. A large quantity of waste glass is being generated in municipalities such as Hong Kong and they are mostly disposed of at landfills due to the lack of a glass manufacturing industry, which induces severe resources and environmental problems. Many researchers tried to recycle waste glass in AAC for use as a construction material. Glass powder (GP) could be used as a partial substitute of GGBS to retard setting time of the alkali-activated slag cement due to the lower alkali reactivity of GP compared to that of GGBS. It can also reduce the drying shrinkage and alkali-silica reaction (ASR) expansion of alkali-activated slag cement [15]. Moreover, some researchers used GP as the replacement of fly ash to improve the performance of AAC mortar as the GP can prevent the transition of the amorphous gel into crystallised zeolites [16,17]. Glass cullet (GC) can be included in AAC mortar as a partial replacement of sand to maximize the recycling of waste glass, but this would
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P. He et al. / Construction and Building Materials 240 (2020) 117983
induce the risk of ASR expansion. In our previous studies, calcium aluminate cement (CAC) was added and successfully mitigated the ASR expansion of AAC mortars prepared with GC [18–20]. The Simplex centroid design method, introduced by Scheffe in 1958 [21], uses only seven batches of experiments to correlate the compositions of ternary composites with different performance. This design method is commonly applied in optimization of industrial product formulations [22,23]. Douglas et al. [24] and Wang et al. [25] successfully used this method to predict the compressive strength of mortars prepared with cement, slag and fly ash. Furthermore, some researchers used this method to study the relationship between materials and properties of cements such as hydration properties [26], ASR expansion [27], chloride-ion permeability [28], drying shrinkage [29], etc. It was found the experimental and predicted results correlated very well [27]. However, limited studies were carried out using this method to investigate the effect of composites of precursors on the properties of AAC materials. But Li et al. [30] proposed an optimal ratio of coarse to fine aggregate of alkali-activated slag-based concrete and Sun et al. [31] optimized the aggregate gradation of alkali-activated metakaolin mortars using this method. The objective of this study is to use the Simplex centroid design method to obtain the optimal ratio of GP, GGBS and CAC in alkaliactivated slag mortars using GC as aggregate. It was reported that the alkali-activated binders had much higher shrinkage compared to the cement-based mortars due to a small volume of water remaining as interstitial water [32,33]. Besides, the high silica phase content in GC could react with alkaline in the pore solution and induce high ASR expansion. Therefore, the most important drawback limiting the use of GC-blended AAC mortars are the high ASR expansion and high drying shrinkage compared to that of the GC-Portland cement composites. The AAC binder compositions affect the characteristics of the products, the pore size distribution and the pore solution, etc. So the effects of the ternary binder on the drying shrinkage, and ASR expansion were discussed in this study. The compressive strength, high temperature resistance of the AAC mortar were also investigated. With the aid of the factorial design method, the optimal binder proportion for AAC mortars with low ASR expansion and drying shrinkage, high resistance to elevated temperature was obtained. The microscopic analysis was presented in another study to investigate the effect of different ratios of precursors on the properties of the AAC mortars. It should be noted that this optimal binder proportion was obtained based on the modulus of the activators and amount and grading of the aggregate used in this study. But influence of the type and dosage of activators, and the dosage and grading of aggregates were not investigated in this study as it was difficult to report all the parameters in a single paper. The optimal proportion may be changed when different modulus or grading of aggregate were used. But this method on assessing the binder was still efficient [27].
2. Experimental program 2.1. Materials The following materials were used in this work: GGBS and CAC, supplied by a commercial source; GP, obtained by milling recycled GC collected from a local glass recycler; a commercially sourced water glass, composed of 28.3% SiO2, 8.6% Na2O and 58.4% H2O; AR grade NaOH. The chemical compositions of GGBS, GP and CAC are shown in Table 1. The particle size distributions of these three powders are shown in Fig. 1. The gradation of the fine aggregate, GC, is shown in Table 2.
2.2. Sample preparation The simplex-centroid design method [24,25] was used to obtain the optimized composition or composition range in GGBS-GP-CAC ternary AAC mortars. When the mixture is composed of three components x1 , x2 and x3 , then
Y ¼ b1 x1 þ b2 x2 þ b3 x3 þ b12 x1 x2 þ b13 x1 x3 þ b23 x2 x3 þ b123 x1 x2 x3
ð1Þ
where Y = response; bi = parameter to be estimated; and x1 ,x2 ,x3 = percentages of GP, GGBS and CAC. The sum of x1 , x2 , x3 were 1. Based on our previous study [34], the proportion range of GP used in this study were 75%-100% in order to maximize the use of waste glass. The proportion ranges of CAC were 0–25% as it was found the AAC mortar containing more than 25% of CAC had a very low compressive strength [19]. The GGBS content was controlled to be lower than 25% so that the setting time was not too short. The seven-batches of designed compositions are listed in Table 3 and plotted in a ternary diagram as shown in Fig. 2. Due to the low reactivity of GP, the mortar containing 100% of GP could not be demolded after 1 day of ambient curing and the ASR expansion and drying shrinkage data could not be obtained. Therefore, another batch of mortar comprising 90% of GP (S5G90C5) was added in the study. The mass ratio of activator/powder, GC/powder, water/ powder and modulus of activator (SiO2/Na2O) were 0.164, 2.7, 0.4 and 1.5, respectively based on our previous study [34]. The powders included GGBS, GP, and CAC. The fresh mixture after mixing by a mechanical mixer was cast into the 40 mm*40 mm*40 mm plastic molds for compressive strength test and the 25 mm*25 mm*285 mm steel molds for drying shrinkage and ASR expansion tests. Then they were cured in laboratory conditions (25 °C and 75% RH) for 1 day before demolding. 2.3. Mix design and experimental methods The compressive strength of cubic specimens was evaluated using a 300 kN capacity compression testing machine with the fixing loading rate of 0.6 MPa/s. After ambient curing for 28 days, some specimens were dried in an oven at 105◦C for 24 h and then placed into an electric furnace which was heated up at a rate of 2.5◦C/min to 800◦C. The maximum temperature was kept for 2 h. After natural cooling to room temperature, the residual compressive strength was tested to evaluate the high temperature resistance of AAC mortars. The drying shrinkage of mortars was evaluated according to the method described in BS ISO 1920--Part 8 [35]. The 25 mm*25 mm*285 mm samples were immersed in a water bath at 25 °C for 48 h and the length was regarded as the initial length. After that, they were moved to a chamber with a temperature of 27 ± 2◦C and a relative humidity of 50%. The length changes at 14 days and 28 days were recorded using a calibrated dial gauge. The measurement of ASR expansion of the mortar bars was conducted following the ASTM C1260 [36]. The 25 mm* 25 mm*285 mm samples were firstly immersed in a water bath at 80 °C for 24 h, followed by placing them in a 1 M NaOH solution contained in a plastic container. The whole setup was then placed in an 80 °C water bath. The length changes of the specimens after 14 days and 28 days of alkaline immersion were recorded. The pore size distributions of the samples were determined using a Poresizer 9320 mercury porosimeter with a maximum mercury intrusion pressure of 210 MPa. The assumed contact angel and a mercury surface tension were 120° and 0.483 N/m, respectively.
3
P. He et al. / Construction and Building Materials 240 (2020) 117983 Table 1 Chemical compositions of GGBS, GP and CAC (% by mass).
GGBS GP CAC
3.5
SiO2
CaO
AlO3
Na2O
Fe2O3
K2O
SO4
MgO
35.14 73.29 2.65
37.79 12.14 38.76
13.24 1.10 57.03
– 10.54 –
0.33 0.30 0.65
0.85 0.84 0.21
3.25 0.24 0.32
7.85 1.25 0.24
GGBS GP CAC
3.0
Volume (%)
2.5
2.0
1.5
1.0
0.5
0.0 0.01
0.1
1
10
100
1000
Diameter (mm) Fig. 1. Particle size distributions of GGBS, GP and CAC. Fig. 2. Composition design for GGBS-GP-CAC ternary AAC composites. Table 2 Particle grading of GC. Sieve size
Percentage (%)
2.36 mm-5 mm 1.18 mm-2.36 mm 0.6 mm-1.18 mm <0.6 mm
42.4 22.2 21.4 14.1
obtained due to the severe deformation resulted from the high GP content after the high temperature exposure. 3.1. Compressive strength The corresponding coefficients can be obtained using Eq. (1) and the compressive strength values in Table 4. The compressive strength prediction equations of the ternary composite mortars at 28 days can be described by Eq. (2):
Table 3 Mix proportions of the ternary AAC mortars. Mixtures Notation
GGBS
GP
CAC
S0G100C0 S25G75C0 S0G75C25 S12G87C0 S12G75C12 S0G87C12 S8G83C8 S5G90C5
0 0.25 0 0.125 0.125 0 0.083 0.05
1 0.75 0.75 0.875 0.75 0.875 0.834 0.9
0 0 0.25 0 0.125 0.125 0.083 0.05
The Vickers micro hardness test was carried out using HVX-1000A micro hardness tester to evaluate the mechanical characteristics of the sample before and after subjecting to the high temperature exposure. A flat surface of the sample was obtained by polishing. The test load was 10 g with a 15 s contact time.
3. Results and discussion The compressive strength, ASR expansion, drying shrinkage and residual strength coefficient after exposure to high temperature of AAC mortars are summarized in Table 4. The effect of ternary binders on the properties of mortars are discussed in the following sections. The residual strength coefficient of S5G90C5 was not
Y S28 ¼ 6:48x1 þ 193:91x2 56:91x3 149:23x1 x2 þ 114:5x1 x3 6746:04x2 x3 þ 9535:615x1 x2 x3
ð2Þ
The compressive strength contours of AAC mortars at 28 days in the ternary diagram are shown in Fig. 3(a). It can be seen that the compressive strength was increased with the increase of both GGBS and CAC contents. This was due to the higher alkali reactivity of GGBS and CAC compared to that of GP. And it seems that the effect of GGBS was more obvious than that of CAC. For example, the compressive strength of the mortar containing 15% GGBS was about 16 MPa, while the value was 12 MPa for that containing the same content of CAC. This might be related to the difference of the alkali reaction products formed with these two systems. According to our previous study, the product of the alkaliactivated GP/GGBS was C-N-A-S-H gel. The product in alkaliactivated GP/CAC was a zeolite phase and C-N-A-S-H gel [18]. The increase of zeolite content decreased the compressive strength of the AAC structure due to the lower strength of the zeolite compared to that of AAC gel [37]. Therefore, the alkali-activated GP/GGBS mortar had a higher compressive strength compared to the alkali-activated GP/CAC mortar when the GP content was the same. Besides, the contour suggested that the compressive strength firstly increased and then decreased with the increase of CAC content when the GP content was fixed at a certain value between 75% and 100%. This was in agreement with our previous
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P. He et al. / Construction and Building Materials 240 (2020) 117983
Table 4 Compressive strength, ASR expansion, drying shrinkage and residual strength coefficient after exposure to high temperature of AAC mortars. No.
Compressive strength (MPa)
S0G100C0 S25G75C0 S0G75C25 S12G87C0 S12G75C12 S0G87C12 S8G83C8 S5G90C5
ASR expansion (micro strain)
Drying shrinkage (micro strain)
Residual strength coefficient (%)
28d
90d
14d
28d
14d
28d
6.48 25.36 12.10 13.59 25.07 11.08 22.69 –
29.59 21.29 17.98 19.19 35.74 16.09 20.83 –
– 1776 1017 3719 389 871 682 2521
– 3216 2632 8404 700 1860 1216 5617
– 1262 1092 2611 2321 1152 1232 735
– 1206 1213 2651 2475 1350 1329 835
– 202 455 31.1 206 405 52 –
Note: Residual strength coefficient was calculated as the ratio between the residual compressive strength of the specimens after high temperature exposure at 800 °C for 2 h to the compressive strength of the specimens after 28 days of air curing.
0.00
2.000 4.000
0.25
6.000 8.000 10.00
0.05
0.20
12.00 14.00 16.00
0.10
18.00
0.15
CA
22.00
BS GG
C
20.00
0.15
24.00
0.10
26.00
12.00
0.20
0.05
10.00
0.25
0.00
0.75
0.80
0.85
0.90
0.95
1.00
GP Compressive strength at 28d
(a) 0.00
0.05
0.20
0.10
0.15
C CA
BS GG
22.00 0.15
0.10
20.00 0.20
0.05
0.85
0.90
0.95
492:48x1 x3 þ 4955:377x2 x3 5232:88x1 x2 x3
ð3Þ
Fig. 3(b) shows the compressive strength contours of AAC mortars at 90 days in the ternary diagram. It can be seen that the 90 days compressive strength of the AAC mortars was higher than that at 28 days. When the GP content was fixed at a certain value between 75% and 100%, the compressive strength of AAC mortar firstly increased and then decreased with the increase of CAC content. Unlike the compressive strength development at 28 days, the strength was firstly decreased and then increased with the increase of GP content when the GGBS content was a fixed value of lower than 10%. A similar strength development trend was also observed for the AAC mortars prepared with a fixed CAC content of lower than10%. These results were attributed to a late development of the alkali activity of glass powder [38]. 3.2. ASR expansion The ASR expansion prediction equations of the ternary composite mortars at 14 days and 28 days can be described by Eqs. (4) and (5):
Y A14 ¼ 1:2 104 x1 þ 3:8 105 x2 þ 2:4 105 x3 5:5105 x1 x2 3:7 105 x1 x3 3:2 106 x2 x3 þ 4:2 106 x1 x2 x3
ð4Þ
1:1105 x1 x2 5:1 105 x1 x3 þ 1:4 106 x2 x3 2:0 0.00
0.80
Y S90 ¼ 29:59x1 þ 296:39x2 þ 352:51x3 400x1 x2
Y A28 ¼ 1:7 104 x1 þ 4:2 104 x2 þ 3:4 105 x3
22.00
0.25 0.75
2.000 4.000 6.000 8.000 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00 26.00 28.00 30.00 32.00 34.00 36.00
0.25
to the formation of zeolite and katoite which had much lower strength compared to geopolymer gel [19]. Moreover, the compressive strength could be higher than 24 MPa when the GP content was lower than 85%. The compressive strength prediction equations of the ternary composite mortars at 90 days can be described by Eq. (3):
1.00
GP Compressive strength at 90d
(b) Fig. 3. Compressive strength (MPa) contours of ternary composite AAC mortars at (a) 28 days (b) 90 days.
study that the high aluminum content in CAC had a positive effect on the development of compressive strength. However, further increase of CAC content decreased the compressive strength due
106 x1 x2 x3
ð5Þ
Fig. 4 shows the contour lines of ASR expansion of the mortars after immersion in the alkali environment for 14 days and 28 days. The area with expansion values higher than 1000 micro strain is regarded as a potentially dangerous composition range as the expansion could lead to severe cracking and damage to the mortar. Some samples showed high ASR expansion after alkaline immersion for 14 days due to the high active silica content in GC compared to that in conventional aggregates such as river sand. When the CAC or GGBS content was fixed, the ASR expansion increased with the increase of GP content at 14 days. This means
P. He et al. / Construction and Building Materials 240 (2020) 117983
0.00
0.000
0.25
500.0 1000
0.05
When the CAC content was higher than 3.75%, the expansion was lower than 1000 micro strain at 14 days. The expansion contour of this ternary composite AAC mortars at 28 days was similar to that at 14 days.
1500
0.20
1000
5
2000
3.3. Drying shrinkage
2500
0.10
3000
0.15
3500
CA
C
BS GG
0.15
4000 1.240E+04
0.10
0.20
0.05
0.25
0.00
0.75
0.80
0.85
0.90
0.95
1.00
GP Expansion in alkali at 14 days
(a)
0.00
0.05
0.20
0.10
C 0.15
BS GG
CA
0.15 1000
1000
2000
0.10
0.20
0.25 0.75
0.000 500.0 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 8000 8500 9000
0.25
0.05
1000 2000 0.80
Y D14 ¼ 1:5 103 x1 þ 1:2 105 x2 þ 3:3 4x3 1:7105 x1 x2 0.00
0.85
0.90
0.95
It was reported that the pore size distribution and the content of calcium had a critical influence on the magnitude of drying shrinkage. The drying shrinkage increased with the increase of mesopore (1.25–25 nm) proportion [14] and the Ca/Si ratio in the raw materials when the Ca/Si ratio was lower than 0.75 assuming that the raw materials were fully soluble [32]. Besides, the drying shrinkage decreased with the increase of compressive strength [29]. The pore size distributions of S12G87C0, S12G75C12, and S0G87C12 are shown in Fig. 5. Even though the porosities of these three samples were similar, the mesopore proportions were quite different. Table 5 summarizes the mesopore proportion, compressive strength, Ca/Si ratio in the raw materials and drying shrinkage of these three samples. It can be seen that when using CAC or GGBS to replace GP, the mesopore proportion increased. That might be related to the higher reactivity of CAC or GGBS compared to that of GP refining the pore matrix. When using CAC to replace GGBS, the mesopore proportion decreased, which might be due to the different characteristics of AAC gel. The sample S0G87C12 had the lowest mesopore proportion and low Ca/Si ratio, therefore it had the lowest drying shrinkage compared to S12G87C0 and S12G75C12. S12G75C12 had the highest mesopore proportion, but the high compressive strength could lead to the higher resistance to drying shrinkage. So the drying shrinkage of S12G75C12 was slightly lower than that of S12G87C0. The drying shrinkage prediction equations of the ternary composite mortars at 14 days and 28 days can be described by Eqs. (6) and (7):
1.00
GP Expansion in alkali at 28 days
(b)
5:6 104 x1 x3 1:6 106 x2 x3 þ 2:0 106 x1 x2 x3 Y D28 ¼ 1:9 103 x1 þ 1:3 105 x2 þ 7:0 104 x3 1:9105 x1 x2 1:1 105 x1 x3 2:0 106 x2 x3 þ 2:6 106 x1 x2 x3
Fig. 4. ASR expansion (micro strain) contours of ternary composite AAC mortars at a) 14 days and b) 28 days.
GP had a negative effect on the ASR expansion when the GP content was higher than 75% [15], which might be due to the higher alkali concentration in the pore solution resulted from the low reactivity of GP compared to that of CAC or GGBS. Besides, the alkalis released from the soda lime glass would also contribute to ASR gel formation. CAC could significantly suppress ASR expansion. For example, the ASR expansion was between 3000 and 3500 micro strain when using 15% of GGBS as the replacement of GP. While this value was lower than 500 micro strain for the AAC mortar using the same content of CAC. According to our previous study [18], the aluminum in CAC was incorporated in the gel matrix, which needed Na to balance the negative charge. Besides, the formation of zeolite absorbed more Na from the pore solution. Therefore, the alkali concentration of the alkali-activated CAC/GP mortars had a lower ASR expansion compared to that of the GGBS/GP mortars. As a result, the ASR expansion decreased with the increase of CAC content when the GP content was a fixed value between 75% and 100%. The AAC mortar containing 25% of GGBS and 75% of GP had an expansion higher than 1500 micro strain.
ð6Þ
Fig. 5. The cumulative porosity of S12G87C0, S12G75C12, and S0G87C12.
ð7Þ
6
P. He et al. / Construction and Building Materials 240 (2020) 117983
Table 5 Compressive strength, mesopores proportion and drying shrinkage of S12G87C0, S12G75C12, and S0G87C12. No.
GP (%)
GGBS (%)
CAC (%)
28 days Compressive strength (MPa)
90 days Compressive strength (MPa)
Mesopores (%)
Ca/Si
Drying shrinkage (micro strain)
S12G87C0 S12G75C12 S0G87C12
87.5 75 87.5
12.5 12.5 0
0 12.5 12.5
13.59 25.07 11.08
19.19 35.74 16.09
2.30 6.19 1.74
0.176 0.246 0.189
2651 2475 861
0.00
200.0 400.0
0.25
600.0 800.0 1000
0.05
0.20
1200 1400
C CA
0.15
2000 2200
0.10
0.20
0.05
1000
0.75
1800
2400
0.15
0.25
1600
BS GG
0.10
0.80
0.00 0.85
0.90
0.95
1.00
GP Dry shrinkage at 14 days
when the GP content increased. When the GGBS content was lower than 15%, the drying shrinkage firstly decreased and then increased with the increase of GP content. The mesopore proportion, Ca/Si ratio and the compressive strength decreased with the increase of GP content. These three factors had opposite effects on the drying shrinkage. Therefore, the drying shrinkage firstly decreased due to the decrease of the mesopore proportion and Ca/Si ratio, and then increased due to the decrease of the compressive strength. When the CAC or GP content was fixed, the drying shrinkage of mortars increased with the increase of GGBS content, which might be related to the increase of mesopore proportion. It was reported that the AAC mortar prepared with a lower water absorption aggregate showed lower drying shrinkage [32]. The water absorption of glass is negligible. Therefore, the drying shrinkage of AAC mortars prepared with glass cullet could be lower than that prepared with conventional aggregates, like river sand. However, the drying shrinkage of most samples in this study was higher than 1000 micro strain after 14 days. The drying shrinkage could be controlled to be lower than 1000 micro strain at 14 days when the GGBS content was lower than 7.5%.
(a) 3.4. High temperature resistance 0.00
0.25
0.05
0.20
1200 1400
0.15
CA
BS GG
C
0.10
200.0 400.0 600.0 800.0 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800
0.15
0.10
1400 0.20
1200
1000
0.05
0.25 0.75
The MIP test results are illustrated in Fig. 7 and Fig. 8. The pore size distribution and micro hardness results are summarized in Table 6. Even though, the micro hardness of the matrix increased after the high temperature exposure, the porosity of S12G87C0 was increased significantly and the size of the major pores was larger than 1 lm. Therefore, the compressive strength decreased dramatically after the high temperature exposure. The reason for the increase of total porosity might be related to the decomposition of the reaction products in the AAC mortar incorporating GP and GGBS (C-N-A-S-H gel) during the high temperature exposure. After 2 h of high temperature exposure, the total porosity of S12G75C12
0.00 0.80
0.85
0.90
0.95
1.00
GP
(b) Fig. 6. Drying shrinkage (micro strain) contours of ternary composite AAC mortars at a) 14 days and b) 28 days.
The contour plots of drying shrinkage of mortars at 14 days and 28 days are presented in Fig. 6. The drying shrinkage of the mortars at 28 days was slightly higher than that at 14 days because the drying shrinkage developed fast at early age and almost reached to a constant value after about 14 days [39]. When the GGBS content was a fixed value of higher than 15%, the drying shrinkage of mortars increased with the increase of GP content. As mentioned above, the compressive strength decreased when the GP content increased. This might explain the increase of drying shrinkage
Fig. 7. Cumulative pore volume of S12G87C0, S12G75C12 and S0G87C12 before and after high temperature exposure.
7
P. He et al. / Construction and Building Materials 240 (2020) 117983
0.00
0.05
0.20
100.0
0.15
CA
BS GG
C
0.10
0.000 50.00 100.0 150.0 200.0 250.0 300.0 350.0 400.0 450.0 500.0 550.0 600.0
0.25
0.15
0.10
0.20
0.05
0.25
0.00
0.75 Fig. 8. Pore size distribution curves of S12G87C0, S12G75C12 and S0G87C12 before and after high temperature exposure.
was decreased and the micro hardness of matrix increased, which might result in the increase of compressive strength after high temperature exposure. The decrease of total porosity might be related to the melting of glass filling the pores in the paste. When comparing S12G87C0 and S12G75C12, it can be found that the AAC mortars incorporating CAC showed much lower porosity after the high temperature exposure. This might be related to the different characteristics of the products, which needs further research. The total porosity of S0G87C12 was significantly decreased and the micro hardness of matrix was increased after the high temperature exposure, which might result in the dramatic increase of compressive strength [40]. S0G87C12 showed a higher residual strength coefficient compared to S12G75C12, which might be related to the higher micro hardness of the matrix after the high temperature exposure. Besides, S0G87C12 had a higher proportion of smaller pores than S12G75C12, which might be because more GP in S0G87C12 was melted which refined the pores. Fig. 9 shows the residual compressive strength of 28 day cured AAC mortars. The residual strength coefficient prediction equation of the AAC mortars could not be obtained due to the missing data in Table 4 as the data of the mortars incorporating a very high percentage of GP could not be obtained due to the severe deformation of the mortars after the high temperature exposure. It was found that the compressive strength of the AAC mortars using other aggregates, like crushed limestone, decreased after high temperature exposure [41]. However, the residual strength of some samples in this study could be higher than that the original strength before the high temperature exposure. The reason for the increase of the compressive strength might be related to the partial melting of the glass (including GP and GC) at the elevated temperature that induced the formation of a stronger bonding between the aggregates and the paste [42]. When the GP content or GGBS content
0.80
0.85
0.90
0.95
1.00
GP Fig. 9. Residual strength coefficient (%) of AAC mortars after exposed to 800 °C for 2 h.
was fixed, the residual strength coefficient increased with the increase of CAC content as the total porosity decreased or the proportion of smaller pores increased when using CAC to replace GP or GGBS as shown in Table 6. The effect of CAC on improving the high temperature resistance might also be related to the dehydration of the CAC hydration products during the high temperature exposure accommodating the expansion of the mortar samples, which will be reported in our another work using differential thermal analysis. When the CAC content was a fixed value of higher than 10%, the residual strength coefficient was higher than 100% and increased with the increase of GP content. The increase of residual strength coefficient might be related to the refinement of the pores in the matrix and the increase of the micro hardness of the matrix after the melting of GP. The residual strength coefficient decreased with the increase of GGBS content, which might be related to the decomposition of products (AAC gel) leaving large pores as mentioned above. 3.5. Optimizing AAC mortar mixture design Based on the above results, the critical values required for the AAC mortars to meet their functions are drawn in Fig. 10. The overlapped area (the grey zone) in Fig. 10 is regarded as the optimal AAC mortar composition which would produce a AAC mortar with low ASR expansion and good drying shrinkage resistance. The optimal composition is an AAC mortar prepared with a GP content in the range of 77% 90%, a GGBS content of lower than 5% and a CAC content of higher than 10%. The 28-day compressive strength of the AAC mortar could reach 16 MPa which can be used as partition wall blocks and the residual strength coefficient after exposure to 800° for 2 h was higher than 500% (gain in strength).
Table 6 Pore size distribution, residual strength coefficient of S12G87C0, S12G75C12, and S0G87C12. No.
S12G87C0 S12G75C12 S0G87C12
Pore size distribution before high temperature exposure (%)
Pore size distribution after high temperature exposure (%)
Micro hardness HV of matrix
porosity
<50 nm
50– 1000 nm
>1000 nm
porosity
<50 nm
50– 1000 nm
>1000 nm
Before high temperature
After high temperature
14.35 15.17 18.04
4.38 10.0 9.49
2.39 1.42 3.36
7.58 3.74 5.19
34.52 6.48 14.00
1.11 1.60 4.77
1.00 2.37 4.86
32.41 2.52 4.38
63.00 57.43 74.99
737.21 384.14 673.74
Residual strength coefficient (%)
31.1 206 405
8
P. He et al. / Construction and Building Materials 240 (2020) 117983
CRediT authorship contribution statement Pingping He: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Data curation, Writing original draft, Visualization. Binyu Zhang: Resources. Jian-Xin Lu: Writing - review & editing. Chi Sun Poon: Writing - review & editing, Supervision, Project administration, Funding acquisition.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement Fig. 10. Optimal binder composition for AAC mortars.
4. Conclusions The main conclusions derived from this work are: (1) The AAC mortars incorporating a higher GP content showed a lower compressive strength at 28 days. When the GP content was higher than 92.5%, the strength was lower than 10 MPa. The compressive strength of AAC mortars firstly increased and then decreased with the increased content of CAC at 28 days and 90 days. (2) When the GP content was higher than 75%, the ASR expansion increased with the increase of GP content. The expansion of the mortars prepared with 85% GP and 15% GGBS could reach 3000 micro strain after 14 days of alkaline immersion. The addition of CAC as the replacement of GGBS could significantly decrease ASR expansion. The expansion could be controlled to lower than 1000 micro strain if the CAC content was higher than 3.75%. (3) When the GGBS content was fixed at a value of higher than 15%, the drying shrinkage increased with the increase of GP contents. When the GGBS content was lower than 15%, the drying shrinkage firstly decreased and then increased with the increase of GP contents. The drying shrinkage of mortars increased with the increase of GGBS contents when the GP or CAC content was fixed. However, the drying shrinkage was lower than 1000 micro strain after 14 days if the GGBS content was lower than 7.5%. (4) The residual strength coefficient after high temperature exposure increased with the increase of CAC content. The residual strength could be higher than that before the high temperature exposure if the CAC content was higher than 10%. When the CAC content was fixed, the residual strength coefficient increased with the increase of GP content. And the residual strength coefficient decreased with the increase of GGBS content. (5) To obtain AAC mortars with acceptable ASR expansion and drying shrinkage, and to maximize the use of waste glass, the GP content should be between 77% and 90%, the GGBS content should be lower than 5% and the CAC content should be higher than 10%. The compressive strength of the mortar in this range could reach 16 MPa, which can be used as partition wall blocks. The residual strength coefficient in this range was higher than 500%.
The authors wish to acknowledge the financial support of the Environment and Conservation Fund.
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