The chapter delves into the basic study of ultra-rapid hardening alkali-activated materials (AAM) using sodium orthosilicate, a substance traditionally used for metal cleaning. The research aims to reduce greenhouse gas emissions by developing sustainable construction materials. The study investigates the effects of sodium orthosilicate concentration and ground granulated blast furnace slag (GGBS) replacement rates on the 3-hour compressive strength of AAM. Additionally, the chapter explores the use of shrinkage reducing agents and expansive additives to improve the length change performance of these materials. The findings reveal that compressive strength increases with higher alkali usage rates and GGBS replacement rates, while the use of expansive additives can lead to expansion and improved moisture absorption. The unique properties of sodium orthosilicate and its potential applications in the construction industry make this chapter a compelling read for professionals interested in sustainable materials and rapid hardening technologies.
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Abstract
For the purpose of developing a repair material that contributes to decarbonization, a basic study is conducted on an alkali activated material that does not require an alkaline solution. Thin study is characterized by the use of sodium orthosilicate as an alkaline source in order to achieve powder premixing, be ultra rapid hardening. Specifically, a basic study on ultra rapid hardening property and an improvement of the length change performance of the material using only ground granulated blast furnace slag were carried out. As a result, it was clarified that sufficient compressive strength at 3 h can be achieved by increasing the alkaline usage rate, and that the length change performance can be improved by using shrinkage reducing agents and expansive agents.
1 Introduction
Reduction of greenhouse gas emissions is required in order to solve global climate change problems. Alkali-activated materials (AAM) are attracting attention as construction materials that contribute to the reduction of CO2 emissions. The AAM studied in Japan so far are mainly classified as geopolymers1). The geopolymer is formulated to have a low calcium content, and the general material composition is a mixture of water glass and an alkaline aqueous solution (alkaline solution), and fly ash and ground granulated blast furnace slag (GGBS) as active fillers [1]. There are cases where only GGBS is used as the active filler [2], and cases where other industrial by-products are used [3]. On the other hand, there are concerns that domestic social infrastructure, which was intensively developed during the high economic growth period, will deteriorate rapidly in the future, and there is a need to accelerate measures to deal with the deterioration. Many of the repair materials used for aging countermeasures are premixed cementitious materials, and various types are available depending on the application, from general-purpose types to ultra-rapid hardening types. However, almost no decarbonized repair materials are on the market. The development of repair applications for AAM is considered to be an effective means of decarbonization, and some practical studies have begun [4]. However, these methods require the preparation of alkaline solutions on-site, which poses a problem of construction safety. In this study, we developed a premix type AAM that can be used on site by simply adding water, and developed an ultra-rapid hardening type that exhibits practical strength after 3 h.
2 Sodium Orthosilicate
To achieve premixing, powdered sodium orthosilicate (Na4SiO4) shown in Fig. 1 was selected as the alkaline source for AAM. Sodium orthosilicate is a substance conventionally used for degreasing and cleaning of metals, and it is considered that sodium metasilicate and NaOH are produced by hydrolysis as shown in the following formula. In other words, by using sodium orthosilicate as an alkaline source, it is possible to approach the state of using water glass and NaOH during mixing, it is the same handling as conventional cementitious repair materials.
The initial strength development of geopolymers is affected by the alkaline concentration in the solution and the proportion of GGBS used as the active filler [1]. Therefore, as Test Series 1, the effects of the amount of sodium orthosilicate used in the solution and the amount of GGBS in the active filler on the 3-h compressive strength of AAM were investigated. Next, it has been reported that the shrinkage property of AAM using GGBS is greater than that of ordinary Portland cement [2]. Therefore, as Test Series 2, the effects of expansive agents and shrinkage reducing agents were investigated for compressive strength and length change characteristics of ultra-rapid hardening AAMs, with the aim of improving the length-change performance.
3.2 Materials, Mix Proportion and Mixing
In Test Series 1, tap water was used as the mixing water. Sodium orthosilicate, symbol AL, its density 2.39 g/cm3 (Fig. 1) was used as the alkaline source. Fly ash equivalent to JIS Class II, symbol FA, its density 2.36 g/cm3 and GGBS equivalent to Blaine 4000 m2/g without gypsum, symbol GGBS, its density 2.91 g/cm3 were used as active fillers. Dry sand for premix, symbol S, its density 2.64 g/cm3 was used as fine aggregate. In Test Series 2, in addition to the above materials excluding FA, powdered shrinkage reducing agent, symbol SR, its density 1.20 g/cm3 and quicklime as expansive agent, symbol EX, its density 3.34 g/cm3, were used.
Table 1 shows the mix proportion of Test Series 1. The volume of AL in the solution and the volume of GGBS in the active filler were taken as parameters with a liquid equivalent of 401 L/m3 as the basic condition. Since AL is water-soluble, the amount. Was set to liquid equivalent. In addition, the fine aggregate active filler ratio S/(FA + GGBS) was set to a constant volume ratio based on the mass ratio when the GGBS replacement rate was 100%. Table 2 shows the composition of Series 2. Under the basic conditions of 401 L/m3 liquid equivalent, 17.3 vol.% alkali content, and only GGBS as the active filler, the usage rates of shrinkage reducing agent SR and expansive additive EX were used as parameters. EX was replaced as part of the active filler volume, and SR was replaced as part of the liquid equivalent. Fine aggregate by active filler ratio S/(GGBS + EX) was set to a same volume ratio.
Table 1.
Mix proportion in Test Series 1
No
AL/(W + AL)
(vol.%)
GGBS/(FA + GGBS)
(vol.%)
Unit quantity (kg/m3)
W
AL
FA
GGBS
S
1
6.49
100
375
62
0
829
829
2
9.73
100
362
93
0
829
829
3
13.0
100
349
124
0
829
829
4
17.3
100
332
166
0
829
829
5
22.7
100
310
218
0
829
829
6
17.3
66.7
332
166
224
553
829
7
17.3
33.3
332
166
448
276
829
8
17.3
0
332
166
672
0
829
※W + AL: 401 L/m3, S/(FA + BFS): Constant volume rate based on the mass rate of the mixture with a BFS replacement rate of 100 vol.%
Table 2.
Mix proportion in Test Series 2
No
SR usage rate
（vol.%）
EX usage rate
（vol.%）
Unit quantity (kg/m3)
W
AL
SR
GGBS
EX
S
4
0
0
332
166
0
829
0
829
9
5
0
312
166
24
829
0
829
10
10
0
292
166
48
829
0
829
11
0
5
332
166
0
788
48
829
12
0
10
332
166
0
746
95
829
※W + AL + SR: 401 L/m3, AL/(W + AL + SR): 17.3 vol.%, S/(BFS + EX): Constant volume ratio based on formulation No. 4
Using a Hobart-type mortar mixer in a room at 20 ℃, the pre-mixed powder material and fine aggregate were added to the water weighed in a mixing bowl and kneaded for 2 min. Immediately after mixing, a fresh test and specimen preparation were performed. The preparation of the specimen conformed to JSCE-F 506 “Method of making cylindrical specimen of mortar or cement paste for compressive strength”.
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3.3 Test Method
In the property test of fresh mortar, the temperature and flow value (JIS R 5201) immediately after mixing were measured. Compressive strength test complied with JSCE-G 505 “Test method for compressive strength of mortar and cement paste using cylindrical specimen”. In Test Series 1, it was tested only at an age of 3 h. In addition, demolding was performed about 2 min before the test. In Series 2 it was tested at 3 h, 1, 7, 28, 56 and 91 days. The curing conditions were sealed in a room of 20 ℃. Series 2 involved a length change test. Using a formwork of φ5 × 10 cm, a simple embedded strain gauge (measurement length 60 mm) was installed in the formwork so that the center of the cylindrical specimen and the center of the measurement part coincided. The kneaded sample was cast and demolded after one day. After that, the specimen was placed in air at 20 ℃ and 60% RH., the length change rate was measured at 0, 1, 4, 7, 14, 28, 56 and 91 days. In order to understand the relationship between the length change rate and the mass loss rate, a specimen for mass measurement was also prepared. Mass measurements were performed on the same day as length change rate measurements. The mass loss rate was obtained by dividing the mass loss due to drying of the specimen by the mass of the specimen at the start of drying.
4 Results and Discussion
4.1 Test Series 1
Table 3 shows the fresh test results. First, focusing on the effect of the AL concentration (No. 1 to 5), the higher the AL concentration, the higher the temperature immediately after mixing and the Flow value tended to decrease. Next, focusing on the effect of the BF replacement rate (No. 4, No. 6 to 8), the higher the replacement rate, the smaller the flow value, and the temperature tended to remain almost unchanged. The flow values of No. 7 and No. 8 were both 300 or more, but No. 8 clearly showed higher fluidity.
Figure 2 shows the effect of AL concentration in solution on compressive strength at 3 h. Mix proportion No. 2 to 5 were demoldable, and the higher the AL content, the higher the compressive strength. Compressive strength increased remarkably from 13.0 to 17.3 vol% of AL concentration, and moderately increased from 17.3 to 22.7 vol%. In addition, in the case of the composition with a low AL concentration (No. 1), demolding was not possible at 3 h, but demolding was possible on the following day. Figure 3 shows the effect of GGBS replacement rate on compressive strength at 3 h. Compressive strength increased with increasing GGBS replacement rate. In addition, no hardening was observed even after the next day in the mix proportion No. 8 that used only FA as the active filler. The GGBS replacement ratio was 33.3% or more in the mix proportion that could measure the compressive strength at 3 h.
Table 3.
Fresh test result in Series 1
No
AL Concentration
（vol.%）
GGBS replacement rate
vol.%）
Temperature
（℃）
Flow value
1
6.49
100
27.3
over 300
2
9.73
100
31.6
298
3
13.0
100
35.3
253
4
17.3
100
40.0
199
5
22.7
100
41.7
133
6
17.3
66.7
39.6
282
7
17.3
33.3
39.6
over 300
8
17.3
0
39.8
over 300
Fig. 2.
Effect of AL concentration on compressive strength at 3 h
Fig. 3.
Effect of GGBS replacement rate on compressive strength at 3 h
×
×
4.2 Test Series 2
Table 4 shows the test results of fresh mortar. First, the mixing temperature was almost constant regardless of the SR usage rate and EX usage rate. As for the flow value, the higher the SR usage rate and the EX usage rate, the smaller the flow value. In particular, when EX was used, the flow value tended to be remarkably small.
Figure 4 shows the relationship between compressive strength and age. The left figure summarizes the effect of SR usage rate, and the right figure summarizes the effect of EX usage rate. First, focusing on the left figure, the compressive strength at 3 h was almost the same for all mix proportions, and the compressive strength tended to increase with the age. However, the higher the SR content, the lower the long-term compressive strength.
Next, focusing on the right figure, the compressive strength of the mixture with an EX content of 5 vol.% was about 60% of that of the mixture with an EX content of 0 vol.% at 3 h. Although the compressive strength tended to increase with age, the rate of increase in strength was small compared to an EX content of 0 vol.%. The compressive strength of the mixture with an EX content of 10 vol.% increased until the age of 1 day, but decreased after that. After 7 days, swelling was clearly observed in the appearance.
Table 4.
Fresh test result in Series 2
No
SR usage rate
（vol.%）
EX usage rate
（vol.%）
Temperature
（℃）
Flow value
4
0
0
40.0
199
9
5
0
39.4
178
10
10
0
39.6
159
11
0
5
39.7
104
12
0
10
40.0
95
Fig. 4.
Relationship between age and compressive strength
×
Figure 5 shows the relationship between drying time and length change. First, focusing on the left figure summarizing the effect of the SR usage rate, the length change became negative from the initial stage of drying, indicating a tendency to shrink. In addition, the shrinkage rate tended to increase with the passage of drying time, regardless of the composition. Furthermore, the higher the SR usage rate, the lower the shrinkage rate, and the shrinkage rate of the mix proportion with SR usage rate of 10 vol.% decreased by about 2000 × 10–6 after drying time 91 days compared to the mix proportion without SR. Next, focusing on the right figure summarizing the effect of EX usage rate, it was confirmed that the length change became a positive value at an EX usage rate of 10 vol.%, indicating expansion. It should be noted that the expansion rate reached about 9000 × 10–6 at the drying time of 4 days, and after that it became impossible to measure. The mix proportion with 5 vol.% of EX showed a negative length change from the initial stage of drying, just like the without EX. However, that with 5 vol.% of EX showed smaller shrinkage of about 700 × 10–6, which was almost the same after 14 days.
Fig. 5.
Relationship between drying time and length change
×
Figure 6 shows the relationship between and drying time and moisture loss. Focusing on the left figure, the moisture loss of the order of several percent was confirmed in hardened cement [5], but the moisture loss in this study was much smaller than that.
Fig. 6.
Relationship between drying time and moisture loss
×
No clear trend due to blending was confirmed. Moreover, no long-term water loss was confirmed, and in some cases, moisture was taken in from the outside of the hardened body, resulting in a negative moisture loss. Next, focusing on the right figure, the higher the EX usage rate, the greater the negative moisture loss rate with the passage of drying time, and the tendency to take in moisture from the outside became noticeable.
From the above, it is considered that the length change of ultra-rapid hardening AAM using sodium orthosilicate may be caused by shrinkage due to polycondensation reaction as experienced in a geopolymer. In addition, it is considered that in mix proportion with EX, the continuous absorption of moisture from the outside reacts with EX and causes expansion. The shrinkage suppression mechanism and the mechanism of the curing reaction itself due to the use of SR have not been elucidated in this study, so they will be studied in the future.
5 Conclusions
The results of basic studies on alkali-activated materials using sodium orthosilicate are as follows. Compressive strength at 3 h tended to increase with increasing alkali usage rate and ground granulated blast furnace slag replacement rate. The effect of powder shrinkage reducing agent on 3-h compressive strength is small. Compressive strength at 3 h tended to decrease with increasing use of expansive additive. In addition, when the expansive additive content was less than 5 vol.%, the strength increased with age. At 10 vol.% expansive additive, the compressive strength decreased after 1 day, and finally could not be measured. The use of shrinkage reducing agents and expansive additives can improve the length change performance. The mass of the hardened alkali-activated material using sodium orthosilicate did not change or tended to increase slightly even after curing in an environment of 20 ℃ and 60% RH.
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