Effective of alkaline additives on the geopolymer cements properties as alternative to Portland cement in order to protect environment

Abstract

Geopolymers are inorganic alumina-silicate materials produced from raw materials, rich in silica (SiO₂) and alumina (Al₂O₃), in combination with an alkaline activator solution. In this study, geopolymer of class C flay ash in ambient curing condition were used form geopolymer mortar and effects of different alkaline activator solutions and variations of associated parameters, were investigated. The obtained results indicated that in ambient curing condition (23±2°C), using sodium hydroxide and sodium silicate as an alkaline activator solution, result in higher 7- and 28-day compressive strength of geopolymer mortar compared to potassium-based (potassium hydroxide and potassium silicate) and combination of sodium and potassium-based alkaline activator solutions, approximately 49% and 145%, respectively. But, in term of 90°C curing condition, potassium-based alkaline activator subject to higher 7- and 28-day compressive strengths. Additionally, simultaneous inclusion of NaOH and KOH led to decline the compressive strength. Also, obtained results of experimental data show that optimal ratio 1.5–2 of SiO₂/Na₂O were highest compressive strength.

Keywords

Geopolymer mortar fly ash compressive strength alkaline activator

1 Introduction

Concrete is the most widely used building material after water due to its special properties such as formability, availability of raw materials and cheapness [1]. Nevertheless, the process of production of Ordinary Portland Cement (OPC), as the main constituent of concrete, has major environmental shortcomings, including: high energy consumption and the production of large amounts of carbon dioxide (CO₂) [2, 3]. Hence, it is essential to use alternatives for Portland cement. Kunt et al were investigated effects of different chemical admixtures on borogypsum containing cement in the compressive and flexural strength tests. Decreasing the water content of the concrete is important in terms of strength, sulfate durability and permeability of concrete [4]. Within the last few years, geopolymer has been suggested as a new and environmentally friendly cement agent as an alternative to Portland cement, which can reduce the environmental problems caused by OPC production because geopolymer cement needs lower energy consumption, produces lower CO₂ emissions, fire resistance and acid resistance characteristics [5 –7]. In 1978, geopolymer was first introduced by Davidovits, a foremost French chemist, as a new binder in the mineral polymer family [8]. In OPC, hydration reaction takes place, while geopolymer cement is the result of geopolymerization reaction, which is the most important difference between OPC and geopolymer cement. In fact, geopolymers are inorganic aluminosilicate materials obtained from the geopolymerization reaction of aluminosilicate source rich in silica (SiO₂) and alumina (Al₂O₃) with an alkaline activator solution that alkaline activator type is one of the influencing factors in the geopolymerization process [9 –11]. Geopolymer having empirical formula of M_n(-(SiO₂)_z-AlO₂)_n.wH₂O (M = Na⁺/K⁺ cation; z = 1,2,3;n = degree of polycondensation) [12]. Geopolymerization is a rapid chemical reaction under alkaline conditions between the mineral elements Si and Al, which results in the formation of three-dimensional Si-O-Al polymer chains [13, 14]. In terms of engineering properties required in civil engineering, Geopolymer Concrete (GPC) has showed enhanced physical and mechanical properties than conventional concrete, which include higher mechanical strength and faster hardening [15], higher resistance to elevated temperatures and fire enhanced durability [16], lower permeability and improved resistance to attack of salts and acids and lower creep [17]. Depending on the required properties, cost and availability, the aluminosilicate source can be natural such as zeolite, industrial such as metakaolin or wastes such as fly ash or smelting furnace slag [18]. Fly ash is a by-product and the waste obtained from burning coal. Fly ash is categorized into two classes C (high CaO) and F (low CaO). Due to its structural nature, fly ash is one of the best sources of aluminosilicate for making geopolymer [19 –23]. Cheng et al. [24] found in their research that alkaline activator solution plays an important role in the polymerization reaction and adding a silicate solution such as Na₂SiO₃ or K₂SiO₃ to NaOH or KOH solution can help increase the reaction rate and achieve better results. They also found that using a solution of NaOH and Na₂SiO₃ had better results than by means of a solution of KOH and K₂SiO₃. Rashed [25] decided in a study on geopolymers that in general, in most cases, the compressive strength of GPC is increased through enhancing the concentration of NaOH solution to a certain concentration. Moradikhou et al. [26] used a combination of NaOH and Na₂SiO₃ as an alkaline activator solution and concluded that increasing the concentration of NaOH to 14M subjects to enhance the compressive strength of concrete. However, further proliferation in concentration up to 16M causes no significant change in compressive strength. This is while, in a similar study, Patel et al. [27] concluded that increasing the concentration of NaOH solution to 12M in metakaolin-based GPC increases the compressive strength, and increasing the concentration to more than 12M decreases the compressive strength. Regarding the weight ratio of silicate to hydroxide solution, Hardjito et al. [28] mulled over the GPC based on Class F fly ash and reported that the optimal weight ratio of Na₂SiO₃/NaOH solution was equal to 2.5 when using NaOH solution with a concentration of 14M. In this study, Class C fly ash was used as the aluminosilicate source. Alkaline activator solution, as one of the two main components of geopolymers, plays an important role in the decomposition and formation of the crystalline structure of Si and Al and is usually selected on the basis of sodium or potassium, which are soluble alkali metals. Considering the fact that in practice the use of concrete and mortar is done in ambient conditions and curing in the oven is only for the laboratory, the study of the role of different alkaline solutions in ambient temperature conditions (actual use conditions) is particularly significant. Therefore, in this paper, the role of different alkaline solutions including: NaOH, KOH and combined states of NaOH and KOH and also Na₂SiO₃ and K₂SiO₃, SiO₂/Na₂O ratio of Na₂SiO₃ solution, some parameters related to alkaline solution and processing conditions on 7 and also 28 days compressive strengths of GM based on Class C fly ash, were deliberated.

2 Experimental and methods

2.1 Materials

Table 1 are shows XRF analysis of alumina silicate of Class C fly ash and Physical properties of sand. It is noteworthy that the amount of sand passed through the sieve number 200 was 0.71%. Water consumption was also tap water in Tehran. Polycarboxylate-based Super Plasticizer (SP) with a specific gravity of 1.1 gr/cm³ was used to improve workability of GM. Chemical analysis of NaOH, KOH, Na₂SiO₃ and K₂SiO₃ solutions with different alkaline activator illustrated in Table 2. SSD (Saturated Surface-Dry) specific gravity and water absorption test were taken from the consumed sand according to ASTM C128 [29], the fineness modulus according to ASTM C136 [30] and sand equivalent according to ASTM D2419 [31].

Table 1
XRF analysis of class C fly ash

Chemical substance SiO₂ Al₂O₃ CaO Fe₂O₃ K₂O TiO₂ Na₂O MnO P₂O₅

Result (w.t. %) 34.70 14.80 33.85 15.10 1.83 0.48 0.17 0.15 0.05

Physical properties of sand

Material SSD specific gravity (gr/cm³) Water absorption (%) Fineness modulus Sand equivalent

Fine sand 2.59 3.2 3.01 73

Chemical substance	SiO₂	Al₂O₃	CaO	Fe₂O₃	K₂O	TiO₂	Na₂O	MnO	P₂O₅
Result (w.t. %)	34.70	14.80	33.85	15.10	1.83	0.48	0.17	0.15	0.05
Physical properties of sand
Material	SSD specific gravity (gr/cm³)	Water absorption (%)	Fineness modulus	Sand equivalent
Fine sand	2.59	3.2	3.01	73

Table 2

Chemical analysis of NaOH, KOH, Na₂SiO₃ and K₂SiO₃ solutions

NaOH			KOH			Na₂SiO₃			K₂SiO₃
Chemical substance	Result	Unit	Chemical substance	Result	Unit	Chemical substance	Result	Unit	Chemical substance	Result	Unit
NaOH	98	%	KOH	90.7	%	SiO₂	30.00	%	SiO₂	30.50	%
Na₂CO₃	1	%	K₂CO₃	0.2	%	Na₂O	14.50	%	K₂O	14.50	%
NaCl	200	ppm	NaCl	0.006	%	Water	55.50	%	Water	55	%
Fe	6	ppm	Fe	0.2	ppm	ROM*	2.06	-	ROM^**	2.1	-
SiO₂	15.7	ppm	NaOH	0.6	%	Appearance	Clear liquid	Appearance	Clear liquid
Appearance	White flake		Appearance	White flake		^*ROM: ratio of model		^**ROM: ratio of model
(SiO₂/Na₂O)	(SiO₂/K₂O)

2.2 Mixed design

2.2.1 Alkaline activator type

Alkaline activator are components of alkali hydroxide solution or a mixture solution of alkali hydroxide and alkali silicate that can Alumino-silicate component dissolve, polymerize and finally hardening to form geopolymer. The mechanism of the geopolymerization reaction is as follows [12] (1) (2)

For investigate the effect of alkaline activator solution type on GM compressive strength used of 3 different types of alkaline activator solution that include relation (1–3) and mix designs related to the type of alkaline activator is show Table 3.

Na₂SiO₃ (with SiO₂ / Na₂O ratio = 2) + NaOH 10M: (SN)

K₂SiO₃ (with SiO₂ / K₂O ratio = 2) + KOH 10M: (SK)

Na₂SiO₃ (with SiO₂ / Na₂O ratio = 2) + KOH 10M: (FK)

Table 3
Mix designs related to the type of alkaline activator [34]

Mix ID Fly ash NaOH 10M KOH 10M Na₂SiO₃ K₂SiO₃ Fine sand SP Extra water Unit

SN 700 112 – 168 – 1070 10 10 Kg/m³

SK 700 – 112 – 168 1070 10 10 Kg/m³

FK 700 – 112 168 – 1070 10 10 Kg/m³

Mix ID	Fly ash	NaOH 10M	KOH 10M	Na₂SiO₃	K₂SiO₃	Fine sand	SP	Extra water	Unit
SN	700	112	–	168	–	1070	10	10	Kg/m³
SK	700	–	112	–	168	1070	10	10	Kg/m³
FK	700	–	112	168	–	1070	10	10	Kg/m³

In the specimens NaOH and KOH solutions with a concentration of 10M were first mixed with Na₂SiO₃, K₂SiO₃ and SP solutions according to the mix designs. It took 24 hours to cool the obtained solutions. On the day of the experiment, first the dry materials including sand and fly ash were poured into the mixer according to the mix designs and mixed for 3 minutes for uniform distribution in the dry form. Alkaline activator solutions, SP and water were then added to the mixer and the mixture was mixed for 2 minutes. Subsequently, compressive (5×5×5cm cubes), specimens were molded and vibrated on a vibrating table for 10 seconds. Then, specimens were cured in laboratory ambient temperature. The GM specimens were subjected to the 7- and 28-day compressive strength test based on BS1881: Part116 [32]. To investigate the effects of NaOH and KOH solutions on GM compressive strength based on fly ash and optimization of this parameter, 8 mix designs according to Table 4 were set. In these mix designs, NaOH and KOH solutions with concentrations of 10, 12, 14 and 16M were used. The GM specimens were subjected to 7 and 28-day compressive strength test.

Table 4

Mix designs related to NaOH and KOH concentration

Mix ID	Fly ash	NaOH	KOH	Na₂SiO₃	Fine sand	SP	Extra water	Unit	NaOH concentration (M)	KOH concentration (M)
N10	700	112	–	168	1070	10	10	Kg/m³	10	–
N12	700	112	–	168	1070	10	10	Kg/m³	12	–
N14	700	112	–	168	1070	10	10	Kg/m³	14	–
N16	700	112	–	168	1070	10	10	Kg/m³	16	–
K10	700	–	112	168	1070	10	10	Kg/m³	–	10
K12	700	–	112	168	1070	10	10	Kg/m³	–	12
K14	700	–	112	168	1070	10	10	Kg/m³	–	14
K16	700	–	112	168	1070	10	10	Kg/m³	–	16

2.2.2 Combination of NaOH and KOH

For investigate effects of different combinations of NaOH and KOH solutions on the compressive strength of GM based on fly ash was scrutinized. For this purpose, 7 mix designs were set up, which are shown in Table 5. In these mix designs, the concentration of NaOH and KOH solutions was 10M. The understudy compounds are as below:

(100% NaOH) + Na₂SiO₃

(80% NaOH + 20% KOH) + Na₂SiO₃

(60% NaOH + 40% KOH) + Na₂SiO₃

(50% NaOH + 50% KOH) + Na₂SiO₃

(40% NaOH + 60% KOH) + Na₂SiO₃

(20% NaOH + 80% KOH) + Na₂SiO₃

(100% KOH) + Na₂SiO₃

Table 5
Mix designs related to NaOH / KOH ratio

Mix ID Fly ash NaOH 10M KOH 10M Na₂SiO₃ Fine sand SP Extra water Unit NaOH/KOH ratio

N100 700 112 – 168 1070 10 10 Kg/m³ 100/0

N80K20 700 90 22 168 1070 10 10 Kg/m³ 80/20

N60K40 700 67 45 168 1070 10 10 Kg/m³ 60/40

N50K50 700 56 56 168 1070 10 10 Kg/m³ 50/50

N40K60 700 45 67 168 1070 10 10 Kg/m³ 40/60

N20K80 700 20 90 168 1070 10 10 Kg/m³ 20/80

K100 700 – 112 168 1070 10 10 Kg/m³ 0/100

Mix ID	Fly ash	NaOH 10M	KOH 10M	Na₂SiO₃	Fine sand	SP	Extra water	Unit	NaOH/KOH ratio
N100	700	112	–	168	1070	10	10	Kg/m³	100/0
N80K20	700	90	22	168	1070	10	10	Kg/m³	80/20
N60K40	700	67	45	168	1070	10	10	Kg/m³	60/40
N50K50	700	56	56	168	1070	10	10	Kg/m³	50/50
N40K60	700	45	67	168	1070	10	10	Kg/m³	40/60
N20K80	700	20	90	168	1070	10	10	Kg/m³	20/80
K100	700	–	112	168	1070	10	10	Kg/m³	0/100

2.2.3 SiO₂/Na₂O weight ratio of Na₂SiO₃ solution

The effect of SiO₂/Na₂O weight ratio of Na₂SiO₃ solution on the compressive strength of GM was studied. In this regard, NaOH solution and 3 types of Na₂SiO₃ solution with SiO₂/Na₂O 2, 2.5 and 3 ratios were used. Similarly, in order to simultaneously dissect the effect of NaOH solution concentration and SiO₂/Na₂O ratio of Na₂SiO₃ solution, 2 concentrations of 10 and 14M were considered for NaOH solution that are shown in Table 6. GM specimens were made at the end, and 28- day compressive strength test was taken from the specimens.

Table 6
Mix designs related to SiO₂ / Na₂O ratio

Mix ID Fly ash NaOH Na₂SiO₃ Fine sand SP Extra water Unit SiO₂/Na₂O ratio of Na₂SiO₃ NaOH concentration (M)

N2–10 700 112 168 1070 10 10 Kg/m³ 2 10

N2–14 700 112 168 1070 10 10 Kg/m³ 2.5 14

N2.5–10 700 112 168 1070 10 10 Kg/m³ 3 10

N2.5–14 700 112 168 1070 10 10 Kg/m³ 2 14

N3–10 700 112 168 1070 10 10 Kg/m³ 2.5 10

N3–14 700 112 168 1070 10 10 Kg/m³ 3 14

Mix ID	Fly ash	NaOH	Na₂SiO₃	Fine sand	SP	Extra water	Unit	SiO₂/Na₂O ratio of Na₂SiO₃	NaOH concentration (M)
N2–10	700	112	168	1070	10	10	Kg/m³	2	10
N2–14	700	112	168	1070	10	10	Kg/m³	2.5	14
N2.5–10	700	112	168	1070	10	10	Kg/m³	3	10
N2.5–14	700	112	168	1070	10	10	Kg/m³	2	14
N3–10	700	112	168	1070	10	10	Kg/m³	2.5	10
N3–14	700	112	168	1070	10	10	Kg/m³	3	14

2.2.4 Curing condition

The role of curing conditions on GM compressive strength was deliberated. For this purpose, specimens made with 2 alkaline activator solutions (NaOH + Na₂SiO₃) and (KOH + K₂SiO₃), in 2 different curing conditions, including: ambient temperature and dry curing in the oven at 90 °C for 24 hours, were used. Toward the end, 7 and 28- day compressive strength test was taken from the specimens. It should be noted that the mix designs of this section are shows in Table 7, as well as fresh GM images and GM specimens in Fig. 1.

Table 7
Mix designs related to curing condition

Mix ID Fly ash NaOH KOH Na₂SiO₃^* K₂SiO₃^** Fine sand SP Extra water Unit KOH concentration (M) NaOH concentration (M)

Na-based 700 112 – 168 – 1070 10 10 Kg/m³ – 16

K-based 700 – 112 – 168 1070 10 10 Kg/m³ 16 –

Mix ID	Fly ash	NaOH	KOH	Na₂SiO₃^*	K₂SiO₃^**	Fine sand	SP	Extra water	Unit	KOH concentration (M)	NaOH concentration (M)
Na-based	700	112	–	168	–	1070	10	10	Kg/m³	–	16
K-based	700	–	112	–	168	1070	10	10	Kg/m³	16	–

^*SiO₂/Na₂O = 2 ^**SiO₂/K₂O = 2.

Fig. 1

Images of fresh GM and GM compressive specimen.

3 Results and discussion

3.1 Effect of alkaline activator solution type on compressive strength of GM

The 7 and 28-day compressive strength of GM with alkaline activator solution type is given Fig2. According to the results, the specimen made with Na-based activating solution (NaOH + Na₂SiO₃), offered a much higher compressive strength than the specimen made with K-based activating solution (KOH + K₂SiO₃) so that the use of Na-based activator resulted in maximum compressive strength. By replacing Na₂SiO₃ with K₂SiO₃ in the FK design (KOH + Na₂SiO₃), the compressive strength of this specimen compared to the SK (K-based) specimen increased approximately 65%.

Fig. 2

Effect of alkaline activator type on 7 and 28-day compressive strength of GM.

These results may be related to differences in the mechanism of reactions of Na and K. The mechanism of action of alkaline activator solution breaks the structure of silica and alumina of aluminosilicate source, forming silicon and aluminum ions and makes geopolymer paste. The type of alkaline activator plays a very important role in the development of the geopolymer production process. In most previous studies in this regard, high temperature curing conditions (60–90°C) have been practiced to process GPC and GM specimens. Therefore, more geopolymer is formed in the case of using K-based activator solution, subjects to the formation of a stronger and more compact system than Na-based activating solution [33], which results in lower 3- and 7-day compressive strength, slower hardening and higher 28-day compressive. On the other hand, in Na-based activator solution, NaOH has the ability to dissolve more minerals in concentrations similar to KOH and NaOH. However, it should be noted that these results are achieved in the case of curing at high temperatures and providing the necessary energy. In this study, ambient temperature conditions are used and as can be seen, the results of this study are different from the previous ones. This could be due to less active K⁺ for the sake of the larger ionic size of K⁺ compared to Na⁺ [34] as well as the lower solubility of Si and Al in K-based activator solution than Na-based at similar molar concentrations [35]. Therefore, more energy is needed in the case of using K-based activator, so the use of ambient temperature conditions in this study did not provide the essential energy for effective K⁺ activity. As a result, the specimen made with K-based activator provided both lower lateral compressive strength and lower initial strength compared to Na-based activator. In fact, for this reason the lateral and initial compressive strength of the specimen increased by replacing part of K⁺ with Na⁺ in the FK specimen. According to the results obtained from this section, two alkaline activator solutions (NaOH + Na₂SiO₃) and (KOH + Na₂SiO₃) were selected to continue the research process.

3.2 Effect of NaOH and KOH concentration on compressive strength of GM

The results of the effects of NaOH & KOH solution concentration on compressive strength given in Figs. 3 4. According to the results, the 7- and 28-day compressive strength were 32.1 and 45.8 MPa in the case of using Na-based activator, related to 10M NaOH. Through increasing the concentration of NaOH solution from 10 to 16M, the 7- and 28-day compressive strength also increased. The results were similar for KOH solution. The lowest compressive strengths of 7 and 28 days were measured in 10M KOH solution at 18.2 and 30.8 MPa. By increasing the concentration of KOH solution to 16M, the compressive strength increased and reached its maximum value. Increasing the molar concentration of hydroxide solutions increases the pH. Also, more amounts of SiO₂ and Al₂O₃ in the aluminosilicate source are dissolved in the alkaline activator solution, resulting in higher amounts of geopolymer gel resulting in an increase in compressive strength by increasing the concentration of hydroxide solutions [36].

Fig. 3

Effect of NaOH concentration on 7 and 28-day compressive strength of GM.

Fig. 4

Effect of KOH concentration on 7 and 28-day compressive strength of GM.

Also increasing trend in the acquisition of initial strength with increasing concentration of NaOH and KOH solutions, was seen in the specimens as the concentration of NaOH and KOH solutions enhanced. This could be related to accelerating pH and dissolution of SiO₂ and Al₂O₃. The reaction process is with increasing concentration.

3.3 Effect of combination of NaOH and KOH on compressive strength of GM

The results of the compressive strength test of the specimens in this section, which were related to the effect of the combination of NaOH and KOH solutions on the compressive strength, are presented in Fig. 5.

Fig. 5

Effect of combination of NaOH and KOH on 7 and 28-day compressive strength of GM.

The results demonstrated that the combination of NaOH and KOH solutions reduced the compressive strength of the specimens compared to both 100% NaOH and 100% KOH solution. Thus, the lowest 7-and 28-day compressive strength was observed in the specimen made of 50% NaOH solution and 50% KOH solution at the rate of 14.5 and 22.7 Mpa. These results can also be due to differences in the mechanisms of Na and K. As a result, the effect of the power and velocity of Na on dissolving Si and Al in the aluminosilicate source is very strong and cannot be balanced with the role of K in the tendency to perform a compression reaction. This brings about interference in the geopolymerization process and consequently reduces the compressive strength of the specimens and according to the results, the higher the percentage of the combination of these two solutions, the greater the interference and the more the compressive strength decreases. On the other, it can be seen that in specimens where the amount of NaOH solution is predominant (≥50%) according to Table 8. Adding KOH solution not only lessens the compressive strength of the specimens compared to 100% NaOH solution, but also diminishes the resistance. In specimens with a predominant amount of KOH solution (≥50%), the addition of NaOH solution increases the initial strength of the specimens to a 100% KOH solution, and as the percentage of NaOH increases, the initial strength of the specimens also upsurges, which can also lead to the differences in Na and K reaction maps are related.

Table 8

Effect of alkaline activator type on compressivestrength of GM

Mix ID	Alkaline activator type	28-day compressive strength (MPa)	28-day strength above SK mix (%)	Level of 28-day strength obtained at 7-days (initial strength) (%)	Initial strength above SK mix (%)
SK	KOH + K₂SiO₃	18.7±0.9	–	54	–
FK	KOH + Na₂SiO₃	30.8±1.1	+65	59	+9
SN	NaO H+ Na₂SiO₃	45.8±0.8	+145	70	+30

3.4 Effect of the weight ratio of SiO₂/Na₂O of Na₂SiO₃ solution on compressive strength of GM

The results of the compressive strength test using 14 NaOH in Fig 6 and 10M NaOH are presented in Fig. 7. The results indicate that the highest 7- and 28-day compressive strength was obtained in Na₂SiO₃ solution with weight ratio of SiO₂/Na₂O = 2 in the case of using 14M NaOH solution. Increasing the SiO₂/Na₂O ratio of Na₂SiO₃ solution to 2.5 and 3 reduced the compressive strength by 3 and 4%, respectively. In the case of using 10M NaOH solution, the 7- and 28-day compressive strength of the specimen made with Na₂SiO₃ solution with SiO₂/Na₂O = 2 were 32.1 and 45.8 MPa, respectively. Unlike the use of NaOH solution 14M, increasing the SiO₂/Na₂O ratio of Na₂SiO₃ solution to 2.5 increased the compressive strength by approximately 7% and achieved the maximum compressive strength at 7 and 28 days (34.9 and 49.2 MPa). By further increasing the SiO₂/Na₂O ratio of Na₂SiO₃ solution to 3, the compressive strength decreased by roughly 10% compared to the optimal state (2.5).

Fig. 6

Effect of SiO₂/Na₂O of Na₂SiO₃ solution on compressive strength of GM (NaOH concentration=14M).

Fig. 7

Effect of SiO₂/Na₂O of Na₂SiO₃ solution on compressive strength of GM (NaOH concentration=10M).

The role of alkaline activator solution, especially NaOH or KOH, is to dissolve Si and Al in the aluminosilicate source, produce SiO₄ and AlO₄ and yield a geopolymer gel. Adding a silicate solution such as Na₂SiO₃ or K₂SiO₃ to the alkaline activator solution increases the amount of SiO₄ and the geopolymerization reaction rate, due to the presence of soluble Si. Consequently, it improves the compressive strength of GPC and GM. As a result, the compressive strength decreases as the Si/Al ratio deviates from the optimal range. But the optimal amount of Si in solution can depend on several factors as: a) SiO₂/Na₂O ratio of Na₂SiO₃ solution. Naturally, Na₂SiO₃ solution with higher SiO₂/Na₂O ratio has higher amounts of SiO₂ as well as soluble Si solution. b) concentration of NaOH solution. the degree of dissolution of the aluminosilicate source of Si is directly related to the concentration of NaOH solution. The use of higher concentrations of NaOH solution increases the solubility of Si of the aluminosilicate source and produces higher amounts of soluble Si. In this case, the use of Na₂SiO₃ solution with high SiO₂/Na₂O beard can lead to a decrease in compressive strength due to excessive increment of soluble Si solution, In fact, this issue can be the reason for the difference in the results of optimizing the SiO₂/Na₂O ratio of Na₂SiO₃ solution in two states of NaOH 10M and 14M in this study. The results of this study presented that in the case of using 10M NaOH solution, the optimal amount of SiO₂/Na₂O is 2.5 in Na₂SiO₃ solution and it is 2 in the case of using 14M NaOH solution.

3.5 Effect of curing condition on compressive strength of GM

The results of effect of curing conditions on GM compressive strength as shown in Fig 8. the curing conditions have a significant effect on GM compressive strength. Under the ambient temperature treatment conditions, the specimen made with NaOH solution provided the final compressive strength (3%) and initial strength higher than the specimen made with KOH solution. However, when using the 90 °C curing conditions, the specimen made with KOH solution provided much higher compressive strength (15%). Likewise, increasing the curing temperature improved the initial strength to the use of ambient temperature conditions. Increasing the curing temperature causes a relative rise in the rate of geopolymerization and a significant increase in the speed of the geopolymerization process [37, 38], which results in an upsurge in GM compressive strength. Besides, as the geopolymerization process accelerates, GM acquires a higher percentage of its strength at a younger age, resulting in a higher rate of initial GM strength. On the other hand, the results indicated that increasing the processing temperature has a greater effect on specimens made with KOH solution than specimens made with NaOH solution, which may be due to the slower K⁺ and higher energy requirements of this ion than Na⁺.

Fig. 8

Effect of curing condition on compressive strength of GM.

3.6 Compared to Portland and geopolymer cements

Compared to Portland cement from limestone and clay that takes place at a temperature about 1400°C and produces CO₂ emissions of 0.9 ton/ton product, geopolymer production needs lower energy consumption and produces lower CO₂ emissions (0.09 ton/ton product). For measurement amount of CO₂ emission, NOx and gases another used of the testo 350 Portable Emission Analyzer State and Local Protocols •EPA methods •CTM 030, 034 •ASTM D6522. Research on geopolymer application as a Portland cement substitute has shown that geopolymer has high mechanical strength, fire resistance, and acid resistance characteristics.

4 Conclusions

The present study deliberated it as an alternative to modified geopolymer cement with alkaline activator solutions in environmental conditions. The results indicated that alkaline activator is one of the parameters affecting the compressive strength of geopolymers in ambient conditions. Using NaOH and Na₂SiO₃ solution leads to greater final compressive strength and greater premature strength than KOH and K₂SiO₃ solution and the dissolved in KOH and Na₂SiO₃. In addition, the concentration of NaOH and KOH solution are other parameters affecting the compressive strength of geopolymer mortar based on fly ash. Increasing the concentration from 10 to 16M enhanced the compressive strength from77% (in the case of using NaOH solution) and 133% (in the case of using KOH solution) as a result of the rise in dissolution of Si and Al (in fly ash) by the alkaline activator solution. Correspondingly, increasing the concentration of NaOH and KOH solutions improved the initial strength of the samples compared to the concentration of 10M. Using the mixture of NaOH and KOH solutions reduced the compressive strength of geopolymer mortar based on fly ash because can produce geopolymer having elastic behavior. Moreover, increasing the replacement ratio of NaOH and KOH solutions from 20–80 to 50–50 %, significantly diminished the compressive strength. The SiO₂/Na₂O ratio of Na₂SiO₃ solution is another parameter influencing compressive strength of geopolymer mortar. The optimal amount depends on various factors, including the concentration of the hydroxide solution. The optimal SiO₂/Na₂O ratio of Na₂SiO₃ solution is 2 in the case of using 14M NaOH solution and 2.5 in the case of using 10M NaOH solution.

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Effective of alkaline additives on the geopolymer cements properties as alternative to Portland cement in order to protect environment

Abstract

Keywords

1 Introduction

2 Experimental and methods

2.1 Materials

2.2.1 Alkaline activator type

Table 3 Mix designs related to the type of alkaline activator [34] Mix ID Fly ash NaOH 10M KOH 10M Na2SiO3 K2SiO3 Fine sand SP Extra water Unit SN 700 112 – 168 – 1070 10 10 Kg/m3 SK 700 – 112 – 168 1070 10 10 Kg/m3 FK 700 – 112 168 – 1070 10 10 Kg/m3

Table 7 Mix designs related to curing condition Mix ID Fly ash NaOH KOH Na2SiO3* K2SiO3** Fine sand SP Extra water Unit KOH concentration (M) NaOH concentration (M) Na-based 700 112 – 168 – 1070 10 10 Kg/m3 – 16 K-based 700 – 112 – 168 1070 10 10 Kg/m3 16 –

3.1 Effect of alkaline activator solution type on compressive strength of GM

4 Conclusions

References

Table 3
Mix designs related to the type of alkaline activator [34]

Mix ID Fly ash NaOH 10M KOH 10M Na₂SiO₃ K₂SiO₃ Fine sand SP Extra water Unit

SN 700 112 – 168 – 1070 10 10 Kg/m³

SK 700 – 112 – 168 1070 10 10 Kg/m³

FK 700 – 112 168 – 1070 10 10 Kg/m³

Table 7
Mix designs related to curing condition

Mix ID Fly ash NaOH KOH Na₂SiO₃^* K₂SiO₃^** Fine sand SP Extra water Unit KOH concentration (M) NaOH concentration (M)

Na-based 700 112 – 168 – 1070 10 10 Kg/m³ – 16

K-based 700 – 112 – 168 1070 10 10 Kg/m³ 16 –