Predicting the 12 Molar Concentration of Activators' pH and the Compressive Strength of Geopolymer Concrete

*Solomon Oyebisi, Anthony Ede and Festus Olutoge* 

#### **Abstract**

 The study forecasts the activators' pH and the compressive strength of the geopolymer concrete (GPC). Sodium silicate and six various products of sodium hydroxide pellets were considered as activators. The industrial and agricultural waste products such as ground-granulated blast-furnace slag (GGBFS) and corncob ash (CCA) were used as binders in the production of GPC. Grade 40 MPa concrete was selected as a design mix proportion. Sodium hydroxide pellets were prepared a day before the casting of fresh concrete and then mixed with sodium silicate to obtain 12 molar concentration. The activators' pH values were determined using HANNA pH (211 microprocessor pH metre). The concrete constituents were thoroughly mixed, cast and cured under the ambient conditions (23 ± 5°C and 60% ± 5% RH). The concrete's compressive strengths were determined at days 7, 28, 56, and 90 of curing using a digital compressive testing machine of 2000 KN capacity. Regression models were also developed for both the compressive strengths and the activators' pH values at 7, 28, 56 and 90 days curing using Minitab 17. The developed models can be employed to predict the correlation between the compressive strength and the activators' pH in the production of GPC.

**Keywords:** pH, sodium hydroxide, sodium silicate, compressive strength, regression model

#### **1. Introduction**

One of the important factors which determine the strength of geopolymer concrete is the activators' pH [1]. Various researches have been carried out on the mechanical property of geopolymer concrete, but the influence of activators' pH on the compressive strength is still limited. Geopolymer concrete is an emerging and innovative product emanating from the yearning for low-cost, durable, sustainable and eco-friendly concrete. It utilises the source materials (industrial and agricultural waste products) such as fly ash, ground-granulated blast-furnace slag, metakaolin, rice husk ash, corncob ash and silica fume as its binding agents activated with the alkaline liquids such as sodium hydroxide, sodium silicate, potassium hydroxide, potassium silicate and sodium carbonate [1–12] to form a hardened product. The alkaline activators are from soluble alkali metals particularly sodium (Na) and

potassium (K), and they are formed from the combinations of sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) or potassium hydroxide (KOH) and potassium silicate (K2SiO3). The alkaline liquids, sodium hydroxide (NaOH) and sodium silicate (Na2SiO3), are mostly used as activators in the production of geopolymer concrete [9–12]. When the sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) solutions are mixed together, polymerisation occurs, and it involves a large amount of heat; that is why it is advisable to prepare the alkaline activator at least 24 h prior to casting of the fresh concrete [13–14]. The silicon oxide (SiO2) and aluminium oxide (Al2O3) present in the source materials react with an alkaline activator to form geopolymer paste which acts as a binder agent for fine and coarse aggregates to form geopolymer concrete [9–10, 13].

 Furthermore, one of the most essential and useful properties of concrete is compressive strength. Concrete is utilised as a construction material to resist compressive stresses. The compressive strength is also employed to determine the needed property at areas where tensile or shear strengths are of paramount importance. The usual pattern in concrete technology is the attribution of compressive strength as a quantitative determinant for other characteristics of hardened concrete.

Moreover, the effective power of hydrogen (pH) of the alkaline activator on the short-term mechanical property of geopolymer concrete cannot be overemphasised as it affects the final product of the concrete. Ref. [15] opined that the higher the pH of the alkaline activator, the higher the compressive strength of the geopolymer concrete. Ref. [15] further stated that the pH in the range of 13–14 is most appropriate for the polymerisation with higher mechanical strength.

Thus, the study aims at evaluating the influence of 12 molar concentration of alkaline activator's pH on the compressive strength of geopolymer concrete using six various sodium hydroxide pellets and a sodium silicate, while Grade 40 MPa concrete was used as a mix design proportion. The correlation between the activator's pH and the compressive strength was predicted to forecast the future data on the compressive strength of GPC. The study was conducted at ambient condition to eliminate heat curing regime and at the same time ensure the field practicability of the concrete produced while the ratio of Na2SiO3 to NaOH solutions was taken as 2.5:1 pertained to the germane researches [1–5]. Moreover, this study bridges gaps in the mix design of GPC by putting into consideration the moisture contents, water absorption capacity and the specific gravities of materials used in the course of concrete mix design. The experimentation was conducted at the Civil Engineering Structural and Materials Laboratory, Covenant University, Ota Nigeria.

#### **2. Materials and methods**

#### **2.1 Materials**

 Granulated blast-furnace slag (GBFS), corncob (CC), sodium hydroxide (NaOH) pellets, sodium silicate (Na2SiO3) gel, water, fine aggregate (FA) and coarse aggregate (CA) were used and locally sourced. GBFS was obtained from Federated Steel Mills, Ota, Nigeria, dried, ground and sieved with BS 90 μm sieve to obtain ground-granulated blast-furnace slag (GGBFS), while the corncobs were obtained from Agbonle, Oyo State, Nigeria, sun-dried for 5 days to aid the burning process. Thereafter, the materials were burnt in a furnace under a controlled temperature (600°C) for up to 3 h to obtain corncob ash (CCA). The ash was then sieved using BS 90 μm sieve to manifest the properties of cement. Furthermore, its oxide compositions were analysed using the X-ray fluorescence analyser (XRF). The result of

oxide compositions is presented in **Table 1**, while the results of the physical properties of the materials used are shown in **Table 2**.

#### **2.2 Methods**

 The mix design based on Grade 40 MPa concrete was determined according to Ref. [16] to arrive at initial mix proportion, and the result is presented in **Table 3**. The GGBFS replacement level was 100, 80, 60, 40, 20 and 0% by volume of CCA, and they are denoted by GPC 1, GPC 2, GPC 3, GPC 4, GPC 5 and GPC 6, respectively. The six various samples of alkaline activators were prepared 24 h prior to use under a standard laboratory condition [17]. The pHs of the alkaline activators were measured with the aid of HANNA pH (211 microprocessor pH metre) (**Figure 1**), and they are connoted by samples A, B, C, D, E and F. Thereafter, the concrete constituents were mixed for about 10 min, while the cube specimens were made, cast and cured under ambient conditions (23 ± 5°C and 60% ± 5% RH). The cube


#### **Table 1.**

*Oxide compositions of GGBFS and CCA.* 


#### **Table 2.**

*Physical properties of materials used.* 


*Note: Coarse aggregate, 12.5 mm size (CA 1); coarse aggregate, 19 mm size (CA 2); sodium silicate solution (SS); sodium hydroxide solution (SH); alkali liquid/binder (AL/B); water-to-geopolymer solid ratio (W/S)* 

#### **Table 3.**

*Mix design quantity (kg/m3 ) for Grade 40 MPa concrete.* 

**Figure 1.**  *HANNA pH (211 microprocessor pH metre).* 

specimens were allowed for a rest period of 72 h before they were removed from the moulds to allow for proper polymerisation and enhance the mechanical property [11–12]. The cube samples were cured, tested and crushed at days 7, 28, 56, and 90 using a digital compressive strength machine with 2000 KN maximum capacity (**Figure 2**). The results of the compressive strengths were modelled with the pH values of alkaline activators using quadratic regression analysis in Minitab 17.

### **3. Results and discussion**

#### **3.1 Oxide compositions**

 The results of the oxide compositions as shown in **Table 1** indicate that the GGBFS satisfied the specifications of [18] which stipulates (SiO2 + CaO + MgO) ≥ 67% and LOI < 3.0%. Similarly, the oxide constituents of CCA fulfilled the specifications of [19] which recommends (SiO2 + Al2O3 + Fe2O3) ≥ 70% and LOI < 10.0%. Thus, it is desirable for use as a pozzolanic material.

#### **3.2 pH and temperature**

**Figures 3**–**6** present the results of the alkaline activators' pH. The results reveal that sample A possessed higher pH value of 13.75 with a percentage increase of 0.81, *Predicting the 12 Molar Concentration of Activators' pH and the Compressive Strength… DOI: http://dx.doi.org/10.5772/intechopen.87836* 

1.02, 1.60, 4.10 and 6.67% than samples B, C, D, E and F, respectively. Similarly, higher temperature was observed for sample A than samples B, C, D, E and F.

This connotes that larger exothermic reaction evolved for a higher pH of alkaline activator [1]. The higher pH value for sample A may be attributed to the increase in OH− ions of NaOH solution, while H+ ions lower as the water equilibrium moves to the left [20].

#### **3.3 Compressive strength**

**Figures 3–6** illustrate the results of the compressive strengths for each sample. The results signify that the compressive strength of GPC increases with increase in GGBFS content. Moreover, the trend in compressive strength of GPC also indicates that the higher the power of hydrogen (pH) of alkaline activator, the higher the compressive strength. By comparison, sample A due to its higher pH value possessed higher compressive strengths at all levels of GPC mixes than samples B, C, D, E and F. This result is in agreement with [15, 20] that higher pH in the range of 13–14 of alkaline liquids results in higher mechanical strength of GPC.

#### **3.4 Quadratic regression model**

Minitab 17 was employed to model and forecast the correlation between the compressive strengths of GPC and the pH of alkaline activators for all mixes as indicated in **Figures 7–10**. The quadratic regression equations for GPC at day 7 of curing for mean compressive strength, fc = 15.00–37.00 MPa and mean pH = 12.80–13.80, are illustrated in Eq. (1), while Eq. (2) illustrates the regression equation for GPC at day 28 of curing for mean compressive strength, fc = 25.00–55.00 MPa and mean

**Figure 3.**  *Correlation of pH and compressive strength for GPC at day 7.* 

**Figure 4.**  *Correlation of pH and compressive strength for GPC at day 28.* 

**Figure 5.**  *Correlation of pH and compressive strength for GPC at day 56.* 

**Figure 6.**  *Correlation of pH and compressive strength for GPC at day 90.* 

pH = 12.80–13.80. Similarly, Eq. (3) represents the regression equation for GPC at day 56 of curing for mean compressive strength, fc = 25.00–55.00 MPa and mean pH = 12.80–13.80, while Eq. (4) signifies the regression equation for GPC at day 90 of curing for mean compressive strength, fc = 25.00–65.00 MPa and mean pH = 12.80–13.80. Thus, the coefficients of determinations (*R*<sup>2</sup> ) for the GPC show that the models are 96.10, 96.20, 96.90 and 95.80% sufficiently fit to forecast the correlation at days 7, 28, 56, and 90 of curing, respectively. Furthermore, compressive strength majorly relies on the pH of alkaline activator at 95% confidence and prediction bands:

$$\text{f.}\_{\text{c.7-day}=4727-730.30 }\text{pH} \text{ + } 28.30 \text{ pH}^2 \tag{1}$$

$$\text{f.}\_{\text{c.28-day}} \text{.} \text{2208 - 357.60 pH} \text{+ 14.61 pH}^2 \tag{2}$$

$$\text{f.}\_{\text{c.56-day}\_{\text{-}}\text{2761 - 441.50 pH} + \text{17.79 pH}^2} \tag{3}$$

$$\text{f}\_{\text{c.90-day}} \text{ 4685 - 73710 } \text{pH} \text{ + 29.15 } \text{pH}^2 \tag{4}$$

 where f c represents the compressive strength (in MPa) and pH denotes the power of hydrogen.

*Predicting the 12 Molar Concentration of Activators' pH and the Compressive Strength… DOI: http://dx.doi.org/10.5772/intechopen.87836* 

**Figure 7.**  *Fitted line correlation between the compressive strength and the pH at day 7.* 

**Figure 8.** 

*Fitted line correlation between the compressive strength and the pH at day 28.* 

#### **Figure 9.**

*Fitted line correlation between the compressive strength and the pH at day 56.* 

 Moreover, the experimental and the predicted compressive strength values reveal that the predicted compressive strength values are in good accordance with the experimental compressive strength values with a percentage margin of 3–4% decrease.

**Figure 10.**  *Fitted line correlation between the compressive strength and the pH at day 90.* 

#### **4. Conclusions**

Consequent upon the experimental findings and results, the study utilises the eco-friendly materials for a sustainable concrete that can be applied in the building construction. Heat curing regime of GPC was also eliminated to allow for the field practicability and the economic purposes. It was also found that the compressive strength of GPC marginally increases with increasing pH of alkaline activator. Moreover, the study developed the mathematical models that could be used to forecast the strength development of GGBFS-based GPC incorporating CCA based on the pH value of alkaline activator at ambient conditions with reasonable accuracy at an early age to later age of concrete samples.

#### **Acknowledgements**

The authors wish to thank the Covenant University Centre for Research, Innovation and Discovery (CUCRID) for the support granted in the course of the study.

#### **Conflict of interest**

The authors declare that there is no 'conflict of interest'. This article is original and contains unpublished material. Any published material is referenced.

*Predicting the 12 Molar Concentration of Activators' pH and the Compressive Strength… DOI: http://dx.doi.org/10.5772/intechopen.87836* 

### **Author details**

Solomon Oyebisi1 \*, Anthony Ede1 and Festus Olutoge2

1 Covenant University, Ota, Nigeria

2 University of the West Indies, Port of Spain, Trinidad and Tobago

\*Address all correspondence to: solomon.oyebisi@covenantuniversity.edu.ng

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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[3] Oyebisi S, Akinmusuru J, Ede A, Ofuyatan O, Mark G, Oluwafemi J. 14 molar concentrations of alkaliactivated geopolymer concrete. In: Proceedings of the International Conference on Engineering for a Sustainable World (ICESW '18); 9-13 July 2018, Ota. Vol. 413. Nigeria, IOP Conf. Series: Materials Science and Engineering; 2018. p. 012065. DOI: 10.1088/1757-899X/413/1/012065

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[12] Fernández-Jiménez A, Palomo A, Sobrados I, Sanz J. The role played by the reactive alumina content in the alkaline activation of fly ashes. Microporous and Mesoporous Materials. 2006;**91**(1-3):111-119

 [13] Davidovits J. Geopolymer Chemistry and Applications. Saint-Quentin, France: Institut Géopolymère; 2008

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**245**

**Chapter 20**

**Abstract**

Effect of Mechanical Properties

Having Optimized Particle Size

Distribution of F-Class Fly Ash

Although gradation is considered in aggregates, it is ignored in powder materials such as cement and fly ash. Unless there is fine gradation, the void ratio will be high, and the products obtained as a result of hydration will not be able to fill all of the cavities. In this study, fly ash was sieved through vacuum sieves to determine gradation of the fly ash at 0–20-, 20–38-, 38–45-, 45–53-, and 53–63-μm intervals. Gradation of fly ash was optimized in these intervals. Using the Fuller-Thompson equation, depending on their particle size distribution and finding most ideal distribution module (n = 0.4), a more rigid structure was formed. The cement mortar was produced by substituting 0–60% of the optimized F-class fly ash with the particle size distribution. For the design of the most ideal "n" particle module of the cementitious composite created, mechanical strengths were investigated for 20% F-class fly ash (FA)-substituted cement mortar according to TS EN 196-1. After obtaining (n = 0.4) that compressive and flexural strengths of F-grade FA-substituted cementitious composite of 0–60% were tested, it has been observed that the same age of optimized fly ash-substitute cement mortar is better than unoptimized ones in

of Cementitious Composites

*İlhami Demir and Ahmet Filazi*

compressive and flexural strengths.

mechanical strength

**1. Introduction**

**Keywords:** F-class fly ash, cement mortar, particle size optimization,

The damage to the environment during cement production is high; approximately 1-ton CO2 gas is released during production of 1 ton of cement [1]. In addition, too much energy is required to produce 1300 C to 1450 C in rotary kilns to produce cement. Cement is the most important material used in concrete production. By replacing the cement with various pozzolanic materials, a more economical

The materials that do not have binding properties on their own but which have the property of binding with another binder such as cement are called pozzolans. In these materials, silica and alumina are present in the colloid, which binds to pozzolans [2]. The most widespread pozzolan is fly ash. This fly ash is a product that is produced by the combustion of ground coal in thermal power plants. The flue gases

and more environmentally sensitive concrete production will be achieved.

#### **Chapter 20**
