Experimental Investigation of 12 Molar Concentration of Activators' Salinity on the Compressive Strength of Geopolymer Concrete

*Solomon Oyebisi, Anthony Ede and Festus Olutoge* 

#### **Abstract**

 This study investigates the influence of the total concentration of all dissolved salts of activators on the compressive strength of geopolymer concrete (GPC). Sodium silicate (Na2SiO3) and six various samples of sodium hydroxide (NaOH) pellets were used as activators. The eco-friendly waste products which are ground-granulated blast-furnace slag (GGBFS) and corncob ash (CCA) were used as binding agents. The study also adopted Grade 40 MPa concrete as a design mix proportion. The activators were prepared to obtain 12 molar concentration, while the salinities were measured with the aid of JENWAY 4510 conductivity metre. The concrete constituents were cast and cured under the ambient conditions, and its compressive strengths were determined at days 7, 28, 56, and 90 of curing. Regression models were also developed using Minitab 17. The experimental investigation indicates that compressive strength increases as the activators' salinity increases. The coefficients of determinations (R2 ) show that the models are 97.10, 96.80, 98.10 and 96.20% sufficiently fit to forecast the correlation between the activators' salinities and the compressive strength of geopolymer concrete at days 7, 28, 56, and 90, respectively. This developed model equations can be used to develop new methods for strength applications that can enhance the short-term mechanical properties of geopolymer concrete.

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

#### **1. Introduction**

One of the important factors which affect the strength of geopolymer concrete (GPC) is the activators' salinity. Various researches have been carried out on the mechanical property of geopolymer concrete, but the influence of activators' salinity on the compressive strength is still limited. 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 combination of sodium hydroxide and sodium silicate solutions are the most alkaline liquids used in the production of geopolymer concrete [1–3]. When the sodium hydroxide and sodium silicate solutions are mixed together, polymerisation occurs and it evolves 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 [4, 5]. The alkaline activator chemically reacts with silicon oxide (SiO2) and aluminium oxide (Al2O3) in the source materials to form geopolymer paste which acts as a binder agent for fine aggregate (FA) and coarse aggregate (CA) to form geopolymer concrete [1, 4–5].

 Reference [6] projected the world population to 8.5 billion, 9.7 billion and 11.2 billion by 2030, 2050 and 2100, respectively, with a higher percentage of the population in developing countries experiencing water crisis. It was further stated that more than half of the population would not have access to drinkable water due to the rapid development in infrastructure due to the yearly needs of clean water. Thus, research on the freshwater economy is very essential.

Furthermore, freshwater has been a problem in coaster areas due to the effect of sea salts (salinity) on the available water. So, in such a location, it is strenuous to provide portable water for civil/building jobs. Thus, it is economically saved to utilise the available seawater closer to the construction site instead of freshwater that has to be delivered to such site from another location. On the other hand, seawater possesses a large number of sea salts (salinity) that may adversely affect the properties of concrete. Thus, water quality plays a significant part in the concrete's preparation. The setting of the cement may be interfered by impurities and may negatively affect the strength properties. The chemical properties of water also partake in the chemical reactions and, therefore, affect the setting, hardening and strength development of the concrete mix. The concrete strength lies on the concrete mix, curing method, water-cement ratio, aggregates and types of binding agents. Moreover the relevant literature and standards of practice signify that the mixing and curing effects of seawater on the durability property of concrete still remain an area needing further investigation more importantly in the application of concrete meant for structural purposes in marine location.

 Many researchers experimentally investigated the effect of salinity on the strength property of Portland cement concrete [7–9]. Reference [7] discovered that the concrete samples prepared and cured with freshwater at 7. 14 and 28 days was found to have the compressive strengths of 27.12, 32 and 39.12 N/mm2 , respectively, while the concrete samples prepared and cured with salt water at 7. 14 and 28 days were found to have the compressive strengths of be 28.45, 34.67 and 41.34 N/mm2 , respectively. These results indicate that there is a minimal increase in the compressive strength of cubes prepared and cured with salt water when compared with the freshwater at all curing days [8]. Similarly, Ref. [9] concluded that there is an increase in concrete strength by 4–8% at 7 days and 9–13% at 14 days in the concrete cubes prepared and cured with seawater when compared with the potable water.

Therefore, this study aims at investigating the effect of alkaline liquids on the short-term property of GGBFS-based GPC incorporating corncob ash (CCA) when mixed and cured with of 12 molar concentration adopting Grade 40 MPa concrete as a design mix proportion and the ratio of Na2SiO3 to NaOH solutions as 2.5:1 based on the germane studies [10–17]. Moreover, the study also fills the gaps identified in the mix design of GPC by putting the physical properties of materials used into consideration when designing the mix proportion.

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

#### **2.1 Materials**

Sodium hydroxide pellets, sodium silicate gel, granulated blast-furnace slag (GBFS), corncob (CC), water, fine aggregate and coarse aggregate were locally

*Experimental Investigation of 12 Molar Concentration of Activators' Salinity on the Compressive… DOI: http://dx.doi.org/10.5772/intechopen.87836* 


**Table 1.** 

*The oxide compositions of GGBFS and CCA.* 

sourced and used. GBFS was obtained from Federated Steel Mills, Ota, Nigeria, dried, ground and sieved with BS 90-μm sieve to obtain ground-granulated blastfurnace 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 hours to obtain corncob ash. 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. [18] 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 Mix 1, Mix 2, Mix 3, Mix 4, Mix 5 and Mix 6, respectively. The six different types of alkaline activators were prepared 24 h prior to use under a standard laboratory condition [19].

 The salinity of the alkaline activators was measured with the aid of JENWAY 4510 conductivity metre (**Figure 1**), and they are connoted by Samples 1, 2, 3, 4, 5 and 6. Thereafter, the concrete constituents were mixed for about 10 minutes while the cube specimens were made, cast and cured under ambient conditions (23 ± 5°C and 60 ± 5% RH). The cube specimens were allowed for a rest period of 3 days prior to removal from the moulds to allow for proper polymerisation and enhance the mechanical property [3, 5, 20].

 The cube samples were cured, tested and crushed at 7, 28, 56 and 90 days using a digital compressive strength machine with 2000 KN maximum capacity (**Figure 2**). The results of the compressive strengths were modelled with the salinity values of alkaline activators using quadratic regression analysis in Minitab 17.


#### **Table 2.**

*The physical properties of materials used.* 


#### **Table 3.**

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

#### **Figure 1.**

*JENWAY 4510 conductivity metre. 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) and water-to-geopolymer solid ratio (W/S).* 

**Figure 2.**  *A digital compressive testing machine.* 

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

#### **3.1 Oxide constituents**

The oxide composition results as shown in **Table 1** indicate that the GGBFS satisfied the specifications of [21] which stipulates (SiO2 + CaO + MgO) ≥ 67% and (LOI < 3.0%). Similarly, the oxide constituents of CCA fulfilled the specifications of [22] which recommends (SiO2 + Al2O3 + Fe2O3) ≥ 70% and LOI < 10.0%. Thus, it is desirable for use as a pozzolanic material.

*Experimental Investigation of 12 Molar Concentration of Activators' Salinity on the Compressive… DOI: http://dx.doi.org/10.5772/intechopen.87836* 

#### **3.2 Salinity**

**Figures 3**–**6** illustrate the results of the alkaline activator salinity. The results reveal that Mix 1 possessed higher salinity value of 1.79 with a percentage increase of 7.19, 20.13, 22.60, 39.84 and 55.65% when compared with Mix 2, Mix 3, Mix 4, Mix 5 and Mix 6, respectively. The higher value of salinity for Sample 1 is due to its higher pH than other samples, and this affirms that the salinity of a solution increases with increasing pH and linearly with temperature [23].

#### **3.3 Compressive strength**

**Figures 3**–**6** illustrate the results of the compressive strengths for each mix. The results indicate that with increasing salinity of alkaline activator, the compressive

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

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

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

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

 strength of GPC also increases. By comparison, Mix 1 due to its higher salinity value possessed higher compressive strengths at all levels of GPC mixes than Mix 2, Mix 3, Mix 4, Mix 5 and Mix 6. This result establishes the findings of [7–9] that there is a minimal increase in the compressive strength of cubes prepared and cured with salt water when compared with the freshwater at all curing ages.

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

Minitab 17 was employed to model and predict the correlation between the compressive strengths of GPC and the salinity of alkaline activators for all mixes as indicated in **Figures 7**–**10**. The regression equations for GPC at day 7 of curing for mean compressive strength, fc = 15.00–37.00 MPa and mean salinity of μ = 1.15–1.79 g/L, 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 salinity of μ = 1.15–1.79 g/L. 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 salinity of μ = 1.15–1.79 g/L, 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 salinity of μ = 1.15–1.79 g/L. Thus, the coefficients of determinations (R<sup>2</sup> ) for the GPC show

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

*Experimental Investigation of 12 Molar Concentration of Activators' Salinity on the Compressive… DOI: http://dx.doi.org/10.5772/intechopen.87836* 

**Figure 8.**  *Fitted line correlation between the compressive strength and the salinity at day 28.* 

**Figure 9.**  *Fitted line correlation between the compressive strength and the salinity at day 56.* 

#### **Figure 10.**

*Fitted line correlation between the compressive strength and the salinity at day 90.* 

that the models are 97.10, 96.80, 98.10 and 96.20% sufficiently fit to forecast the correlation at at days 7, 28, 56, and 90 curing, respectively. Furthermore, compressive strength majorly relies on the salinity of alkaline activator at 95%

 confidence and prediction bands where f c represents the compressive strength (in MPa) and μ represents salinity of alkaline activator (in g/L):

$$\text{f}\_{\text{c7-day}} - 46.61 + 71.07 \,\upmu - 13.49 \,\upmu^2 \tag{1}$$

$$\text{f}\_{\text{c.28-day}} = \text{-118.60} + \text{181.0 } \text{\mu} - \text{47.70 } \mu^2 \tag{2}$$

$$\text{f}\_{\text{c.56-day}} - 108.40 + 167.10 \text{ \textmu - 42.57 } \text{\textmu}^2 \tag{3}$$

$$\text{If } \text{f}\_{\text{c.90-day}} \text{ - 117.80 + 175.20 } \text{\textmu - 41.81 } \text{\textmu}^2 \tag{4}$$

 Moreover, the experimental and the predicted compressive strength values signify that the predicted compressive strength values are in good agreement with the experimental compressive strength values with a percentage margin of 1.0–3.0% higher.

#### **4. Conclusions**

 Based on the experimental findings and results, the study utilises the ecofriendly materials for a sustainable concrete that can be applied in the building construction. Heat curing regime of GPC was also removed to accommodate the field practicability and the economic purposes. It was also found that the compressive strength of GPC increases with increasing salinity of alkaline activator. Moreover, the study developed the mathematical models that could be employed to forecast the future data trend of strength development of GGBFS-based GPC containing CCA based on the salinity value of alkaline activator at ambient conditions with reasonable accuracy at an early age to later age of concrete samples.

#### **Acknowledgements**

The authors acknowledge the Centre for Research, Innovation and Discovery of Covenant University for the support offered at the time of the study.

#### **Conflict of interest**

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

*Experimental Investigation of 12 Molar Concentration of Activators' Salinity on the Compressive… 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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Construction Conference (ISEC-10); May 20-25, 2019; Chicago, Illinois, USA: ISEC Press; ISBN: 978-0-9960437-6-2

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**203**

**Chapter 16**

**Abstract**

sample.

compressive strength, SEM

**1. Introduction**

Examination of Mortars Exposed

to High Temperatures Containing

Although the concrete containing Portland cement is resistant to high temperatures, its structure shows some physical and chemical changes due to fire or other temperature increases caused by different reasons. The effect of high temperatures on the concrete should be firstly examined for the repair of fire-damaged concrete. This study was carried out to investigate microstructure and strength changes in concretes exposed to high temperatures. For this purpose, mortars containing normal Portland cement and two types of aggregates were prepared. Quartzite and pumice were used as aggregates in the mixture in this study. The prepared mortar samples for compressive strength tests were subjected to a temperature of 100, 200, 500, 700, and 850°C for 4 h, including the temperature rise period. The same region of the same sample was examined before being subjected to temperature and after being subjected to temperature for SEM analysis. It is an advantage of this study to show the effects of temperature on microstructure in the same region of the same

Concrete is known as a material that is more resistant to high temperature and fire effects compared to many building materials. Some characteristics of components of the concrete such as thermal expansion coefficient and thermal conductivity play an important role for resistance to high temperatures. The performance of concrete exposed to high temperatures depends mostly on changes of the mechanical and physical properties of concrete [1]. Portland cement pastes, different types of aggregates, and water in the concrete exhibit completely different behaviors against high temperatures. When the cement paste is heated, it first shows normal expansion; however, meanwhile, hydrates containing water lose the water in their structure and these cause shrinkage in the structure. Thus, the opposite interaction creates stresses in the concrete. One of the hydrates, calcium hydroxide, loses its water at 400–450°C and calcium oxide is increased. When calcium oxide is rehydrated by the moisture, calcium hydroxide is increased again; this can lead to crack formation by creating expansion in the hardened cement paste [2]. Also, calcium silicate hydrate gels are broken down partially at 400–600°C [3]. High temperatures

Different Types of Aggregates

*Kenan Toklu, Osman Şimşek and Can Demirel*

**Keywords:** different types of aggregates, mortar, high temperature,

#### **Chapter 16**
