A Study on Coefficient of Thermal Expansion of Geopolymer Mortars

N. P. Rajamane, Head, Concrete Composites Lab, Nataraja M C, Professor, SJ College of Engg, Mysore N Lakshmanan, Former Director, and P S Ambily, Scientist, Structural Engg Research Centre, CSIR, Chennai

Conventional Cement Concrete (CCC) consists of Portland Cement (P-C) as binder which binds the inert aggregate system. However, geopolymer composites (GPCs) have geopolymeric (GP) material as binder instead of P-C. GP is obtained by chemical activation of alumino-silicates present in minerals such as fly ash and Ground Granulated Blast furnace Slag (GGBS). Besides strength characteristics, it is necessary to understand the thermal properties of these new composites. In this connection, coefficient of linear thermal expansions (CLTEs) of two typical GPCs (in the form of mortars) were investigated using the computer controlled Dilatometer installed at Structural Engineering Research Centre (SERC), Chennai. The first GPC had GGBS and the second had a combination of fly ash and GGBS as the ‘starting materials’ for geopolymerisation reactions.

Introduction

Geopolymers are inorganic materials of cementitious nature derived from the alkaline activation of readily available natural or industrial by-product aluminosilicate materials. The term geopolymer was introduced by Davidovits to represent the mineral polymers. Geopolymer-based materials are environmentally friendly since they need less energy to produce. Their performance as a construction material has to be comparable with that of Portland cement in order to accept them as in practical applications. The present paper is concerned with the thermal expansion studies of geopolymer composites. The Coefficients of Linear Thermal Expansion (CLTE) of geopolymer mortars derived from Ground Granulated Blast furnace Slag (GGBS) and fly ash (FA) were measured as per ASTM E 228 on beam specimens using a computer-controlled dilatometer.

Scope of the Study

The knowledge of thermal transport properties of construction materials involved in the process of heat transfer is required for predicting the temperature profile and heat flow through the material. It is also needed in many civil engineering applications such as high rise building subjected to variation of temperature, blast furnace, and pressure vessels. Some information has been generated on the mechanical, microstructure and chemical properties of geopolymer materials. However, the test data on the thermal properties of this novel material is not widely available. The present work was therefore taken up to investigate the thermal properties of the GGBS and FA based typical geopolymer mortars (GPMs), particularly ‘linear thermal expansion coefficients’ (CLTEs). The CLTEs were measured at different temperatures, on prismatic specimens (size : 25 mm*25 mm *100 mm).

Figure 1: Dilatometer for thermal expansion measurement

Details of Experimental Work

Materials

GGBS from steel plants and Class F FA from coal based thermal power station (near Chennai) were used as the ‘source materials’ (SMs) for the geopolymers (GPs). A combination of sodium silicate solution and sodium hydroxide solution was used as the alkaline activator. GPMs were prepared using locally available river sand as inert filler material. The properties of the ingredients of GPMs are described in Table 1.

Figure 2: Specimen inside the dilatometer Specimen size: 100*25*25 mm (prism)

Preparation of Test Specimens

Figure 3: Specimens before testing

Two different GPMs were studied; one with GGBS and the other having a combination of GGBS and FA, mixed in the ratio of 1:4 by mass. The fresh mixes (prepared in an electrically operated mortar mixer) were cohesive and there was no sign of segregation. Conventional table vibrator was sufficient to effect the compaction in the steel moulds. The moulds after casting operations were initially covered with plastic sheets for about 24 hours, after which the specimens were demoulded. Thereafter, the specimens were allowed to cure at ambient conditions inside the laboratory. After 28 days of casting, the cube specimens were taken for compression test and the prismatic specimens were subjected to a temperature of 100°C in a hot air oven for a period of about 24hours before using them for expansion studies.

Measuring Technique

Figure 4: Specimens after testing

The thermal expansion test was carried out using equipment called Dilatometer (Fig.1). It consists of specimen holder (Fig.2), furnace, transducer, recorder and temperature sensor. After taking out the specimen from the hot air oven, the specimens were allowed to cool to room temperature. The initial specimen length in the direction of the expansion was measured at room temperature. The specimen (Fig.3) was then placed in the specimen holder of the Dilatometer. The furnace portion of the equipment was moved to enclose the specimen holder. After positioning the specimen accurately, the AC power was switched on. An appropriate force was applied to the sensing probe to ensure that it was in contact with one end of the specimen. The initial displacement was set to zero. The specimen was heated at a constant rate of 3°C/min up to a temperature of 300°C and the changes in specimen length at every 100°C were recorded. After the desired maximum temperature was reached, the furnace was put off to allow the specimen (Fig.4) to cool. Three specimens were tested for each GPM.

The CLTE is calculated as follows:

Linear thermal expansion =[Change in length (linear expansion)] / Original length

The coefficient of linear thermal expansion, α, is computed using the equation

α = (Linear Thermal Expansion / 100) / (T_Max - T_Min)

T_max = Maximum temperature (ºC), T_min = Minimum temperature (Ambient Temperature, ºC)

Test Results And Discussions

Coefficients of Linear Thermal Expansion

The variation of CLTE of geopolymer mortars with temperature is shown in Fig.6. The GPM made from mixture of GGBS and FA indicated a lower CLTE value compared to that GPM made from GGBS alone. The CLTE of GPM with GGBS alone was about 4.3x10^-6/°C at 100°C and this value increases to 12.3 x10^-6 /°C at 300°C; however, in case of GPM containing both GGBS and FA, the CLTEs were 3.3 x 10^-6 /°C and 12.25 x 10^-6 /°C at 100 °C and 300 °C respectively. It may be noted that in estimating the range of thermal movements (e.g., highways, bridges, etc.), the use of lower and upper bound values of 8.5 x10^-6/C and 11.7 x10^-6/C has been suggested to be more appropriate for conventional concretes, by ACI 209. Thus, GPMs can be considered to possess comparatively lower values of CLTEs in general at near ambient temperature exposure conditions. It is also reported that for Portland cement paste specimens, though the CLTE may increase initially with temperature, but, beyond about 200 °C, the CLTE could actually decrease with temperature and it may even become negative at higher temperatures; this behaviour may be related to the various chemical changes occurring in the cement matrix (Neville, 1996). However, in case of Portland cement based concretes, the CLTE is reported to be increasing with temperature as seen in the present case of GPMs also.

Compressive Strength

Figure 5: Compressive Strength of geopolymer mortar at 28 Days

The 28 day compressive of about 63 MPa was obtained for GPM with GGBS alone and this value was 50 MPa for GPM containing GGBS and FA. (Fig.5). These strengths indicate that GPMs develop satisfactory strength levels making them suitable for many structural applications. However, before adopting the GPMs in important practical applications, there is a need to study various aspects of durability of these composites including their potential to provide protection to the embedded steel reinforcement.

Density

The density of GPM with GGBS alone was 2277 kg/m³ and it was 2271 kg/m³ for mix with GGBS and FA. Thus the GPMs have marginally lower density than conventional concretes.

Concluding Remarks

Figure 6: Comparison of CLTE of geopolymer mortars

1. Ground Granulated Blast furnace Slag (GGBS) and fly ash (FA) could be used as ‘starting materials’ for initiation of geopolymerisation reactions so that geopolymeric binder system is evolved.
2. Densities of Geoplymer mortars (GPMs) prepared with GGBS and FA are lower than those of conventional Portland based concretes.
3. The 28 day strengths of 63 and 50 MPa achieved for GPMs described in this paper are high enough o conclude that the GPMs can be used as structural materials.
4. The GPMs had higher CLTE values at higher temperatures. The difference in CLTE values between 300°C and 100°C was about 65% for GPM with GGBS; this difference for GPM having GGBS and FA was 73%.
5. The lower and upper bound values of 8.5 x10^-6/C and 11.7 x10^-6/C has been suggested by ACI 209 to be more appropriate for estimating the range of thermal movements in transportation structures such as highways, bridges, etc. The geopolymer composites with lower values of CLTEs (at near ambient temperature exposure conditions) can be considered to be good candidate materials for construction of infrastructures .

Abbreviations

• GP – Geopolymer, GPM – Geopolymer mortar, GPCs- Geopolymer Composites
• P-C -Portland cement, CCC-Conventional Cement Concrete
• GGBS – Ground Granulated Blast furnace Slag, FA - Fly Ash
• CLTE - Coefficient Linear Thermal Expansion
• SERC-Structural Engineering Research Centre

Acknowledgement

This paper is published with the kind consent of Director, SERC, Chennai. The co-operation and help received from the scientific and technical staff of Concrete Composites Laboratory, SERC, in creating test data and preparation of paper is gratefully acknowledged

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