Compressive Strength of Palm Ash Geopolymer Mortar against High Temperatures

Palm ash geopolymer mortar is an alternative material that can be used in construction materials. This study to analyze the physical characteristics of palm ash geopolymer mortar against the effects of high temperatures. The mortar sample will be designed with a geopolymer mixture consisting of sand, palm ash, alkaline activator solution (NaOH and Na2SiO3) as an adhesive paste to replace cement. The geopolymer composition will take into account the effect of palm ash with an alkaline activator solution, so the optimal composition will be a sample size of 5x5x5 cm. Then the sample is put into the oven furnace to give high temperature effect on the mortar with temperature variations of 100°C, 200°C, and 300°C, and a mixture of 10%, 20%, 30% portland composite cement substitution by weight of palm ash. The results on the temperature variation of the maximum value of compressive strength occurred at 200°C with 20% cement variation and the minimum compressive strength results occurred at 100°C for 10% cement variation. The effect of high temperature will have an effect on visually different physical characteristics. These resulted in the composition of the palm ash geopolymer components which would affect high temperatures which resulted in changes in the physical characteristics of the palm ash geopolymer mortar.


Introduction
Mortar is a compotition consisting of fine aggregate, adhesive material (cement) and water.The function of mortar is as a binder for structural and non-structural parts of a construction.The use of mortar for structural construction is for example masonry mortar for foundation structures, while nonstructural is masonry mortar for infill walls.
Geopolymer mortars are an alternative to conventional mortars based on silica and alumina activated by alkaline solutions [1].The properties of geopolymer mortars are affected by the type and dosage of activator, modulus of activator, curing temperature, length of curing time, and moisture content in the solution.Activator dosage and modulus also affect the compressive strength and mechanical properties of geopolymer concrete.The use of different base materials provides different activator compositions to obtain high compressive strength.
These elements are found in many waste materials such as palm ash.The use of these waste materials as a replacement for portland cement in conventional concrete to increase the strength and durability of concrete has been demonstrated in previous studies [2].Geopolymer concrete provides an advantage in terms of utilizing factory waste as a usable material, although differences in the place of origin of the material source affect the level of strength results.
The increasing use of mortar demands the strength of the material under the influence of high temperatures.Concrete and mortar are able to withstand high temperatures, but still experience a decrease in strength [3] .Higher temperatures in concrete and mortar weaken their performance and increase the risk of failure.The increase in temperature due to exposure to fire will reduce durability and service life, increasing the risk of unexpected structural failure [4].
Based on research, mortar is able to maintain its characteristics at high temperatures.Several ways to improve material properties to withstand high temperatures include using additives instead of cement.Previous research used palm ash because it has high pozzolanic activity and is resistant to high temperatures.The use of palm ash has several disadvantages, namely in terms of cost, ease of supply and its effect on the environment [5].The high content of palm ash is expected to improve mortar properties due to exposure to high temperatures.There has not been much research on the repair of mortar exposed to high temperatures.Therefore, it is necessary to conduct further research on the physical properties of palm ash geopolymer mortar at high temperatures by conducting physical form testing and compressive strength testing.

Methods and Materials
Mortar is part of the load-bearing construction, so the use of mortar must be in accordance with the standard specifications [6].The mortar specification standard refers to the compressive strength value, which is the ability of the mortar to accept the load.

Specification of mortar properties
The specification of mortar properties is in accordance with the provisions of material requirements and laboratory testing of mortar, which consists of cementitious binders, aggregates and water that have met the requirements of mortar according to SNI 03-6882-2002.The stirring process, tools, materials, and mortar testing followed the requirements of SNI 03-6882-2002.A good stirring process will give good results.The effect of maintenance also has a very vital role.This is because the better the treatment results in a complete hydration reaction, so that the strength of the mortar will increase as the treatment life increases.The use of mortar must be adjusted to the type of mortar.Mortar is able to withstand the pressure of the load even if it is only as a wall coating, mortar in casting and glue ceramics or granite [7].Based on SNI 03-6882-2002, mortar is classified into 4 types as in the table 1.

Effect of Increased Temperature on Mortar
Exposure to very high heat results in changes in material performance due to changes in the nature of the material.Mortars heated to 600°C experience degradation in the form of a significant reduction in strength that may not recover after the cooling process [4].The severity and duration of heating can be seen from the physical shape of the mortar after heating.

600° -800°
Decarbonation of carbonates, such as aggregates containing lime, causes contraction of the concrete due to the release of carbon dioxide.The contraction of the concrete volume will cause micro-cracks in the concrete.

800° -1200°
The complete release of lime compounds due to cement-aggregate segregation and extreme temperature stress, causes a gray discoloration of the concrete and visible micro-cracks.Lime particles will turn into white color.

1200°C
Concrete begins to crumble

1300° -1400°C
Crushed concrete Mortar curing was done after opening the mold.At the initial treatment time, the mortar was kept at room temperature for 28 days.This aims to provide an opportunity for the hydration process to occur completely, so that the mortar provides the desired compressive strength value.After the mortar curing period, it was baked in the oven for 1 hour with temperature variations of 100° -300°C.With a mixture of 10%, 20%, 30% PCC substitution by weight of palm ash.

Methods and samples
Mortar testing was carried out at 28 days of age.The room temperature mortars were removed and allowed to air dry before firing.Mortar testing was carried out after reaching room temperature.High temperature modeling was carried out by burning the mortar for 1 hour.
Based on SNI 03-6825-2002 testing the compressive strength of mortar to determine the maximum force per unit area acting on mortar cubes with (5x5x5 cm).In the compressive strength test, 9 pieces of each temperature variation were used.Mortar compressive strength : Compressive strength (1)

Material
The palm ash tested passed the No. 100 sieve.The palm ash composition test was carried out by sending some of the palm ash to the Laboratory of the Padang Industrial Research and Standardization Agency.
The chemical composition of palm ash from Palm Oil Mill, Riau Province is shown in Table 6.The total of SiO 2 , Al 2 O 3 and Fe 2 O 3 is 50,63%, which shows that the palm ash has met the chemical requirements of pozzolan which can then react with Ca(OH) 2 , which is the hydration product of portland cement which will form a calcium hydrate silicate gel and can be used as a geopolymer base material in this study.

Sample
Trial mix 1 was used to determine the solid composition of sand and palm ash.Trial mix with a ratio of alkali and palm ash of 0.8, using a percentage of 80% sand and 20% palm ash.This mixture did not use cement, so the samples made were very fragile when opened from the mold.Trial mix 2 used a percentage of cement equal to 30% of the weight of palm ash, using a molar variation of 18M with a ratio of lye and palm ash of 0.9 using a percentage of 80% sand and 20% palm ash.This mix looks perfect.Trial mix 3 used a cement percentage of 30% by weight of palm ash, using a molar variation of 18M with a ratio of alkali activator and palm ash of 1.2, using a percentage of 75% sand and 25% palm ash.This mix shows a more watery texture.

Compressive Strength Testing Result
Mortar compressive strength testing consists of 27 test objects made based on the composition of mix design with geopolymer constituent materials.Mortar compressive strength test objects are square with a size of 5x5x5 cm.The selection of these dimensions is based on the shape of the mortar that is widely produced with tested at the age of 28 days.The results on the temperature variation of the maximum value of compressive strength occurred at 200 ° C with 20% cement variation and the minimum compressive strength results occurred at 100 ° C for 10% cement variation.
Coefficient of thermal expansion, specific heat, and thermal conductivity are factors that affect mortar when exposed to high temperatures or heated [8].The increase in temperature will cause the limestone and silica stone aggregate mixture to experience a decrease in compressive strength.At temperatures above 400°C, the compressive strength of mortar only drops 10% from the compressive strength at room temperature and a maximum of 40% when the combustion reaches temperatures above 600°C [9] .Mortar durability can only be maintained for 1 hour at 100°C.Temperature 300°C, the compressive strength of the mortar decreased to 30% of the normal strength.Exposure to high temperatures changes the chemical composition and physical structure of mortar.Water dehydration occurs significantly in CSH at temperatures above 100°C [10].At temperatures above 300°C internal stresses increase resulting in cracks on the concrete surface.

Acknowledgments
Our gratitude goes to those who have helped in this research, Faculty of Engineering, Pasir Pengaraian University and Riau University, PT Ara Abadi, Tambusai Utara Rokan Hulu Regency, Environmental Service, Rokan Hulu Regency, Industrial Research Center Padang, West Sumatra, Faculty of Engineering, Sultan Agung Islamic University Semarang.

L
= Area of the compressive plane, mm 2

Figure 4 .
Figure 4. Compressive strength results for high temperature variation

Table 1 .
Classification of mortar types.

Table 2 .
Portland composite cement chemical composition.

Table 3 .
Chemical and mechanical changes in mortar due to high temperature.

Table 4 .
Testing Reference Standard of Fine Aggregate.

Table 7 .
Fine Aggregate Characteristics Testing Results.