Evaluation of fly ash concrete in salt environment

. Indonesia is an archipelagic country with many buildings located in the coastal area. Seawater contains 3.5% salt, which can eat away at the strength and durability of concrete. Sodium salts can be dangerous when combined with reactive alkaline aggregates, and the crystallization of salts in concrete cavities can destroy them due to crystallization pressure. Concrete porosity is important to study, especially in beachfront buildings and buildings that intersect with the ground. In Riau Province, several companies operate AMP (Asphalt Mixing Plant) or paved mixture production units, which produce a large amount of fly ash or AMP waste. Research suggests that using only alkaline cement, using only alkaline cement, a mixture of 15% and 20% fly ash, can be more durable than concrete. This study used fly ash additives with seawater curing to determine the compressive strength of concrete with a curing life of 7, 14, and 28 days. This research was conducted because there was limited compressive strength research on concrete using a mixture of fly ash with seawater curing.


Introduction
The State of Indonesia is an archipelagic country, which means that at every location point, there are buildings located in coastal areas such as piers, harbors, canals, and other buildings that we often find in accordance with the needs of community activities.Seawater contains 3.5% salts which can undermine the strength and durability of concrete.The main salts present in seawater are chloride (55%), sodium (31%), sulfate (8%), magnesium (4%), calcium (1%), potassium (1%), and the rest (less than 1%) consist of bicarbonate, bromide, boric acid, strontium, and fluoride [1].Sodium salts in seawater can be hazardous when combined with reactive alkaline aggregates and cement.In addition to chemical reactions, salt crystallization in concrete cavities can destroy due to crystallization pressure [2].
The crystallization occurs at the near-surface pores, which can pressure concrete to damage or spalling [3].Concrete porosity is significant to study, especially in coastal buildings.In coastal buildings, the concrete will come into contact with saltwater containing NaCl which can seep into the concrete.Concrete damage occurs when NaCl evaporates so that crystals appear in the pores of the concrete, which can push through the pores of the concrete walls.Thus, the concrete can be damaged and broken into pieces [4].
The alternative solution to improve concrete strength is using fly ash.It is well known that adding fly ash to concrete increases construction materials' performance and decreases cement consumption in construction projects [5].In Riau Province, especially in Kampar Regency, several companies operate AMP (Asphalt *Corresponding author: ahmad.zaki@umy.ac.idMixing Plant) or asphalt mix production units, for example, PT.Vira Jaya can produce a large amount of fly ash or AMP waste.
Fly ash is an industrial waste from coal combustion that rises above [6].Over 70% of waste coal ash is categorized as fly ash (FA), ranging from 0.5 to 300 mm [7].The surface area range of fly ash is 300-500 m 2 /kg, and the specific gravity varies from 2.1 to 3.0 [8].Fly ash is considered as pozzolanic material with consist of SiO2, Al2O3, Fe2O3, and CaO.Also the smaller amount of K2O, Na2O, TiO2, MnO, MgO, SO3, and unburnt carbon [9].
Two types of fly ash are used as a concrete mixture, namely class C and class F. Fly ash class C (calcareous) with oxides exceeding 50% is usually derived from subbituminous coal or lignite.While, fly ash class F (siliceous) is commonly produced using anthracite or bituminous coals, with the sum of SiO2, Al2O3, and Fe2O3 exceeding 70% [10].Because class C fly ash contains more calcium than class F. Concrete mixture containing class C fly ash develops strength quicker than class F [11].
The application of fly ash in cement concrete has many advantages, not only increasing concrete strength and reducing cement consumption but also improving the workability of concrete mixture, reducing creep, hydration heat, and thermal expansion of concrete, reducing the expansion of alkali-silica reaction, increase sulfate resistance, reduce chloride penetration, and decrease the porosity of concrete [6] [12].
An example of research has been done in utilizing fly ash waste by Naibaho [13], concrete using a mixture of 0 %, 10 %, 20 %, and 25 % fly ash at 28 days of age.The 10% fly ash mixture has a higher compression strength than another sample.This study used type F of fly ash obtained from the Paiton steam power plant through PT.Merak Concrete in Malang.
The other study about burning coal waste as a substitute for cement by Mangi et al. [14] shows that replacing cement with 10% of coal bottom ash significantly develops compressive strength, around 11.32%, then the control mix.Applying coal bottom ash in concrete also increases the resistance against an aggressive environment.
Meanwhile, Oktaviani et al. [15], in their study about the flexural behavior of reinforced fly ash concrete, show that the normal and fly ash concrete beams were generally equivalent in load-deflection response, crack pattern, and failure mode.However, the load-carrying capacity of the fly ash beam was 22% higher than the normal beam, while the deflection of the fly ash beam was found to be significantly higher.
The study about compressive strength and porosity concrete using fly ash and alkaline activators by Kartini et al. [16] used fly ash variations of 0%, 70%, 80%, 90%, 100%, Sodium Silicate (Na2SiO3), and Sodium Hydroxide (NaOH), with molarities of 6M and 8M.The result shows that using fly ash and molarity in concrete cause the compressive strength to decrease significantly.However, the porosity of concrete also decreases because the mixture is more concentrated, and the small particle size of fly ash can fill voids in the concrete.
Pangestuti et al. [17], in an experiment on using fly ash as an additive material to high-strength concrete, show that the greater the percentage of use of fly ash as a substitute for cement, the more compressive strength decreases.The best fly ash used as an additive is 20-30%.The excessive use of 30% fly ash causes not all fly ash waste to react with water and cement in a concrete mixture.Fly ash which cannot respond, causes the binding strength of the concrete mix to decrease, and the result is the strength of the concrete decreases.
The effect of adding fly ash and rice husk on concrete was also studied by Mardiaman and Dewita [18].Partial replacement of cement with fly ash and rice husk ash did not linearly increase the compressive strength of concrete.On the other hand, adding rice husk ash as much as 10%, 15%, and 20% reduces the compressive strength of the concrete.So, replacing cement with 10%, 15%, and 20% fly ash, its compressive strength is better than rising husk ash.
Darmawan et al. [19], in the study of shear strength concrete beams using high calcium content fly ash in the marine environment, show result that specimens cured at room temperature of 34 0 C had 40% higher average compressive strength than specimens cured in a seawater environment 25 0 C. Specimens cured at room temperature also showed lower porosity and high concrete resistivity.The loading test showed no significant differences between the two series' crack patterns and development.However, specimens cured in seawater showed a higher proportion of the cracking load to the ultimate load than the specimens cured at room temperature.
The effect of tidal zone and seawater attack on highvolume fly ash with metakaolin and quartz powder in marine environments has been studied by Rashad and Ouda [20].The result shows that deterioration in the specimens exposed to the tidal zone is more severe than their counterparts exposed to seawater attack.The 20% quartz powder on fly ash concrete shows the lowest deterioration.The high-volume fly ash blended with 20% quartz powder gives superior compressive strength and microstructure stability than other specimens.This mixture can also be applied as coverings in structures close to coastal areas to cope with harsh conditions.
Rameshkumar, on the review of fly ash concrete using seawater [21], concludes that the concrete's strength is increased when fly ash is added to the mixture and the salt water for the curing method.The strength of the concrete also increases when seawater is used as mixing water with the addition of fly ash.Besides that, seawater curing also aims to reduce the scarcity of drinking water or normal water as concrete material.
From the evidence above, this study will focus on determining the effect of adding fly ash mixture with variations of 0%, 5%, and 10% by seawater curing method on the compressive strength of concrete.This study also aims to determine the behavior of concrete structures when exposed to salt and salt environments.The other purpose is also to reduce normal water as a curing method because of the scarcity of drinking water.This study used local materials like coarse aggregate (gravel) from the Kampar River area and fly ash from PT. Vira Jaya, Kampar Regency, Indonesia.At the same time, the seawater used for the curing method comes from the sea flow in the Selat Baru, with a curing age of 7, 14, and 28 days.

Fly ash
The main material used in this study is fly ash waste from AMP PT.Vira Jaya, Kampar Regency.The size and shape of the fly ash particles are round, like small amorphous balls and intertwined clusters.The size of fly ash is between 1µm to 1mm [22].The fineness of fly ash will affect the performance of concrete, especially in the strength, resistance to abrasion, and density of concrete [23].The specific gravity of fly ash is around 2.0 but varies greatly-3.1 [7].
Fly ash contains chemical elements including SiO2, Al2O3, MgO, Fe2O3, CaO, K2O, Na2O, TiO2, MnO, P2O5, and SO3 [7].These elements greatly affect the chemical reactions in the concrete.Fly ash is a pozzolanic substance that forms CSH gel, fuel in refund concrete, which responds when it reacts with CH produced by cement hydration [11].The AMP waste fly ash shows in Fig. 1.

Other material
The other materials used in this study are as follows: Baru stream, Riau.e.Oil is a lubricant in the mold, so the concrete mix does not stick to it when opened.f.Sulfur to the level of the specimen after being removed from the mold.g.The cement used is PCC (Portland Cement Composite) Semen Padang.h.Coarse aggregate uses crush 2/3 from Kampar River, Kampar Regency, Riau.i. Fine aggregate, the aggregate used is Pasir Danau, Kampar Regency, Riau.j.The water used for curing is seawater in the Selat Baru stream, Riau.k.Oil is a lubricant in the mold, so the concrete mix does not stick to it when opened.l.Sulfur to the level of the specimen after being removed from the mold.

Mix design
The preparation of the specimen is obtained from mixed design planning.Concrete mix design refers to the rules of SNI 03-2834-1993 [24].The number of specimens to be made in this study is described in Table 1 meanwhile, the concrete mix calculation is based on the mix design used by PT.Vira Jaya Riau Putra in Kampar Regency, Riau.The data and calculations can be seen in Table 2 below.
Table 1.Details of the number of specimens to be cured.

Cement Type
Concrete Age (Days) 7

Concrete mixing stage
After the data/value of the concrete mix mixture is obtained, do the thing by mixing the aggregate (coarse and fine), cement, water, and fly ash at a predetermined percentage.The mixture is poured into a large steel tray to test the slump value, and after obtaining the slump value of the concrete mixture, a mix of 15 cm x 15 cm x 15 cm cube mold is poured.The mold filling was carried out in 3 layers, and each layer was pounded 25 times, with the aim that the mixture fills the cavities in the cube.Fig. 2 shows a mold filled with fresh, freshly concrete.

Curing
The curing method of the specimens was carried out by immersion in a soaking bath.The first curing method uses seawater for a design age of 7, 14, and 28 days.Then, the second method uses normal/plain water for a design age of 28 days.The curing process is shown in Fig. 3.

Fig. 3. Curing process.
The compressive strength of specimens was tested at the age of 7, 14, and 28 days in seawater immersion.
Before carrying out the compressive strength test, the soaked concrete was removed and left to stand for 24 hours at room temperature.The concrete that has been set aside is weighed to find out the maximum load after being weighed after the compressive strength test.After obtaining the compressive strength value, it is necessary to find the conversion value of the specimen for each age.The conversion rate for the age of the specimen shows in Table 3.Meanwhile, the compressive strength is shown in Fig. 4. Table 3. Age conversion figures test objects [25].

Slump test results
Fig. 5 shows the slump test graph, where the variation of 0%, 5%, and 10% fly ash experienced workability of fresh concrete of 11.3 cm to 5.5 cm from all curing ages.These results show that the more fly ash is added to the concrete mix, the workability of fresh concrete will decrease.

Compressive strength test results
From the compressive strength result shown in Fig. 6-8, the graph for normal concrete shows a decrease in compressive strength graph from variations of fly ash 5% to 10%.This might happen due to factors such as mixing concrete or pouring concrete into cube molds.From this case, molds do not know that there is a fine or coarse aggregate in one cube mold than fresh concrete in another.The results of observations decreased the compressive strength of concrete with increasing levels of fly ash mixture, but the more use of fly ash.Fig. 9 explains that specimens of normal concrete age (0%) increased from 7 days to 28 days, while for 5% and 10% fly ash concrete from the concrete age of 7 days to 28 days, there was a decrease.The results of these observations decreased the compressive strength of concrete with increasing fly ash mixture.This might happen because fly ash cannot fully bind aggregates such as cement function.Result of Normal Water and Seawater Curing.The comparison between the curing method in normal water and seawater shows that the average compressive strength of concrete curing in normal water is higher than that of curing in seawater.This happens because the more fly ash is used in the concrete mix, the lower the compressive strength.Seawater curing also greatly affects the compressive strength of concrete because seawater contains 3.5% salts which can undermine the strength and durability of concrete.The compressive strength comparison between normal water and sea curing is shown in Fig. 10.
The result shown in Fig. 11 explains that the percentage of compressive strength with normal water immersion is 9.33%, the addition of 5% fly ash is 18.40%, and 10% fly ash is 32%.Whereas for the curing method in seawater, the percentage of normal concrete compressive strength is 10.24%, 5% fly ash is 22.93%, and 10% fly ash is 32.91%.Due to several factors, the percentage of curing in normal and seawater has not reached 100% of K-250 concrete.The main factor happens when fly ash is not evenly distributed in each mold.Thus, fly ash cannot be fully added as a concrete ingredient.

Conclusion
From the results of research and discussion, it can be concluded as follows: a. Variation in the factors of adding 5% fly ash and 10% fly ash gives a different effect on the compressive strength of concrete.b.The effect of curing in seawater for 28 days and curing in normal water for 28 days give different compressive strength results.For variations, 5% to 10% of fly ash shows a significant decrease in each curing method.This happens because the more fly ash is used in the concrete mix, the lower the compressive strength.Seawater curing also greatly affects the compressive strength of concrete because seawater contains 3.5% salts which can undermine the strength and durability of concrete.c.Comparison of curing in normal water and seawater also give different average compressive strength of concrete.The longer curing duration using normal water will give higher compressive strength to concrete.But seawater is inadequate for concrete curing because salt contains as much as 3.5% more than normal water.d.For normal water curing, the percentage decrease in compressive strength is 9.33%, while for seawater curing, the percentage decrease in compressive strength is 10.24%.e.The decreasing compressive strength of the concrete specimens occurs because when the fresh concrete is poured into the cube mold, the concrete mixture is not evenly distributed between one specimen and another.

Fig. 1 .
Fig. 1.AMP waste fly ash.a.The cement used is PCC (Portland Cement Composite) Semen Padang.b.Coarse aggregate uses crush 2/3 from Kampar River, Kampar Regency, Riau.c. Fine aggregate, the aggregate used is Pasir Danau, Kampar Regency, Riau.d.The water used for curing is seawater in the SelatBaru stream, Riau.e.Oil is a lubricant in the mold, so the concrete mix does not stick to it when opened.f.Sulfur to the level of the specimen after being removed from the mold.g.The cement used is PCC (Portland Cement Composite) Semen Padang.h.Coarse aggregate uses crush 2/3 from Kampar River, Kampar Regency, Riau.i. Fine aggregate, the aggregate used is Pasir Danau, Kampar Regency, Riau.j.The water used for curing is seawater in the Selat Baru stream, Riau.k.Oil is a lubricant in the mold, so the concrete mix does not stick to it when opened.l.Sulfur to the level of the specimen after being removed from the mold.

Fig. 10 .
Fig. 10.Graph of compressive strength comparison between normal water and sea.
Fig. 2. Fresh concrete in the mold.