Mechanical properties of concrete with hybrid binder fly ash and thermally activated alum sludge as partial cement replacement

Different binders affect the mechanical properties of the concrete differently. Previous results show that optimum 15% of thermally activated alum sludge (TAASA) replacing cement (C) increase the compressive strength of concrete, and reversed for fly ash (FA) at 45% and more. This paper investigates the optimum binders’ ratio (total binder 25%) to produce highest compressive strength at 7, and 28 days. The ratios of FA: TAASA are S0-Control (0:0), S1-(4:0) S2-(3:1) S3-(2:2) S4-(1:3) and S5-(0:4). The cement used is a CEM II/B-L 32.5 N Portland composite. Results show that the slumps of all mixes decreased, with lowest for S1 (single binder FA). For the compressive strength (7 and 28 days), hybrid FA and TAASA with equal amount of 12.5% (S3) shows the highest, and all mixes’ strengths are above the control (S0). The strength increase of S3 is 58%, that is, from 22 MPa at 7 days to 29 MPa at 28 days. The flexural and split tensile of S3 are also higher than control (S0) at both ages. Hybrid FA and TAASA binders can produce superior mechanical properties concrete when the correct ratios are used compared to single binder replacement or all-cement binder concrete.


Introduction
Various additives / supplementary cementitious materials can be used to replace part of the cement content in concrete.Several materials are available and can be used in the cement mixture, such as fly ash (FA), alum sludge (AS), metakaolin (MK), silica fume (SF), rice husk ash (RHA) and a few other types of materials.These materials can be mixed into concrete as a single or more combinations and result in a multi-blended cement.This paper focuses on the possible usage of two types of cement supplementary/ replacement materials, namely, FA and thermally activated alum sludge ash (TAASA, heat treatment of AS) in a mixture of concrete.These two materials are each a by-product of coal combustion from electric power plants and deposits from the drinking water treatment process [1], [2].About 900 to 1000 million tons of FA are produced worldwide [1] and more than 2 million tons of water treatment process waste has been produced within one year in Malaysia [2].These materials are to be disposed of in landfills.Disposing of these waste materials has become a serious issue due to their ever-increasing quantities.This not only damages the environment but also affects people's health.Therefore, various research have been carried out to find alternative solutions to the usage of FA and AS as an additional/ cement replacement component in concrete [3].
It was shown that slump value increased with an increasing percentage of AS added in concrete mixes.Addition of 15% AS produced the highest value of slump, 80 mm, compared to the control concrete mixture, with 60 mm [4].A similar test was done later to analyse the influence of TAASA on the properties of multi-blended binders of cement with samples having TAASA content varies from 0% up to 25% [5].With a constant amount of water and other components, the slump shows a decreasing value with an increasing percentage of TAASA.The slump at 5% material substitution is 97 mm while the control concrete has a value of 103 mm.
On the other hand, the workability of concrete with the addition of FA has also been studied by numerous researchers.It is proved that there is an increase in the slump of the samples which are 84 mm and 88 mm comparing 0% of fly ash content and 10% respectively in mixes of concrete with an addition of fly ash and pumice powder [6].Another research shows an increasing slump value from 50 mm to 200 mm in samples of 0% fly ash and 60% fly ash respectively [7].Finally, a study by Yoon et al. [8] also shows an increased value of the slump test with a difference of 50 mm at 50% fly ash compared to the fresh control sample, from 150 mm to 140 mm.Table 1 summarizes the review of the research done on the effect on the workability of fresh concrete with either FA or AS and TAASA.For the mechanical properties, research on the strength characteristics of cement comprising lowcalcium and high-calcium fly ash (HCFA) and also granulated blast furnace slag (GBFS) was explored [1].A test was done after a curing process of 2, 7, 28, 90 and 180 days and the results of control 100% OPC and mixture 65% OPC + 20% FA + 15% HCFA were compared.After curing for 180 days, concrete with a total addition of 35% fly ash shows a 2.4 MPa increasing deviation of strength from the control concrete mixture.Another study of fly ash concrete strength was using 50% of cement replacement by Nikhil T. R. [9], and it shows the opposite of the previous study results.At 28 days, the compressive strength decreases from 49 MPa for the control mixture to 33 MPa.But, the value is still increasing as the age increase and it achieved 50 MPa on day 90.The same experiment by N. Kabay et al. [6], shows that a 10% addition of fly ash is optimum compared to 0% and 20% after curing for 180 days.The results are comparable to 85 MPa, 89 MPa and 84 MPa as the percentage increase.It is revealed in another study that 0% fly ash concrete has the highest strength to resist pressure before breaking than the other samples of 40%, 50% and 60% [7].This analysis is contradicting the study by Z. Giergiczny [1] and N. Kabay et al. [6].
Apart from fly ash being used as cement substitution, research on having alum sludge either raw or ash, in a concrete mixture is also available.Owaid et.al. [4], analysed the high-performance concrete with raw alum sludge addition on the effect of its physical and mechanical properties.Results show that the drinking water sludge is best to be used at 6% replacement of cement across all ages of the curing period.The strength and hydration rate are also positively increased in that mixture rather than in the control concrete such that at 28 days, the compressive strength increase from 68 MPa to 72 MPa.This is said could be due to the fine size of the particles of the material.Another study also by H. M. Owaid et.Al. [5], [10] was about the durability properties of multi-blended binder concrete and the influence on the engineering properties when using a thermally activated drinking water treatment sludge ash.Both results show an optimum percentage of 15% sludge ash after curing for 90 and 180 days.On day 90 it increases from 80 MPa to 89 MPa with and without the ash, while on day 180 the values are 86 MPa and 93 MPa accordingly.
Sahu et.al. [11], have proven analysis of the use of both fly ash and raw drinking water treatment residue together.A percentage of 40% FA and 40% lime sludge are showing promising results on the strength of concrete.Its durability increases by 27% at 28 days compared to a mixture consisting of 50% fly ash and 30% sludge.A comparison can also be made with a binder that contains 70% FA, 30% sludge and 0% cement, it shows decreasing in the value of strength which is 14 MPa and 4 MPa at day 28.Hence it is why a blend of 40% FA and 40% lime sludge is the finest choice.
The summary of the review for the studies of the impact of using FA and drinking water treatment plant residue on the compressive strength of hardened concrete is included in Table 2.The split tensile strength of concrete is tested on a cylinder sample that indicates tensile durability.N. Kabay et al. [6], tested the split tensile strength on samples containing pumice powder, fly ash and both materials at days of curing days of 7, 28, 90 and 180.The results show the split tensile strength of both mixes exhibited approximately the same strength value of 5.25 MPa at 180 days between control mix and 20% substitution of FA Table 3. Blended mix containing 10% FA exhibited the lowest value of durability with a value of 5 MPa.Nikhil [9] research shows decrease in the strength value comparing both 0% and 50% FA content, from 4.2 MPa to 2.9 MPa at 28 days of curing.But, these value are increasing as the age increased and the strength did exceed the control concrete's value with a 4.4 MPa at 90 days.
In 2013 and 2014, H. M. Owaid et al [4], [5] analysed the split tensile strength of concrete samples containing 6% drinking water sludge and 15% thermally treated drinking water sludge ash (Table 3).Both studies have shown positive results in increasing durability values.The first analysis indicates at strength at 28 days increases from 4.6 MPa to 4.8 MPa while the other experiment results show improvements from 5.3 MPa to 5.6 MPa at 90 days.Even though the deviation between the 2 compared results is not much, it still indicates improvement in durability to resist the pressure of the concrete.Table 3 shows the summary of the studies on how the usage of FA and AS in concrete mixture affects the split tensile strength.Very little research on flexural testing of concrete mixtures consisting of a drinking water treatment residue.Owaid et al. [4], in the study of physical and mechanical properties of high-performance concrete with AS addition, found that flexural strength improves on the usage of 6% raw AS (Table 4).They compared the results with a mixture of 0%, 6%, 9%, 12%, and 15% a curing period of 3 to 28 days.The strength values of day 28 increase from 7.75 MPa, maximum at 8.1 MPa and end at 4.4 MPa across all percentages of substitution.
Meanwhile, flexural strength on fly ash concrete was analysed by Nikhil [9], for research on substituting high-volume fly ash concrete (HVFAC) for pavement developments.It was found that the replacement percentage of high-volume fly ash, 50% does not give a good impact on the strength of concrete.It has indicated on day 28 of the curing process where the test was conducted, the flexural strength values are 4.8 MPa and 3.2 MPa respectively for 0% and 50% fly ash content.The study concluded that high-volume fly ash concrete (HVFAC) can be used only for the construction of new pavements but not for overlay.
A summarised review of the effect of using fly ash and alum-derived water treatment sludge on the strength of the flexural of concrete samples is stated in Table 4. Results shows that both FA and AS or TAASA produce properties that are different from each replace cement at different percentages.Also when both are combined as hybrid binders to replace cement, the properties changes, sometimes complimenting each other best pproperties and can produce superior properties.This research is to determine the optimum ratio of FA and TAASA that will complement each other properties and produced superior concrete properties compared to if only single complimentary binder is used.
According to study [5], [10] and [13] shows result of improvement on mechanical properties of concrete when respective FA and TAASA binders which is not only reducing the density but eventually increase the strength of concrete at lower percent of replacement.Therefore, a further study in hybrid of both binders is a potential finding on various mechanical properties of concrete.

Methodology 2.1 Materials used
The materials are coarse aggregate, fine aggregate, cement and water together FA and TAASA.
2.1.1Aggregates, water and cement.Sieve analysis of the aggregate was done to determine their particle size distribution coarse and fine aggregate (BS 00822).The coarse aggregate used is a crushed stone with a maximum size of 20 mm while the fine aggregate is sand with a maximum size of 4.75 mm.Figures 1 and 2 show the results of sieve analysis for both fine and coarse aggregate respectively.The results confirm that the size distribution of the coarse aggregate is between 20 mm and 10 mm while the fine aggregate is in the majority of the size distribution of the majority 4.75 mm to 1.18 mm.Most of them behave similarly to the size recommendations from BS 822: part 11.  2.1.2Fly ash.The class F FA was obtained from the pulverized coal combustion in the electric power generation plant where the ashes were filtered and collected from the exhaust gases chamber before they can be release into the atmosphere.The FA as in Figure 3  2.1.3Thermally activated alum sludge ash.Alum sludge is a by-product of when aluminium sulfate Al2(SO4)3 is as a coagulant to clot the sediment in drinking water treatment plants.The AS was taken from Sungai Semenyih Water Treatment Plant, Putrajaya.To produce an activated alum sludge ash, the raw component will oven dried for 24 hours at a temperature of 105 o C, then it was placed in an electric furnace for 2 hours at 800 o C and a rate of heating of 5 o C/min following reference [5]. Figure 4 shows the TAASA used in this study.

Concrete mix design
The concrete mixed proportion was designed based on the design of normal concrete by the Building Research Establishment (BRE) [14].A concrete mixture of grade M30 was designed with the replacement of both materials combined to be up to 25% and the percentage of materials replacement by weight varies from 0%, 6.25%, 12.5%, 18.75% and 25%.The w/c ratio used was determined from the design and each mixture was set to be 0.5.The details of mix proportion are shown in Table 5.Three types of sample shapes will be used which are a cube with size 100 mm x 100 mm x 100 mm, a cylinder of 100 mm diameter and 200 mm height as well as a beam in size 100 mm x 100 mm x 500 mm for tests of compression, tensile and flexural strength respectively.All cube and cylinder samples were tested after curing periods of 7 days and 28 days while beam samples was tested after 28 and 56 days.

Slump test
The workability of the wet concrete mixture were determined by slump test based on the procedure given by ASTM C143 [15].

Compression test
Samples of cubes with sizes 100 mm x 100 mm x 100 mm were prepared prior to this test.It was carried out at the ages of 7 and 28 days based on the procedure outlined in BS 1881 [16].

Splitting tensile strength test
Cylinder-shaped samples with a diameter of 100 mm and 200 mm height were used for testing after curing in water for 7 and 28 days.The tensile strength test carried out in this study was based on standard code ASTM C496 [17] and only performed on control and optimum mixture samples.The optimum concrete mixture was determined based on compression strength test results.

Flexural strength test
A flexural strength test was performed on samples of beam-sized 100 mm x 100 mm x 500 mm following the standard procedure of ASTM C78 [18].Similar to the tensile strength test, this study only determines the flexural strength of the control and optimum concrete mixture but after the ages of both 28 and 56 days.

Workability
The workability of each concrete mix was tested with a slump test and the results are shown in Figure 5 below.It can be seen that the slump of multi-blended concrete shows similarity between each other but it is slightly reduced when compared to normal control concrete mixture.The mix of all hybrid concrete was found to be drier during mixing with the w/c ratio kept constant at 0.5.Figure 5 shows the overall workability results of wet concrete, the derived value for each mixture is above the set target value which are in between 30 mm to 60 mm.The highest slump value is from the S0 control concrete sample with 170 mm while the other hybrid concrete mixtures on average show almost the same value between 130 mm, 140 mm and 150 mm.The workability of sample S2 was the closest to the control sample with a value of 150 mm which experienced a decrease of -11.76% with ratio of FA: TAASA at 3:1.Next, samples S1 and S5 have the lowest workability with a descent of 130 mm which decreases by -23.53% compared to the control sample S0.However, the slump values for all hybrid concrete mixes are below the values of the control concrete.
The workability shown is in line with the study from Owaid et.al.[5] which had shown a decrease in value when added with TAASA.An increase in the replacement of TAASA increase the demand for water use in mixing.On the other hand, the study from Yoon et.al. [8] has obtained an increase in the slump when part of the cement is replaced with FA which is likely to increase due to the use of superplasticizer to achieve the desired derivative value, and the ratio of water per binder used is also different from this study.It can be seen that the decrease in slump was the highest when either FA or TAASA is engaged as single cement replacement concrete, but when both are combined at some ratio, the workability improved (highest at S2, FA: TAASA of 3:1).Fa and TAASA have a higher water demand due to their particle fineness, combining them together compensate the workability loss.

Compression strength
The compressive strength of the cube samples was tested for each hybrid combination as as at 7 and 28 days of curing.For normal concrete (control), the compressive strength will reach 60 to 75% on the 7th day of curing and 98% on the 28th day.This strength will also increase with increasing curing period.
Figure 6 shows that the hybrid concrete mix samples exhibit higher compressive strength compared to control concrete at both curing periods of 7 days and 28 days.The concrete mix S3 containing a proportion FA: TAASA of 2:2 was found to have the highest average compressive strength at both curing periods respectively with values of 21.7 MPa and 28.8 MPa compared to the control concrete mix, which on average obtained strengths of 10.6 MPa and 18.3 MPa at both 7 and 28 days of curing.The percentage of compression strength increment compared to control (S0) is as shown in Table 6.A remarkable strength increase of S3 is at 105% and 58% higher than control, that is, from 22 MPa at 28 days, respectively, followed by sample S4 with slightly less values.Results shows that hybrid FA and TAASA at equal moment and higher amount of TAASA in the hybrid combination produced nearly twofold strength from all Portland cement concrete at early age (7 days).Generally, all hybrid TAASA + FA mixes, or single replacement had shown higher early strength gain compared to control.With this, it can be concluded that the S3 concrete mix is the optimum concrete mixture for this study.The improvement in compressive strength shown for all hybrid concrete samples is mainly due to the pozzolanic reaction that occurs between the materials.FA and TAASA are pozzolanic materials that react with calcium hydroxide from the cement hydration process, and this reaction produces additional hydration components such as calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H).The resulting C-S-H and C-A-H components fill the voids between the concrete making it much denser.In addition, the rate of permeability to water and harmful substances such as Slump, mm Sample chloride ions and sulfate ions can also be reduced which lead to an increase in strength and durability of the concrete.However, the compressive strength values in this study did not successfully reach the M30 grade that was targeted at the beginning of the preparation of the mix design.This is due to the use of CEM II/B-L 32.5 N Portland composite cement in the sample mix which is the main affecting factor.The type of cement that should be used is ordinary Portland cement to obtain more accurate test results since Portland composite cement contain other additives that results in inaccurate proportions of the concrete mix design that has been provided.A study from Shaker et al. [19], confirmed that the compressive strength of concrete using cement type CEM I 42.5 N is much higher when compared to mixing cement type CEM II (B-L).A difference of 44.87% was shown with the strength of 38.2 MPa and 24.2 MPa respectively for the two types of cement.Table 7 shows the percentage of 7 days compression strength increment compared 28 days strength of the mixes.The strength increase of S3 is 58%, that is, from 22 MPa at 7 days to 29 MPa at 28 days, and sample S4 with slightly less values at second place.Results shows that hybrid FA and TAASA at equal moment and higher amount of TAASA in the hybrid combination produced nearly twofold strength from all-Portland cement concrete at early age (7 days).Generally, all hybrid POFA + FA mixes, or single replacement had shown higher early strength gain compared to control.

Tensile strength
As it is known that concrete is weak in terms of tension therefore the expected strength results are low.For this study, the tensile strength was only tested on control concrete samples and optimum concrete samples only.Based on the results of the compressive strength test, sample S3 with an FA: TAASA content of 2:2 was found to be the optimal mixture.Both mixtures were tested at curing periods of 7 days and 28 days.Figure 7 displays a summary of the test results that have been recorded.It was found that the tensile strength of multi-blended concrete S3 were higher 39.33% and 31.62% compared to the control concrete S0 at both curing periods of 7 and 28 days, respectively.The optimum sample obtained the highest average strength at 28 days with a value of 1.54 MPa while the tensile strength of the control concrete was 1.24 MPa.The improvement of strength can be seen is in line with the study of the replacement of cement to alum sludge ash by Owaid [5], who found that the tensile strength of concrete increased when the compressive strength increased which are caused by the fine particles of the ash material used that had started the pozzolanic reaction.C-S-H and C-A-H components resulting from the combination of these materials had filled in the space between the concrete and subsequently cause an increase in strength.
The results obtained are found to be very low and align with the nature of concrete which is inherently brittle and the notion that the sample cannot withstand tension well is true.In addition, the use of Portland composite cement (CEM II/ B-L 32.5 N) has also given a great influence on the results of this test.This tensile strength is expected to increase with increasing curing time.

Flexural strength
The flexural strength test is to identify the modulus of rupture or the maximum load that the beam sample can withstand without any reinforcement bars.Since concrete is brittle and easy to break, the results for this test are also expected to be at a low value.This test was carried out after a curing period of 28 days and 56 days for both concrete mixes of control and optimum.
The optimum hybrid concrete S3 with a mixture of 12.5% fly ash and 12.5% alum sludge ash shows a higher modulus of rupture which is 0.79 MPa and 0.9 MPa when compared to control concrete samples that only have a flexural strength of 0.71 MPa and 0.88 MPa respectively after testing on days 28 and 56.With this, the enhancement is 11.27% on day-28 and 2.27% on day-56 respectively, compared to control.
Studies from Ibrahim et al. [12] have also shown an increase in the flexural strength of concrete with the addition of fly ash when compared to control concrete.This strength can also be reduced if the replacement rate has exceeded the optimum level.The flexural strength test on the alum sludge ash replacement sample could not be identified from any previous study.However, Owaid et.al. [4] has proven that the replacement of raw alum sludge that is replaced as part of the cement content in the concrete mix can increase the flexural strength of the concrete.As described in the compressive and tensile strength tests that the increase in strength that occurs is the result of the pozzolanic reaction between the materials that produce additional components such as C-S-H and C-A-H.This proves that concrete with additional elements of FA and TAASA can withstand bending loads on unreinforced beam samples better than plain concrete.However, this result has also been influenced by the use of Portland composite cement (CEM II/ B-L 32.5 N) as discussed previously.Due to this, the test results obtained are lower when compared to the results from other studies.

Conclusion
This research study is concerned with the possibility of using replacement materials in the concrete mix namely FA and TAASA as a partial substitute for cement and how this mixture will affect the mechanical properties of both wet and hardened concrete.
It exhibits that these two materials can be used as a single component or as a multi-blended substitution to the cement content in the production of concrete.The results obtained from each research whether from this study or previous studies are not the same because they are influenced by many factors and elements that will cause the results to diverse depending on different situations and conditions.The slump test show that all the hybrid concrete samples have obtained a lower slump value compared to the control concrete, especially in samples S1 and S5 which respectively contain 25% FA and 25% TAASA.The decrease shown is -23.53% from 170 mm to 130 mm.It can be seen that the decrease in slump was the highest when either FA or TAASA is engaged as single cement replacement concrete, but when both are combined at some ratio, the workability improved (highest at S2, FA: TAASA of 3:1).FA and TAASA have a higher water demand due to their particle fineness, combining them together compensate the workability loss.This is due to the ability of this cement replacement material to absorb water effectively.
The replacement of both FA and TAASA materials had improved the mechanical strength of concrete compared to control concrete.All of the hybrid concrete samples have managed to achieve strength exceeding the strength of the control concrete.
The compressive strength test of the optimum concrete mixture, S3 with a replacement rate FA: TAASA of 2:2 had shown remarkably higher strength (57.52%) of 28.81 MPa at 28 days curing when compared to S0 concrete which has the strength 18.29 MPa.A remarkable strength increase of S3 is at 105% and 58% higher than control that is, from 22 MPa at 7 days to 29 MPa at 28 days, respectively, followed by sample S4 with slightly less values.Results shows that hybrid FA and TAASA at equal amount and higher amount of TAASA in the hybrid combination produced nearly twofold strength from all-Portland cement at early age (7 days).Generally, all hybrid FA + TAASA mixes, or single Flexural Strength, MPa Sample 7 days replacement had shown higher early strength compared to control In addition, this result was also strengthened by the tensile strength test which gave an increase of 24.19% from 1.24 MPa to 1.54 MPa at 28 days curing on the same concrete comparison which is between the S3 and S0 samples.The flexural strength test has also shown a 2.27% increase in modulus of rupture at the same comparison from 0.88 MPa to 0.9 MPa.The compressive strength obtained did not reach the specified strength target of 30 MPa.In conclusion, hybrid FA and TAASA binders can produce superior mechanical properties concrete when the correct ratios are used compared to single binder replacement or all-cement binder concrete.

Recommendations
Some recommendations can be suggested for the purpose of improving and as a guideline for future studies.Among the suggestions for improvement that can be considered including: 1. Change the type of cement used from CEM II/B-L 32.5 N portland composite cement to CEM I 42.5 N / 52.5 N ordinary Portland cement which does not contain other composite materials to ensure better results.

2.
Testing the sample at a longer curing period because the strength of compression, tensile and flexural will increase after a longer curing period.

Figure 2 .
Figure 2. Analysis of coarse aggregate.The water used is normal tap water, free from any inappropriate impurities.The water per cement ratio was determined through the concrete mix design calculation.The cement used in this study is Castle general-purpose cement produced by YTL Cement Marketing Sdn Bhd.This type of cement is a CEM II/B-L 32.5 N Portland composite cement that follows the standard of MS EN 197-1:2014, Portland limestone cement.

Figure 6 .
Figure 6.The compression strength of concrete.

Figure 7 .
Figure 7.The tensile strength of concrete.

Table 1 .
Workability of concrete mixes with fly ash and drinking water treatment sludge.

Table 2 .
Compressive strength of concrete with FA and alum sludge.

Table 3 .
Split tensile strength of concrete with FA and AS as cement replacement.

Table 4 .
Flexural strength of concrete with FA and AS as cement replacement

Table 5 .
Percentage of replacement and mix proportion of concrete.

Table 6 .
Percentage of compression strength increment compared to control (S0).

Table 7 .
Percentage of 7 days compression strength compared 28 days strength.