Development of sulfuric-acid resistant concrete with reduced carbon emissions

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INTRODUCTION
Corrosion and erosion of concrete structures by sulfuric acid at sewerage facilities in Japan has recently become a social problem.The deterioration means that facilities can fail to reach their expected service life.On the other hand, in the face of global warming, the development of low-carbon concretes using blast furnace slag and fly ash, which are industrial by-products, is progressing.These concretes use less cement, the manufacture of which emits large quantities of carbon dioxide.Against this background, a new low-carbon concrete that is highly resistant to sulfuric acid is developed in this study.

Outline of the experiment 2.1 Materials
The binder used in this experiment was a conventional Portland cement (C; Density: 3.16 g/cm 3 ), Fly ash (FA); density: 2.20 g/cm 3 ) and blast furnace slag fine powder with a specific surface area of 4000 cm ² /g (BS4; density: 2.90 g/cm 3 ).Crushed sand was used as fine aggregate.Tap water was used as mixed water.In addition, high performance AE water reducing agent (SP) and resin acid AE agent (AE; Density: 1.06 g/cm 3) was used.

Compounding, kneading and mixing
Tests were carried out on the formulations listed in Table 1.The binder compositions of these formulations are shown in Table 2. Three water-binder (W/B) ratios were used: 30%, 25% and 20%.The mass ratio of cement in the binder was kept constant at 20% based on past research results 1) .The replacement ratios of FA in the binder were 0%, 10%, 20% and 30%, with the remainder replaced by BS4.In total, 12 formulations (numbered 1 to 12) were examined, consisting of these binder ratios along with three W/B ratios.A forced biaxial mixer with a nominal capacity of 60 L was used for mixing.To begin with, the binder and fine  aggregate were placed into the mixer and air-kneaded for 30 seconds, after which any material adhering to the inner wall was scraped off.Next, the kneading water including the high-performance AE water reducing agent and resin acid AE agent was added.After kneading for 120 seconds, the coarse aggregate was added, with the resulting mix discharged after a further 90 seconds.The blended formulations were then used for various tests.

Tests (1) Fresh property tests
Concrete slump tests were carried out according to ISO 4109 [3] and air quantity tests according to ISO 4848 [4].
The target values of fresh properties were slump 15±2 cm and air volume 4.5±1.5%. (

2) Compressive strength tests
Compressive strength tests were performed according to ISO 1920-4 [5].The mould was removed at the age of 1 day and then curing was carried out in water at 20±3°C.Compressive strength was measured at the age of 7, 28, and 91 days.The target value was made to be over 60 N/mm 2 which was the target level as a high strength concrete in the measurement of material age of 28 days. (

3) Sulfuric acid immersion tests
After demoulding, water curing was carried out at 20±3°C until 28 days of age, at which point the sample was immersed in 5% sulfuric acid solution at 20°C.Mass change and sulfate neutralization depth were measured up to 91 days of immersion.
The actual ionic concentration of sulfuric acid in sewerage is known to be about 30 to 80 ppm.The 5% sulfuric acid used in this experiment represents an ionic concentration of 50,000 ppm, which makes it a very severe test condition.

a) Mass change rate
Measurements of mass change were taken after 7 days, 28 days, 56 days and 91 days of immersion.Test samples were washed in flowing water for 1 minute and excess water was removed before measurement, according to the method specified by the Sewage Works Agency.Similar measurements were carried out using 3 samples for each formulation and the average value was used to calculate the mass change rate.

b) Sulfate neutralization depth
Sulfate neutralization depth was measured after the same immersion periods: 7 days, 28 days, 56 days, and 91 days.Samples were cut in half crosswise using a dry diamond cutter and 1% phenolphthalein solution was sprayed on the cut surface.The non-coloured area was regarded as having been eroded by the sulfuric acid.The diameter of the coloured area was measured at 5 places and the average was subtracted from the 100 mm initial diameter of the sample.Half of this value was taken to be the neutralization depth of the sample.Similar measurements were carried out using 3 samples for each formulation and the average value was taken to be the sulfate neutralization depth.
The target values of mass change rate and neutralization depth for 28 days of sulfuric acid immersion were ± 10% and 3 mm or less, respectively, based on "Corrosion prevention and corrosion prevention technology manual of sewerage concrete structure" of Japan Sewage Works Agency.

Fresh property test results
The results are shown in Table 3.All formulations met the fresh property target values.

Compressive strength test results
Figure 1 shows the results of compressive strength tests on days 7, 28 and 91 for each formulation.The strength of the fly ash immiscibility was comparable to or slightly higher than those of the mixture at 7 and 28 days when compared to the formulation with a miscibility ratio of 30% for each water binder ratio.In a previous study (2) , the initial strength of concrete was found to be lower as the proportion of fly ash increased.However, the pozzolanic reaction of fly ash means that a compressive strength equal to or greater than that of concrete without fly ash can be attained as the material ages.Formulations 5 and 9, in which the water binder ratio was 30% and 25% and the FA ratio was 0%, and formulations 8 and 12, in which the FA ratio was 30%, showed the same or slightly greater strength than the other blends at 91 days.When blast furnace slag fine powder and FA are used together, the blast furnace slag quickly consumes Ca(OH) ₂ because its reaction rate is faster than that of FA.This limits the amount of calcium hydroxide available for the slower pozzolanic reaction of FA and prevents the blend reaching sufficient strength.In the formulation with 30% FA, there is a lower proportion of blast furnace slag fine powder needing calcium hydroxide to harden, so more is available for the slower pozzolan reaction, as seen in the results at 91 days.In formulations without FA, there was sufficient calcium hydroxide to react with the blast furnace slag fine powder, so strength is seen to improve in comparison with other blends.

Sulfuric acid immersion test
The degradation by sulfuric acid immersion was observed as a crack in hardened concrete, which was caused by the expansion pressure of dihydrate gypsum and ettringite formed by the reaction of sulfuric acid with Ca (OH) 2 formed by the cement hydration reaction.FIG. 2 shows the degradation of formulation 1 in this experiment.The degradation progressed with age. Figure 6 shows the mass change rate plots for formulations with different W/B ratios.Figure 7 shows the state of deterioration of the sample of formulation 1 after 28 days of immersion.The target of no more than 10% mass change was satisfied by all formulations even at 91 days, demonstrating that erosion by sulfuric acid is suppressed under all blending conditions.In Figs. 3, 4 and 5, it can be observed that the mass change rate is suppressed as the ratio of FA increases, and the mass change rate continues to be suppressed up to an immersion period of 91 days.It was surmised that the pozzolanic reaction of fly ash causes densification of the hardened concrete, which inhibits sulfuric acid erosion.This is particularly notable when the W/B ratio is 30%.In FIG.
-3, as a reason why the mass change rate of No. 1, which is not miscible with fly ash, does not become maximum during the immersion period of 28 to 91 days, the mass increase due to the product by the reaction with sulfuric acid and the mass decrease due to the deterioration breakdown of the expansion pressure due to the product occur simultaneously in the sample reacted with sulfuric acid.As shown by the red circles in FIG.-7, it was confirmed that the decomposition of the No. 1 sample by sulfuric acid proceeded in the immersion period of 28 days, and the increase in the mass was suppressed by a large decrease in the mass in the immersion period of 56 days.In the 91 day immersion, the decrease exceeded the increase, and the mass seemed to decrease From FIG.
-6, focusing on the water binder ratio, only the mixture with the highest water binder ratio (4: FA 30) showed a decrease in mass after 91 days of sulfuric acid immersion.The mass change of the mixture containing W/B = 25% was significantly changed compared to W/B = 20%.According to research 3) , it has been reported that in general concrete, the higher the water-cement ratio, the more the expansion pressure at the time of dihydrate gypsum formation is relaxed at the pores, and the higher the sulfur resistance.However, in this study, a decrease in mass was confirmed at a higher W/B ratio.The larger the W/B, the larger the pore volume and the greater the penetration rate of sulfuric acid.Therefore, it is considered probable that the pressure of expansion due to the formation of gypsum dihydrate increased, causing peeling.Therefore, it is considered that No.4 with a large water binder ratio suppressed the mass change rate more than No.8 and No. 12 as a result of measurement.

Sulfate neutralization depth
Figures 8, 9 and 10 show the results of the sulfate neutralization test for each formulation up to 91 days of immersion, for each W/B ratio respectively.Figure 11 shows the results for formulations with different W/B ratios.From Figures -8, -9 and -10, the target range for sulfate neutralization depth testing at 28 days of immersion was met for all formulations.The sulfate neutralization depth of the immiscible fly ash formulations No. 1, No. 5 and No. 9 was greater than that of the mixed formulation with the same water binder ratio during the 28 day immersion period.This is considered to be because the mixing ratio of BS4, which has a slow reaction rate, is larger than that of other formulations, so that sulfuric acid easily penetrates into the hardened body and the neutralization depth of sulfuric acid is promoted.Regarding the mixing ratio of fly ash, the value of sulfate neutralization depth was similar during the immersion period of 28 days, but the sulfate neutralization depth was suppressed when the mixing ratio of fly ash increased during the immersion period of 91 days.As this factor, the hardened body was densified by the pozzolanic reaction which appeared over the long term of fly ash, and it seemed to prevent the erosion of sulfuric acid.

Conclusion Fresh propertiesy
By adjusting the addition of AE and SP agents, the target air volume of 4.5±1.5% and slump of 15±2 cm were satisfied for all formulations in the tests of fresh properties.

Compressive strength
Formulations with a low water-binder ratio satisfied the target compressive strength of 60 N/mm 2 , which places them in the high-strength concrete category.High compressive strength can be ensured by ensuring sufficient calcium hydroxide for full reaction of the blast furnace slag fine powder in specimens with no FA.

Mass change
Mass change is shown to be suppressed in formulations containing fly ash.It is thought that the pozzolanic reaction of fly ash results in densification of the hardened concrete, which prevents the intrusion of sulfuric acid.

Sulfate neutralization depth
After immersion in sulfuric acid, the neutralization depth was increase with the proportion of blast furnace slag.Blast furnace slag develops strength slowly, so the concrete remains more porous and allows easy permeation of sulfuric acid.

Table 1
Formulations of mixes

Table 2
Binder compositions

Table 3
Measured fresh properties