Carbonation and chloride penetration of repair mortars with water treatment plant sludge and sugarcane bagasse ash sand

. Water treatment plant sludge (WTPS) and sugarcane bagasse ash sand (SBAS) (wastes from water treatment and sugar/ethanol industries) can be used as replacements of natural sand in concrete. Thus, this paper aims to evaluate carbonation depth and chloride penetration of cementitious repair mortars produced with WTPS and SBAS. Three mortars compositions were analysed: plain reference sample (REF); sample with 3% of WTPS (3WTPS); and sample with 30% of SBAS (30SBAS). They were subjected to tests of accelerated carbonation and immersion in NaCl solution up to 84 days (12 weeks). The results showed that SBAS mortars had the best performance in relation to carbonation and chloride penetration tests. 3WTPS mortars had similar results to the reference sample. This is due to refinement of pores given by incorporation of fine waste materials making it difficult for aggressive agents to penetrate cement matrices. Therefore, 3WTPS and 30SBAS composites can be satisfactorily used in buildings repair services for more sustainable and durable construction.


Introduction
One of the major problems in civil construction is related to steel corrosion in reinforced concrete [1].The main causes of reinforcement corrosion are related to carbonation and chloride penetration.
Carbonation is a physical-chemical reaction in which carbon dioxide (CO2), present in the atmosphere, reacts with calcium hydroxide (Ca(OH)2), present in cementitious composites, forming water and calcium carbonate (CaCO3).Although CaCO3 increases mechanical strength by filling micropores, the formation of this salt occurs with consumption of alkali from the cement paste.As consequence, pH of concrete pore solution is reduced to levels close to 9. As the reinforcement passivation depends on highly alkaline medium (pH between 12.5 to 13), carbonation reaction leads to depassivation of steel bars and, therefore, facilitates the beginning of corrosive process.
In the presence of sea air/water, for example, chloride ions (Cl -) can penetrate in concrete and reach the reinforcement.Electrochemical reactions between chlorides and steel result in dissolution of iron and formation of more Cl -, contributing to propagation of the corrosion process.This corrosion occurs only where free chlorine ions are found, which breaks steel passivation.
In an attempt to increase sustainability of cementitious composites, alternative aggregates from industrial wastes have been successfully incorporated in concrete constructions [2,3].Some studies have shown the feasibility of using water treatment plant sludge (WTPS) and sugarcane bagasse ash sand (SBAS), wastes from water treatment and sugar/ethanol industries, respectively [4][5][6][7][8].However, there is still a lack of consensus on the durability and corrosion initiation modified by these by-products in concrete structures.
Therefore, the present study aims to evaluate the durability parameters of repair mortars with water treatment plant sludge (WTPS) and sugarcane bagasse ash sand (SBAS), used as partial replacements of natural sand.It was investigated by tests of accelerated carbonation and chloride penetration up to 84 days.This paper is part of an ongoing research project designed to evaluate steel corrosion processes in concrete with industrial wastes.

Materials
For the experimental study, Portland cement CP V ARI [9] containing 90-100% in mass of clinker + calcium sulphate (similar to cement CEM I [10]) was used.Coarse and fine natural sands was obtained in the region of São Paulo state, Brazil [11].The fine portion was partially replaced by water treatment plant sludge (3% WTPS) and sugarcane bagasse ash sand (30% SBAS).Physical characteristics of aggregates are shown in Table 1.WTPS was collected from a water treatment plant in the state of São Paulo, Brazil.The sludge was collected during cleaning of decantation tanks.In laboratory, WTPS samples were completely oven-dried at 110 ± 5 ºC for 24 h, homogenized by sieving (# 4.8 mm) and grinded in a fine aggregate mill [4,12].
SBAS was collected from a sugarcane plant in the state of São Paulo, Brazil.It was also oven-dried at 110 ± 5 ºC for 24 h, homogenized by sieving (#4.8 mm) and grinded by a mechanical mill for 3 minutes.Table 2 shows chemical characterization of CP V ARI, WTPS and SBAS.

Mortar preparation
Three cementitious mortars (REF, 3WTPS and 30SBAS) were produced with different fine aggregates, as shown in Table 3.The level of substitution of natural fine sand by industrial wastes was 3% for WTPS and 30% for SBAS, in mass [4,6].Cylindrical and reinforced prismatic specimens were prepared according to the subjected test.Cylindrical samples (50 mm diameter and 100 mm height) were casted by manual compaction (four layers with 30 strokes in each layer) [13], as shown in Figure 1.They were demoulded after 24h and cured in saturated calcium hydroxide solution for 28 days [14].Cylindrical specimens were subjected to carbonation and chloride penetration tests.

Carbonation depth measurements
For the accelerated carbonation test [15], three cylindrical specimens of each mixture were moulded for each age, totalizing 45 specimens.After curing, the samples were subjected to pre-conditioning: drying in oven at 50 ± 5 ºC for seven days (until mass constancy), followed by moisture stabilization in laboratory environment at 23 ± 1 ºC and relative humidity of 60 ± 5% for five days [6].Finally, the specimens were kept in a carbonation chamber, with CO2 content of 15 ± 5% and the relative humidity between 60% and 85% [16] until the ages of tests (at 7, 14, 28, 56 and 84 days).
To determine carbonation depth, the broken faces of each specimen (obtained by diametrical compression) were treated with phenolphthalein indicator [15].A digital calliper was used to measure the width of the unchanged colour area (Figure 2).

Chloride penetration measurements
For chloride penetration test, two cylindrical specimens of each mixture were moulded for each age, totalizing 42 samples.After curing, specimens were dried in oven at 50 ± 5 ºC for 7 days [6].Afterwards, they were subjected to two different conditions until the test commencement: The highlighted region at the borders of the tested specimens (after colorimetric treatment) represents the depth reached by chlorides [19].A digital calliper was used to measure the width of this area.

Carbonation depth
Figure 3 shows the typical carbonated region after the colorimetric analysis for all samples at different ages of CO2 exposition.
Figure 4 shows the evolution of carbonation depth (average and standard deviation) for the studied samples.
The average for all mixtures was lower than 10.00 mm at 84 days.This value is considerably lower compared to regular covers for reinforced concrete structures [20][21][22].The results showed a reduction of 26.28% between the REF and 30SBAS samples, while the results of 3WTPS were similar to REF.This difference is not only observed at final ages, but for all ages tested: 30SBAS always presented lower values of carbonation depth compared to other samples.Moreover, incorporation of WTPS provides a statistically similar behaviour compared to REF.In both cases, there is evidence of high efficiency of the analysed by-products as buildings repair materials, which provides a protective environment against carbonation effects.In special, 30SBAS significantly reduces the advance of carbonation front compared to REF and 3WTPS samples.

Chloride penetration
Figure 5 shows the profile of chloride penetration front in both exposure conditions.It was different for samples subjected to partial immersion (cone-shaped) and those exposed to total immersion (uniform for all sides).
As the specimens with partial immersion were conditioned to wetting/drying cycles, they had greater depth of Cl -penetration than those totally immersed in saline solution (without any accelerated aging cycles).This is because wetting/drying cycles intensify capillary absorption and suction mechanisms on the surface of specimens, as well as, ionic diffusion inside the samples [23].Thus, it promotes greater penetration of chlorides compared to specimens in static conditions of immersion.The results of chloride penetration are presented in Figure 6, considering two types of exposition.On the other hand, Figure 6b (results from wetting/drying cycles and partial immersion) shows the complete chloride penetration at 28 days (for REF and 3WTPS specimens), and 56 days (for 30SBAS mixes).It means that all samples reached the maximum level of Cl-penetration (specimens' radius of 25 mm) before the age of test (84 days).It was also earlier than the test without aging cycles and total immersion in NaCl solution (Figure 6a).This outcome denotes the higher aggressiveness promoted by wetting/drying cycles and partial immersion of the structure in saline environment, as observed in tidal zones in real situations.
Similarly, to carbonation results, REF and 3WTPS samples showed the same trend over the time of chloride exposition.It confirms the possibility of using 3% of this by-product without compromising mortar performance for structural repair purposes.Moreover, incorporation of 30% SBAS can increase resistance to chloride penetration, due to its lower rate of ingress related to modified microstructure.
The change of mass measured in each wetting and drying semi-cycle up to 12 weeks is shown in Figure 7.
It was observed that all mixtures showed similar behaviours, with a slight mass increment throughout the test.This is due to deposition and accumulation of NaCl salts into microstructure of the specimens.
Additionally, higher variation of the masses is notable for drying conditions.This is associated with hygroscopic property of the salt remaining inside the cementitious composites.This factor leads to increased capacity of water absorption, resulting in higher level of moisture in wetting semi-cycles.Also, there was a decrease in the amplitude between masses measured in each half cycles.This is due to filling of micropores by NaCl deposition and formation of Friedel's salt (Ca2Al(OH)6(Cl,OH)•2H2O) (by reaction between Cl - and cement paste).The lowest masses were recorded for 3WTPS mortars.It is because this by-product has lower specific mass in relation to the natural fine sand (Table 1) [11,24].From the experimental outcomes, it is possible to conclude that mortars with 3% WTPS in replacement of fine natural sand have similar behaviour to a reference sample when exposed to aggressive environments with CO2 and Cl -.In turn, substitution of the same aggregate by 30% SBAS leads to increased resistance against carbonation and chloride penetration.
Therefore, the use of WTPS and SBAS in cementbased materials can indicate a sustainable and durable solution with increased resistance against aggressive environments.Nevertheless, SBAS incorporation seems to be more efficient for repair mortars than WTPS.Moreover, further studies should be carried out to investigate other durability parameters, as well as different contents of sand replacement by the studied byproducts.

Fig. 2 .
Fig. 2. Diametral rupture (a) and measurement (b) of specimens for carbonation tests (i)half of the samples were conditioned to wetting/drying cycles (WC): three days partially immersed in 3.5% NaCl solution (wetting semi-cycle), followed by four days in oven at 50 ± 5 ºC (drying semi-cycle), based on ASTM C876-15[17].The mass was recorded after each semi-cycle; (ii) another half of the samples were kept totally immersed in 3.5% NaCl solution without any wetting/drying cycle (NC), following ASTM T529[18].To determine chloride penetration depth, AgNO3 solution was sprayed on each broken face of the specimens (obtained by diametrical compression).The measurements were obtained at 3, 7, 14, 21, 28, 56 and 84 days.The highlighted region at the borders of the tested specimens (after colorimetric treatment) represents the depth reached by chlorides[19].A digital calliper was used to measure the width of this area.

Fig. 5 .
Fig. 5. Specimens right after application of AgNO3 solution Figure 6a (chloride penetration without cycles and total immersion) shows the depth measures were 17.69 mm, 15.95 mm and 14.41 mm, respectively for REF, 3WTPS and 30SBAS samples at 84 days.Thus, for all composites, chloride penetration depths did not reach the full size of the specimens' widths (i.e., radius of 25 mm).On the other hand, Figure6b(results from wetting/drying cycles and partial immersion) shows the complete chloride penetration at 28 days (for REF and 3WTPS specimens), and 56 days (for 30SBAS mixes).It means that all samples reached the maximum level of Cl-penetration (specimens' radius of 25 mm) before the age of test (84 days).It was also earlier than the test without aging cycles and total immersion in NaCl solution (Figure6a).This outcome denotes the higher aggressiveness promoted by wetting/drying cycles and partial immersion of the structure in saline environment, as observed in tidal zones in real situations.Similarly, to carbonation results, REF and 3WTPS samples showed the same trend over the time of chloride exposition.It confirms the possibility of using 3% of this by-product without compromising mortar performance for structural repair purposes.Moreover, incorporation of 30% SBAS can increase resistance to chloride penetration, due to its lower rate of ingress related to modified microstructure.The change of mass measured in each wetting and drying semi-cycle up to 12 weeks is shown in Figure7.It was observed that all mixtures showed similar behaviours, with a slight mass increment throughout the test.This is due to deposition and accumulation of NaCl salts into microstructure of the specimens.Additionally, higher variation of the masses is notable for drying conditions.This is associated with hygroscopic property of the salt remaining inside the cementitious composites.This factor leads to increased capacity of water absorption, resulting in higher level of moisture in wetting semi-cycles.Also, there was a decrease in the amplitude between masses measured in each half cycles.This is due to filling of micropores by NaCl deposition and formation of Friedel's salt (Ca2Al(OH)6(Cl,OH)•2H2O) (by reaction between Cl - and cement paste).The lowest masses were recorded for 3WTPS mortars.It is because this by-product has lower

Fig. 6 .Fig. 7 .
Fig. 6.Evolution trend of chloride penetration of cementitious composites: (a) without cycles and total immersion; and (b) with wetting/drying cycles and partial immersion

Table 1 .
Physical characteristics of aggregates

Table 2 .
Chemical composition of Portland cement CP V ARI, SBAS and WTPS (by FRX)