Consistency of Mixtures
The workability of each concrete was determined by using the slump test;34 the results for all the treated and control mixes are presented in Table II. Increasing the amount of ASAc added to the concrete mix increased its consistency and flow properties. Despite the high slump of the 46/4 K mix, no apparent cracks were observed after aging for 7 days, 14 days, or 28 days. The 32/2 K and 32/4 K mixes resulted in hard concrete with zero slump. Regardless of the low slump values and the difficulty of compacting the mixes, very good compacted mixes were created with no visible cracks.
Table II Slump values for all treated and control mixes Morphological Analysis
Microstructure Analysis
EDX analysis was carried out to clarify the morphology of the ASAc material, revealing sodium, oxygen, and carbon as the main elements, with small percentages of silicon and calcium. The formation of crystals after mixing this material with concrete may correspond to the presence of sodium acetate in its composition; water activates its reaction with concrete to form crystals inside the pores.32 Also, it is believed that silicon and sodium, which were also present in this material, will react with calcium hydroxide present in the concrete, in presence of water, to form silica gel that adheres to the walls of the pores.43,44 This gel will develop into solid crystals after its hydration. The structure of crystals formed from the aforementioned reactions can be recognized in the SEM micrographs in Fig. 1a and b, where they appear as needle-shaped crystals. The formation of crystals of this shape may support easy integration of the material into the concrete mix, acting to improve its density and reduce the size of pores, making them finer.44,45 This can be seen in the interaction between the material and concrete in Fig. 1c and d, which show graphs of the cross-sectional area of the internal parts of the concrete. It is noted that the crystals are well distributed within the concrete mix, appearing with very small sizes (below 200 nm), which facilitates their implantation inside capillary pores.30,46 The presence of crystals within the concrete structure in such a dense and well-distributed way would give the treated concrete rigidity and strength, as discussed below in Compressive Strength Analysis section.
After running the salt ponding test, all the concrete samples were investigated to evaluate the presence of chloride at depth of 20 mm (Table III). The cross-sections of all the concrete mixes revealed the presence of chloride as white spots with different densities. White spots of chloride could be found adhered to some areas of concrete with either minimum or no presence of ASAc. The micrographs at 50,000× , as outlined in Table III, showed insignificant presence of chloride in the 37/4 K and 32/4 K mixes, indicating their high robustness to chloride compared with the other mixes. The presence of chloride at such low densities in those two mixes may correspond to two factors: their low w/c ratio, which contributes to reducing the size of the pore structure,47 and the presence of ASAc in proportions compatible with the w/c ratio, which does not negatively affect the hydration process.30 The first attack of chloride on concrete, when its carrier solution starts to permeate through the pores, results in combination of hydration products with chloride until a steady state between free and combined chloride is achieved.48 In the case of the mixes with low w/c ratios, this process would help to reduce the chloride penetration further, especially for additive amounts compatible with the w/c ratio, in contrast to other mixes with higher additive amounts which would hinder the hydration process and decrease the presence of hydration products that are necessary to achieve the previously mentioned stability.
Table III Chloride residues in concrete texture at 20-mm depth after testing for chloride penetration (×50,000) Functional Groups of the Protective Material
FTIR spectra of the protective material and its interaction with concrete are shown in Fig. 2 by three spectra corresponding to the concrete powder, ASAc powder, and concrete integrated with ASAc. The characteristic peaks at 2969 cm−1, 2881 cm−1, and 875 cm−1 remained almost steady and unchanged for ASAc and the concrete mixed with ASAc. Amongst these, the peak at 875 cm−1 mostly corresponds to -CO3 bond,49,50 whereas the peaks in the range from 2969 cm−1 to 2881 cm−1 are believed to correspond to -CH and -OH stretching vibrational bonds resulting from anharmonic resonances of -CH and -OH bonds in the crystal of the acetic acid chain (CH3COOH) forming sodium acetate.50 The most considerable variation in the transmittance peaks was observed at 1599 cm−1, 1479 cm−1, 1380 cm−1, 1320 cm−1, 1116 cm−1, and 1060 cm−1. The peak at 1599 cm−1 corresponds to -COO bond, the peaks in the range from 1479 cm−1 to 1380 cm−1 may correspond to stretching vibrations of C–H bond,51,52 and those in the range from 1060 cm−1 to 1116 cm−1 correspond to stretching vibrations of C-O bond in the -COOH carboxylic acid part. Finally, the peak at 1320 cm−1 corresponds to C-O doublet bond in the same -COOH part.53,54
An increase in the intensity is noticed for the peaks in the ranges from 1479 cm−1 to 1380 cm−1 and 2969 cm−1 to 2881 cm−1 (C-H and O-H) when ASAc is mixed with concrete (Fig. 2). This increase in the hydrogen bond can be ascribed to the chemical reaction between ASAc and cement in the presence of water, where anhydrous sodium acetate dissociates in water, forming CH3COO− and Na+ ions 55,56:
$$ {\text{H}}_{3} {\text{COONa}} \to {\text{CH}}_{3} {\text{COO}}^{ - } + {\text{Na}}^{ + } $$
It is believed that CH3COO− ions will form a linkage with cement through their reaction with sodium already present in the cement, forming sodium acetate crystals again, which contributes to the increase in the intensity of the signals corresponding to C–H bonds (Fig. 2). In turn, the dissociated Na+ ions will react with free water, forming sodium hydroxide (NaOH) composite, which increases the intensity of the signal corresponding to O-H bonds. Moreover, some of the free CH3COO− may react with water to form acetic acid (CH3COOH), which will also contribute to increasing the intensity of the signal corresponding to O-H bonds. On the other hand, the reduction in the intensity of the signals corresponding to the C-O (1320 cm−1) and -COO (1599 cm−1) could indicate the ionic reaction between CH3CO− and H+ ions, forming small amounts of volatile CH4 and CO2.57,58
The presence of small quantities of acetic acid in the mix may act to delay the hydration process and increase the workability of the concrete mixes, especially if a large amount of sodium acetate admixture is added to the mix.59 In addition, the presence of sodium hydroxide (NaOH) in the concrete mixes at reasonable amounts may further increase its workability, reduce the segregation, and accelerate C3S hydration. However, large amounts of NaOH may contribute to decreasing the final strength of the concrete, as a result of the formation of metastable C-S-H with reduced bonding strength.60,61 Another reason for the increase in C-H bonds is believed to be the replacement of -OH groups by -CH3 groups, which in turn bond with silicon atoms in the cement, resulting in the formation of a hydrophobic component integrated within the mix (organosilicon bonds).62,63,64,65,66,–67 This could result in improving the hydrophobicity of the concrete treated with ASAc (Supplementary Fig. S4). Notwithstanding the modest contact angle resulting from the treatment, a clear increase in hydrophobicity resulted from integrating ASAc into the mix, especially for the 32/4 K mix, where the contact angle almost doubled compared with control. Meanwhile, some of the mixes, especially those with high w/c ratio, suffered from a reduction in hydrophobicity, which could be due to their relatively high water content. Excess water may contribute to decreasing the organosilicon bonds that are responsible for the modest hydrophobicity of the ASAc–concrete composite.
Water Absorption
The efficacy of ASAc for reducing the water absorption by concrete was evaluated by the ISAT method. The results from the ISAT after 7 days, 14 days, and 28 days of curing are shown in Fig. 3a, b and c. These graphs show the accumulative water absorption rate of all the treated and untreated samples after testing for periods from 10 min to 60 min.
At 7 days, the mix with a w/c ratio of 0.32 achieved the lowest water absorption rate among all the mixes, when treated with either 2% or 4% admixture (Fig. 3a). The same performance was observed after aging for 14 days, where the mix with a 0.32 w/c ratio and treated with 4% admixture absorbed the least amount of water (Fig. 3b). After 28 days of curing, the 32/4 K and 37/4 K mixes exhibited the best performance among all the mixes, showing water absorption close to zero with efficacy of more than 50% and 60%, respectively, compared with their control (Fig. 3c).
Increasing the w/c ratio of the mix above 0.37 was seen to have a negative effect on the performance of the treated concrete. Treating mixes with high w/c ratio (0.40 and 0.46) was shown to have a negative effect after aging for 7 days, as recognized by their high water absorption rate exceeding that of the corresponding control (Fig. 3a). On increasing the curing age to 14 days, the 40/4 K and 46/2 K mixes performed better than their control. After aging for 28 days, the 40/2 K and 46/2 K mixes showed the best performance among all the mixes with w/c ratio of 0.40 or 0.46, whereas the same mixes treated with 4% ASAc exhibited worse performance than control.
The negative effect of adding 4% admixture into the mixes with w/c ratio of 0.46 or 0.40 resulted from the higher amount of water used in these mixes, compared with the concretes with w/c ratios of 0.32 and 0.37. Adding 4% ASAc to the concretes with relatively high w/c ratios contributed to increasing the slump value and the consistency of these mixes due to the formation of large amounts of NaOH (Table II), which in turn participated in the increasing air voids and microcracks.59 Another reason for this negative effect is that the interaction between ASAc and cement in the presence of water will most probably contribute to the formation of a hydrophobic organosilicon-based composite (as suggested in Functional Groups of the Protective Material) section. Increasing the amount of water added to the mix with a high dose of ASAc will increase the rate of formation of the hydrophobic content. Formation of large quantities of this hydrophobic content, at early aging times, will contribute to exclusion of excess water from the pores in the concrete, decreasing the quantity of water required to progress the hydration process, which leads to the formation of some microcracks within the structure of the concrete.29,30 As a result, more water will be absorbed into the concrete, at later aging times, for the w/c ratios of 0.40 and 0.46 and treated with 4% admixture. In contrast, the concretes with w/c ratio of 0.32 or 0.37 treated with 4% admixture, or even 2% admixture, showed very high ability to combat water absorption. This could result from the use of a compatible amount of water and admixture, with no excess water present in the mix, and an amount of water suitable to initiate the reaction between ASAc and cement, and at the same time to continue the hydration process.
Concrete Resistance to Chloride Penetration
Figure 4 shows the results of testing the chloride diffusion through the treated and untreated concretes with different w/c ratios. The chloride content was calculated based on the equation39
$$ {\text{CC}} = 3.545*f*\left( {V2 - V1} \right)/m $$
where CC is the chloride content (%), f is the molarity of the silver nitrate solution, V1 is the volume of ammonium thiocyanate solution used in the titration (ml), V2 is the volume of ammonium thiocyanate solution used in the blank titration (ml), and m is the mass of the concrete sample (g).
The results of the unidirectional chloride ponding test show a general reduction in chloride content with increasing depth. The 37/4 K concrete (Fig. 4b) showed the most reduced chloride diffusion among all the mixes through the depth from 10 mm to 50 mm; the chloride content at depths of 20 mm to 50 mm was reduced by more than 90% with respect to the control mix. The 32/4 K mix (Fig. 4a) showed high resistance to chloride diffusion, albeit less than that of the 37/4 K mix (Fig. 4b), with efficacy above 70% in the depth range from 20 mm to 50 mm. On the other hand, adding 4% of admixture to the concrete with w/c ratio of 0.46 (46/4 K) (Fig. 4d) negatively affected the performance, resulting in an increase of the chloride absorption compared with control (7% increase in the depth range from 20 mm to 50 mm). The poor performance of the 46/4 K mix also correlates with the high workability of the mix, as discussed in Water Absorption section. Increasing the w/c ratio contributed to increasing the voids and the size of the pore structure of the concrete, which in turn increased the rate of capillary suction and permeation through the pores.48 In addition, at the beginning of chloride transport from the surface into the concrete, chloride from the saline solution will be amalgamated by the hydration products until a balanced state between free and combined chloride is achieved. This should contribute to decreasing the concentration of chloride with increasing depth.48 However, in the case of the mixes with high w/c ratio and high amount of additive, the additive will reduce the hydration process, preventing such a balanced condition between free and combined chloride from being reached. Moreover, the resistance of concrete to chloride diffusion followed a similar trend to that observed for the water absorption, where long-term testing revealed that it is possible to produce durable concrete when low w/c ratios are used along with the ASAc material.
Compressive Strength Analysis
After 7 days, 14 days, and 28 days of curing, the compressive strength of all the treated and control mixes was determined. The results of this test are shown in Fig. 5. A remarkable negative effect of adding the admixture to all the concrete mixes was noticed at the early aging durations of 7 days and 14 days (Fig. 5a and b). After aging for 7 days, the strength of all the mixes dropped by 3–36% with repsect to their control mix, the lowest drop being noticed for 32/4 K and the highest for 46/4 K. Further strength reduction was observed for all the treated mixes after aging for 14 days compared with their control; a drop of 16–38% in strength was noticed, with 32/2 K being the lowest and 46/4 K the highest. However, after aging for 28 days (Fig. 5c), the treatment increased the compressive strength of the mixes with w/c ratio of 0.32 or 0.37, ranging from 13% to 42%, with the maximum strength gain seen for the 37/4 K mix. On the other hand, mixes with w/c ratio of 0.40 or 0.46 treated with either 2% or 4% admixture continued to lose strength, reaching 32% in the case of 46/4 K.
A negative effect of adding the admixture to all the concrete mixes was noticed after shorter aging periods of 7 days and 14 days (Fig. 5a and b). After aging for 7 days, the strength of all the mixes dropped by 3–36% with respect to their control mix, the least drop being noticed for 32/4 K and the greatest for 46/4 K. Further reduction in strength was seen for all the treated mixes after aging for 14 days compared with their control; a drop of 16–38% in strength was noticed, with 32/2 K being the lowest and 46/4 K the highest. However, after aging for 28 days (Fig. 5c), treatment increased the compressive strength of the mixes with w/c ratio of 0.32 or 0.37, ranging from 13% to 42%, with the maximum strength gain seen for the 37/4 K mix. On the other hand, the mixes with w/c ratio of 0.40 or 0.46 treated with either 2% or 4% admixture continued to lose strength, reaching 32% in the case of 46/4 K.
The strength reduction seen after aging for 7 days and 14 days may correspond to the large amount of water present at that time, especially in the case of the mixes with w/c ratio of 0.40 or 0.46. Adding the admixture to the concrete contributed to increasing the consistency of these mixes, as seen in Table II. This increased level of workability along with the presence of activated hydrophobic crystals may combine to delay the hydration process, resulting in the reduction in the compressive strength at these early aging times. Furthermore, after aging for 28 days, the mixes with relatively low w/c ratios managed to complete the hydration process and achieve higher compressive strength than their corresponding control. These results, when combined with the ISAT results, prove that ASAc participated in forming a denser concrete structure (refer to Morphological Analysis section) with minimum cracks or microcracks, resulting in higher compressive strength than control (in the case of the mixes with w/c ratio of 0.32 or 0.37). Moreover, concrete with high w/c ratio sustained its strength loss after aging for 28 days, since the hydration process was inhibited by the dual effect of adding the admixture and increasing the w/c ratio.