A Study on Mechanical Properties of Porous Concrete Using Cementless Binder

In order to analyze the physical and mechanical properties according to the mixing factors of porous concrete using cementless binder, the target void ratio was selected as 12 and 15 %, the water-binder ratio was set to 22.5 % and the type of binder was changed to OPC and cementless binder. In order to analyze the durability, mixing was carried out with 0.3 M KOH, an alkali activation agent, and the table of mix proportion is as shown in Table 9. Also, in order to improve the dispersibility of binder paste, the split injection method was used for mixing by adding the binder and aggregates first using a 30 l Omni mixer and then mixing them with a mixer for 30 s at 50 rpm and then for 60 s at 200 rpm, and then adding mixing water and mixing them again for 180 s.

The test specimens were produced by mixing the materials in accordance with the porous concrete specimen production methods of the Technical Research Committee on Ecological Concrete in Japan, filling half of each required mold with porous concrete and compacting the concrete (JCI 2003). The test specimens were air-dry cured for 24 h, removed from the molds, and cured with a thermostat-humidistat at 20 °C in accordance with the experimental plan.

Figures 1 and 2 show the results of the SEM images of OPC and cementless binder according to material age. At early ages, a number of new crystalline structures in the mineral kingdom were found in cementless binder rather than in OPC; these were: Gismondine, with the chemical formula CaAl₂Si₂O₈·4[H₂O]; Hatrurite, with the chemical formula Ca₃(SiO₄)O; and Wollastonite, with the chemical formula NaAlSi₃O₈. These structures are believed to have been formed during the process of chemical properties such CaO, Al₂O₃ and SiO₂ contained in BFS, CP, and quicklime that comprise cementless binder, forming an Al₂O₃·SiO₂·6H₂O (ASH₆) hydrate, which is a film about 0.2 μm thick, and which then ruptures upon contact with the OH properties of slaked lime, thereby eluting \( {\text{Si}}_{4}^{ + } \) and \( {\text{Al}}_{3}^{ + } \) ions (Pacheco-Torgal 1991; Brough and Atkinson 2002; Puertas and Fernández-Jiménez 2003; Kumar et al. 2010; Kathirvel 2016). This result is presumably attributable to the fact that, in cementless binder Porous concrete, 2CaO·Al₂O₃·SiO₂, an amorphous substance, undergoes a hydration reaction unlike the hydration reaction of existing OPC. This kind of result is also believed to affect the formation of Ettringite. In particular, OPC’s Ettringite’s chemical formula is C₃A·3CaSO₄·32H₂O, whereas cementless binders Ettringite chemical formula is Ca₆Al₂(SO₄)₃(OH)₁₂(H₂O)₂₆, and its hydration reaction of amorphous substance is believed to have led to this outcome. It takes a longer time for cementless binder to form Ettringite, Ca(OH)₂ and C–S–H gel than it does for OPC, but the formation starts to become more active after the age of 14 days; and, from 28 days, the density of the microstructures and structures reaches a level similar to that of OPC.

Figures 3, 4, 5, 6, and 7 shows the results of the XRD analysis of OPC and cementless binder according to material age. The crystalloids of porous concrete using cementless binders were fewer than those of porous concrete using OPC by approximately 12 %. However, at the age of 28 days, the crystalloids of porous concrete using cementless binders amounted to 62 %, thus increasing by approximately 10 % in comparison with the early stage, and differed quantitatively from those of porous concrete using OPC by approximately 8 %, thus exhibiting outstanding crystalloid generation. In the case of porous concrete using OPC, Ettringite, Ca(OH)₂, and C–S–H gel were actively generated from an early age, and the amounts of these materials generated were confirmed to increase with the passage of time (Juenger et al. 2011; Bakharev 2005). In the case of porous concrete using cementless binders, as with the results of SEM analysis, substances including Gismondine, Hatrurite, and Wollastonite were detected in considerable amounts from an early age, while the amounts of Ettringite, Ca(OH)₂, and C–S–H gel were relatively smaller. With the passage of time, however, the materials generated at first were no longer detected, and the amounts of Ettringite, Ca(OH)₂, and C–S–H gel increased.

Figure 8 shows the result of target void ratio measurements of the porous concrete according to the binder type. In the mix design of porous concrete, the void ratio is designed as the total void ratio (target void ratio) combining the continuous voids and separate voids but continuous void become the most important factor for the main performances of porous concrete such as permeability and the provision of habitation base for living organisms. Therefore, the target void ratio and the actual void ratio (continuous voids) were compared. In the case of porous concrete using cementless binder, void ratio increased toward the target void ratio by approximately 1.12 and 1.42 % in comparison with porous concrete using OPC under all mixing conditions, the maximum difference between the actually measured void ratio and the target void ratio amounted to approximately 1.11 %, thus satisfying the target void ratio; when cementless binder with high viscosity were used, it was easier to attain the target void ratio.

Figure 9 shows the test result of compressive strength of OPC and cementless binder according target void ratio porous concrete. 80 % of cementless binder used in this study consists of BFS and FA. Therefore, the initial strength of cementless binder was lower than the case of using OPC due to the latent hydraulic property of these elements. However, the strength development increased after the aging of 7 days due to improvement of alkali activation and Pozzolanic reaction between BFS and FA. When the compressive strength properties were examined, it was found that as the target void ratio increased, the compressive strength decreased. As the target void ratio increased, strength seems to have decreased due to a decrease in the amount of paste with respect to the amount of unit aggregates. For the same target void ratio, porous concrete using cementless binder tended to decrease in compressive strength in comparison with porous concrete using OPC, but the difference was negligible, at 0.6–1.4 MPa. Meanwhile, under all mixing conditions. The compressive strength of porous concrete was estimated over 15 MPa at all mixing conditions, satisfying 10 MPa which was the specific concrete strength of porous concrete.

The freezing–thawing test result of porous concrete according to the type of binder is as shown in Fig. 10. Relative dynamic elastic modulus of concrete using cementless binder was lower than that of porous concrete using OPC. Especially, the relative dynamic elastic modulus of porous concrete decreased to 60 % or less at over freezing–thawing 80 cycles. The porous concrete with cementless binder which has relatively larger void than the porous concrete with OPC features relatively more free moisture transport and relatively more slow formation of hydration products due to the latent hydraulic property such as BFS and FA included in the binder and Pozzolanic reaction (Fu et al. 2011). Since the strength development is delayed due to such reasons, the result showed somewhat deteriorated freezing–thawing resistance.

Because a large number of continuous voids formed inside the porous concrete, chemical corrosion simultaneously occurred both inside and outside the specimens; chemical resistance was lower in comparison with that of conventional normal concrete. Specimens were produced in accordance with the types of binders, immersed in 5 % HCl and 5 % H₂SO₄ aqueous solutions, and measured to determine their respective mass change rates and surface conditions; the test results are shown in Figs. 11, 12, 13, 14, 15, and 16.

All specimens immersed in the 5 % HCl aqueous solution decreased in mass with the passage of time. In particular, after immersion for 84 days, the mass reduction rate of specimens of porous concrete using cementless binder amounted to 68.53 %, which value is relatively higher than those of specimens of porous concrete using OPC, which had values of 65.46 %. KOH, which is a binder material, engaged in a strong exothermic reaction (-49 kcal/mol) with HCl and generated KCl. Because its solubility in water amounts to 360 g KCl/1 kg H₂O, KCl came to exist in the 5 % HCl aqueous solution in an ionized form. Consequently, there occurred a phenomenon in which KOH hydrates dramatically reacted with HCl, were dissolved, and came to be deleted. Because the surfaces created by the deletion phenomenon exposed CaO to HCl and generated CaCl₂, which is a material with high solubility in water, the deletion phenomenon in the specimens was accelerated. For these reasons, porous concrete using cementless binder exhibited a high mass reduction rate.

Porous concrete using OPC that had been immersed in the 5 % H₂SO₄ aqueous solution continuously decreased in mass with the passage of time, exhibiting a mass reduction rate of 38.87 % after immersion for 84 days. SO₄ ²⁻ ions infiltrated the porous concrete fabricated using OPC, reacted with Ca(OH)₂ generated by the hydration reaction of OPC, and generated CaSO₄·2H₂O (separated gypsum), thus causing volume expansion and leading to their elution. In addition, because SO₄ ²⁻ ions quickly reacted with the C₃A·CaSO₄·12H₂O (monosulfate) generated by hydration and continuously generated C₃A·3CaSO₄·32H₂O (Ettringite), bulging failure occurred (Bakharev et al. 2001). In contrast, porous concrete using cementless binder generated K₂SO₄ because of KOH and H₂SO₄ (Gómez-García et al. 2015). Although this reaction is also a strong exothermic reaction (−49 kcal/mol), unlike KCl, which exists as ions, the materials were generated as solid substances and came to exist within all specimens as expansive hydrates. Consequently, while the specimens’ masses increased for up to 7 days of immersion, as the immersion period continued, the hydrates became less solid and were gradually deleted from the main bodies, thus leading the mass to decrease to 26.74 % of its original value by the age of 84 days. OPC secured slightly higher chemical resistance performance than cementless binder at 5 % HCl solution, but there was no significant difference. Also, the cementless binder secured a relatively higher chemical resistance performance than OPC at 5 % H₂SO₄ solution. Therefore, it is considered that there would be no problem in the chemical resistance performance if the cementless binder is used in future.