Effect of Rheology of Fresh Paste on the Pore Structure and Properties of Pervious Concrete Based on the High Fluidity Alkali-Activated Slag

Abstract

Pervious concrete is made of cementitious materials, coarse aggregate, water and additives, with characteristic macro- and meso-connected pore structure, which enables the acceptable mechanical properties and high water permeability for pavement and road applications. In this study, the effect of rheology of fresh alkali-activated slag paste on the sedimentation of paste on the bottom of pervious concrete, meso-structure, connected porosity, mechanical properties and water permeability was investigated by a range of analytical techniques through varying the equivalent alkali content to control the rheology of fresh paste in the pervious concrete. The compressive strength of pervious concrete was related to the percentage area of paste and the average thickness of paste on the surface of coarse aggregate. The tensile strength and water permeability were correlated to the connected porosity of pervious concrete and the rheology of fresh paste. A relative lower fluidity, higher viscosity and shear stress of fresh alkali-activated slag paste favoured lower sedimentation of paste on the bottom of pervious concrete, higher connected porosity, tensile strength and water permeability. There was no correlation between compressive strength and tensile strength of pervious concrete.

1. Introduction

Pervious concrete is an eco-friendly construction material that consists of cementitious materials, coarse aggregate, water and additives [1]. The characteristic connected macropore structure of pervious concrete enables the high water permeability and acceptable mechanical properties as pavement materials, which are widely used in car parks, gardens and sometimes roads [2,3]. The porosity of pervious concrete is usually between 15% and 35%, with a water permeability of 2–70 mm·s⁻¹, compressive strength of 5–30 MPa and tensile strength of 2–5 MPa [4,5,6,7,8,9,10]. Pervious concrete could also immobilize heavy metals in the water, absorb noise and isolate heat, according to previous studies [11,12,13,14,15].

Alkali-activated materials are made of alkali-activators and aluminosilicate materials as precursors, which react to form Si–O–Al bonding in the materials [16,17,18,19,20]. The most common alkali activators include sodium hydroxide, sodium silicate, sodium sulphate and their potassium counterparts. The aluminosilicate precursors are usually natural minerals or industrial by-products, such as calcined clay, metakaolin, fly ash, bottom ash, slag and rice husk ash. Due to the industrial by-products’ nature as the precursors of alkali-activated materials, pervious concrete is usually treated as an eco-friendly construction materials that could potentially partially or even fully replace Portland cement in the future [21]. High early and late age strength, low creep and shrinkage and high sulphate resistance are the characteristic advantages of alkali-activated materials [20,22,23,24,25,26]. There are also disadvantages associated with the alkali-activated materials, such as efflorescence during the service life and higher risk in handling alkaline solutions during preparation.

The fluidity, viscosity of the fresh paste, setting time and mechanical properties of alkali-activated materials were affected by the equivalent alkali content, modulus of water glass and water:binder ratio [19,27,28,29,30,31,32]. A previous study on the equivalent alkali content and modulus of activator on the properties of fresh paste suggested that the fluidity changed with the increase in equivalent alkali content [27]. Another research on the relationship between the fresh and hardened properties and the equivalent alkali content suggested that the fluidity and compressive strength increased with the equivalent alkali content [33]. The differences of rheology between various cementitious materials were also investigated and the results indicated that the shear stress of water glass-activated slag was higher than that of Portland cement and sodium hydroxide-activated slag, which could be related to the formation of C–S–H in the former [30].

Preparing pervious concrete with alkali-activated materials has been investigated previously [13], and the mechanical properties, water permeability, durability and heavy metal removal efficiency of alkali-activated fly ash-based pervious concrete have been reported. There are also researches focused on the macro- and mesopore structures of pervious concrete [34], which is vital for the properties. The macropores are in the size of centimetres and the mesopores are in the size of millimetres. The macro- and mesopore structure of pervious concrete defines the mechanical properties and water permeability. Due to the nature of mesostructure of pervious concrete, where layers of cementitious materials cover the surface of coarse aggregate [34], the rheology properties of cementitious materials are important for the pervious concrete as they control the homogeneity, thickness of paste on the surface of aggregate and the connected pore structure, which in turn affects the mechanical properties and water permeability of pervious concrete [4,5,8,15]. However, there are few literatures on the effect of fluidity and rheology of alkali-activated materials on the mesostructure of pervious concrete, particularly the properties of it.

In this study, aiming to utilize the industrial by-product so as to reduce the energy demand of Portland cement production and save resources, pervious concrete was designed and prepared by using alkali-activated slag as cementitious materials, and the effect of equivalent alkali content (as Na₂O content) of paste in pervious concrete on the rheology of paste, sedimentation of paste on the bottom of pervious concrete, mesostructures, mechanical properties and water permeability of pervious concrete were investigated by a range of analytical techniques. The relationship between the rheology of paste and the sedimentation of paste on the bottom of pervious concrete, the mesostructure and the thickness distribution of paste were discussed. The aim of this study is to shed some light on the relationship between the rheology of paste and the meso- and macrostructure of pervious concrete, particularly the mechanical properties and water permeability.

2. Materials and Methods

2.1. Materials

Ground granulated blast furnace slag of grade S95 from Huaxin Cement Co., Yichang, China, was used as precursor in this study. The chemical composition and particle size distribution are shown in

Table 1. Chemical composition of slag.

Oxide	CaO	Si2O	Al2O3	MgO	SO3	TiO2	Fe2O3	Na2O	L.O.I. *
wt%	38.7	35.5	14.8	6.7	2.2	0.79	0.27	0.26	0.88

* L.O.I.: loss on ignition.

and Figure 1, respectively. The particle size distribution of slag was characterized by Malvern Mastersizer 2000 laser diffraction particle size analyser (Malvern Panalytical, Malvern, UK) through dispersing slag particles in water.

The particle size distribution of slag is similar to that of Portland cement. Chemical and physical properties of water glass used are shown in

Table 2. Chemical and physical properties of water glass.

Modulus	Solid Content	Na2O Content	SiO Content	Density	Transparency	Fe Content
2.2	53 wt%	24.5 wt%	53.9 wt%	1495 kg/m3	88%	0.08%

, which were provided by the supplier. Industrial grade sodium hydroxide with purity of 99% from Yihua Chemicals Co., Xining, China, was used in this study. AR grade calcium gluconate from Sinopharm, Shanghai, China, was used as retarder in this study. Limestone coarse aggregate in the size of 4.75–9.5 mm from Wuhan, China, was used in this study (Figure 2), and the properties are shown in

Table 3. Properties of coarse aggregate.

Type	Bulk Density	Apparent Density	Crushing Value	Water Absorption	Moisture Content
Limestone	1560.5 kg/m3	2680 kg/m3	4.57%	3.2%	0.3%

, which were tested according to Chinese standard GB/T 14685-2011 [35].

2.2. Experimental Details

2.2.1. Specimen Preparation

The mix design of pervious concrete with various equivalent alkali content is shown in

Table 4. Mix design of pervious concrete (kg/m³).

Mix	Slag	Equivalent Na2O Content	NaOH	Water Glass	Retarder	Water	Aggregate	DVR §
A4	305	4%	5.01	49.11	0.3	68.42	1529	22%
A6	305	6%	7.51	73.66	0.3	56.88	1529	21.5%
A8	305	8%	10.02	98.22	0.3	45.34	1529	21%
A10	305	10%	12.52	122.77	0.3	33.80	1529	20.4%

^§ DVR: designed void ratio.

. The water:slag ratio was set to 0.3. The method of mix design of pervious concrete followed the previous study by Sun [36]. The microstructure of pervious concrete was separated into various components, from which the volumes of components were calculated, as shown in Figure 3.

The mix design of pervious concrete in this study can be calculated by the following equation according to the method suggested above:

$$\frac{M_G}{{\rho }_G}+\frac{M_{Sg}}{{\rho }_{Sg}}+\frac{M_{Wg}}{{\rho }_{Wg}}+\frac{M_{NaOH}+M_W}{{\rho }_{NaOH,liquid}}+V_{Void}=1,$$

(1)

where M_G, M_Sg, M_Wg, M_NaOH and M_W are the weight of coarse aggregate, slag, water glass, sodium hydroxide and water per cubic meter of pervious concrete (kg), respectively; ρ_G, ρ_Sg, ρ_Wg and ρ_NaOH,liquid are the apparent density of aggregate and slag, density of water glass and sodium hydroxide solution, respectively; and V_Void is the designed void ratio (%).

The activator was prepared by thoroughly mixing water glass, water, sodium hydroxide and retarder. A horizontal concrete mixer was used to prepare fresh concrete. During the preparation of pervious concrete, the coarse aggregate and slag were dry mixed at revolution speed of 62 ± 5 rpm and rotation speed of 140 ± 5 rpm for 1 min before the activator was added and mixed at revolution speed of 125 ± 10 rpm and rotation speed of 285 ± 10 rpm for another 2 min. Then, the pervious concrete was transferred into moulds sized 100 mm × 100 mm × 100 mm for compressive strength test and water permeability test, and 100 mm × 100 mm × 400 mm for tensile strength test, sealed with polyethylene film and stored in the laboratory at 20 ± 2 °C before being demoulded after 1 day of curing. The demoulded specimens were transferred to a curing room to be cured under standard curing conditions at a temperature of 20 ± 2 °C and relative humidity of more than 95% RH. The preparation of tensile strength test specimen was shown in Figure 4.

A laboratory cement mixer was used to prepare fresh paste. Pastes of the same mix design as those in Table 4 but without coarse aggregate were also prepared by mixing slag with activator at revolution speed of 125 ± 10 rpm and rotation speed of 285 ± 10 rpm for 2 min and transferring the paste into the moulds sized 40 mm × 40 mm × 160 mm for compressive and tensile strength tests. The specimens in the moulds were sealed with polyethylene film and stored in the laboratory under 20 ± 2 °C before being demoulded after 1 day of curing. The demoulded specimens were transferred to a curing room to be cured under standard curing conditions at a temperature of 20 ± 2 °C and relative humidity of more than 95% RH.

2.2.2. Analytical Techniques

The fluidity of fresh pastes was measured according to Chinese standard GB/T 2419–2005 [37] and the method used by Puertas [30]. Truncated cone for mortar fluidity test was used to measure the fluidity of alkali-activated slag in this study. Firstly, a cone sized 70 mm in inner diameter on top, 100 mm in inner diameter on bottom and 50 mm in height was placed on the even glass surface. Secondly, fresh paste was filled in the cone immediately after mixing. Thirdly, the cone was quickly and steadily lifted to let the paste flow on the glass surface. Finally, the diameters of two cross directions of paste were measured and the average was calculated as the fluidity of fresh paste.

The viscosity and shear stress of fresh pastes were evaluated by a Brookfield R/S-SST rotary viscosimeter with a V20×10 vane spindle rotor from Brookfield, Middleboro, MA, USA. The pastes were firstly pre-sheared under shear speed of 15 s⁻¹ for 30 s, and then the measurement of rheological properties was performed as follows. The shear speed was gradually increased from 0 s⁻¹ to 100 s⁻¹ in 60 s, and then the shear speed was gradually decreased from 100 s⁻¹ to 0 s⁻¹ in 60 s. The viscosity and shear stress were analysed by Rheo2000 software (Version 2.8, Brookfield, Middleboro, MA, USA). The shear history of paste for rheology test is shown in Figure 5.

The sedimentation of paste on the bottom of pervious concrete sized 100 mm × 100 mm × 100 mm was evaluated by calculating the percentage of area of paste on the bottom of pervious concrete after scanning and binarizing the images on the bottom of pervious concrete as shown in Figure 6.

The mesostructure of pervious concrete was evaluated by vertically cutting the pervious concrete sized 100 mm × 100 mm × 100 mm (Figure 7). Then the images of cut surfaces were taken by optical microscope and analysed in ImageJ and Matlab to obtain the images of aggregate, paste and pore distribution.

The thickness distribution of paste on the top and bottom of pervious concrete were analysed by horizontally cutting the pervious concrete at one-third and two-thirds of the height (Figure 7). Then, the thickness of paste on the coarse aggregate was measured by hand on the top and bottom cut surfaces. The methodology of measurement was the same as that reported by Torres [38]. Approximately 300 points were measured for each surface. The average thickness of each surface was calculated, and the thickness distribution was analysed.

The total porosity and connect porosity of pervious concrete were measured according to the methodology suggested by Jang [34] and multiple standards [39,40] and reports [41,42], where the total porosity, i.e., all the porosity in the pervious concrete, was suggested to correlate to the strength of pervious concrete, and the connect porosity, i.e., the pores with at least one open to the surface of pervious concrete, was suggested to correlate to the permeability of pervious concrete. Presumably, a pervious concrete with higher connect porosity could result in higher water permeability. The total porosity and connect porosity were calculated as follows:

$$V_{connect}=\left[1-\frac{\left(W_2-W_1\right){\rho }_W}{V_1}\right]\times 100\%,$$

(2)

$$V_{total}=\left[1-\frac{\left(W_3-W_1\right){\rho }_W}{V_1}\right]\times 100\%,$$

(3)

where W₁ is the weight of specimens in water, W₂ is the weight of specimens after being dried in air for 24 h, W₃ is the weight of specimens after being dried under 105 °C for 24 h, V₁ is the volume of specimens and ρ_w is the density of water.

The compressive and tensile strength tests of pervious concrete were performed on the specimens cured under standard curing conditions for 28 days, according to Chinese standard JTG E30-2005 [43]. Specimens sized 100 mm × 100 mm × 100 mm were used for the compressive strength test, and those sized 100 mm × 100 mm × 400 mm were used for the tensile strength test. Three specimens were tested for each batch. The compressive strength and tensile strength were calculated by the average of three specimens.

According to Chinese standard JTG E30-2005, during the compressive strength test, the specimens were placed in the middle of the loading part of testing machine. The loading rate was set to 0.3–0.5 MPa·s⁻¹. Then, the force at the point of broken of specimen was recorded and the compressive strength was calculated as follows for standard-sized 150 mm cubic specimens:

$$f_{cu}=\frac{F}{A},$$

(4)

where f_cu is the compressive strength (MPa), F the force at the point of broken (N) and A the cross section area of specimen (mm²).

For nonstandard-sized 100 mm cubic specimens, the compressive strength calculated from Equation (3) should be timed at 0.95 as the compressive strength of pervious concrete.

For the tensile strength test, the specimens were bended with three-point bending equipment and the setup is shown in Figure 8. The specimens were bended with a loading rate of 0.02–0.5 MPa·s⁻¹. Then, the force at the point of broken of specimen was recorded and the tensile strength was calculated by Equation (4) for standard-sized 150 mm × 150 mm × 600 mm specimens.

$$f_t=\frac{FL}{b{h}^2},$$

(5)

where f_t is the tensile strength (MPa), F is the force at the point of broken (N), L is the distance between the unmovable and moving supports (mm), b is the width of specimen (mm) and h is the height of specimen (mm).

For nonstandard-sized 100 mm × 100 mm × 400 mm specimens, the tensile strength calculated from Equation (4) should be timed at 0.85 as the tensile strength of pervious concrete.

Specimens sized 100 mm × 100 mm × 100 mm at 28 days were used for the water permeability test. The diagram of water permeability test is shown in Figure 9. In this study, two types of water head, namely constant head and falling head, were used to evaluate the water permeability of pervious concrete. The detailed methodology is outlined in Ref. [44].

In this study, the constant and falling head were set to 160 mm. The water permeability of pervious concrete under constant head was calculated as follows:

$$K=\frac{L}{h}\times \frac{Q}{A\left(t_2-t_1\right)},$$

(6)

where K (mm·s⁻¹) is the water permeability of pervious concrete under constant head, L (mm) is the length of specimen, Q (mm³) is the weight of water collected from the outlet during time of t₂−t₁, h (mm) is the water head, t₂−t₁ (s) is the time of test and A (mm²) is the area of top surface of specimen. In this study, t₂−t₁ was set to 30 s.

The water permeability of pervious concrete under falling head was calculated as follows:

$$K=\frac{A_{tube}\times L}{A\times t}\times \ln \left(\frac{h_1}{h_2}\right),$$

(7)

where K (mm·s⁻¹) is the water permeability of pervious concrete under falling head, L (mm) is the length of specimen, A_tube (mm²) is the area of cross section of tube on top of specimen, A (mm²) is the area of cross section of specimen and t (s) is the time duration from the water head of h₁ (mm) to h₂ (mm). In this study, h₁ and h₂ were set to 160 mm and 50 mm, respectively.

3. Results and Discussion

3.1. Workability of Fresh Paste

The effect of the amount of water glass on the workability of fresh paste in this study is shown in Figure 10. The fluidity gradually increased with the amount of water glass (as Na₂O content) from 214 mm with 4 wt% Na₂O to 338 mm with 8 wt% Na₂O, then slightly decreased to 322 mm with 10 wt% Na₂O (Figure 10a). The increase in fluidity with the water glass is due to the adsorption of silicate anions from sodium silicate on the surface of slag particles, resulting in the formation of an electric double layer separating slag particles with repulsive forces [45,46]. With the addition of 10 wt% Na₂O, the pH of the liquid phase increased and became supersaturated, resulting in the formation of C–S–H gel, which in turn reduced the fluidity of the paste [45].

The development of apparent viscosity and shear stress of paste with the amount of sodium silicate addition was due to the similar reason (Figure 10b,c). The viscosity and shear stress gradually decreased with the amount of sodium silicate due to the formation of an electric double layer between the slag particles, and then decreased with the addition of 10 wt% Na₂O due to the supersaturation of liquid phase and formation of C–S–H gel [47]. The decrease in viscosity with the increase in water glass content could also be due to the lower solid content, resulting from the higher degree of dissolution of slag with higher content of activator [48]. The sharp change at the beginning of the curve of 6 wt% and 10 wt% composition could be due to the delay during the experiments when setting the shear rate from 100 to 0 s⁻¹ at the end of 0 to 100 s⁻¹.

3.2. Sedimentation of Paste

The fresh paste on the surface of aggregate in the pervious concrete could flow down to the bottom of the specimen due to the gravity. With a suitable rheology of fresh paste, the distribution of paste in the pervious concrete could reach homogeneity, which in turn results in the ideal mechanical properties and water permeability. The fluidity, viscosity and shear stress of fresh paste were vital to control the ideal and homogenous mechanical properties and water permeability of pervious concrete. The ideal fresh paste should form a homogenous thin layer of paste on the coarse aggregate, resulting in acceptable mechanical properties and high water permeability.

To evaluate the effect of water glass amount on the sedimentation of fresh paste in pervious concrete, the area percentage of paste and aggregate on the bottom of pervious concrete with the same amount of fresh paste was measured to identify the suitable water glass content for the pervious concrete. The optical and binary images of bottom of pervious concrete with different water glass content (as Na₂O content) are shown in Figure 11, and the area percentage of paste and aggregate on the bottom of pervious concrete prepared with different water glass content is shown in Figure 12.

The optical and respective binary images of the bottom of pervious concrete with different water glass content are shown in Figure 11. It seems that both the pores and paste were well connected in the bottom of the pervious concrete with 4 wt% Na₂O content (Figure 11a,e). In addition, the sedimentation of fresh paste on the bottom of pervious concrete increased with the Na₂O content (Figure 11f–h and Figure 12). The area percentage of paste and aggregate increased with the water glass content, from 58% for 4 wt% of Na₂O to 80% for 10 wt% of Na₂O (Figure 12). It seems that the degree of sedimentation is not directly related to the fluidity or viscosity but could be related to yield stress of paste. With a higher fluidity, the fresh paste is easier to flow through the space between coarse aggregates and fully cover the surface of coarse aggregate, and with a higher yield stress, the paste does not flow by gravity to the bottom of pervious concrete to result in a higher degree of sedimentation. The yield stress of fresh alkali-activated slag paste depends on the suspension composition, such as the composition of activator, the content of activator, the particle shape and grading [49,50,51,52,53]. Fresh water glass-activated slag paste can be described as Herschel–Bulkley liquid instead of Bingham liquid for fresh Portland cement paste [30]. A lower content of slag in the fresh paste could result in a lower yield stress [48], and the fresh paste with 10 wt% Na₂O equivalent content of water glass has the lowest slag/activator ratio, which in turn resulted in the highest sedimentation.

3.3. Mesostructure

The typical mesostructure of pervious concrete with 8 wt% Na₂O as water glass content is shown in Figure 13. The specimens of pervious concrete were vertically cut and the percentages of aggregate, paste and pore were characterized by analysing the grey scale of image of surface.

The percent areas of aggregate, paste and pore are shown in Figure 14a. The areas of aggregate were similar for all pervious concrete with different water glass content, which agrees well with the designed aggregate content, indicating that the aggregate was evenly distributed. The percent areas of paste slightly increased and those of pores decreased with the increase in water glass content in the pervious concrete. The trend of pore areas agrees with the designed void ratio, which slightly decreased with the increase in water glass content, although the calculated pore areas were higher than the designed void ratio, possibly due to the wall effect where the specimens contacted the mould, resulting in the increase in pore volume.

The distribution of paste on the top, middle and bottom of vertically cut surface of pervious concrete was evaluated (Figure 14b). With the increase in water glass content, so as to the fluidity according to Figure 10a, firstly the middle areas were the highest percent areas and increased with the water glass content (4 wt% and 6 wt% Na₂O content), then the bottom area was the highest percent area of paste due to the increased fluidity, resulting in more sedimentation (8 wt% Na₂O content) as seen in Figure 10 and Figure 12. For 10 wt% Na₂O content, the percent areas of paste on the top and middle were similar and that on the bottom was slightly lower because of the decrease in fluidity and increase in viscosity and shear stress of fresh paste according to the results from rheology of paste.

3.4. Thickness of Paste on the Surface of Coarse Aggregate

The thickness of paste on the surface of coarse aggregate in the pervious concrete was characterized to evaluate the homogeneity of paste and aggregate distribution. For each specimen, the thickness of paste on the top and bottom sections of pervious concrete was measured. A total of more than 300 points were measured for each specimen. According to the results in

Table 5. Thickness of paste on the surface of aggregate.

	Part	N	Average Thickness (mm)	Thickness Distribution
				0.2–0.6 mm	0.6–1 mm	1–1.4 mm	1.4–1.8 mm	1.8–2.2 mm	2.2–2.6 mm	2.6–3 mm	3–5 mm	5–9 mm
A4	Up	144	0.919	15%	19%	22%	14%	11%	4%	4%	1%	3%
	Dn	212	0.929	16%	20%	19%	18%	7%	10%	3%	2%	2%
A6	Up	120	0.941	3%	27%	20%	22%	8%	3%	3%	5%	0%
	Dn	182	1.003	8%	20%	15%	29%	11%	7%	3%	7%	0%
A8	Up	158	1.123	8%	22%	6%	22%	9%	8%	6%	15%	0%
	Dn	174	1.151	15%	29%	11%	13%	8%	6%	5%	9%	0%
A10	Up	132	1.089	15%	17%	21%	9%	6%	8%	6%	11%	3%
	Dn	172	0.965	10%	27%	20%	15%	10%	5%	2%	7%	1%

Note: Up and Dn: top and bottom of the specimens; N: number of thickness measured; Thickness distribution: percentage of points measured with thickness in the ranges.

, for each mix, the average thickness of paste on the top and bottom sections was similar, with a slight increase in thickness of paste on the bottom section, which agrees well with the results from sedimentation and mesostructure analysis. The slight increase in thickness of paste could be due to the gravity which caused the flow of paste from top to bottom. These results also indicated the favoured rheology of paste, which improved the homogeneity of the thickness of paste.

The average thickness of paste firstly increased with the water glass content and reached the maximum of approximately 1.1 mm at 8 wt% Na₂O content, and then decreased slightly at 10 wt% Na₂O content (Figure 15). For all mixes, the thickness of paste on the coarse aggregate ranged from 0.919 mm to 1.151 mm, indicating the similar average thickness of paste on the surface of aggregate. A previous study by Jang indicated that the average paste thickness on the surface of coarse aggregate was in the range of 1.08–1.29 mm for pervious concrete prepared with Portland cement and 0.63–4.31 mm for those with geopolymer by analysis under optical microscope under 35× magnification [34]. The results obtained in this study had a narrow range of distribution for paste thickness.

Although there was no big difference between the average thickness of paste between the mixes with the various Na₂O content, the results of thickness distribution seem to reveal more information on the homogeneity of paste distribution. It seems that the mix with 6 wt% Na₂O content had better homogeneity of thickness of paste than other mixes. There are more points with thickness of paste close to the average thickness and less thick or thinner paste in the mix with 6 wt% Na₂O content. For the mix with 4 wt% Na₂O content, there were 15% and 16% of points measured with much thinner thickness (0.2–0.6 mm) on the top and bottom of the specimen, respectively. For the mixes with 8 wt% and 10 wt% Na₂O content, there were more thinner (0.2–0.6 mm thickness) and thicker (3–5 mm thickness) layers of paste on the surface of coarse aggregate in the top and bottom of specimens. These results indicate that the homogeneity of paste in the pervious concrete is complex and controlled by many factors, such as rheology of paste. There was no clear relationship between the fluidity and homogeneity of paste in the pervious concrete.

3.5. Strength and Water Permeability

The compressive strength and tensile strength of alkali-activated slag prepared with different water glass content (as alkali equivalent) at 28 days are shown in Figure 16. The paste with 8 wt% Na₂O content had the highest compressive strength at 28 days, followed by those with 4 wt%, 6 wt% and 10 wt% Na₂O content. The tensile strength decreased with the increase in Na₂O content. The reason could be due to the excess amount of SiO₂ from the overdosed water glass at high water glass content. The mechanical properties of alkali-activated slag could reach the maximum when the optimum Si/Al was satisfied [54]. Either higher or lower Si/Al could result in lower mechanical properties. Another reason could be that the high water glass content resulted in the high OH⁻ concentration in the paste, which in turn resulted in the thicker layer of calcium hydroxide formation on the surface of slag particles, preventing further resolution of slag particles and the formation of more hydration products [32,55,56]. These two reasons resulted in the decrease in mechanical properties of alkali-activated slag with higher equivalent Na₂O content.

The compressive and tensile strength of pervious concrete prepared with pastes with different equivalent Na₂O contents are shown in Figure 17. The trend of compressive strength of pervious concrete at 28 days was similar to that of paste. The pervious concrete prepared with paste of 4 wt% equivalent Na₂O content had the highest compressive strength among the specimens studied, which is followed by those of 6 wt%, 8 wt% and 10 wt% equivalent Na₂O content. For the tensile strength of pervious concrete with 4 wt% Na₂O, the specimens showed similar tensile strength of approximately 3.5 MPa at 28 days. The mechanical properties of pervious concrete were controlled not only by the properties of cementitious materials but also by the pore structures, especially macro- and mesopore structures. There was correlation between the mechanical properties of paste and that of pervious concrete. It is interesting that there is no correlation between the compressive strength and the tensile strength of either paste or pervious concrete, indicating a different mechanism behind the mechanical properties between pervious concrete and normal concrete. The relationship between the thickness distribution of paste on the coarse aggregate and the mechanical properties of pervious concrete warrant further study.

For normal cement-based pervious concrete, the porosity of pervious concrete greatly affects the mechanical properties, and the other differences in properties/components also result in the variation of mechanical properties, such as the type and size of aggregate, the type and content of cement and admixtures, the formula of cementitious materials and the water/cement ratio. Generally, for cement-based pervious concrete, the permeability is 0.2–7.0 cm/s, the porosity is 15–35%, and the compressive strength is 5–30 MPa [1,2,6,7,8,9].

The DVR, total porosity and connect porosity of pervious concrete at 28 days are shown in Figure 18. There are similar trends for the DVR and total porosity measured, with a slightly higher porosity for the latter. The total porosity decreased with the increase in equivalent Na₂O content of paste. It is interesting that the trend of total porosity is similar to that of compressive strength of pervious concrete. Usually, the higher porosity indicates lower compressive strength for concrete. These results suggested that the relationship between the compressive strength, total porosity and thickness distribution of paste on the surface of coarse aggregate is more complex than expected. The total porosity from other previous studies ranges from 31% to 45%, with the compressive strength of 1.1–6.45 MPa [55,56]. The pervious concrete in this study had lower total porosity but much higher compressive strength. As suggested by Barnhous and Srubar III [55], the permeability could be sharply increased with porosity when the porosity is over 30% but together with a decrease in compressive strength. The design of pervious concrete is basically a balance between the compressive/tensile strength and permeability.

The connect porosity of pervious concrete had a similar trend of total porosity, except that the connect porosity of pervious concrete with paste of 10 wt% Na₂O content was slightly higher than that of 8 wt% Na₂O content. The connect porosity is a defining factor for the water permeability of pervious concrete. According to the water permeability results of pervious concrete in Figure 19, the water permeability of pervious concrete under both falling head and constant head decreased with the increase in equivalent Na₂O content of paste, which agrees well with the trend of connect porosity of pervious concrete. These results are reasonable because water can only permeate through the pervious concrete by the connected pore, which was characterized as connect porosity. There is a correlation between the connected porosity, water permeability and tensile strength of pervious concrete, which all decreased with the increase in equivalent alkali content by up to 8 wt% Na₂O and slightly decreased when Na₂O content reached 10 wt%. A previous study [34] on the porosity of pervious concrete prepared with geopolymer indicated that the pervious concrete with a total porosity of 24.1–40.2% had connect porosity of 18.4–34.1%, together with a lower compressive strength compared with this study. A higher total porosity usually results in a lower compressive strength for concrete.

The intrinsic permeability based on Darcy’s law can be descripted as the following equations:

$$Q=-KA\frac{\Delta h}{L},$$

(8)

$$K=k\frac{\rho g}{\eta },$$

(9)

where Q is the total discharge rate (m³/s); K is the hydraulic conductivity (m/s); A is the area of cross section of the flume (m²), which is the cross section area of specimen of pervious concrete in the manuscript; Δh is the height difference (m), which is the water head in the manuscript; L is the length of specimen of pervious concrete in the manuscript (m); k is the intrinsic permeability (m²); ρ is the density of fluid (kg/m³); g is the gravity (m/s²); η is the dynamic viscosity of the fluid (Pa·s).

The term “water permeability” used in this study is hydraulic conductivity in Darcy’s law, which is used to descript how easily the water can move through the connected pores in the pervious concrete. The intrinsic permeability can be calculated from hydraulic conductivity according to Equation (9). Considering that only water permeates through pervious concrete under normal atmospheric pressure and the temperature for the test was 20 °C, the hydraulic conductivity is usually used to describe the permeability of pervious concrete, which is termed as water permeability in the field of cement and concrete research. The relationship between these two is as follows:

$$K=9.75\times {10}^6\cdot k$$

(10)

4. Conclusions

In this study, pervious concrete was prepared with high fluidity alkali-activated slag of various alkali equivalent. The effect of alkali equivalent on the fluidity and sedimentation of paste, pore structure, mechanical properties and water permeability of pervious concrete was characterized by a range of analytical techniques. According to the results obtained, the following conclusions can be drawn.

The fluidity of water glass-activated slag increased with the equivalent alkali content by up to 8 wt% Na₂O content and slightly decreased by 10 wt% Na₂O content, while the effect of equivalent alkali content on the viscosity and shear stress of fresh pastes was more complex, where the viscosity and shear stress firstly decreased with the increase in equivalent alkali content and then increased when equivalent alkali content increased to 10 wt% Na₂O content under a shear rate of 20–80 s⁻¹. With a shear rate over 80 s⁻¹, the viscosity and shear stress drastically decreased for paste with 4 wt% Na₂O content.

The sedimentation of fresh paste on the bottom of pervious concrete was related to the rheology of fresh paste, which had a similar trend to that of fluidity. A higher fluidity and lower viscosity and shear stress will result in a more severe sedimentation of fresh paste on the bottom of pervious concrete, which worsens the homogeneity of mesostructure of pervious concrete, resulting in lower tensile strength and water permeability.

The percentage area of paste and the average thickness of paste on the surface of coarse aggregate had a similar trend to that of fluidity, which increased with the equivalent alkali content and slightly decreased when the Na₂O content reached 10 wt%. There was a relationship between the compressive strength with the percentage area of paste or the average thickness of paste on the surface of coarse aggregate, but there was no obvious relationship for the tensile strength with the latter.

The water permeability of pervious concrete was governed by the connected porosity, which had good correlation with the tensile strength. A relative lower fluidity, higher viscosity and shear stress of fresh alkali-activated slag paste favoured lower sedimentation of paste on the bottom of pervious concrete, higher connected porosity, tensile strength and water permeability.

The findings in this study are potentially beneficial for the optimized design of pervious concrete prepared with alkali-activated materials, especially in the application of pavement, where the tensile strength is more important than compressive strength.