Production Process of an Eco-Friendly One-Part Alkali-Activated Binder

Abstract

One-part alkali-activated binder (AAB) is a product with lower environmental impact than Portland cement, and can be a feasible commercial alternative. The main objective of this study was to propose and test a method of obtaining one-part AAB with metakaolin and an alternative powder sodium silicate from rice husk ash (RHA). To evaluate the AAB properties, mortars were produced with different SiO₂/Al₂O₃ molar ratios (2.7, 3.1, and 3.3), and tests were conducted to characterize the binder. The results indicated that the process was efficient in generating a non-crystalline sodium silicate powdered material. With high SiO₂/Al₂O₃ molar ratio the mortars achieved high compressive strength and it increased with age, had low permeability, and capillary water absorption. Compressive strength results varied from 6 MPa to 12 MPa at 28 days, indicating the possibility of using the proposed method for one-part AAB production. AAB produced can be applied in different building materials.

Keywords
: One-part alkali-activated binder ; production process; cement; alternative binder; cleaner production; sustainability

1. Introduction

Alkali-activated binder (AAB) can be considered one of the breakthroughs in concrete technology of the last century¹. AAB is an alternative to Portland cement (PC), and it is formed by a chemical reaction between an alkali-activator with an aluminosilicate precursor².

The environmental benefits of AAB have driven the interest in this cement. AAB production does not demand high calcination temperatures and the chemical reaction does not release CO₂^3,4. According to Carreño-Gallardo et al.⁵, the replacement of PC by AAB can provide a reduction of up to 1480 million tons of CO₂ per year. Accordingly, the reduction of CO₂ emissions when compared to PC is one of the main facts that direct the researchers to pay attention to AAB⁶.

There are two methods to AAB production. The two-part mixing is more usual in the literature, but the one-part mixing is a recent proposal of growing interest because of the commercialization feasibility and facility of using^7,8. In two-part mixing, the aluminosilicate precursor is a solid material, and the alkali-activator is an aqueous solution. These two parts are mixed to form the AAB. Differently, in one-part AAB the aluminosilicate source and the alkali activator are powders. One-part AAB is used in the same way of conventional PC: the dry materials are mixed with water, starting the binding reaction^7-10.

The discovery of one-part AAB was a significant event in the development of eco-friendly cement¹¹. The one-part AAB does not need storage, management, and use of highly corrosive alkali-solutions in the worksite, which are very dangerous to the workers involved. This favors the commercialization and application of AAB in civil construction activities^1,12,13.

One-part AAB made with anhydrous sodium metasilicate as activator presented high compressive strength; however, the production of this chemical product has environmental impacts^12,14,15. The extra energy necessary to produce the solid activators makes one-part AAB more environmentally impacting than a two-part binder¹⁶.

Within this scenario, researches have been conducted on alternative activators. Abdel-Gawwad et al.¹⁷ proposed a new user-friendly activator powder obtained by thermal treatment at 60 °C of a mixture of sodium hydroxide and ethylene glycol. Granulated blast-furnace slag or slag-lead bearing sludge was mixed with the activator to originate one-part AAB. Pastes achieved considerable mechanical strength, with one-part AAB presenting a compressive strength 157% higher than two-part AAB at 180 days. In another research, Ma et al.¹⁵ evaluated the partial replacement of sodium silicate anhydrous by sodium sulfate, aiming to achieve a cleaner one-part AAB. They found the use of sodium sulfate generated material with compressive strength higher than 15 MPa at 1 day and reduced the carbon footprint of AAB. Samarakoon et al.¹⁸ obtained an alternative activator to commercial sodium silicate by mixing soda-lime glass powder and sodium hydroxide. They tested the solid activator with fly ash/ground granulated blast-furnace slag precursors in mortar production, and the results indicated compressive strength higher than 30 MPa at 28 days. The alternative solid activator showed to be cheaper and more environmentally friendly than commercial options.

The use of agro-industrial wastes, such as rice husk ash (RHA) due to the high volume generated and high reactive silica content, has a growing interest recently^19-21. RHA has been applied in civil construction as supplementary cementitious material, and it has also been used to obtain alternative sodium silicate solutions to AAB production^19,22-32. Commercial sodium silicate has a high carbon footprint and the use of alternative dissolved silica sources is important in environmental aspects²¹. The activators are considered as the main problem related to AAB production because they are produced using expensive and polluting processes³³.

In this study, an alternative sodium silicate powder from RHA was proposed to make one-part AAB. The research aimed to test an innovative production method to produce a binder with cleaner production.

2. Materials and Methods

2.1. Materials

In the present investigation, it was used the precursor materials to make the AAB (metakaolin – MK, rice husk ash – RHA, and sodium hydroxide – NaOH) and natural river quartz sand to make the mortar to evaluate the AAB properties to building components.

Metakaolin (MK = “HP ultra” from Metacaulim do Brasil Indústria e Comércio Ltd.) and rice husk ash (RHA = “Silcca Nobre” from Sílica Verde do Arroz Ltd.) were the aluminosilicate sources. The two solid precursors are commercial products, and they were employed without any additional processing.

Chemical composition, physical characteristics, and particle size distribution of the MK and RHA are shown in Table 1 and Figure 1. The chemical compositions were determined using X-ray fluorescence (XRF) (equipment: Bruker FRX S8-Tiger). Particle size distribution was carried out with a laser diffraction particle size analyzer (Malvern Mastersizer 3000).

Table 1
Chemical composition and physical properties of metakaolin (MK) and rice husk ash (RHA).

Oxides (mass %)	MK	RHA
SiO2	49.36	93.19
Al2O3	41.07	1.27
K2O	2.45	1.51
Fe2O3	2.08	0.05
TiO2	1.40	-
MgO	0.82	0.33
CaO	0.07	0.58
Loss on ignition	2.45	2.08
Others	0.30	0.99
Physical properties
Bulk density (kg/m3)	439	479
Specific mass (kg/m3)	2775	2388
Specific surface area – BET (m2/g)	21.71	15.37
Mean particle diameter (μm)	11.32	13.83

Figure 1
Frequency (a) and cumulative (b) distribution of the solid precursor.

MK is mainly composed of SiO₂ and Al₂O₃, which represent more than 90% of the total composition. According to Poon et al.³⁴, MK is typically composed of 50–55% SiO₂ and 40–45% Al₂O₃. TiO₂ is a common impurity presented in MK and the content found was similar to other researches^35,36. Considering the physical properties, Rashad³⁷ presented that typical values of bulk density and specific mass of MK are 2600 kg/m³ and 300-400 kg/m³, respectively. The obtained results indicated that MK used in the present study is slightly denser. The high content of SiO₂ in RHA (> 90%) was also observed by Tong et al.³² and indicates the feasibility of using it as a silica source, which tends to be reactive and suitable to AAB production³⁸.

X-ray diffraction (XRD) patterns of MK and RHA are shown in Figure 2. Equipment Bruker – S8 x-ray diffractometer with CuKα radiation at 40 kV and 40 mA, from 5-60° 2θ by scanning at 0.02° seg^-1 was employed for XRD measurements. The identification of phases was carried out with X'Pert HighScore Plus version 3.0.5 and GSAS II version 3913 software. Inorganic Crystal Structure Database (ICSD) and Crystallography Open Database (COD) were used to identify the crystalline peaks.

Figure 2
XRD patterns of MK (a) and RHA (b).

XRD patterns indicate the mineral composition of MK (Figure 2a) is kaolinite (Al₂O₃·2SiO₂·2H₂O, COD ID: 9009234), muscovite (KAl₃Si₃O₁₀(OH)₂, COD ID: 9005014), and quartz (SiO₂, ICSD PDF: 01-085-0457). The range between 18° and 35° 2θ is related to non-crystalline phases with some peaks attributed to impurities as quartz²³. RHA (Figure 2b) had a peak at 21.8° 2θ attributed to cristobalite (SiO₂, COD ID: 9008225), a quartz polymorph formed during combustion²⁸.

Sodium hydroxide (NaOH) in flakes (Unipar Carbocloro S.A., 97.5% purity) was employed as activator, and water from the municipal supply was used for its dissolution.

Natural river quartz sand locally available was used as fine aggregate with specific mass of 2609 kg/m³, bulk density of 1890 kg/m³, fineness modulus of 2.38, and maximum particle size of 2.4 mm. Figure 3 shows the particle size distribution of the sand.

Figure 3
Particle size distribution of sand.

2.2. Mortars: mix design, production, and curing method

The mortar mix proportions are presented in Table 2.

Table 2
Mix design (in weight).

Sample		Molar ratio (m/m)			APSS2 (g)	MK (g)	NaOH1 (g)	RHA1 (g)	Sand (g)
	SiO2/ Al2O3	Na2O/ SiO2	Na2O/ Al2O3	H2O/ Na2O
A_2.7	2.7	0.1	0.6	16	30.7	91.3	8.0	-	244.0
B_3.1	3.1	0.2	0.6	16	42.0	80.0	-	-	244.0
C_3.3	3.3	0.2	0.6	16	42.0	80.0	-	3.6	244.0

• ¹
  Additional material used to adjust molar ratio.
• ²
  Alternative powder sodium silicate.

The theoretical SiO₂/Al₂O₃ molar ratio was varied to compare the influence of chemical composition on the properties of one-part AAB. In the literature, one-part AAB has been produced with SiO₂/Al₂O₃ molar ratios ranging between 0.75 – 6.02⁸. In the present study, SiO₂/Al₂O₃ molar ratios of 2.7, 3.1, and 3.3 (considering total solubility of raw materials) were used. The range employed was close to 3.0, considering the results of Bernal et al.²⁵, which used MK and RHA to produce AAB. Na₂O/Al₂O₃ and H₂O/Na₂O molar ratios were kept constant in all the specimens by using extra NaOH or RHA in some mixtures to adjust the initial chemical composition. The binder/sand ratio was equal to 1:2.

To prepare the mortars, the solid one-part AAB and sand were put in a mechanical mixer (equipment: Perfecta; capacity: 20 L) for 2 min at slow speed until homogenization. Thereafter, water was gradually put in the mixture followed by continuous mixing (medium speed) for more 5 min. The homogenized fresh mortar was immediately transferred to the molds, which were subjected to vibration (vibrating table) for 1 min to eliminate the entrapped air.

After casting, the molds were placed in a laboratory room with temperature of 25 °C ± 2 °C and relative humidity of 60% ± 5%. The specimens were demolded after 3 days of curing and stored in a sealed container in laboratory conditions until testing.

2.3 Methods

2.3.1. Thermogravimetry (TG), derivative thermogravimetry (DTG), and differential thermal analysis (DTA)

Thermal analysis (TG/DTG/DTA) was carried out in a DTG 60H Shimadzu simultaneous TG/DTA apparatus using about 10 g of the sample in a platinum crucible. The heating rate of 10 °C min^-1 was used from 25 °C to 1000 °C under a nitrogen flow of 50 mL min^-1.

2.3.2. Specific mass, bulk density, and pH

Specific mass (Le Chatelier flask) and bulk density of the powders were determined by adapting Brazilian Standards NBR 16605³⁹ used for PC and NBR 12127⁴⁰ used for gypsum plaster.

The pH of the powdered materials was estimated employing the method described to determine soil pH⁴¹. A 10 mL sample was mixed with 25 mL of distilled water. The mixture remained to stand for 1 h, before determining the pH (equipment: Thermo Orion 410).

2.3.3. Flow and heat release

A flow table test was used to determine the workability of the mortars. The test was made following NBR 13276⁴².

The heat released was registered immediately after the addition of water in the mortar. The specimens were molded in expanded polystyrene (EPS) thermal cup of 120 mL. The material was placed inside an EPS container filled with expansive polyurethane foam, aiming to achieve higher temperature insulation from the internal environment. In the central part of the specimens was inserted a thermocouple (K-type) to register the temperature through a data acquisition system (equipment: datalogger Testo 177-T4). The container with the mortars was kept in ambient conditions (average temperature registered during all the test time of 20.9 °C ± 2 °C). Data were saved each 22 s during the total time of 2000 min.

2.3.4. Mechanical properties and statistical analysis

To evaluate the importance of RHA dissolution (step 1 of one-part AAB synthesis, as presented in results), cubic mortars (40 mm × 40 mm × 40 mm) were molded (mix proportion in grams = 42 alternative powder sodium silicate (APSS); 80 MK; 244 sand; and 66.3 water) and subjected to compressive strength test. Specimens were prepared with and without the previously RHA dissolution system proposed (heating and stirring) and the test was carried out in a Versa Tester machine (maximum load of 150 kN) at the ages of 3, 7, 28, 90, and 180 days.

Mechanical properties of mortars were evaluated by flexural and compressive strength tests at the ages of 3, 7, 28, 90, and 180 days (compressive strength) or 3, 28, and 90 days (flexural strength). The specimens were tested in a Versa Tester machine (maximum load of 150 kN). Cubic samples (40 mm × 40 mm × 40 mm) were molded to the compressive strength test, which was obtained by the maximum load divided by the sample area⁴³. Prismatic samples (40 mm × 40 mm × 160 mm) were molded to flexural strength test, obtained by the three-point-bending test⁴⁴.

Analysis of variance (ANOVA) (Microsoft Excel^® software) and Least Square Difference (LSD) (Statgraphics Centurion^® software) were made with a 95% confidence limit to compare the treatments.

2.3.5. Air permeability and capillary water absorption

Air permeability of the mortars was evaluated according to Thenoz method, such as presented by Geraldo et al.³⁰.

Cylindrical specimens (50 mm diameter and 100 mm height) were prepared and cured at room temperature for 25 days (average temperature of 25 °C ± 2 °C and relative humidity of 60% ± 5%). At this age, the specimens were cut into three parts and the central part slice (50 mm height) was oven-dried (60 °C ± 2 °C) for 48 h and after cooling they had their lateral surfaces sealed to guarantee the flow was uniaxial and perpendicular to the cross-sectional area. The test was developed at 28 days in a varying charge apparatus. The average values of three specimens of each mortar composition were reported. Air permeability was calculated with Equation 1, which is based on Darcy’s law.

$$k=\left[\mu .s.1/\left(\rho .g.t.S\right)\right].ln\left(h_0/h_1\right)$$ (1)

Where: k = air permeability coefficient (m²); μ = air viscosity at the test temperature (Pa.s); ρ = density of water (kg/m³); g = gravity acceleration (m/s²); S = cross-sectional area of the sample (m²); s = cross-sectional area of capillary tube (m²); h₀ = start water level (m); h₁ = final water level (m); 1 = sample height (m); and t = testing time (s) (time to water pass from h₀ to h₁).

A capillary water absorption test was performed on cylindrical specimens (50 mm diameter and 100 mm height) at 28 days. The average values of three specimens for each mortar composition were reported and the test was based on NBR 9779⁴⁵ with some modifications, as follows.

The specimens were oven-dried (60 °C ± 2 °C) for 48 h. Thereafter, the samples were weighed and had their lateral surfaces sealed before testing to have the water flow perpendicular to the cross-sectional area. The specimens were partially immersed in water (5 mm ± 1 mm) and the water uptake into the specimens was evaluated at 5 min, 10 min, 15 min, 30 min, 1 h, 2 h, 4 h, 6 h, 8 h, and 24 h. Capillary water absorption was calculated with Equation 2.

$$Cwa=\left(H_m–D_m\right)/S$$ (2)

Where: Cwa = capillary water absorption (kg/m²); H_m = humid mass (kg); D_m = oven-dried mass (kg); and S = cross-sectional area of the specimen (m²).

2.3.6. Scanning electron microscopy (SEM) observations

Scanning electron microscopy (SEM) by secondary electron images were obtained on fractured surfaces from compressive strength test specimens. The images were observed on LEO Electron Microscopy/Oxford 440i with 20 kV and 100 pA. The equipment contained X-ray dispersive energy detector (EDS) and chemical analysis was performed. The samples were coated with gold before SEM observations.

3. Results and Discussions

3.1. Synthesis of one-part AAB

The method to one-part AAB obtaining was divided into steps as shown in Figure 4 and described, as follows.

Figure 4
Steps to production of one-part AAB proposed.

3.1.1. Alternative powder sodium silicate (APSS) production

The APSS production comprises steps 1 to 3 (Figure 4).

Step 1-RHA silica dissolution: RHA was blended into a NaOH solution. The mix proportion employed was equal to (in mass) 1 NaOH : 1.605 water : 1.284 RHA. The mixture was put in a magnetic stirrer (equipment: Fisatom 752A) with heating (90 °C ± 5 °C) for 30 min, following the process described by Geraldo et al.³⁰ to produce two-part AAB. Due to the high viscosity of the material, it was used another mechanical stirrer (equipment: Fisatom 713D, rotation range: 500-600 rpm) equipped with a stainless-steel stem to maintain the mixing.

The liquid material originated in this step had high viscosity and density = 1600 kg/m³, pH = 14, total solid content = 65%, and humidity content = 35%.

The main objective of this step is to use RHA as a source of silica to synthesis an alternative sodium silicate (ASS)^21,27, first in liquid phase (ASS) and in powder (APSS) in this work. Different results in the literature indicate the suitability of using RHA to obtain an ASS³².

Step 2-Oven-drying: The material from step 1 was collected and spread in a rectangular aluminum tray (thickness = ~5 mm). This viscous liquid was oven-dried (temperature of 200 °C ± 10 °C for 3 h) in a stationary kiln with air circulation. Different preliminary trials were carried out with lower drying temperatures (50 °C, 100°, 150 °C); however, during this process, a film was formed over the material layer, which prevented water evaporation. Because of it, a temperature of 200 °C was employed. To compare, Vinai and Soutsos⁴⁶ made the oven-dry process to obtain a reactive powder using the temperature of 330 °C for 2 h, revealing higher temperatures may decrease the total dry time.

Step 3-Grinding and homogenization: during this process, the dried material from step 2 was grounded in a porcelain ball mill until reaching a fineness to pass through a 0.075 mm sieve opening. The resultant material was manually homogenized using a plastic container, and it was designed as APSS.

Table 3 and Figure 5 show the chemical composition of APSS obtained by XRF and the XRD pattern of APSS, respectively.

Table 3
Chemical composition of alternative sodium silicate (APSS) in powder.

Sample	Composition (wt.%)
	SiO2	Na2O	Al2O3	K2O	CaO	MgO	Fe2O3	Others	LOI1
APSS	53.80	32.80	1.72	1.01	0.36	0.16	0.05	0.52	9.58

• ¹
  Loss on ignition

Figure 5
XRD results of APSS (a) and XRD of RHA and APSS (b).

XRF revealed that APSS is predominantly composed of silica, derived from RHA, and sodium, derived from NaOH. SiO₂ and Na₂O represent more than 85% of the APSS total chemical composition. The APSS silica modulus Ms SiO₂/Na₂O molar ratio was equal to 1.69, a value higher than the silica modulus of 1.25 stated by Gado et al.⁴⁷ as an optimum condition to alkali-activation of waste clay bricks. APSS had a pH of 13.26.

XRD shows a low-crystalline material, indicating a reactive powder. Step 1 (dissolution of NaOH + RHA) was efficient in breaking the cristobalite peak, as indicated by Mejía et al.²⁸. This crystal has a high dissolution susceptibility to being removed by an alkaline source. The crystalline phases in APSS revealed by XRD peaks were identified as NaOH(H₂O) (COD ID: 1532133), Na₂(Si₂O₅) (COD ID: 1533028), and Na₂SiO₃ (COD ID: 2310858). Considering the chemical stoichiometry and the products identified by XRD, the products were formed following the overall chemical reactions in solution (Reactions 1 to 4).

$$2NaOH\to N{a}_{2}O+H_{2}O$$ (1)

$$N{a}_{2}O+H_{2}O+Si{O}_2\to N{a}_{2}Si{O}_3+H_{2}O$$ (2)

$$N{a}_{2}Si{O}_3+Si{O}_2\to N{a}_{2}S{i}_2{O}_5$$ (3)

$$N{a}_{2}O+2H_{2}O\to 2NaOH•H_{2}O$$ (4)

Reaction 1 represents NaOH in flakes dissolved in water⁴⁸. NaOH solution dissolves the RHA silica to form sodium silicate, as shown in Reaction 2^21,46. The Na₂SiO₃ and Na₂(Si₂O₅) phases are sodium silicate from RHA and NaOH. It indicates that the procedure for RHA silica dissolution was efficient.

TG curves (Figure 6) indicated a continuous mass loss, more pronounced at 200 °C – 350 °C, with a DTG peak at 282 °C, due to the water loss. There was a low endothermic DTA peak at 800 °C, which can be attributed to calcium carbonate decomposition⁴⁹. Another exothermic DTA peak at 858 °C can be related to the β-tridymite formation derived from the silica heating at the range of 870 °C⁵⁰.

Figure 6
TG/DTG/DTA curves of APSS.

3.1.2. One-part AAB

The production of one-part AAB, after the APSS production, comprises steps 4 and 5.

Step 4-MK addition and homogenization: MK was added to APSS in determined proportions, and the powders were manually mixed in a sealed container until complete homogenization.

Step 5-One-part AAB: after the process described, the one-part AAB was obtained, and it was ready to be used only with the water addition. Considering the three different SiO₂/Al₂O₃ molar ratios tested in this study and Table 4 shows some characteristics of the obtained binders.

Table 4
Characteristics of the one-part AAB produced considering the theoretical SiO₂/Al₂O₃ molar ratio.

Sample	Molar ratio (m/m)	pH	Bulk density (kg/m3)	Specific mass (kg/m3)
A_2.7	2.7	13.13	552	2525
B_3.1	3.1	12.81	573	2513
C_3.3	3.3	12.81	586	2513

The results indicated that the physical properties of the binders B_3.1 and C_3.3 were similar. A_2.7 presented a slightly higher pH due to the incorporation of extra NaOH, used to correct the molar Na₂O/Al₂O₃ ratio. The values of pH are very close to that presented by PC, which is around 13⁵¹.

3.2. Importance of RHA silica dissolution (step 1)

Figure 7 shows the effect of step 1 (RHA silica dissolution with heating and stirring) on compressive strength of mortars, comparing results with mortars made with an equivalent system, but just adding RHA into NaOH solution and going on with other processes.

Figure 7
Influence of RHA dissolution step in the compressive strength of mortars made with one-part AAB.

The results indicated that the RHA dissolution by heating and mixing is important in strength development. The mortar made with one-part AAB produced with RHA dissolution (step 1) achieved a compressive strength 2.8 times higher than the mortar made just by mixing RHA into NaOH solution and following the next steps. The difference is because the steps (heating and mixing) to prepare APSS supplied a higher amount of amorphous silicate, which contributed to the mechanical strength development, as explained by Autef et al.⁵².

RHA dissolution also provided a higher strength increases over time. The chemical mechanism of APSS to dissolve the aluminosilicate precursor (metakaolin) is expected to be similar to the activator containing only NaOH, the difference is that APSS accelerates the reaction by supplying soluble silicate from the activator²¹.

3.3 Mortars made with one-part AAB containing different SiO₂/Al₂O₃ molar ratios

The flow table test indicated A_2.7, B_3.1, and C_3.3 with flows of 130 mm, 129 mm, and 124 mm, respectively. The results indicated that all mortars had low workability. The workability of mortars was similar, with a low spread diameter. This can be explained by the physical characteristics of the powder binder used (e.g. water absorption, grain size) and the water content that was not sufficient to reach good workability.

Figure 8 shows the results of heat evolution curves of mortars immediately after water addition.

Figure 8
Heat evolution curves of the mortars made with the one-part AAB immediately after water addition (a) with magnification from 21 °C to 33 °C (b) (average ambient temperature during the test = 20.9 °C ± 2 °C).

The temperature registered with time indicated an exothermic chemical reaction (heat release). The heat curves of B_3.1 and C_3.3 presented a similar pattern and reached the maximum temperatures registered after about 1 h. The use of additional NaOH in A_2.7 did not result in an acceleration of chemical reactions, differently, the curves indicated that in this sample there was a lower heat releasing and the reaction occurred slower, where the maximum temperature delayed 4 times more than the other AAB mix designs.

Each curve presented only one maximum temperature peak, a result similar to that obtained by Longhi et al.⁵³ and Geraldo et al.³⁰ with two-part AAB. The peak presented in the curves is related to the chemical reactions of dissolution and polycondensation from alkaline activation, which happen almost at the same time^53,54.

Figure 9 shows the results of compressive strength (Figure 9a) and flexural strength (Figure 9b) of the mortars, respectively. ANOVA indicated there was a significant difference among the data set considering flexural or compressive strength results (p-value ≤ 0.05 and Fcrit < F). Table 5 shows the analysis obtained by the Least Square Difference (LDS) to compare the treatments.

Figure 9
Compressive (a) and flexural (b) strengths of mortars made with the one-part AAB.

Table 5
LSD test with a 95% confidence limit.

Sample	Age (days)
	3	7	28	90	180
Compressive strength (MPa)
A_2.7	2.89aA	3.47aA	6.11bA	6.89bA	6.66bA
B_3.1	4.70aB	9.03bB	10.35cB	12.14dB	15.69eB
C_3.3	6.89aC	8.42bB	12.79cC	15.31dC	17.99eC
Flexural strength (MPa)
A_2.7	0.83aA	-	1.17bA	1.67cA	-
B_3.1	2.17aB	-	4.35bB	4.63bB	-
C_3.3	2.35aB	-	4.24bB	4.02bB	-

• Row data: the same lower-case letters indicate similar statistics results (strength evolution with curing time). Column data: the same upper-case letters indicate similar statistics results (strength values among the groups).

The results indicated that compressive strengths increased with time. The increase was more evidenced in B_3.1 and C_3.3, while A_2.7 did not obtain a significant increase after 28 days. The compressive strength from 3 to 90 days of A_2.7, B_3.1, and C_3.3 increased in the order of 2.4, 2.6, and 2.2 times, respectively. The strength development may be related to the slow RHA silica dissolution when the binder is mixed with water. As showed by Luukkonen et al.⁵⁵, the slow silica availability results in continuous gain strength by increasing the degree of reaction.

The mechanical strength of the mortars also increased with the higher SiO₂/Al₂O₃ molar ratios used, indicating the importance of the initial composition. Statistics indicate a significant difference in compressive strength according to SiO₂/Al₂O₃ molar ratio in most of the curing times. Considering flexural strength, the specimens B_3.1 and C_3.3 did not present significant differences.

In the literature, other authors reported strength increasing with higher SiO₂/Al₂O₃ molar ratio to AAB production^56,57. The strength increase is associated with higher Si-O-Si bonds and lower Si-O-Al bonds formation, derived from the more significant degree of SiO₂ available. The three-dimensional networks Si-O-Si bonds have more strength than Si-O-Al bonds, resulting in a material with improved mechanical strength with denser and more homogeneous microstructures^56-58.

The maximum compressive strength achieved was ~18 MPa at 180 days, which was lower than other literature values about one-part AAB. Mortars produced with AAB achieved lower compressive strength than mortars in a two-part method produced by Geraldo et al.³⁰ and Teixeira et al.³¹ using similar materials. The authors reported compressive strength in the range of 29 MPa – 40 MPa at 28 days. Nevertheless, the data indicate that the one-part AAB proposed generated material with binding properties and compressive strength higher than required for non-structural applications, such as mortars for components and rendering⁵⁹. According to Brazilian standards, the minimum compressive strengths for some applications are: ceramic bricks = higher than 1.5 MPa, soil cement brick = higher than 2.0 MPa, concrete blocks for sealing = higher than 2.5 MPa, ceramic blocks for structural masonry = higher than 3 MPa, and hollow concrete blocks of C category = higher than 4 MPa.

The use of more reactive precursors and a thinner APSS could improve the AAB reactivity and, as a result, the mechanical strength.

Figure 10 exhibits the results of air permeability (Figure 10a) and water absorption (Figure 10b) of the hardened mortars.

Figure 10
Air permeability (a) and capillary water absorption (b) of the AAB mortars.

The results indicated that A_2.7 was a more permeable mortar. The additional water used to dissolve the extra NaOH and to adjust the molar ratio H₂O/Na₂O contributes to the porosity increasing in hardened mortar, as reported by Cheng et al.⁶⁰.

Air permeability results obtained in a two-part AAB mortar made with APSS from RHA were 1 x 10^-13 m²³⁰, while the results obtained in this research with one-part AAB made with APSS were 10 times higher. The high permeability can be related to entrapped air in the mortars, which is derived mainly by the difficulty of mixing and molding the mortars in the fresh state due to the low workability.

The capillary water absorption results indicated that within the first 480 min the differences between the capillarity of A_2.7 and other specimens increased with time. After 24 h, A_2.7 had capillary water absorption 30% higher than the other mortars. The specimens B_3.1 and C_3.3 presented similar behavior during all tests, which indicates that they have a similar internal capillary pores network¹¹.

The results suggest that in sample A_2.7 there was a broader number and connection of pores, resulting in a higher capillary suction⁶¹. The data are in accordance with the air permeability tests that indicated A_2.7 was a more permeable material. B_3.1 and C_3.3 provided lower internal water transport by capillary forces, indicating they are materials of higher durability than A_2.7⁶².

Figure 11 shows the morphology of the mortars obtained by SEM. The numbers 1, 2, 3, and 4 presented in the images indicated the EDS point of detection. Table 6 presents the results of the chemical composition obtained by EDS (average value obtained at least two reading points in diverse regions of specimens).

Figure 11
Microstructure of mortars obtained by SEM.

Table 6
Elemental composition of mortars (obtained by EDS).

Sample	Element (%)							Molar ratio (m/m)
	O	Si	Al	Na	K	Fe	Others	Na/ Al	Na/ Si	Si/ Al
1 A_2.7	53.63	12.25	6.68	20.99	0.91	0.75	4.79	3.69	2.09	1.76
2 A_2.7	51.34	23.05	14.59	6.11	1.24	1.01	2.66	0.49	0.32	1.51
3 B_3.1	50.43	27.50	13.66	5.66	0.95	0.72	1.08	0.48	0.25	1.93
4 C_3.3	51.32	26.00	12.28	6.47	1.87	1.13	0.93	0.61	0.30	2.03

Figure 11 revealed that the specimen microstructures were homogeneous, dense, and presented micro-cracks and voids usually with spherical shapes distributed by the matrix (porosity).

The observed samples were fractured from the specimens of compressive strength test. This can cause some microcracks observed in the microstructure. In the literature, other studies reported microcracks in one-part AAB which may also be due to some expansive reactions in the hardened material¹⁰. Spherical voids reveal the presence of trapped air bubbles that contribute to total porosity³⁶.

Mortar matrix A_2.7 microstructure revealed the presence of elongated needle-shaped crystals. This is due to the additional NaOH and water in the mixture, necessary to keep a constant Na₂O/Al₂O₃ and H₂O/Na₂O molar ratios, resulting in unreacted and free alkalis. The unreacted and free alkalis react with CO₂ to form efflorescence⁶³. Temuujin et al.⁶⁴ stated that efflorescence is mainly composed of Na and O, with a chemical composition ranging in according to the precursor employed.

The high Na/Si and Na/Al ratios obtained from EDS point 1 (Figure 11 – A_2.7, images C and D) revealed the concentration of sodium in the needle-like crystals, while the concentrations were considerably lower in the A_2.7 matrix indicated by EDS point 2 (Figure 11 – A_2.7, image D). During the crystal formations, internal pressure can be generated ranging according to pore diameter and, in some cases, it causes cracking⁶⁵.

Mortar matrix B_3.1 presented a continuous and dense microstructure with pores and cracks. Figure 11 – B_3.1, images B and C showed small zeolite crystals formed on the matrix surface as secondary products in the reaction process (on the N-A-S-H gel) which is the precursor of zeolites formation^66,67. Sample B_3.1 had a Si/Al molar ratio lower than 0.3. Ma et al.⁶⁸ results showed that in these cases the N-A-S-H gels may have been few geopolimerized which results in the formation of the zeolitic phase.

Mortar matrix C_3.3 presented a continuous and dense microstructure. Figure 11 – C_3.3, image C, present crystals similar to the zeolitic phase, but EDS measurements were not made on these surfaces, and it makes difficult to confirm this phase formation. Mortar C_3.3 presented the Na/Si and Si/Al molar ratios very close to the results available in the literature.

The elemental compositions obtained by EDS showed the molar ratios: Na/Al varying from 0.48 to 0.61; Na/Si varying from 0.25 to 0.32; and Si/Al varying from 1.51 to 2.03. The EDS results in point 1 were not considered because it was made on the alkaline crystal. Guo et al.⁶⁹ reported Na/Si molar ratios between 0.28 and 0.51, and stated that the best ratio was close to 0.5 where occurred the greatest formation of N-A-S-H gel. Duxon et al.⁵⁸ studied the Si/Al molar ratio between 1.15 and 2.15, and reported that the higher mechanical properties were achieved with Si/Al molar ratio of 1.90. The mechanical properties of B_3.1 and C_3.3 mortars had better results.

The results of one-part AAB obtained in this work confirm that the process to obtain this one-part binder is a viable economic alternative in diverse applications, such as fire-proof building materials, protective coatings, renderings, and building components.

4. Conclusions

A method to produce one-part alkali-activated binder (AAB) was carried out by the following steps: 1- production of alternative powder sodium silicate solution; 2- oven-drying the solution; 3- grinding to obtain a sodium silicate as a thin powder (APSS); 4- addition of metakaolin to the APSS; and 5- the binder is ready (one-part AAB).

The SiO₂/Al₂O₃ molar ratio used in mortar production affected the final properties. The increase in SiO₂ content generated mortar with higher strength and lower porosity.

The highest compressive strength (~18 MPa) was achieved at 180 days by mortar produced with a theoretical SiO₂/Al₂O₃ molar ratio of 3.3. Despite the relatively low mechanical results, the mortars achieved enough compressive strength for different applications in civil construction (renderings and components).

Data indicated the effectiveness of the proposed method of production one-part AAB. The use of a solid silicate soluble source from rice husk ash (agricultural waste) dissolution makes the AAB production cleaner when compared with other commercial activators, resulting in environmental benefits.

5. Acknowledgments

This work was supported by the Coordination for the Improvement of Higher Education Personnel - Brazil (CAPES, Finance code: 001), PROCAD/CAPES, and National Council for Scientific and Technological Development - Brazil (CNPq).

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Publication Dates

• Publication in this collection
  06 Dec 2021
• Date of issue
  2022

History

• Received
  28 Aug 2021
• Reviewed
  19 Oct 2021
• Accepted
  04 Nov 2021