Potassium-rich calcinated waste coffee husk ash as an alkali activator in one-part geopolymer concrete

Abstract

The study explores the possibilities of using calcinated waste coffee husk ash (WCA) as a natural alkaline activator in one-part geopolymer concrete. WCA was calcinated at 1000 °C for 2–14 hours at 2-hour intervals, and physicochemical characteristics were determined, including pH, water solubility, and Loss on Ignition (LOI). The effect of calcination period on the reactivity of WCA and its impact on the mechanical performance of geopolymer concrete was evaluated by replacing part of GGBS with WCA. These findings suggest that the 12-hour calcination of WCA shows better performance with a maximum pH (13.98), water solubility (54.5%), and calcination efficiency (LOI = 23.15%). Concrete mixes that used 15 percent replacement with 12-hour calcinated WCA (C12H15) noted the highest mechanical performance, which has 16 percent increased compressive strength at 28 days, relative to 10-hour calcinated WCA mixes. The optimized mixes were further proved to be superior in terms of fresh and mechanical performance, as confirmed by workability, compressive strength, and split tensile strength tests. The research confirms that properly calcinated WCA has the potential to replace traditional chemical activators, which would be a green, cost-effective, and environmentally friendly option in a massive building. This also solves the environmental issue of waste disposal of coffee husks, which would ensure sustainable construction without affecting the structural performance. Based on reported literature, one-part geopolymer systems can reduce carbon emissions by about 30–40% compared to conventional OPC concrete.

Keywords:
Waste Coffee Husk Ash; Geopolymer; Alkaline Source; Calcination; Alternative Activator.

1. INTRODUCTION

Global warming due to the activities of different human beings is the most important issue that needs to be solved by researchers worldwide. The construction industry is one of such industries and a top contributor to environmental degradation because its material manufacturing processes have a high carbon footprint [¹,²,³,⁴]. Cement is an inevitable substance that has been utilized as a foundational component in construction projects around the world. Researchers are also working on the reduction of environmental effects of cement and the construction industry in general by finding creative methods that can help enhance sustainability [⁵]. International researchers have already tried to substitute cement in part and entirely with alternative cementitious admixtures. It establishes that the substitution of cement in the construction sector with alternatives is urgent in the reduction of environmental impact [⁶]. The partial replacement of cement has not yielded effective results in mitigating the outgassing of carbon, but full cement replacement can prove to be an effective means of mitigating the adverse effects of cement [⁷, ⁸]. Among these options, geopolymer technology is one such alternative technology that can substitute cement entirely, with numerous benefits over Portland cement use [⁹]. Geopolymer concrete mainly has the best features of superior properties in that it has a smaller carbon footprint and improved performance than conventional concrete technologies [¹⁰]. Geopolymer concrete exists in two types: one-part and two-part geopolymers, possessing different characteristics and uses. One-part and two-part geopolymers are also critical to promoting sustainable construction methods through decreasing the consumption of conventional cement-based products and lowering the environmental impact. One-part geopolymers are ready-to-wear mixtures of all required components, and are usually composed of an alkali activator and raw materials, including fly ash or slag and only need the addition of water to begin geopolymerization [¹¹, ¹²]. Conversely, two-part geopolymers are those where source materials are first mixed independently with an activator solution and mixing ratios and curing conditions must be carefully controlled [¹³]. Comparatively, one-part geopolymer is more affordable and easier to handle. However, in both variants of geopolymer concrete, the solid precursors and activators remain conventional.

Industrial wastes and by-products, such as high and low-calcium fly ash, GGBS, metakaolin, wood ash, and biomedical waste ash, which contain alumina-silica composition, have been used as solid precursors in geopolymer concrete [¹⁴]. Hydroxide, silicate, carbonate and fluoride-based chemicals such as sodium or potassium hydroxide, sodium or potassium silicate, and sodium or potassium carbonate have been used as alkaline activators in the production of geopolymer concrete [¹⁵]. In these chemical-based activators, heat evolution occurs while the chemicals react with water, which can harm the handlers. Even in one-part geopolymers, chemical activators are used in powder form, which also needs to be addressed [¹⁶]. The production process of these chemical activators involves high energy consumption and a large carbon footprint [¹⁷]. Specifically, the production of silicate-based activators releases CO2 emissions nearly equal to half of those generated by cement production [⁹]. Meanwhile, the properties of geopolymer concrete are influenced by various parameters such as the quantity of aluminosilicate source in the precursor, ratios of NaO/SiO2, SiO2/Al2O3, NaOH/Na2SiO3, solution to binder ratio, and concentration of NaOH [¹⁸]. This indicates that activators play a crucial role in geopolymer concrete. However, the use of alkaline activator solutions is often cited as a major practical drawback in geopolymer technology due to their high cost, corrosiveness, and lack of environmental friendliness [¹⁹]. Hence, there is a need to identify a sustainable and cost-effective activator to replace chemical activators and significantly enhance the feasibility of using geopolymer concrete.

Researchers have utilized various industrial by-products such as rice husk ash, silica fume, bottom ash, wood ash, micro silica, waste glass powder, and nano silica as activators in geopolymer concrete [²⁰,²¹,²²,²³,²⁴,²⁵]. However, these alternatives are silica-rich materials that can replace silicate-based activators when pre-mixed with hydroxide-based activators. Subsequently, even with these alternatives, hydroxide-based chemical activators are still required, which are again harmful to handle [²⁶]. FERNÁNDEZ-JIMÉNEZ et al. [²⁷] utilized cleaning solution (a washing liquid used for cleaning moulds), which is a waste residue obtained from the aluminium industry, as an alkaline activator. The cleaning solution performed well as an alkaline medium for dissolving the precursors; however, the performance was notable in silica-rich precursors. Most previous studies are still based on chemical activators such as NaOH and Na2SiO3 that are not only expensive, corrosive to handle, and have associated high carbon emissions with their production [⁹, ¹¹, ¹⁶, ¹⁹]. Several researchers have studied the use of agro-waste ashes like rice husk ash, wood ash, and silica fume as an alternative material, however, these materials mainly contain silica and require hydroxide-based chemical activators for effective geopolymeriztion [²⁰, ²¹, ²⁶]. This inclusion of chemical activators causes the overall sustainability and practical applicability of the geopolymer technology to be limited. Very few studies have explored potassium-rich natural ashes as complete replacements for chemical activators in one-part geopolymer concrete systems [¹¹, ¹², ¹⁶, ²⁷]. Potassium-rich wastes such as coffee husk ash are preferred because the activators based on potassium are less corrosive and easier to handle and are potentially more environmentally benign than conventional activators based on sodium. There is a clear research gap for developing a fully natural, potassium-based activator derived from waste materials, for the one-part geopolymer concrete.

Therefore, in this study, chemical alkaline activators have been entirely substituted with a waste residue product, namely Waste Coffee Husk Ash (WCA), which contains a high potassium content. Potassium-based activators play a significant role in precursor dissolution, geopolymerization, reaction mechanism, and strength development. Waste coffee husk ash is a waste residue produced from the coffee powder production industry, with an annual production volume of 18.29 million metric tonnes (MMT). The coffee husk needs to be converted into ash by burning with the help of a blower. The ash obtained from burning cannot be directly used as an activator because of its poor alkaline nature. This research aims to explore the utilization of WCA as alkaline activators and investigate their activation mechanism. The optimum percentage of these natural activators is also examined with the help of the mechanical characterization of concrete cubes. Further, microstructural investigation has been carried out to justify the reaction mechanism and the formation of geopolymer gel.

The objectives of this study are:

1. To calcinate the waste coffee husk ash (WCA) at different durations and evaluate its physico-chemical properties (pH, solubility, LOI, XRD).

2. To determine the optimum calcination time of WCA to achieve the maximum alkalinity and reactivity.

3. To use WCA as a natural alkaline activator used in one-part geopolymer concrete by partially re-placing GGBS (5–20%).

4. To assess the fresh and mechanical properties of the concrete using calcined WCA.

5. To determine the optimum percentage of WCA replacement based on strength performance.

2. MATERIALS AND METHODS

2.1. Materials

GGBS is a waste by-product of the iron and steel industry, which typically contains oxides of calcium, silica, aluminium and magnesium, exhibiting properties similar to cement. GGBS is considered one of the most promising aluminosilicate source materials and has already been widely used by various researchers in geopolymer concrete. GGBS was selected as the primary binder because it is highly reactive in alkaline environments and widely used in geopolymer systems. The waste coffee husk ash (WCA), procured from coffee processing industries, was oven-dried and calcinated at a constant temperature for various durations to be used as a natural alkaline activator. WCA was selected because of its high potassium content that can function as a natural activator. In the present study, WCA was to act primarily as a natural activator rich in potassium. However, as WCA also contains small quantities of reactive silica and alumina, the partial substitution of GGBS with WCA unavoidably affects the composition of the aluminosilicate binder. The mix design was therefore formulated to represent a practical one-part geopolymer concrete system wherein WCA both provides alkalinity and minor reactive oxides similar to possible real-field application conditions. After calcination, WCA was crushed in ball mill to improve its fineness. The fineness of the processed WCA was found and was determined to be 85% retained on a 90um sieve. The specific gravity of GGBS is 3.23, while that of WCA is 2.24. The bulk density of the GGBS and WCA is 1250 and 260 kg/m³, respectively. Figure 1 shows the variation in colour of WCA before and after calcination.

Figure 1
WCA before and after calcination.

For this study, manufactured sand (M-sand) passing through a 4.75 mm sieve was used as fine aggregate. Locally available angular coarse aggregates with a nominal size of 12.5–20 mm were used. The material characteristics of M-sand and coarse aggregates are presented in Table 1. Particle size distribution of M-sand and coarse aggregate is shown in the Figure 2. A constant water-to-binder ratio of 0.5 was adopted to reduce the effect of WCA replacement on concrete properties.

Table 1
Physical properties of fine aggregate.

S.NO	DESCRIPTION	TEST VALUES
		FA	CA
1	Specific gravity	2.63	2.8
2	Water absorption (%)	3.1	1.2
3	Fineness modulus	3.4	7.3
4	Zone	Zone-3	–
5	Loose bulk density	1650 kg/m3	–
6	Rodded bulk density	1420 kg/m3	–

Figure 2
Gradation curve for M-sand and coarse aggregate.

Initial burning of coffee husk was carried out using a hand blower for 1 hour, after which the ash was transferred to a muffle furnace for calcination. The obtained ash was then subjected to calcination at a constant temperature of 1000°C for different time intervals to activate the alkaline compounds present in the coffee husk. Calcination was carried out in a muffle furnace at a heating rate of approximately 10°C per minute until 1000°C was reached. After completion, samples were allowed to cool inside the furnace to room temperature to avoid thermal shock. The optimum time required to produce efficient WCA as an activator was determined through its physicochemical material characterization. The physico-chemical characterization included testing WCA for pH, water solubility, loss on ignition (LOI), consistency, setting time, and microstructural characteristics. The pH and water solubility tests were used in this study as comparative screening test of alkalinity and dissolution behaviour of calcined WCA and not as a direct measurement of pore solution chemistry. These tests provide a preliminary understanding of alkaline availability and reactivity trends, while detailed pore-solution analysis was outside the scope of the present investigation. The microstructural examination using X-ray Diffraction (XRD) was carried out to identify the major elements present in the WCA. Table 2 presents the chemical compounds present in GGBS and WCA before calcination.

Table 2
Comparison of compound content in GGBS and WCA.

SI.NO	COMPOUND	GGBS (%)	WCA (%)
1	Al2O3	19.35	0.88
2	Fe2O3	0.56	4.20
3	CaO	38.97	9.44
4	MgO	8.64	4.92
5	SiO2	32.46	18.22
6	K2O	ND	61.93

2.2. Mix design and methodology

The study has been categorized into two major phases, viz., (i) physical and chemical characterization of WCA calcinated for various time intervals and (ii) fresh and mechanical characterization of concrete specimens prepared using WCA at different proportions. In the first phase, the waste coffee husk ash was calcinated for different durations of 2, 4, 6, 8, 10, 12 and 14 hours at a constant temperature of 1000°C in a muffle furnace. A constant temperature of 1000°C was maintained to reduce the effect of calcination duration on the reactivity of WCA. This temperature was selected to ensure the complete removal of organic matter and adequate activation of potassium-rich mineral phases present in the ash. Properties such as pH, water solubility and loss on ignition (LOI) were determined. Microstructural analysis using XRD was also carried out to identify the major elements and crystalline phases present in the WCA and to study the variation of these elements with calcination duration.

In the second phase, concrete mixes with a suitable mix proportion and a constant water-to-binder ratio were prepared to investigate the fresh and mechanical properties. The quantity of materials is illustrated in Table 3. The optimum percentage of WCA utilization was determined by partially replacing GGBS at 5%, 10%, 15%, and 20%. WCA replacement levels of 5, 10, 15 and 20% were selected based on preliminary trials and literature on agro-waste ashes, which indicate that 10–20% replacement is practical without major loss in workability and strength. A fresh property evaluation was carried out using the slump cone test as per IS 1199:1959 [²⁸]. Mechanical characterization included compressive strength (IS 516:2021 [²⁹]), and split tensile strength (IS 5816:1999) [³⁰], at different curing ages, were done on concrete specimens prepared in standard sizes with different proportions of WCA. Three specimens were tested at each age (3, 7, and 28 days), and the average value is reported. The influence of varying calcination durations (10, 12, and 14 hours) on the mechanical characteristics of concrete was also examined. For the control specimen, 15% of potassium carbonate was taken since the WCA contains potassium as an alkaline medium. A total of 13 concrete mixes were made in order to systematically study the effect of three calcination durations (10, 12, 14 hours) and four WCA replacement levels (5, 10, 15, 20%). One control mix containing Potassium Carbonate (PC) was also cast for the comparison. Three specimens each were tested for each of the strength tests at each curing age for reliability. For the control specimen 15% PC was used since WCA is predominantly potassium based and it provides a chemically comparable reference system but does not suffer with the handling and safety problems of highly caustic KOH or K2SiO3 solutions. Potassium carbonate provides an alkaline environment; but KCO3 activation mechanism is not the same as found in the case of hydroxides, and silicates generally used in conventional geopolymer systems [⁸, ¹⁵].

Table 3
Material quantities and slump value of all the specimens.

SAMPLE ID	CALCINATION DURATION IN Hrs	GGBS		PC		WCA		FA	CA	WATER	SLUMP mm
		%	kg/m3	%	kg/m3	%	kg/m3	kg/m3	kg/m3	kg/m3
CGPC	NA	85	382.5	15	67.5	–	–	705	1006	225	85
C10H05	10	95	427.5	–	–	5	18.5	705	1006	225	82
C10H10	10	90	405	–	–	10	36.9	705	1006	225	78
C10H15	10	85	382.5	–	–	15	55.5	705	1006	225	75
C10H20	10	80	360	–	–	20	73.9	705	1006	225	70
C12H05	12	95	427.5	–	–	5	18.5	705	1006	225	83
C12H10	12	90	405	–	–	10	36.9	705	1006	225	79
C12H15	12	85	382.5	–	–	15	55.5	705	1006	225	76
C12H20	12	80	360	–	–	20	73.9	705	1006	225	72
C14H05	14	95	427.5	–	–	5	18.5	705	1006	225	82
C14H10	14	90	405	–	–	10	36.9	705	1006	225	78
C14H15	14	85	382.5	–	–	15	55.5	705	1006	225	74
C14H20	14	80	360	–	–	20	73.9	705	1006	225	70

For comparison with conventional systems, the performance of the present mixes was discussed in terms of GGBS-based geopolymer concrete activated by NaOH and Na2SiO3 because the same were widely reported in the literature. In sodium-based systems, early age high strength is generally observed because of faster dissolution of aluminosilicate precursors [¹⁴, ²¹]. In the current Potassium based system, strength development is relatively more gradual yet becomes comparable at 28 days, especially the C12H15 mix. This suggests that potassium rich WCA can serve as a viable alternative activator with comparable long-term performance with the added benefit of sustainability and handling.

3. RESULTS

3.1. Physical and chemical characterization of WCA

3.1.1. pH Value

The pH values of waste coffee husk ash (WCA) after calcination at different durations are presented in Figure 3. It can be seen that the alkalinity of WCA is enhanced gradually with the time of calcination, and it reaches the peak at 12 hours with the pH of about 13.98, which is almost 99.9%. This is an indication that calcination can effectively mobilise the existing alkaline content in the coffee husk, making it more appropriate to use in the role of an activator of geopolymer binders.

Figure 3
Variation of pH of WCA with calcination duration.

The increase in pH due to calcination can be explained by the breakdown of organic material and the development of alkali-rich phases, mainly calcium hydroxide and potassium carbonate, which make it highly alkaline. This finding is in accord with the previous reports regarding agro-waste ashes, whereby long-term calcination increased the level of reactive alkali species, which increased their possibility to dissolve aluminosilicate raw materials. After 12 hours, the pH starts decreasing slightly, and it can be attributed to phase changes that take place at a higher temperature. Long-term calcination causes the creation of slower-reacting phases, including β-tricalcium phosphate (β-TCP) and other stable calcium compounds, which decrease the concentration of the hydroxide ions on the surface. Also, excessive calcination can lead to sintering of the particles of the ash, decreasing surface area and limiting the availability of alkaline species.

Therefore, 12 hours of calcination is an effective compromise between the mobilization of the alkali population and preservation of phase reactivity and surface area. The 12-hour calcinated WCA has a high level of alkalinity, which is enough to enhance the dissolution of aluminosilicate precursors in geopolymer systems that is capable of causing efficient geopolymerization and consequent formation of mechanical strength. This observation is in line with the findings of another research on fly ash and rice husk ash, where optimum calcination or thermal activation increased the pH and reactivity of the ash, which enhances binder activation. Results of the pH analysis show that the optimal time of the calcination of WCA to give the maximum alkalinity and make it a natural activator in geopolymer is 12 hours, with maximum applicability to sustainable geopolymer use. Similar improvement of the alkalinity using thermal treatment of waste-derived materials has been reported in recent papers on the use of potassium-activated geopolymer systems in which increasing pH may result in the dissolution of the aluminosilicate precursors and the formation of gels [³¹]. The optimum pH noted at 12 hours of calcination is also in accordance with the results from studies carried out with coffee-waste-based geopolymers, where controlled thermal activation was shown to improve the alkaline availability without excessive sintering [³²].

3.1.2. Solubility

The solubility test was conducted to test the dissolution behaviour of the chemical components of the calcinated coffee husk ash (WCA) in water. To prepare the test solution, 2 g of calcinated ash was dissolved in 20 mL of distilled water. This was then filtered with filter paper, and the material left behind on the filter paper dried under the oven at 100 °C for 1 hour. The dried residue was weighed, and the difference was obtained as the solubility of WCA. Figure 4 below represents the findings of the test, indicating the change in water solubility as a function of calcination time.

Figure 4
Variation of water solubility of WCA with calcination duration.

It is noted that the WCA that has been calcinated over a period of 12 hours has the highest level of solubility in water. Water solubility is a trend that follows the change in pH mentioned above closely. Solubility rises with the length of time of calcination to a point of 12 hours, after which it declines. In particular, of 2 g of WCA that is calcinated for approximately 12 hours, approximately 1.09 g of it dissolves in distilled water, which means that its solubility is greater than 50%. The solubility of calcinated WCA, which is primarily composed of calcium hydroxyapatite, is highly affected by the period of calcination. This increased solubility at 12 hours may be explained by the fact that a smaller crystallite size formed a metastable hydroxyapatite phase that exposes a higher surface area and allows ion exchange. The dissolution is further assisted by the controlled release of hydroxide ions to sustain an alkaline environment. Above 12 hours of calcination, solubility declines as the hydroxyapatite is converted to lower solubility phases, including β-tricalcium phosphate (β-TCP) and calcium carbonate (CaCO3). Such stages decrease the ion availability to dissolution, whereas the development of the surface layers on particles further limits access to water. The experimental findings are clear that the 12-hour calcinated WCA is best in applications that need quickness in the liberation of ions like in cementitious binders.

The improved water-solubility of calcined 12 h WCA and its correlation with reactivity is similar to what has been noted when studying coffee waste-based geopolymer systems where increased dissolution of alkali activators improved geopolymerization kinetics [³²]. Similar correlations between solubility, alkalinity and binder performance have also been noted in recent reviews on agro-waste based geopolymer systems [³³].

3.1.3. Calcinating efficiency and loss of ignition of WCA

The calcining efficiency is determined by comparing the initial and final weight of WCA before and after Calcination. Table 4 below shows the initial and final weight of WCA before and after calcination. The loss of Ignition value is determined based on the following equation,

(1) $$LossofIgnition=\frac{\text{(W1-W2)}}{\text{W1}}\times 100$$

where

W1-Mass of sample before calcination.

W2-Mass of sample after calcination.

Table 4
Calcination efficiency and loss of ignition.

SL No	CALCINATION DURATION	W1 (gm)	W2 (gm)	CALCINATION EFFICIENCY IN %	LOSS OF IGNITION IN %
1	2	108	102	94.44	5.56
2	4	108	99	91.67	8.33
3	6	108	95	87.96	12.04
4	8	108	91	84.26	15.74
5	10	108	85	78.70	21.30
6	12	108	83	76.85	23.15
7	14	108	79	73.15	26.85

The calcination behaviour of WCA at 1000°C was analysed in terms of LOI and calcination efficiency. LOI represents the percentage of unburnt carbon and volatile matter remaining in the ash, while calcination efficiency indicates the effectiveness of thermal treatment in decomposing organics and stabilizing mineral phases.

Based on the results (Table 4 and Figure 5), it can be noted that the LOI is gradually growing with the duration of calcination. LOI of 2 hours is approximately 5.6, which means that a significant portion of organics decomposes even during the first stage. An increase in time by 4 hours yields LOI of 8.3, and after 6 hours, LOI is 12.0, indicative of further decomposition of hemicellulose, cellulose and lignin fractions. The additional time of calcination elevates LOI to 15.7% after 8 hours, 21.3% after 10 hours and 23.2% after 12 hours. The highest LOI of 26.8 is seen at 14 hours, implying that the longer the calcination time, the higher the proportion of the volatile matter removed and the stabilization of the ash. This behaviour is also parallel to the progression of ash colour to dark brown at shorter times and greyish-white at longer times, which is a sign of less carbon being burnt.

Figure 5
Variation of loss on ignition (LOI) of WCA with calcination duration.

There is also an improving trend of the calcination efficiency values with an increase in the duration of the calcination. When the duration is short, the combustion is not completed, resulting in lower values of efficiency. When the duration is long, the thermal decomposition of organics is increased, resulting in increased efficiency. The efficiency at 2 hours is relatively low at 73.6, as there is a large proportion of unburnt material. The efficiency increases progressively with time: it is 81.3 percent at 4 hours, 87.6 percent at 6 hours, 92.1 percent at 8 hours, and 94.5 percent at 10 hours. The highest efficiency of 97.5 is obtained at 14 hours, and the efficiency at 12 hours is 96.3. This is a clear indication that the longer the time taken in the calcination process, the higher the rate at which organics are destroyed, and the ash generated is much cleaner and more stable.

Trends in LOI and calcination efficiency have been observed to be consistent with the analysis of XRD. WCA at 6 hours has more amorphous silica and moderate crystalline phases, which enhances the pozzolanic reactivity. At 12 hours, the LOI of 23% and efficiency of 96.3 is associated with the formation of the crystalline phases, which include akermanite, CaO and magnesium aluminate, which means that mineral structures have stabilized. Therefore, calcination at 1000°C for 6–12 hours can be considered optimal depending on the intended application. Calcination for 6 hours produces ash with higher amorphous content and good pozzolanic activity, while 12-hour calcination yields ash with lower carbon content and greater thermal stability. Further calcination to 14 hours, although achieving the highest LOI and efficiency, does not offer significant additional benefits and may not be energy-efficient for large-scale production.

The gradual increase in LOI with calcination duration and its influence on reactivity aligns with previous studies, where thermal treatment removed unburnt carbon and stabilized mineral phases, improving binder performance [³²]. Recent reviews on agro-waste and industrial by product based geopolymers also suggest that optimum calcination time is required to find a balance between carbon removal and amorphous phase retention for effective geopolymerization [³³]. Although, the LOI value at the calcination duration of 12 hours was relatively high (23.15%), the corresponding WCA still had a good performance in concrete. This has been attributed to the fact that there were enough reactive potassium-rich phases and amorphous silica, which compensated for the effect of unburnt carbon and played a role in effective alkali activation and strength development.

3.1.4. XRD

The crystalline phase composition of calcinated WCA was investigated through X-ray diffraction (XRD) analysis after calcination at 1000°C for two different durations, i.e., 6 hours and 12 hours. The diffraction patterns of both samples are shown in Figures 6 and 7. The identified crystalline phases mainly include K2O (K, PDF No. 41-1366), CaO (Ca, PDF No. 37-1497), Quartz (Q, PDF No. 46-1045), Akermanite (Ak, PDF No. 35-0590), Barium peroxide (B, PDF No. 06-0062), Forsterite (F, PDF No. 34-0189), and Magnesium aluminate (Ma, PDF No. 21-1152). The nature and intensity of the peaks clearly indicate the influence of calcination duration on the crystallization process and phase stability.

Figure 6
XRD pattern of WCA calcinated for 6 hours at 1000°C.

Figure 7
XRD pattern of WCA calcinated for 12 hours at 1000°C.

The XRD pattern in 6-hour calcination demonstrates the prevalence of potassium-based crystalline phases (K2O) and the presence of quartz (Q) and minor amounts of akermanite and CaO. The sharp peaks at 2θ ≈ 26–32° are associated with quartz and potassium compounds, implying that the partial crystallization took place during this period. The presence of amorphous silica is also suggested by the relatively wide humps at low angles that are characteristic of biomass ashes. This amorphous silica is preferable in pozzolanic reaction, because it increases the reactivity of the ash when mixed with cementitious materials. The fact that crystalline quartz is present, however, suggests that within 6 hours, full conversion of silica to the amorphous state is not attained.

In the case of 12 hours of calcination, the XRD pattern depicts that the crystallinity is further enhanced. There are more separate and sharp peaks of abundance of K2O, CaO, Akermanite and Magnesium aluminate, and other smaller phases like Forsterite and Barium oxide also exist. The maximum intensity of quartz is relatively low and indicates that the inflexible calcination results in the increased decomposition of organic matter and changing silica phase. The enhancement of the peaks that represent the presence of akermanite (Ca2MgSi2O7) and magnesium aluminate means that the formation of stable crystalline compounds occurs at the extended exposure to high temperature. This implies that after 6 hours, phase stabilization and crystalline mineral growth are promoted at the cost of amorphous material due to calcination. When comparing both samples, it can be seen that the time of calcination is critical in developing the phases. Although the sample of 6 hours has a larger fraction of amorphous silica, the crystal nature of the 12-hour sample has high crystallinity with a definite mineral phase. It means that the 6-hour calcined WCA can be more appropriate to pozzolanic reactivity because amorphous silica content promotes the development of secondary hydration products of cementitious systems. The 12-hour calcined WCA, conversely, comprises more stable crystalline oxides like akermanites and CaO, which would inhibit pozzolanic activity, although may aid in long-term strength building and durability.

Altogether, microstructural analysis proves that a careful selection of the time of calcination is crucial to maintain a balance between the amorphous and crystalline phases of waste coffee husk ash. When high pozzolanic reactivity is required, it is advantageous to have reduced calcination (6 hours) at 1000°C, and long calcination (12 hours) gives a more crystalline structure, which can be useful in various functional applications of blended cement systems. Even though the 6-hour and 12-hours calcined ashes demonstrated better amorphous and crystalline phases balance as confirmed by XRD and other physical characterizations, the 14-hour calcined ash was also taken into consideration to test the concrete cubes to investigate whether the superior extent of crystal phases and thermal stability acquired at the long periods of calcination would have some impact on the compressive strength performance. The phase evolution observed in WCA was consistent with results obtained in studies on potassium activated steel slag-based geopolymer systems in which de-crystallization of crystalline phases with formation of new mineral phases was reported indicating structural reorganization during alkali activation [³¹]. Similar mineral transformations have also been reported in coffee-waste-based geopolymer systems, that calcined WCA contributes to geopolymer gel formation and structural development [³²].

The XRD result supports the formation of the geopolymers in WCA-GGBS systems [¹⁴, ²²]. The change in peak intensity of quartz and the presence of akermanite and calcium-rich phases indicate partial dissolution and reorganization of the aluminosilicate structures. The development of such phases, as well as the great alkalinity of WCA, indicates the formation of a potassium based geopolymer gel. This microstructural evolution is consistent with this observed mechanical perfor-mance improvement, especially for the C12H15 mix. Also, the presence of a broad hump in the XRD patterns between 20° and 35° (2θ) attributed to the existence of amorphous silica in calcinated WCA is beneficial for the alkali activation and formation of geopolymer gel. Detailed SEM and FTIR analysis will be performed in the future work for further confirmation of the nature of the geopolymer gel and pore structure.

3.1.5. Activation mechanism of potassium-rich WCA

The mechanism of activation of calcined WCA is dominated by the high potassium content (K2O), as shown from Table 2. During mixing with water, potassium compounds in WCA dissolve and release K and OH ions into the pore solution. These alkaline ions raise the pH of the system and have provided a favourable environment for the dissolution of aluminosilicate phases from GGBS. The dissolved silica and alumina then take part in the geopolymerization process and potassium-based aluminosilicate hydrate gel (K-A-S-H) is formed.

Compared to conventional sodium hydroxide (NaOH) activators, it is relatively slower in the initial dissolution rate of WCA as it is a solid, natural derived activator and not a highly concentrated chemical solution [³⁴]. However, WCA generates a sustained release of alkaline species over a period of time, which promotes geopolymerization over time and continuous strength development [¹²]. This behaviour is reflected in the gradual increase in compressive strength from 3 to 28 days, particularly in the C12H15 mix.

The higher solubility and pH of 12-hour calcined WCA gradually release potassium in the mixing water that enhances the alkalinity and improve the dissolution of GGBS helps in the formation of binding gels (K-A-S-H and C-A-S-H) responsible for strength development. Over-calcination (14 hours) causes a slight reduction in amorphous and a slow reactivity which accounts for slight decrease in strength compared to 12-hour calcined WCA.

3.2. Fresh and mechanical characterization of calcined WCA

3.2.1. Workability

The slump cone test was used to determine the workability of concrete mixes with various proportions of WCA, and the findings are summarized in Table 3. The control mix (CGPC) had a slump of 85 mm, which is a medium workability as per standard classification (IS 1199-1959), and this was used as a reference. The workability of the mixes in which WCA was calcined for over 10 hours reduced gradually with increasing replacement levels. The slump decreased to 82 mm of C10H05 and 70 mm of C10H20. According to the standard classification, C10H05 and C10H10 belong to medium workability, C10H15 is at the border of medium and low workability, and C10H20 is low workability. The decreased slump is explained by the large surface area and size of fine particles of WCA that enhance water requirement and decrease flow.

A similar trend was observed with the mixes with WCA calcined in 12 hours. The slump was reduced by 83 mm (C12H05) to 72 mm (C12H20). In this case, C12H05 and C12H10 had a medium workability, C12H15 had a low workability, and C12H20 had a low workability. This increase in the value of the slump at lower replacement levels relative to the C10 series could be explained by the longer period of calcination, which lowers the amount of residual organic material in WCA, slightly increases particle dispersion and flowability. Concrete samples with WCA that were calcined for 14 hours experienced a reduction in slump from 82 mm (C14H05) to 70 mm (C14H20). Similar to the other series, 5–10 replacement gave moderate workability, 15% replacement was close to the lower medium range, and 20% replacement gave low workability. The minor decrease in workability relative to C12H15 can be attributed to the higher crystallinity of silica in over-calcined WCA, and silica captures water faster and reduces free water to lubricate.

The slump test indicates that workability decreases with increasing WCA replacement, irrespective of calcination duration. Longer calcination (12–14 hours) slightly improves slump at lower replacement levels due to reduced organic content, but high replacement (≥20%) leads to low workability in all cases. Therefore, practical mix design should consider water-adjustment or superplasticizer addition to maintain sufficient workability, especially for mixes with 15–20% WCA replacement. The reduction in workability with increasing WCA content is in line with recent studies on one-part alkali-activated mortars based on waste materials, where finer particles and higher surface area increased water demand and reduced flowability Similar observations have been reported for coffee husk based geopolymeric composites in which the proportion of solid activator produced higher specific water and mix proportion has to be carefully controlled for workability [³²]. The better flowability observed for 12-hour calcinated WCA, even with higher LOI, is mainly due to the finer particle size achieved after longer calcination and ball milling. The finer particles improved packing in the mix and reduced resistance to flow.

3.2.2. Compressive strength

The compressive strength of concrete cubes (150 × 150 × 150 mm) containing different percentages of waste coffee husk ash (WCA) was determined at 3, 7, and 28 days, and the results are presented in Figures 8 to 10. Various combinations of WCA and GGBS were used as partial replacements for GGBS. The mix ID notation, such as C10H10, indicates 10% replacement of GGBS with WCA calcinated for 10 hours. The specimens were demoulded after 24 hours and cured at ambient temperature until testing. In the concrete specimens containing WCA calcined for 10 hours, compressive strength increased with replacement levels up to 15%, beyond which a slight reduction was observed. The control mix (CGPC) recorded compressive strengths of 18.5 MPa, 29.8 MPa, and 44.3 MPa at 3, 7, and 28 days, respectively.

Figure 8
Compressive strength of concrete containing 10-hour calcined WCA at 3, 7 and 28 days.

Figure 9
Compressive strength of concrete containing 12-hour calcined WCA at 3, 7 and 28 days.

Figure 10
Compressive strength of concrete containing 14-hour calcined WCA at 3, 7 and 28 days.

Meanwhile, C10H15 recorded the highest strengths of 19.8 MPa, 26.4 MPa, and 39.3 MPa at 3, 7, and 28 days, respectively, in the C10 series. Within the C10 series, C10H15 exhibited the highest strength; however, when compared with the control CGPC mix, C10H15 showed a 7.0% increase at 3 days but lower strength at 7 days (−11.4%) and 28 days (−11.3%). Although the 28-day strength of C10H15 was lower than the control, the slight improvement at 3 days suggests a faster early reaction, which may be associated with the presence of reactive phases in calcined WCA. At 20% replacement (C10H20), compressive strength decreased to 17.6 MPa, 24.0 MPa, and 35.3 MPa at 3, 7, and 28 days, respectively. This represents a reduction of 4.0%, 19.5%, and 20.3% compared to C10H15, confirming that 15% is the optimum replacement level for 10-hour calcined WCA.

Significant enhancement in compressive strength was observed in the concrete specimens that contained WCA that was calcinated within 12 hours. The C12H15 mix had a strength of 22.3 MPa, 32.8 MPa and 45.5 MPa at 3, 7 and 28-days, respectively. This is an improvement of 20.5, 10.1 and 2.7 percent on the control at the respective ages, which indicates a positive effect of 12-hour calcination on the reactivity of WCA. The 3-day strength enhancement also introduces the enhanced early hydration as a result of the effective release of the reactive amorphous silica. Beyond 15% replacement (C12H20), strengths slightly decreased to 20.6 MPa, 30.7 MPa, and 43.8 MPa, corresponding to 7.6%, 6.4%, and 3.7% reduction relative to C12H15, likely due to the dilution effect and agglomeration of ash particles reducing effective binder content.

In the mixes with WCA calcined for 14 hours, compressive strength also increased with replacement up to 15%, though slightly lower than the C12 series. The mix C14H15 showed strengths of 21.5 MPa, 29.9 MPa, and 42.8 MPa at 3, 7, and 28 days, corresponding to a 16.2% increase at 3 days, a marginal increase of 0.3% at 7 days, and a 3.4% decrease at 28 days compared to the control. The slight decline compared to the C12 series may be attributed to over-calcination, resulting in partial crystallization of silica and reduced pozzolanic reactivity. At 20% replacement (C14H20), strengths reduced to 19.3 MPa, 27.7 MPa, and 40.9 MPa, indicating 10.2%, 7.3%, and 4.4% reduction relative to C14H15. It implies that over replacement makes WCA less of a reactive binder and more of a filler, which lowers effective cementitious bonding.

The findings show that a 15 percent substitution of binder with WCA gives the best compressive strength regardless of the time of calcination. Comparing all the calcination periods, 12-hour calcination yielded the highest strengths, followed closely by 14-hour calcination. There is increased early-age strength development in mixes containing 12-hour calcined WCA, indicating higher early reactivity compared to other calcination durations. Although 14-hour calcination has been found to be effective, a hint of over-calcination will also decrease the amorphous silica content and hence reduce strength slightly. A replacement of over 15 percent tends to cause the effects of dilution and less bonding efficiency. The above observations show that both the duration of calcination and the replacement level have a critical role in maximizing mechanical performance of WCA-blended concrete. The optimum replacement level of 15% WCA and the superior performance of 12-hour calcined ash are comparable to results reported in recent studies which states an optimum activator content helps to balance the reactivity and dilution effects to achieve maximum strength [³²]. Similar trends of gradual strength development in potassium-activated systems have also been observed in structural characterization studies of KOH-activated slag-based geopolymers [³⁵].

3.2.3. Tensile strength

Figures 11 to 13 illustrates the split tensile strength of the concrete specimen with different percentages of WCA calcinated at different periods at 3, 7, and 28 days. The control mix (CGPC) had tensile strengths of 1.7 MPa, 2.9 MPa, and 4.3 MPa at 3, 7 and 28 days, respectively, which can be used as a basis of comparison. Mix containing WCA calcined for 10 hours, tensile strength was enhanced with the replacement till 15 percent, then a marginal decrease was noted. The mix C10H15 obtained tensile strengths of 1.8 MPa, 2.4 MPa, and 3.9 MPa in 3, 7 and 28 days, respectively. Compared to the control, these values indicate a 5.9% increase at 3 days, 17.2% decrease at 7 days, and 9.3% decrease at 28 days, which is evidence of early-age densification of the microstructure as a result of pozzolanic reaction. At higher replacement, the marginal loss at higher replacement (C10H20: 1.5 MPa, 2.2 MPa, 3.5 MPa) is 16.7% loss, 8.3% loss and 10.3% loss when compared with C10H15. This is because of reduced Interfacial Transition Zones (ITZ) between WCA particles and surrounding binder paste as levels of their replacement increase, and restricts the transfer of tensile stress.

Figure 11
Split tensile strength of concrete containing 10-hour calcined WCA at 3, 7 and 28 days.

Figure 12
Split tensile strength of concrete containing 12-hour calcined WCA at 3, 7 and 28 days.

Figure 13
Split tensile strength of concrete containing 14-hour calcined WCA at 3, 7 and 28 days.

The mix has WCA calcined for 12 hours, showed the most substantial change in all age groups in tensile strength. A mix C12H15 was obtained with 2.0 MPa, 3.3 MPa and 4.5 MPa at 3, 7 and 28 days, respectively, although 17.6, 13.8 and 4.7 percent higher than the control mix. The development of tensile behaviour can be explained by the fact that the optimization of the calcination and the 15% replacement increase the reactive amorphous silica content and lead to the better bonding within the matrix and the pore structure refinement. Even early-age strength at 3 days increased noticeably, indicating accelerated crack-bridging capability and more uniform stress distribution. Beyond 15% replacement, a slight decline of 5–10% relative to C12H15 was observed, likely due to the aggregation of ash particles causing local weak points and reduction of continuous cementitious matrix connectivity.

The mix with WCA calcined for 14 hours, tensile strength also improved up to 15% replacement, but slightly lower than the C12 series. The mix C14H15 showed tensile strengths of 1.9 MPa, 3.0 MPa, and 4.3 MPa at 3, 7, and 28 days, representing 11.8%, 3.4%, and 0% increase over control, respectively. The small decrease compared to the C12 series may result from over-calcination, which reduces the amorphous fraction of silica and slightly weakens the ITZ, thus affecting tensile stress transfer. At 20% replacement (C14H20: 1.8 MPa, 2.8 MPa, 4.1 MPa), a further reduction of 5.3%, 6.7%, and 4.7% relative to C14H15 was observed, confirming that excessive ash content diminishes tensile performance, likely because ash particles act more as fillers than reactive binders, reducing cohesion under tensile loading. The findings show that 15 percent GGBS-WCA substitution offers the best tensile strength, whereas the 12-hour calcination is the most efficient because a fine and compact microstructure is formed, and the bonding in the ITZ is enhanced. The tensile strength of the early ages is positively impacted by the presence of WCA, which is manifested in the increase of microstructural integrity and crack resistance. Over-replacement or over-calcination will decrease tensile performance, and it is important to optimize the length of calcination and the degree of replacement on tensile behaviour. The improvement in tensile strength at optimum WCA content is consistent with observations in potassium-activated geopolymer systems, where better interfacial bonding and pore refinement enhanced tensile performance [³¹]. Recent studies also reported that improved crack resistance and stress distribution is observed when the activator content and microstructure were properly optimized [³⁶].

3.2.4. Practical implications

Calcination of WCA at 1000°C for a longer duration is energy-intensive and may affect sustainability at an industrial scale. In this laboratory study, this temperature was adopted to remove organic matter completely and to activate the potassium-rich phases in WCA under controlled conditions. However, for practical large-scale use, the calcination temperature and duration need to be optimized. Continuous and controlled calcination of coffee husk ash at 1000°C may require dedicated furnaces and higher energy input. The availability of coffee husk may also vary with region and season. Proper grinding of WCA is necessary to obtain a uniform particle size. At higher WCA content, workability may reduce, and the use of superplasticizer may be required in real construction applications [³⁷].

4. CONCLUSIONS

The present study was primarily focused on the physico-chemical characterization of calcined WCA and its influence on the fresh and mechanical properties of one-part geopolymer concrete. The study clearly illustrates that the calcinated waste coffee husk ash (WCA) can be successfully used as a natural alkaline activator in the manufacturing of one-part geopolymer concrete. The study proves the potential of WCA as a substitute for the chemical activators commonly used in the construction industry that are typically hazardous, corrosive, and less environmentally friendly. The physical and chemical properties of WCA were investigated through systematic experimentation by changing the period of calcination with 2hr intervals, and its influence on alkaline activity and performance of geopolymer was carefully examined. Based on the results, it is concluded that 12 hours of calcination at 1000°C is the most effective when compared with other durations. The 12-hour calcinated WCA showed a better performance in the alkalinity (pH), water solubility and loss on ignition (LOI) performance. The pH value increased with calcination time and reached its highest at 13.98 after 12 hours, indicating a highly alkaline nature suitable for polymer activation. After this period, a minimal decrease in the pH was recorded, which indicated that excessive calcinations might weaken the active ingredients. The solubility of the 12-hour calcinated WCA was maximum at 54.50 percent, which reveals the existence of water-soluble alkaline compounds, particularly potassium carbonate, which is essential in the activation of the binder system. In addition, the optimal time of calcination (calculated as loss on ignition) was identified as 12 hours, with an LOI of 23.15, which is within acceptable limits, indicating that the majority of organic matter is removed. The highest compressive strength was recorded with the WCA mix designated as C12H15, which contains 15% replacement with 12-hour calcinated WCA blended with GGBS. It was an increment of 20 percent in 28-day compressive strength over that of the mix with 10-hour calcined WCA. This increase is directly related to the increase in the pH and increased reactivity of the ash in 12 hours of calcination. In summary, it is clear that the study shows that WCA that undergoes 12 hours of calcination has the potential to substitute the chemical activators of geopolymer concrete systems. It provides a green, sustainable and economical product that can be used in the construction industry on a large-scale basis and at the same time, provides the solution to the disposal of agricultural waste products such as coffee husk.

The present study was primarily focused on the physico-chemical characterization of calcined WCA and its influence on the fresh and mechanical properties of one-part geopolymer concrete. Durability investigations such as acid resistance, sulphate attack, chloride penetration, and long-term performance will be addressed in future studies. Future research may also explore energy optimization of the calcination process and particle size refinement of WCA for improved performance.

5. ACKNOWLEDGMENTS

The authors would like to acknowledge the Mangalam College of Engineering and APJ Abdul Kalam Technological University, Thiruvananthapuram, for providing moral and infrastructure support to do this research.

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Publication Dates

• Publication in this collection
  15 May 2026
• Date of issue
  2026

History

• Received
  27 Nov 2025
• Accepted
  01 Apr 2026