Comparison of the Effectiveness of Reducing the Leaching of Formaldehyde from Immobilized Wool in Geopolymer and Cement Mortar

Abstract

Innovative building materials should also be pro-environmental. This article discusses the environmental footprint of geopolymer and cement-based mortars. It describes the methodology for preparing geopolymer and cement mortars using mineral wool waste. The phenol–formaldehyde resin used in mineral wool is a source of phenol and formaldehyde emissions to the environment. The prepared mortar samples were subjected to durability tests to assess the correlation between the amount of mineral wool and the flexural and compressive strength of the samples. The key element of the paper is to test whether immobilisation of mineral wool in the geopolymer will reduce leaching of phenol and formaldehyde into the environment. The results revealed that cements prepared with mineral wool showed higher compressive strength, whereas geopolymer samples had better flexural strength. The study also proved that immobilisation of the wool in the geopolymer reduces phenol and formaldehyde leaching significantly.

1. Introduction

This paper discusses the problem of leaching of hazardous substances from mineral-wool-based geopolymer and cement mortars. The partial substitution of wool waste with metakaolin and CEM II cement is investigated as a method to prevent leaching. The earliest information regarding geopolymers goes back to ancient times, when it had various applications. Some sources state that the ancient Egyptians used similar solutions in the construction of the pyramids [1]. The concept of geopolymers was introduced into the literature by Professor Joseph Davidovits in 1978 [2]. What exactly is a geopolymer? Geopolymers are synthetic, nonorganic, i.e., not containing carbon atoms in their structures, polymers of aluminosilicates. The basic reaction behind the formation of this material is the synthesis of silicon and aluminum. In other words, geopolymers are synthetic aluminosilicate polymers characterized by an amorphous internal structure. The definition geopolymer can be presented by analysing its two parts: “geo” and “polymer”, the former indicating that t is a material with a structure that imitates raw materials found in nature, hence, the ‘geo’ in the name [3]. Furthermore, like a polymer, it is formed of multiples of repetitive minor particles, hence, ‘polymer’.

The base of a geopolymer composition is a powdered material whose chemical composition is dominated by aluminosilicates. The other major component of a geopolymer is an activator—a chemical mixture whose task is to create an alkaline environment in which the binding reaction will take place [3]. Geopolymer is an innovative environmentally friendly building material, commonly referred to as ‘green concrete’. According to the sources, the production of geopolymer uses up to three times less energy compared with classic Portland cement concrete. The same is true for CO₂ emissions. Geopolymers are regarded as the ecological successor to Portland cement as classical concrete production generates several times more harmful greenhouse gases. The production of one tonne of Portland cement emits approximately 1 tonne of CO₂ into the atmosphere. The synthesis of geopolymer generates 4–8 times less CO₂ [4,5]. Mineral wool is a widely used construction material, performing the function of cold, fire, and noise insulation. Waste from this building material is generated during both the demolition and production process [6]. Mineral wool waste poses a significant environmental challenge. In the age of thermomodernisation of buildings, massive amounts of this waste are being generated, reaching an estimated annual of 2.5 million tonnes in the European Union. Due to the difficulty of recycling, most of this ends up in landfills. The management of mineral wool waste is a difficult issue. Due to the specific characteristics of the waste, it is stored in industrial landfills and, thus, contributes to environmental degradation. Nevertheless, due to its low bulk density, high volume, poor compressibility, and the consequential high elasticity, mineral wool causes problems, even in landfilling. Its characteristics adversely affect the landfill stability [7]. The synthesis of geopolymer may offer a method to counteract this occurrence. Geopolymers can be obtained from various raw materials, including fly ashes [8,9], bottom ashes [10], glass powder [11], natural rocks [12], and mineral wool [13] and metakaolin [14], which are the subjects of research in this article. A problem that the production of wool-based geopolymers can generate is the leaching of environmentally hazardous substances. This issue was covered in a previous article of the team [15], where acceptable concentrations of sodium and strong alkalinity were found to be exceeded. An additional challenge is the reduced leachability of phenol and formaldehyde. This paper addresses the issue found in that article regarding the exceedance of permissible leaching standards for geopolymer mortars. The replacement of mineral wool with metakaolin aims to reduce the leaching of exceeded parameters. The article also investigates the effect of using metakaolin on the compressive strength of the geopolymer and determining the ratio of wool to MK that will result in the best durability.

2. Materials and Methods

2.1. Materials

The materials used for the research were recycled stone wool waste and metakaolin, which acted as bonding agents. The following materials were used to bond the mortar: in three series of samples, geopolymer activation was used. The other three samples were bonded with Portland cement (CEM II) multicomponent cement. The final geopolymer mass was created and examined using these materials.

2.1.1. Mineral Wool

The stone wool waste sample was sourced from production waste, so it is characterised by its homogeneity and low moisture content. The chemical composition of the wool was supplemented with alumina to equalize its chemical composition to metakaolin, which is aluminium-rich. Ten weight percent Al₂O₃ was added to the wool. The fraction of wool before milling is presented in Figure 1.

The stone wool waste sample was pulverised using a Los Angeles type ball mill according to EN 1097-2 [16]. The drum was set at 3000 rpm. After milling, the product was the residue as presented in Figure 2.

The granulometry of the ground wool was examined. The granulometric curve is illustrated in Figure 3.

The modal value of the milled sample was 21.20 μm, and the median was 18.31 μm. The measured specific surface area of the wool after milling was 25,700 cm²/g. Determination of specific surface area was carried out on a Gemini 2360 apparatus from Micromeritics. The principle of measurement is to measure the adsorption of gas (nitrogen) on the surface of the adsorbate.

2.1.2. Metakaolin

Metakaolin is a product, or more precisely a by-product, created by firing kaolin at high temperatures. Chemically, it is an aluminosilicate. It is a very efficient pozzolanic material that can be successfully used in mortars [16]. In contrast to kaolin, which has a crystalline structure, metakaolin is amorphous. Due to its amorphous structure, metakaolin is very reactive when in contact with an alkaline solution [17]. For this reason, metakaolin is ideally applicable as a raw material for the production of geopolymer [18,19] The metakaolin used for this study (Figure 4) comes from the producer ASTRA MK-40 [20]. The relative density of the metakaolin used in the study is 2.30–2.80 g/cm³. The specific surface area of the material, as specified by the supplier, is 20,000 cm²/g.

2.1.3. Comparison of Stone Wool and Metakaolin

When comparing the chemical compositions of mineral wool and metakaolin, similarities are observed.

Table 1. Comparison of chemical compositions [15].

Chemical Composition	SiO2	Al2O3	Fe2O3	TiO2	CaO	K2O	MgO	SO3	Na2O	P2O5
Mineral wool	43.8	16.4	5.4	0.5	21.83	0.21	9.7	0.0	1.9	0.1
Metakaolin	52.00	45.00	0.50	1.50	0.40	0.05	0.20	0	0.15	0

presents a comparison of the chemical compositions of the most commonly used raw materials, i.e., stone wool and metakaolin, for the preparation of the geopolymer.

By analysing the chemical composition of metakaolin, it can be concluded that it is an ideal aluminosilicate [21]. There are high contents of silicon oxide and aluminium oxide with very low proportions of residual oxides. In the case of stone wool, a dominance of silicon oxide, calcium oxide, and aluminium oxide is observed. However, a lower proportion of alumina than that in metakaolin can cause difficulties in geopolymer synthesis, which is why the stone wool was enriched during the study by adding 10% alumina in powder form. The chemical composition after enrichment is shown in

Table 2. Chemical composition of stone wool after enrichment.

Chemical Composition	SiO2	Al2O3	Fe2O3	TiO2	CaO	K2O	MgO	SO3	Na2O	P2O5
Mineral wool	33.9	29.9	6	0.5	19.4	0.18	6.8	0.5	2.1	0.1

.

2.1.4. Geopolymer Mortar and Cement Mortar for Continued Research

Six recipes were prepared as described in

Table 3. Summary of recipes prepared for testing.

		NSD (g)	MSW (g)	MK (g)	CEM II (g)	NaOH (g)	H2O (g)
Geopolymer mortars	WM1	1350	135	315	0	212	212
	WM2	1350	225	225	0	187	187
	WM3	1350	315	135	0	162	162
Cement mortars	WM4	1350	135	0	315	0	225
	WM5	1350	225	0	225	0	225
	WM6	1350	315	0	135	0	225

. In all six, the base was norm sand (NSD), according to PN EN 196-1, consisting mainly of quartz with granulation of 0.05–2 mm. The individual recipes contained different proportions of ground stone wool (MSW) and metakaolin (MK): three where the binder was sodium hydroxide (NaOH) and three where the binder was cement (CEM II). The recipes were designed on the basis of increasing proportions of mineral wool against decreasing proportions of MK and CEM II. The method enabled the determination of the amount of leachate from the substances in the mortar depending on the proportion of mineral wool. It also served to determine which better retains leaching contaminants—MK or CEM II.

Mortar samples were then prepared on the basis of the above formulations. Geopolymers made with stone wool waste were tested (Figure 5).

The purpose of preparing the mortars was to further test them for compressive and flexural strength and leaching tests [22]. The geopolymer was forged in 40 × 40 × 160 mm moulds and heat-treated for 48 h at 70 °C for geopolymer mortars and for 72 h at 45 °C for cement mortars. The test samples were then cured under laboratory conditions for an additional 26 days at an air temperature of approximately 20 °C and a humidity of approximately 50%. After this time, flexural and compressive strength tests were performed on the crushed samples (Figure 6).

2.2. Methods

2.2.1. Compressive Strength Test of Mortars

Compressive strength testing is an overall assessment of mortar performance [23]. This parameter is determined by many factors, such as the amount of water, cement, various additives, and their proportions [24]. Durability tests of the flexural and compressive strength of the wool-based geopolymers and mortars were tested according to EN 196-1:2016 [25]. A Controls Model 65-L27C12 Cement Compression and Flexural Machine was used for the tests. The tests were carried out after 28 days of hardening under air conditions of approx. 20 °C and humidity of approx. 50%.

2.2.2. The Leachability of the Wool-Based Geopolymer and Wool-Based Concrete

The test procedure involved preparing aqueous extracts from the prepared geopolymer mortars and estimating the leaching rates from them. This was followed by a comparison with the limits specified in the regulation [26,27]. The procedure for the preparation of water extracts from solid waste was carried out in accordance with PN-EN 12457-2:2006 [28]. Water extracts for waste were prepared at a liquid to solid ratio of L/S = 10 L/kg (basic test). The washing liquid was distilled water with pH 7.1 and specific electrical conductivity of 61.18 µS/cm. The prepared water extracts were shaken on a laboratory shaker for 24 h, after which time, the obtained extracts were filtered. Analysis of the water extracts from the waste included a number of indications. The pH of the aqueous extracts was determined using the potentiometric method in accordance with PN-EN ISO 10523:2012 [29]. Concentrations and contents were determined by a variety of methods, as detailed in the following: Concentration and elemental content of Zn, Cd, Cu, Pb, Cr, Ni, Ba, As, Mo, Se, Sb, and Na by inductively coupled plasma atomic emission spectrometry (ICP-OES) according to EN ISO 11885:2009 [30]; concentration and content of mercury by atomic absorption spectrometry with cold vapour generation (CVAAS) according to EN ISO 12846:2012 [31]; concentration and content of anions: fluorides, chlorides, and sulphates by ion chromatography (IC) according to PN-EN ISO 10304-1:2009+AC:2012 [32]; dissolved organic carbon (DOC) concentration and content by high-temperature combustion with infrared (IR) detection according to PN-EN 1484:1999 [33]; dissolved substances (TDS) by weight method according to PN-EN 15216:2010 [34]; phenol index (volatile phenols) by spectrophotometric method according to PN-ISO 6439:1994 [35]; and formaldehyde concentration spectrophotometric method according to EPA Method 316:2020 [36].

3. Results

The results of the research work are described below.

Table 4. Compressive and flexural test strength results.

Sample No.	Flexural Strength (Mpa)		Compressive Strength (Mpa)
	Average Value	Standard Deviation	Average Value	Standard Deviation
WM1	2.96	0.97	11.34	1.73
WM2	3.12	0.27	9.24	1.28
WM3	3.68	0.40	8.53	1.79
WM4	1.48	0.51	17.78	1.77
WM5	2.58	0.14	17.07	1.07
WM6	1.90	0.18	5.99	1.16

presents the results of the compressive strength, and

Table 5. Leachability of selected contaminants from geopolymer mortar, expressed as mg/kg dry weight compared with maximum acceptable waste disposal rates.

					Criteria for Waste Landfilling Acceptance [26]
					Neutral	Other Than Neutral and Hazardous	Hazardous
PL	Unit	WM1	WM2	WM3
Arsenic, As	mg/kg	0.42	0.39	0.42	0.5	2	25
Barium, Ba	mg/kg	0.040	0.020	<0.01	20	100	300
Cadmium, Cd	mg/kg	<0.01	<0.01	<0.01	0.04	1	5
Chromium, Cr	mg/kg	0.24	0.19	0.13	0.5	10	70
Copper, Cu	mg/kg	2.54	1.59	0.99	2	50	100
Mercury, Hg	mg/kg	0.003	0.003	0.009	0.01	0.2	2
Molybdenum, Mo	mg/kg	<0.20	0.22	0.3	0.5	10	30
Nickel, Ni	mg/kg	0.49	0.58	0.46	0.4	10	40
Lead, Pb	mg/kg	4.08	2.03	0.92	0.5	10	50
Antimony, Sb	mg/kg	<0.20	<0.20	0.21	0.06	0.7	5
Selenium, Se	mg/kg	0.24	<0.20	<0.20	0.1	0.5	7
Zinc, Zn	mg/kg	0.13	0.050	0.040	4	50	200
Chlorides, Cl−	mg/kg	17.0	15	13.8	800	15,000	25,000
Fluorides, F−	mg/kg	28.4	20.6	13.1	10	150	500
Sulphates, SO42−	mg/kg	117	45.3	175	1000	20,000	50,000
Dissolved organic carbon, DOC	mg/kg	633	965	1000	500	800	1000
Total dissolved solids (TDS)	mg/kg	72,760	70,000	67,540	4000	60,000	100,000
Phenolic index	mg/kg	0.870	1.35	0.620	1	-	-

,

Table 6. Leachability of selected contaminants from cement mortar, expressed as mg/kg dry weight compared with maximum acceptable waste disposal rates.

					Criteria for Waste Landfilling Acceptance [26]
	Unit	WM4	WM5	WM6	Neutral	Other Than Neutral and Hazardous	Hazardous
Arsenic, As	mg/kg s·m	<0.01	<0.01	<0.01	0.5	2	25
Barium, Ba	mg/kg s·m	1.80	2.36	2.52	20	100	300
Cadmium, Cd	mg/kg s·m	<0.01	<0.01	<0.01	0.04	1	5
Chromium, Cr	mg/kg s·m	3.23	2.04	1.33	0.5	10	70
Copper, Cu	mg/kg s·m	0.14	0.15	0.17	2	50	100
Mercury, Hg	mg/kg s·m	0.0004	0.0004	0.0002	0.01	0.2	2
Molybdenum, Mo	mg/kg s·m	0.73	0.41	0.37	0.5	10	30
Nickel, Ni	mg/kg s·m	<0.01	<0.01	<0.01	0.4	10	40
Lead, Pb	mg/kg s·m	<0.01	<0.01	<0.01	0.5	10	50
Antimony, Sb	mg/kg s·m	1.03	0.91	0.94	0.06	0.7	5
Selenium, Se	mg/kg s·m	<0.20	<0.20	<0.20	0.1	0.5	7
Zinc, Zn	mg/kg s·m	<0.01	<0.01	0.01	4	50	200
Chlorides, Cl−	mg/kg s·m	31	31	46.6	800	15,000	25,000
Fluorides, F−	mg/kg s·m	2.50	2.50	3.40	10	150	500
Sulphates, SO42−	mg/kg s·m	1870	1600	1550	1000	20,000	50,000
Dissolved organic carbon, DOC	mg/kg s·m	37.4	38.5	44.8	500	800	1000
Total dissolved solids (TDS)	mg/kg s·m	5400	4210	4040	4000	60,000	100,000
Phenolic index	mg/kg s·m	0.870	1.35	0.620	1	-	-

,

Table 7. Leachability of selected contaminants from geopolymer and cement mortar, expressed as mg/kg dry weight compared with maximum acceptable waste disposal rates.

PL	Unit	WM1	WM2	WM3	WM4	WM5	WM6
Arsenic, As	mg/kg	0.42	0.39	0.42	<0.01	<0.01	<0.01
Barium, Ba	mg/kg	0.040	0.020	<0.01	1.80	2.36	2.52
Cadmium, Cd	mg/kg	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01
Chromium, Cr	mg/kg	0.24	0.19	0.13	3.23	2.04	1.33
Copper, Cu	mg/kg	2.54	1.59	0.99	0.14	0.15	0.17
Mercury, Hg	mg/kg	0.003	0.003	0.009	0.0004	0.0004	0.0002
Molybdenum, Mo	mg/kg	<0.20	0.22	0.3	0.73	0.41	0.37
Nickel, Ni	mg/kg	0.49	0.58	0.46	<0.01	<0.01	<0.01
Lead, Pb	mg/kg	4.08	2.03	0.92	<0.01	<0.01	<0.01
Antimony, Sb	mg/kg	<0.20	<0.20	0.21	1.03	0.91	0.94
Selenium, Se	mg/kg	0.24	<0.20	<0.20	<0.20	<0.20	<0.20
Zinc, Zn	mg/kg	0.13	0.050	0.040	<0.01	<0.01	0.01
Chlorides, Cl−	mg/kg	17.0	15	13.8	31	31	46.6
Fluorides, F−	mg/kg	28.4	20.6	13.1	2.50	2.50	3.40
Sulphates, SO42−	mg/kg	117	45.3	175	1870	1600	1550
Dissolved organic carbon, DOC	mg/kg	633	965	1000	37.4	38.5	44.8
Total dissolved solids (TDS)	mg/kg	72,760	70,000	67,540	5400	4210	4040
Sodium, Na	mg/kg	29,710	27,330	28,940	826	346	249
pH (T)	- (°C)	12.7 (22.3)	12.7 (22.4)	12.7 (22.5)	11.2 (23)	11.1 (22.9)	11.1 (22.5)
Formaldehyde	mg/kg	5.56	8.30	12.6	5.18	7.22	8.41
Phenolic index	mg/kg	0.870	1.35	0.620	0.210	0.350	0.470

,

Table 8. Leachability of phenol and formaldehyde from geopolymer, cement mortar, and mineral wool, expressed as mg/kg dry weight compared with maximum acceptable waste disposal rates.

	Unit	WM1	WM2	WM3	WM4	WM5	WM6	Mineral Wool
pH (Temperature)	°C	12.7 (22.3)	12.7 (22.4)	12.7 (22.5)	11.2 (23)	11.1 (22.9)	11.1 (22.5)	8.9 (21.9)
Formaldehyde	mg/kg	5.56	8.30	12.60	5.18	7.22	8.41	578
Phenolic index	mg/kg	8.70	1.35	0.62	0.21	0.35	0.47	7.80

and

Table 9. Correlation between wool quantity and formaldehyde leachate expressed as mg/kg dry weight.

Sample No.	Wool Quantity (%)	Formaldehyde (mg/kg)
WM1	7.5	5.56
WM2	12.5	8.30
WM3	17.5	12.60
WM4	7.5	5.18
WM5	12.5	7.22
WM6	17.5	8.41
Mineral wool	100	578

presents the comparison of the results of the leaching of environmental pollutants from the samples tested with emphasis on the emission of formaldehyde.

3.1. Compressive and Flexural Strength Test Results

The results of the durability test showed that the highest compressive strengths were those of the WM 4 series of samples based on the CEM II cement binder, in which the amount of cement predominated relative to the amount of wool. Their average compressive strength was 17.78 MPa. The series of samples with the lowest compressive strength was the series also based on cement binder (CEM II); in this case, however, the highest intake of wool was accompanied by the lowest intake of cement. Regarding the flexural strength, the samples based on geopolymer mortar performed best with a high proportion of mineral wool. The series of samples with the lowest compressive strengths was the WM6 series, with a high proportion of mineral wool and a low proportion of CEM II cement, the latter being insufficient to adequately consolidate the material. The research led to the conclusion that the fiber structure of the wool improved the compressive strength of the mortar only with alkaline activation. Another important aspect was the very high ratio of flexural strength to compressive strength in geopolymer formulations of up to 1:2. In the case of cementitious mortars designated WM4, WM5, and WM6, the flexural strength was low. In this respect, geopolymer mortar had an advantage over cement mortar.

3.2. Results of Geopolymer Mortar Leaching Tests Compared with the Limit Values

Waste generated from the geopolymer formulation materials is not qualified for landfills for neutral waste due to exceeded acceptable concentrations of copper, nickel, lead, fluoride and dissolved organic carbon. For the WM2 recipe, the phenolic indicator concentration was also exceeded. In addition, waste from the WM1, WM2, and WM3 geopolymer formulations may not be disposed of in landfills for other than neutral and hazardous waste due to exceeded concentrations of total dissolved solids (TDS). Waste from geopolymer formulation materials may be disposed of only in landfills designated for hazardous waste.

3.3. Results of Cement Mortar Leaching Tests Compared with the Limit Values

Waste from cementitious formulation materials, such as geopolymer materials, cannot be disposed of in landfills for neutral waste because concentrations of chromium, molybdenum, antimony, sulphates, and dissolved substances are exceeded. On the other hand, they can be successfully disposed of at other than neutral and hazardous landfills for non-hazardous or inert waste, as they meet all the requirements.

3.4. Comparison of the Results of Geopolymer and Cement Mortar Leaching Tests

The WM1 geopolymer recipe and WM4 cement recipe had in their composition the least mineral wool and the most bonding material metakaolin and CEMII cement, respectively. In contrast, the WM3 and WM6 recipes had the highest wool content ratio to the bonding material (MK, CEMII). The proportional increase in mineral wool content was visible in the leaching results. In fact, for both geopolymer and cementitious formulations, the concentration of formaldehyde increased in direct proportion to the amount of wool in the formulation. The main differences that could be observed between the geopolymer WM1-3 and the cement WM4-6 recipes were the sodium and sulphate concentrations. Due to the use of sodium hydroxide in the activating solution, the sodium concentration in the geopolymer formulations was very high relative to the cement formulations. On the other hand, due to the presence of cement in the cement formulations, the sulphate concentration was very high.

3.5. Comparison of Phenol and Formaldehyde Leaching Results from Geopolymer, Cement Mortar, and Mineral Wool

The summary shows the differences between the leaching of formaldehyde and phenol from the mortars tested and the mineral wool waste sample. The study showed that the use of geopolymer or cement mortar significantly stopped phenol and formaldehyde leaching from the wool.

4. Discussion

The test results presented in Table 4 for the compressive and flexural strengths of the mortars showed the properties of the mortars and the differences between them. The cement-based recipes with the highest cement-to-wool-content ratio showed the highest compressive strength. The combination of high cement content and low wool content resulted in the samples presenting the highest compressive strength. A logical conclusion is that the lower the amount of cement and the higher the amount of mineral wool, the lower the compressive strength. A similar trend can be observed with geopolymer mortars. Recipes in which mineral wool dominated over aluminosilicate (MK) had lower compressive strengths. It is also important to note the relation in which geopolymer mortars have a better flexural strength compared with cement mortars. The durability of the geopolymer is also influenced by the homogenisation and granulation of the wool [37]. The purity of the wool waste used for recycling is also a significant factor; it is crucial that it is uncontaminated by gypsum, adhesive mortar, or other construction contaminants [38]. Emphasis should also be placed on balancing the aluminium/silicon ratio. Therefore, aluminium oxide was added to the mineral wool sample to raise the proportion of aluminium to silicon. The results of leaching tests on cement mortar samples in Table 6 showed that waste from these materials cannot be disposed of in inert landfills. Nevertheless, they can be disposed of in landfills for non-hazardous and neutral waste, as can mineral wool [15]. In the case of geopolymer mortars, the concentrations of copper, nickel, lead, and fluoride were exceeded in relation to the limits set out in the regulation on landfill of inert waste. In addition, the dissolved organic carbon limit was exceeded in relation to the limits set out in the regulation on landfills for non-hazardous and inert waste [26,27]. The comparison made in Table 8 is the most important observation resulting from the study. By comparing the leaching of formaldehyde and the phenolic index from mineral wool and geopolymer mortars, it is concluded that the geopolymer mortar effectively inhibits the leaching of hazardous substances from the wool contained in the mortar. The table below shows the effect of the amount of wool on the formaldehyde content of the water extract.

The above results indicate a correlation between the amount of wool and the amount of formaldehyde in the water extract. On the basis of the tests described in the article, it is not possible to conclude whether the geopolymer or cement mortar is a better solution for formaldehyde immobilisation. The leaching of formaldehyde with both geopolymer and cement mortar is inhibited to a comparable, impressive significant level.

5. Conclusions

The identified research problem was the leaching of formaldehyde from mineral wool. It is common to use phenol–formaldehyde resin in the production of mineral wool [39]. Over the years, major manufacturers have set targets for VOC reduction, which is derived from the use of phenol–formaldehyde resin. A number of patents have been issued, with the goal of reducing these substances, based mainly on two methods of scavenging formaldehyde molecules during heating in the curing oven and the development of new resin formulations that are not based on formaldehyde [40]. These issues are an area of research, while formaldehyde leaching is high in the case of wool with phenolic formaldehyde binders. The method used by the team to immobilise the resin-soaked mineral wool fiber was successful. The leaching of formaldehyde for the mineral wool sample was 578 mg/kg, while the maximum value for the geopolymer mortar was 12.60 mg/kg. The difference is significant. The results clearly state that the use of wool in geopolymer mortar significantly reduces its leaching into the environment. Given the increasing environmental awareness in society and the tightening of environmental requirements, the research team unanimously believes that the building materials of the future must be ecological. A research niche that is emerging from the work described in this article is undoubtedly the re-use of geopolymer waste to produce geopolymer to close the mineral wool cycle.