Study on the Use of Glass By-Products for Sustainable Polymer-Modified Mortars

The disposal of waste materials presents a complicated issue around the world [1]. An increasing interest in the use of recycled materials and more environmentally friendly alternative sources of building materials is the aim of promoting sustainable and green construction [2]. The growing environmental requirements, increasing scarcity of landfill sites and depleting sources of raw materials is leading to an increased use of recycled materials [3]. The goal of the current ambitious environmental challenge, known as the European Green Deal, is to achieve net zero emissions of greenhouse gases by 2050 [4]. The measures needed to achieve this goal will need to apply to all industries, including construction. The use of recycled materials as supplementary cementing materials (SCM) with lower carbon dioxide emissions can reduce environmental impact and promote sustainable construction [5].

Millions of tons of waste glass are being produced worldwide every year. Although glass can be easily recycled, and has been recycled for many years, it is still disposed of in significant quantities as waste glass [6]. Once the glass becomes designated as waste it is disposed of in landfill sites, which is not only environmentally destructive but very expensive as well. The use of milled waste glass as partial replacement of cement could be an important contribution to sustainable development [7]. Supplementary cementing materials like fly ash, silica fume or fine glass powder are useful in improving the mechanical and durability properties of cement-based composites [8]. Being amorphous and containing large quality of silicate, finely ground glass can be used as a pozzolanic material [9, 10]. A particle size of 75 µm or less is reported to be favorable for pozzolanic reaction [8]. Several studies confirm that fine glass powder can even be used as a suppressor, as well as SCM, to mitigate alkali-silica reactions (ASR). To avoid cracking due to deleterious ASR it is recommended the grain size of the glass is reduced to 300 µm or less [6, 9]. Patel et al. [11] reported that very fine glass powder (particle size below 45 µm) replaced cement at 20 wt.% without compromising the quality parameters. According to Parghi and Alam [12], recycled glass powder can be used as an effective supplementary material in cement mortar with 25% optimal replacement of cement. Calcium silicate hydrate gel (C-S-H) formed during the pozzolanic reactions leads to the refinement of pores and the reduction of porosity [13]. Zheng [9] found that the pozzolanic reaction of soda-lime glass not only consumes portlandite to form C-S-H but also leads to a decrease in monosulfate. Dehghan et al., [14] investigated the use of glass fibers from waste glass fiber reinforced polymers (GFRPs). Among GFRPs, E-glass is the most common reinforcement and represents the vast majority of the commercial market. The chemical composition for E-glass used in general applications such as GFRPs are outlined in ASTM D578, see Table 1.

Some of the recyclable glass shards that do not meet the requirements of the glassworks and would end up in landfill sites can be used for different applications, e.g., for the production of foam glass for thermal and acoustic insulation [15]. This material is also produced in the Czech Republic and used in the building industry as foam glass boards and granulates.

In order to get closer to the circular economy, the government of the Czech Republic approved a secondary raw materials policy, that is in line with the European raw materials strategy. The main vision of the secondary raw materials policy of the Czech Republic is “turning waste into resources” and is focused on ten important commodities and sources of secondary raw materials, including waste glass [16]. Unfortunately, despite support for the conversion of waste into resources, part of the untreated waste glass still remains in the waste category and is landfilled. Container glass accounts for approximately 25% of the glass marketed in the Czech Republic.

The disposal of waste materials presents a complicated issue around the world [1]. An increasing interest in the use of recycled materials and more environmentally friendly alternative sources of building materials is the aim of promoting sustainable and green construction [2].

The aim of this experiment was to investigate changes in the properties of polymer-modified cement mortars caused by an addition of two types of recycled glass. This study uses waste E-glass (fibers) and the rest is from foam glass production. Both types of waste glass have to be ground to particle sizes below 63 μm. The pozzolanic activity was measured based on the amount of calcium oxide reacting with milled E-glass and foam glass, see Table 2. Oxide contents are specified in Table 3.

Specimens were prepared with ordinary Portland cement CEM I as binder (OPC), CEN standard sand (according EN 196-1) and redispersible polymer powder on the basis of ethylene/vinyl acetate (EVA). OPC was substituted with waste glass by 10 wt.%, 20 wt.% and 30 wt.%. Table 4 shows the composition of the mixtures.

Specimens of PMMs were made with the dimensions of 40 × 40 × 160 mm and were tested for physical-mechanical properties and examined to study their microstructure. The determination of flexural and compressive strength of PMMs was performed according to the standard EN 1015–11:2019 – methods of test for mortar for masonry – Part 11: determination of flexural and compressive strength of hardened mortar. Specimens were tested after 28, 90, and 90 days + 25 freezing cycles (FCs). Prior to cyclic freezing and thawing, the test specimens were saturated with water for 24 h by immersion in a water bath at +20 ℃ ± 3 ℃ so that the water was at least 3 cm above the surface of the samples. They were frozen immediately after saturation with water. Freezing and thawing of test specimens was performed in FCs based on the standard ČSN 72 2452 – mortar frost resistance test. One FC consisted of four hours of freezing at −20 ℃ ± 3 ℃ and two hours of defrosting. At the end of 25 FCs, the test specimens were measured, weighed and bulk density determined. The set of beams was then tested for flexural and compressive strength.

Another variable that was monitored immediately after the preparation of the samples was the course of temperatures during the hydration of mortars, which indicates the course of hydration reactions during the change of structure and the formation of hydration products. To monitor the temperatures, comparability and possible repeatability, a constant ambient temperature is necessary. This was ensured by the air conditioning cabinet and was 20 ℃. The temperature monitoring itself was performed using a Testo 177-T4 measuring control panel using thermocouple probes type K - nickel-chromium-nickel [17].

The microstructure of selected specimens was examined, using differential thermal analysis (DTA), high-pressure mercury intrusion porosimetry and a scanning electron microscope (SEM), in order to gain a better understanding of the obtained strength test results.

The results of compressive and flexural strength performed at an age of 28 days, 90 days and 90 days + 25 FCs are shown in Figs. 1, 2, 3 and 4. To determine the flexural strength, 3 test beams with the dimensions of 40 × 40 × 160 mm were produced and subsequently the compressive strengths of the 6 halve-beams were determined.

Changes in compressive and flexural strength of PMMs is dependent on the age and FCs showing an improvement up to 20 wt.% substitution of Portland cement by grounded waste E-glass. According to authors [11], very fine glass powder (particle size below 45 µm) can replace cement at 20 wt.% without compromising the quality parameters. This study confirmed that glass particle size below 63 µm is sufficient in the case of 20 wt.% substitution of Portland cement and leads to increased compressive and flexural strength, especially after 90 days and 90 days + 25 FCs. The maximum improvement in the compressive strength of approximately 12% was achieved when 10% replacement was used for PMM (marked as E/FG10) compared to the reference mixture (marked as E/REF) at 90 days. The use of waste glass powder as a partial replacement of Portland cement contributes to the long-term strength development of mortars and PMMs due to the slower rate of pozzolanic reaction compared to the hydration of Portland cement.

The results of hydration temperature evolution during first 24 h are shown in Fig. 5. It is seen that the hydration peak was reduced and retarded by the partial cement substitution. Glass, similar to other SCMs, generates lower temperatures during the early stages of curing. This effect can be beneficial in the design of the mortar to avoid shrinkage.

According to the results of DTA analysis shown in Table 5, PMMs with the use of grounded waste E-glass contained the highest amount of calcium silicate hydrate gel. The pozzolanic reaction of grounded glass consumes portlandite to form C-S-H phases [8], thus enhancing resistance to aggressive chemicals.

The selected materials were examined using SEM imaging. Figure 6 shows an example of mortar REF, Fig. 7 shows an example of polymer-modified mortar E/REF and Fig. 8a–b the sample of PMM containing grounded waste E-glass (marked as E/E-G20 with 20 wt.% substitution of Portland cement). The glass powder converts calcium hydroxide into a calcium silicate hydrate gel, and therefore the newly formed C-S-H phases were observed as a benefit of the pozzolanic behavior of the grounded E-glass used.

It can be seen from Fig. 9 that the addition of ground waste E-glass shows the reduced porosity compared with reference mixtures (without cement replacement). A reduction in porosity and pore refinement is expected to enhanced durability such as improving freeze–thaw resistance and increasing resistance to aggressive environments.

In this study, changes in the properties of cement mortars and polymer-modified cement mortars (PMMs) using two types of glass powder were investigated. The following findings were recorded:

The findings show that finely ground recycled glass, that no longer meets the requirements of glassworks for the reproduction of glass, has high potential to be used as an effective cement replacement for PMM materials, that are currently used in large amounts, mainly in the rehabilitation of concrete structures. Using waste glass in this way not only supports the circular economy but improves important properties such as durability of cementitious mortars and especially polymer-modified cement mortars as well.