Reactivity of Recycled Glass Powder in a Cementitious Medium

The reactivity of the recycled glass powder (GP) in a cementitious medium has been studied over time by means of X-ray diffraction and thermal gravimetric analysis. Two different mixtures based on cement/glass powder (0 or 20 wt% GP) and lime/glass powder (70 wt% GP) were considered. Analysis revealed the coexistence of both hydration and pozzolanic reaction during the hardening of the mortars. At young age, the cement hydration would prevail over the pozzolanic one resulting in a decrease of physico-chemical and mechanical properties of the material due to the dilution effect. The pozzolanic reaction that predominates from 91 days, would induce the formation of supplementary C-S-H leading to improve the material properties.


Introduction
Bottle glasses, essentially composed of amorphous SiO 2 , Na 2 O and CaO, are stable non-biodegradable materials. Residues of mixed glass (mixture of several colors) that are difficult to dispose are found in landfills and constitute a major environmental issue. Recycling them into building materials arouses a lot of interest and is the subject of numerous studies [1] [2] [3] [4]. They can be used as aggregates in mortars or architectural concrete [5] [6] [7].
The durability of these structures is affected by the alkali-aggregate reaction (AAR) favored by the high alkali content in this material [5]. However, it has How to cite this paper: Tognonvi, M.T., Tagnit-Hamou, A., Konan, L.K., Zidol, A. and N'Cho, W.C. (2020) Reactivity of Recycled Glass Powder in a Cementitious Medium. New Journal of Glass and Ceramics, trability of chloride ions results from the refining of pores and grains, and even more from the pozzolanic reaction of additions [13].
Unlike the hydration of the cement which has been the subject of several studies [14] [15] [16], that of cementitious additions, in particular glass powder, remains less known. This work aims to study the hydration reaction of the glass powder in the presence of hydrated lime or cement through structural (XRD) and thermal (TGA-DSC) characterization of the prepared mixtures. In addition, the properties in the fresh and hardened state of mortars containing glass powder as a partial replacement for cement were evaluated to demonstrate the effectiveness of the pozzolanic activity of glass powder in construction materials. This study enables to better understand the physicochemical mechanism of the glass powder reactivity in concrete resulting in the optimum management of such residues in building materials.

Materials
Waste mixed glasses collected by a sorting center (Tricentris, Quebec-Canada) were crushed using a ball mill in 6 sequences of 25 min. The obtained glass powder possesses amorphous structure (Figure 1(a)) and is mainly composed of silicon oxide (72%), sodium oxide (13%) and calcium oxide (11%) ( Table 1). The cement used is an ordinary Portland cement (OPC) provided by HOLCIM company. The mean diameter of GP particles is D50 = 17 µm whereas the one of the cement is 15 µm.  Hydrated lime, (Ca(OH) 2 ) (purity of 92%) with 8% CaCO 3 content (characterized by TGA) commercialized by Anachemia is also utilized. Its limit of solubility is 1.73 g/l at 20˚C.
The physico-chemical and mineralogical properties of the different used materials are provided in Table 1 and Table 2 and Figure 1.

Methods
The chemical analysis of raw materials was performed through X-ray fluorescence spectrometry by means of PANalytical WD-XRF type apparatus. Pellets containing a mixture of 6.3 g of the powdered sample and 0.7 g of a Licowax binder were manually pressed at 25,000 psi.
Structural analysis of raw materials and pastes has been performed through X-ray diffraction using a X'pert Pro MRD type diffractometer provided by PA-Nalytical company in CuKα configuration. Data recorded between 5˚ and 60˚, were treated using JADE 2010 software for peak identification.
The weight losses associated with the water departure between 400˚C -450˚C and the release of CO 2 between 450˚C -750˚C enable to determine lime, L(t), and carbonated lime, L cc (t), contents at a given time (t) respectively according to equations (3) and (4) The flow of mortar specimens was measured in accordance with ASTM C1437. The diameter of the specimen after the test was determined according to ASTM C 230. The flow was the resulting average diameter recorded to the nearest millimeter. The compressive strength tests were conducted on 50 × 50 × 50 mm cubes in line with ASTM C109. A total of three specimens, in each case, were characterized and the average value of the compressive strength nearest 0.1 MPa was determined.
The normal consistency of the studied mortars was obtained according to ASTM C187. The amount of water required for normal consistency was calculated to the nearest 0.1%. A total of three measurements were performed and the resulting average of normal consistency was determined. The initial and final setting times were measured according to the procedure specified in ASTM C191 on paste mixtures proportioned and mixed to normal consistency. The setting time was determined to the nearest 1 min. Two measurements were conducted for each mortar and the average setting time was calculated.

Reactivity of GP in the Presence of OPC
1) X-ray diffraction characterization New Journal of Glass and Ceramics Figure 2 shows the variation of XRD patterns during the hydration of both pure cement and the one containing 20 wt% GP for various periods. A decrease in peak characteristic of C 3 S, C 2 S, C 3 A and gypsum, present in anhydrous cement is observed for hydrated compound. This decrease is related to the formation of hydration products such as portlandite or lime (L), ettringite (Aft) and calcium silicate hydrate (CSH) from the first day of the cement hydration. The peaks attributed to CSH are less resolved due to its amorphous character. The cement paste containing GP shows the same spectra as those of pure cement paste whatever the curing time. These observations suggest that the presence of GP does not impede the cement hydration. However, since the analysis in this study is only qualitative, it is difficult to isolate the role of GP in the cement hydration process. Otherwise, studies carried out on GP reactivity in an aqueous medium have revealed that because of its amorphous character, the glass powder hydrates very well [17]. This hydration would be favored by the dissolution of the surface alkalis making the medium basic (pH > 11). In addition, it has been shown that the degree of this hydration increased with the contact time of GP with water. Likewise, other authors [1] have shown that an increase in pH and temperature of the medium accelerated GP hydration reaction. The cement hydration is an exothermic reaction with the release of portlandite which renders the medium more basic, promoting thus GP hydration.
2) Thermal analysis In fact, the hydrate layer which coats cement grains gradually becomes thick inducing the decrease in ions and water diffusion towards the anhydrous components of the system. Hydration slowdowns more and more, but can continue for months or even years [18]. Also, unlike C 3 S which has a fairly rapid hydration This insinuates that from 91 days, the pozzolanic reaction would prevail over the cement hydration. The pozzolanic reaction only begins when there is enough lime in the medium. This result is in accordance with that observed by other researchers [11] [12] who showed that the beneficial effects of the pozzolanic reaction in cementitious systems are noticeable from curing time greater than or equal to 56 days.

1) X-ray diffraction characterization
The hydration of GP in the presence of hydrated lime was monitored by XRD over a period of 365 days. Figure 4 shows the X-ray diffraction spectra of sam-  is likely to carbonate. However, unlike the 28-day sample which seems not to contain any more lime, the 1-day sample is less carbonated because it still contains lime, according to XRD analysis. These results show the importance of minimizing the contact time of the sample with ambient air before characterization tests when the latter still contains lime. These observations suggest that the pozzolanic reaction would be negligible up to 28 days. This is similar to previous results obtained in a cement system [3] [11] [12].
From 91 days, a decline in the intensity of the peaks of lime as a function of time, is observed. This decrease is accompanied by the appearance of peaks of C-S-H that are less resolved due to its amorphous nature. In addition, the quasi absence of calcium carbonate peaks suggests that the lime did not carbonate, thus confirming the reaction of GP with the lime, namely the pozzolanic reaction. The disappearance of peaks attributed to lime and carbonate at 365 days indicates an almost total pozzolanic reaction, confirmed by the unique presence of CSH peaks. The pozzolanic reaction is therefore a slow reaction which appears to be better revealed beyond a sufficiently long time (t > 28 days) when GP is put into contact with lime in aqueous media. It emerges from these observations that the more time the sample spends in a controlled atmosphere, the less it is subject to carbonation because of the pozzolanic reaction which is raised.
Carbonation, therefore, seems to take place after putting out the sample when it still contains lime (as in the case of 1 and 28 days) and this, during the preparation for the various characterization tests, hence the importance of taking precautions to minimize carbonation as previously outlined.
2) Thermal analysis with 27.546 g corresponding to the total amount of pure lime in the mixture.
The consumed lime (L reacted ) represents the lime content which has reacted with the glass powder. It corresponds to the difference between the initial lime days and a low carbonation for older ages. Accordingly, carbonation is linked to the presence of a high quantity of vulnerable element such as lime in the mixture. The longer the sample is maintained in a controlled atmosphere, the more the lime present reacts and the less the sample is exposed to carbonation.
Also, unlike the almost non-existent or hardly detectable pozzolanic activity at young age in a cementitious medium, a GP reaction is observed as early as 24 hours when the latter is brought directly into contact with lime. This suggests that in the presence of cement, the observed delay in GP pozzolanic reaction could be due to both the low availability of the portlandite and the predominance of the cement hydration reaction. Indeed, studies showed that in cement-based materials, the beneficial effects of the pozzolanic reaction of cementitious additions, especially GP, on the physico-chemical properties of the material can only be detected after 56 days of reaction [12].
The pozzolanic reaction of glass powder with lime can be subdivided into three distinct stages: from 0 to 1 day, from 1 to 91 days and from 91 to 365 days.   Table 4 shows the flow, setting time and water demand of mortars containing 20% GP in comparison with the reference (without GP). According to these results, the presence of GP in cement mortar slightly decreases the flow. The water demand was similar in both cases, since the normal consistency of paste was 29.7% regardless of the paste considered. For the setting time, the reference paste began setting 7 min before the GP-based paste and recorded the longest final setting time. These results suggest that the setting time is shortened by replacing a part of the cement with the glass powder. This could be due to the alkali ions, which are known to speed up the reaction mechanism [19].

Mortar Characterization
Changes in the compressive strength were also assessed as a function of curing time. The results reported in Table 4 show an increase in the compressive strength over time regardless of the mortar type, expressing the normal progression of the hydration process. However, the compressive strength of GP-based mortar was lower than the one of the control. This is due to the well-known dilution effect, as a part of the more reactive product, namely the cement, was replaced by GP. Besides, the pozzolanic activity of GP was well illustrated through the pozzolanic index determined from the compressive strength of mortars (Table 4)

Pozzolanic Reaction Mechanism
The analyzes carried out on the 70GP-L paste revealed a progressive consumption of lime by GP. Thus, the evolution of the pozzolanic reaction of the glass powder in the presence of hydrated lime can be described in three stages as follows: -The presence of OH − ions in solution leads to a strongly basic medium (pH > 12) promoting the partial dissolution of the amorphous silica of GP and thus the dissolution of silicates following the reaction (8).
The concentration of dissolved silicates will increase with the amount of water.
-Then, reaction of silicates 2 2 4 H SiO − with calcium ions (Ca 2+ ) to form CSH according to Equation (9) [20]: Therefore, from 0 to 1 day, there is mainly the lime dissociation reaction ac-  (10) and (11): In the cement paste containing GP, the lime or portlandite (Ca(OH) 2 ) formed during the hydration of C 3 S and C 2 S will make the medium more basic (pH > 12) due to the presence of OH − ions in the interstitial solution. Also, the dissolution of alkali ions present on the glass surface will significantly increase the pH of the system. In fact, it has been demonstrated that in aqueous medium, alkalis on GP surface readily pass into the solution resulting in the pH enhancement [17] [19]. This basic medium associated with the increase of the temperature due to the exothermic reaction of the cement hydration fosters the partial dissolution of amorphous silica of GP according to reaction (8). Then, soluble silicates will New Journal of Glass and Ceramics react with calcium ions following the reaction (9) to form supplementary C-S-H.
Moreover, in cementitious system, the GP pozzolanic reaction would take place after the beginning of the cement hydration when enough lime will be available to favor the dissolution of GP silicates. Thereafter the two reactions would take place simultaneously with a predominance of one over the other depending on the reaction time. Although the pozzolanic reaction would start at young age, it would not be perceptible before 91 days (in line with the results obtained on the studied mortars), owing to the continuous production of  (Table 4).
All these results reveal that in a cementitious environment, the presence of pozzolanic material such as GP would cause competition between the cement hydration and the GP pozzolanic reaction. At young age, the cement hydration would prevail over the pozzolanic reaction resulting in a decrease in the physico-chemical and mechanical properties of the material due to the dilution effect.
Afterward, at advanced ages, especially from 91 days, the predominance of the pozzolanic reaction would lead to the formation of additional C-S-H, which would contribute to improve the material properties. These results are well in line with those observed on the evolution of the mechanical properties of concrete incorporating GP. Studies have shown that at early age, the presence of GP induces a reduction in the compressive strength of concrete [12]. In contrast, from 56 days, an increase in the compressive strength of such a concrete to reach the one of the control concrete (100% OCP) was observed. At advanced ages (from 91 or 180 days), depending on the water-to-binder ratio, the concrete containing GP exhibited higher resistance than that of the control concrete [12].

Conclusions
The reactivity of the glass powder with lime was analyzed in mixtures of cement pastes containing 0% and 20% GP and pastes of 70% GP and 30% lime. The