During my work experience in the waste treatment industry I’ve practiced that, although a lot of materials (powders, muds, sands) can only be disposed in landfills or incinerators (because excessively contaminated by oil, organics, hydrocarbons, solvents, etc..), there are a lot of other categories that could be reused as new raw materials for other industrial processes. Focusing on byproducts suitable for a “new life” as supplementary cementitious materials or artificial aggregates in concrete, apart from the very well known fly ashes, silica fume, ground granulated blast furnace slags (studied from decades), in the scientific literature a vast amount of papers have been published promoting the reuse and recycling of solid wastes on the agenda of sustainable waste management: FCC catalyst, rubber, foundry sands and slags, MSWI ashes, etc…
My research in the scientific literature of “alternative” materials to be reused in concretes and my curiosity drove me to pay attention to glass wastes. Glass is an amorphous material containing a great amount of SiO2 and, theoretically, can be infinitely re-melted without degradation of its physical properties: so, the glass manufacturing industry could use 100% recycled glass as a primary feedstock. However, not all the waste glasses are suitable for new glass production because of contamination (referring to glass bottles by papers, labels, corks and other substances) or color mixes, which make these glasses not suitable for glass bottle re-production. Hence, research has been needed to find other outlets for the waste glass: in the construction industry, glass is considered a good substitute for natural sand due to similarities in physical properties and chemical compositions. If this could be accomplished, economical, ecological, and even technical advantages would be realized in the concrete industry, resulting in more savings in energy and raw materials, and reduction in landfills. The majority of studies concentrate on soda-lime glass, commonly used in the production of containers, bottles and windows, which typical chemical composition is 73% SiO2, 5% CaO, 16% Na2O, with small amounts of additives added during the production of colored glass. The major colors for waste glasses are brown, green, and clear. The diverse properties of concrete containing glass as aggregate at various contents have been studied by several investigations. Two major ways have been pointed out by researchers to use this type of artificial aggregate as replacement of natural ones (previous cleaning and removal of contaminants):
1. Glass milling up to obtain a “powder” < 100 µm, to use as partial cement replacement (in order to employ its probable pozzolanic activity promoted by the amorphous silica of the glass);
2. Glass crushing up to a typical sand granulometry for the substitution of the corresponding natural aggregate.
Due to its sharper (because of crushing and milling operations) and smoother surface (practically with no water adsorption) than natural aggregates, scientists have studied fresh and hardened concrete behavior case by case. Now I try to resume benefits and drawbacks of both the material classes.
GLASS POWDER
GLASS POWDER AS CEMENT REPLACEMENT. A lot of studies show how glass “powders” promote secondary C-S-H (pozzolanic reaction) when used as cement replacement. Chemically, pozzolanic reaction is:
SiO2 + Ca2+ + 2OH- → n1CaO ∙ SiO2 ∙ n2H2O
Under alkali attack, the destruction of silica networks releases silica that combines with calcium from Portlandite, forming secondary C-S-H, which can improve the concrete properties.
The kinetics of C-S-H reaction is more favourable (so faster) than the kinetics of the formation of the similar-gel substance due to alkali-silica reaction (ASR), one of the major concerns regarding the use of glass in concrete: ASR is a chemical reaction that takes place between the silica-rich glass particles and the alkalis (Na2O e K2O, that as a consequence of hydration reactions expand their volume) in the pore solution of concrete. This reaction leads to cracks and other detrimental effects. Chemically, ASR reaction is the following:
SiO2 + 2 Na+(K+) + 2 OH- → Na2(K2) ∙ SiO3 ∙ H2O
Being the pozzolanic reaction faster, it compensates the eventual drawbacks of alkali-silica reaction. Benefits are: compressive strengths at longer ages than reference concretes with only Portland cement; better durability properties, as resistance to aggressive agents (chlorides, sulphates, freeze-thaw cycles, alkali-silica reaction itself). These last properties have been intensively investigated by researchers who, on the one hand, experimented a lot of combinations between “glass powder” and other pozzolanic materials in substitution of cement, such as ground blast furnace slag [“Studies on mortars containing waste glass bottles and industrial by-products” by O. Özkan e I. Yüksel - Construction and Building Materials 22 (2008), 1288-1298] or fly ash [“The combined use of ground waste glass with an industrial by-product in manufacturing Portland Cement mortar” by A. Yilmaz e N. Degirmenci - Advances in Cement Research 22 (2010), 73-80]. On the other hand, combinations of glass “powders” and glass “aggregates” in replacement of natural aggregates (both a fine sand - 0 ÷ 4 mm – or a coarser one - 4 ÷ 16 mm) to develop more eco-friendly concrete mix designs [“Study on the effect and mechanism of alkali-silica reaction expansion in glass concrete” by D. Huang, P. Sun, P. Gao, G. Liu, Y. Wang, X. Chen - Sustainability 13 (2021) 10618].
GLASS POWDER AND RECYCLED AGGREGATES. Scientists have investigated the possible synergy between glass powders and recycled aggregates [“Strength and durability of recycled aggregate concrete containing milled glass as partial replacement for cement” by R.U.D. Nassar, P. Soroushian - Construction and Building Materials 29 (2012), 368-377], discovering evident benefits for these concretes: glass powder balances the well-known drawbacks of recycled aggregate concretes, such as higher water absorption (two to three times that of normal aggregate), increased shrinkage of the resulting recycled concrete, lower mechanical strengths, durability troubles. These drawbacks result largely from the old mortar/cement paste clinging to the surface of recycled aggregates.
GLASS AGGREGATE
PRELIMINARY TREATMENT. P. Spiesz, S. Rouvas, H.J.H. Brouwers in “Utilization of waste glass in translucent and photocatalytic concrete” [Construction and Building Materials 128 (2016), 436-448] try to produce mortars also with glass aggregates crushed “as received” (so neither washed nor dryed), obtaining some “disastrous” results. This class of aggregates requires more water to maintain workability (causing a complete distortion of cement hydration reactions), an excessive delay in setting times and a collapse in both flexural and compressive strengths: these last at short ages are not detectable, while at 28 days vary from -60% to -80% (compared to a concrete mix without any glass aggregate).
INFLUENCE OF COLOR. H. Du e K.H. Tan in “Use of Waste Glass as Sand in Mortar: Part I – Fresh, Mechanical and Durability Properties” [Cement & Concrete Composites 35 (2013), 109-117] study the properties of mortars containing aggregates derived from crushing of glass bottles of various colors (green, clear and mixed), discovering that clear glass is weaker than the others. After crushing activities it suffers from cracks propagation phenomena, penalizing the mechanical properties of correspondent mixes with respect to the others containing the green or the mixed colored glass aggregates.
Furthermore, clear glass is more alkali reactive, so the authors suggest it is better to use clear glass as artificial aggregate only mixed with other colored types, not “alone” [please read also “Enhancing the performance of pre-cast concrete blocks by incorporating waste glass – ASR considerations” by C.S. Lam, C.S. Poon, D. Chan - Cement & Concrete Composites 29 (2007), 616-625].
INFLUENCE OF GLASS SOURCE. If soda-lime glass seems suitable for being used in concretes or mortars, we must consider also other sources of glass. S.Maschio, G. Tonello, E.Furlani in “Recycling Glass Cullet from Waste CRTs for the Production of high Strength Mortars” [Journal of Waste Management 4 (2013)] report on the results of some experiments dealing with the recycling of mixed cathode ray tube (CRT) glass waste in the production of high-strength mortars. CRT glass waste includes that from TVs, PC and other monitors used in special applications, and waste from the original assembly process. Mixed CRT glass (funnel, neck and screen glass) contains less silica (60%) and alkalis (Na2O 7% and K2O 6%), but high amounts of heavy metals oxides (SrO 5%, PbO 8%, BaO 6%). Waste CRT glass cullet was previously milled, and sieved, and the only fine fraction was added to the fresh mortar in order to replace part of the natural aggregate. Glass containing samples showed a more rapid increase of strength with respect to the reference compositions when subjected to long-term ageing (90 and 180 days). The addition of more than 10wt% of CRT glass powder did not lead to the production of materials with the best mechanical performances. The results obtained in that research are reasonably due to the favourable influence of the small glass particles which interact with the hydraulic phases promoting pozzolanic reaction, limiting ASR and increasing the amount of hydrated silicates produced during long term ageing.
SYNERGY WITH OTHER POZZOLANIC MATERIALS. A good compromise between performances (as previously seen for glass powder) and drawbacks could be the combined use of a glass aggregate (as sand replacement) and a pozzolanic material (as fly ash, silica fume) as partial cement replacement. This could lead to control the deleterious ASR reaction and to get the benefits of pozzolanic materials to cement matrix (less porosity, resistance to aggressive agents) without the concern of the color of the glass aggregate [“Use of waste glass as sand in mortar: Part II – Alkali-silica reaction and mitigation methods” by H. Du e K.H. Tan - Cement & Concrete Composites 35 (2013), 118-126].
J-X. Lu, B-J. Zhan, Z-H. Duan, C.S. Poon in “Using glass powder to improve the durability of architectural mortar prepared with glass aggregates” [Materials and Design 135 (2017), 102-111] investigate the applicability of a soda-lime glass mix, derived from recycling activities, for architectural purposes, milled up to different powder fineness (28.3 ÷ 204 µm) to cast mortars replacing 20wt% of cement. In addition, mostly of the natural aggregate is substituted by the respective glass aggregate, in order to obtain at least 70% of the total amount of the aggregate from artificial sources. The synergistic effects obtained are very interesting! Firstly, the high temperature resistance, part of the study that could simulate a possible fire resistance or this aggregate combination. Despite the unavoidable collapse of mechanical performances after a thermal treatment at 800°C (due to C-S-H decomposition and to the different behavior of glass and cement paste at high temperature), the pozzolanic activity of glass powder (< 50 µm) balances mortar’s pore dilatation: the denser microstructure of glass powder mortars allows the majority of pore’s diameters to be stable in the range 50-200 nm, while mortars without glass powder show the pore diameters enlarged over 1000 nm. The second advantage is the resistance to H2SO4 attack (that leads to both C-S-H and aluminate phases decomposition). Mortars with glass powder do not suffer from mass loss due to acid exposure (+2% after 8 weeks), while mortars with “coarser” powders register a moderate mass loss (-2%) respect to -12% of mortars without glass powders. The authors explain that behavior both with the contribution of the more favorable kinetics of the reaction between Ca(OH)2 (more susceptible to acid attack) and glass silica (pozzolanic reaction), and to minor formation of gypsum and ettringite (secondary products more voluminous generated in the first 3 weeks of curing) on mortar’s surface, with a consequent reduction of expansive phenomena.
PRACTICAL APPLICATIONS
A lot of studies are focused on architectural purpose of these kind of materials in order to benefit of translucent glass properties: as example check “Using glass powder to improve the durability of architectural mortar prepared with glass aggregates” by J-X. Lu, B-J. Zhan, Z-H. Duan, C.S. Poon [Materials and Design 135 (2017), 102-111]; “Recycled glass as aggregate for architectural mortars” by F. Tittarelli, C. Giosué, A. Mobili [International Journal of Concrete Structures and Materials (2018) 12:57] or the paper by P. Spiesz, S. Rouvas e H.J.H. Brouwers cited above.
If you want to find publications on more field applications of glass use in concrete mixes, I suggest “Enhancing the performance of pre-cast concrete blocks by incorporating waste glass – ASR considerations” by C.S. Lam, C.S. Poon, D. Chan [Cement & Concrete Composites 29 (2007), 616-625], where the authors study the applicability of glass crushed aggregates to cast concrete pavement blocks, or “Performance of glass-powder concrete in field applications” by A. Omran e A. Tagnit–Hamou [Construction and Building Materials 109 (2016), 84-95], where are studied concretes containing from 10 to 30% of glass powders (≤ 40 µm) as cement replacement used to cast various structural elements (slabs, sidewalks and walls) exposed to different environmental conditions (indoors and outdoors) in Canadian field sites, obtaining very promising results for all the mechanical and durability properties considered.
FINAL CONSIDERATIONS
Researchers and scientists agree that you can use a 30% of both glass powder or glass aggregates as cement or natural aggregates replacement without compromising the concrete properties. More eco-friendly but more “extreme” mixes should be evaluated case by case, but in none of the papers the authors have priorly excluded a use of glass aggregates in “structural” concretes. Logically, on the balance you should weigh also material’s availability, transport costs, the expenses for a preliminary treatment, the costs of crushing/milling to obtain the aggregates or the powders of the desired fineness. It is promising that, technically, the material seems to offer very encouraging results for a hypothetical use on an industrial scale.
Dr. Simone Capra