Literature Review

A total of 4.2 million tons of glass are used annually in home settings, with sheet glass for windows accounting for approximately 36% of the amount, glass fiber goods for 6%, and throwaway glassware for 58% [6]. Comparing EG to several other forms of lightweight aggregate, it seems to be pretty promising [7]. EG is a frost- and chemical-resistant construction material that is hygienic, light, finely porous, flame-retardant, efficient, and simple to use [8]. The compressive strength of EG aggregates, which range from 0.45 to 1.55 MPa [9], is quite low due to the low density of these aggregates. When EG is sintered at 750°C with a 7 wt % Li₂CO₃ content, a greater compressive strength may be achieved [10]. In EG aggregates produced of sintered amber glass, compressive strength may be increased more significantly (16.7-29.7 MPa) [11]. The experiments show [12] that the LWAC density reduced from 685 to 561 kg/m3, thermal conductivity from 0.163 to 0.140 W/ (m·K), and compressive strength from 5.8 to 4.1 MPa with an increase in EG aggregates (from 300 to 400 kg/ m³).

Several industrial waste materials have shown potential for improving the properties of LWAC:

Metakaolin (MKA) is a highly reactive aluminosilicate, which makes it an excellent alternative to Portland cement (PC) for building concrete. MKA can enhance structural integrity, compressive and flexural strength, and reduce density, capillary water absorption, and alkali-silica reaction [13].

Lignosulfonates act as dispersants, improving workability and reducing the water-to-cement ratio, which enhances the mechanical properties and durability of concrete, and serve as self-curing agents, reducing the need for additional water during curing [14].

Polycarboxylate-based superplasticizers (SP) are high-range water reducers that improve the flowability and workability of concrete without compromising its mechanical properties [15].

Polyacrylates are used in cementitious composites to enhance grout properties. The interaction between calcium ions from cement hydration and carboxyl groups from polyacrylate hydrolysis leads to the formation of a dense, complex network structure that boosts overall performance [16].

Air-Entraining Admixtures (AM) are added to concrete to introduce stable air bubbles, improving its workability and freeze-thaw resistance, and reducing density [17].

Since the volume fraction of aggregate has the greatest impact on the thermal conductivity characteristics of LWAC, EP and its waste are possible aggregates in LWAC [18]. The findings demonstrate that increasing EP significantly lowers the heat conductivity, fire resistance, and compressive strength of concrete [19]. For a given porosity, porous LWAC's compressive strength increases with increasing EP aggregate fineness [20].

The surface-active agents AM for lowering concrete density and increasing porosity, in addition to improving segregation resistance, have a direct detrimental effect on the mechanical characteristics of structural concrete. The ideal dose for AM must be selected based on the physical, mechanical, and pore structure of porous LWAC that has been identified [17]. It is possible to create PC and fly ash-based ultra-lightweight aerated concrete [21] by combining hydrogen peroxide with chemical admixtures. A thermal conductivity of 0.054 W/m·K and a compressive strength of 0.74 MPa were reached for the obtained samples [22]. It was discovered that SP had an impact on pore formation, volume, structure, and stability, in addition to improving the strength of concrete [15]. However, when an SP was present, the effects of various AM kinds (with protein or synthetic origin) on cement paste varied [23]. The compressive strengths of the protein-based AM were around 3-10 times greater than those of the synthetic ones.

The purpose of this study is to investigate the effect of various industrial waste additives (lignosulfonates, polycarboxylates, polyacrylates, and air-entraining admixture) on the quality of lightweight concretes. To achieve this goal, it is necessary to solve the following tasks: 1. Determine the effect of various additives on the initial and final setting time of Portland cement pastes.

2. Determination of dynamic viscosity, density, and porosity of Portland cement pastes. 3. Determination of density, strength, and coefficient of thermal conductivity of lightweight concretes.

Materials

For the investigations, a local manufacturer's PC CEM I 42.5 R was employed. The PC had a specific surface area of 4220 cm²/g, a bulk density of 1.15 g/cm³, an initial setting time of 135 min, a final setting time of 185 min, and a maximum alkali content of 0.75%. The percentages of the minerals are as follows: C₃S = 56.64, C₂S = 16.72, C₃A = 8.96, and C₄AF = 10.59. PC particles varied in size from 1 to 100 m, with 15 to 30 m particles making up 50% of the total.

EG aggregate, created from waste glass, has a fraction of 0 to 2 mm and a bulk density of 210 kg/m³, whereas crushed waste EP aggregate, acquired from the packaging tare of home appliances, has a fraction of 0.5 to 2.0 mm and a bulk density of 20 kg/m³. In the porous LWAC forming mixes, pozzolanic MKA, which is commercially manufactured, was employed to substitute some of the PC in the composition. MKA's chemical make-up is as follows (in mass percent): 52.1 SiO₂, 45.0 Al₂O₃, 0.5 Fe₂O₃, 0.2 CaO, 0.2 MgO, 0.3 K₂O+Na₂O, 0.6 TiO₂, and 1.1 additional elements. According to the particle size distribution, particles between 6 and 13 m in diameter made up around 40% of MA. The primary minerals in MKA, according to an X-ray diffraction phase study, are quartz and kaolinite.

All three of the following forms of SP (in liquid form) were utilized in the study: modified lignosulphonate LS, polymer-based synthetic polycarboxylate ester PCX, and polyacrylate-based PA. PC water solution has 27% of the active ingredient, PA contains 27%, and LS contains 31%. In the combinations, the quantities for SPs were constant (0.6% of the PC amount), and they were recalculated in accordance with the concentration of the dry material in the solution of each SP. It was distilled water with a pH of 5.8 and an electrical conductivity of 8 S. The Content of SP was chosen according to a deep review [24] and works [25] in which the estimations of origin and industrial applications of lignosulfonates with a focus on their use as superplasticizers in concrete were performed.

AM is a white powder that is created using sodium alkene sulfonate. It makes sure that the cement paste develops tiny air holes and that the crushed waste EP is hydrophilized. The manufacturer's suggested paste has 0.01 to 0.06% of the PC mass in AM. According to [26], 0.02% of AM should be used for testing in order to prevent the strength qualities from degrading. Table 1 provides more specific information regarding used SPs and AM.

Creating Mixes and Sample Preparation

The molds used were made of steel to ensure rigidity and non-reactivity with the concrete mix. The dimensions of the molds were 100x100x100 mm, which is a standard size for creating concrete cubes for compressive strength testing.

The mixing process began with blending the Portland cement (PC) and superplasticizers (SPs) with two-thirds of the total water for 3 min to ensure a homogeneous mixture. The air-entraining admixture (AM) was then dissolved in the remaining one-third of the water and gently mixed for 30 s to avoid excessive foaming. After the initial mixing, metakaolin (MKA) was added, and the mixture was further mixed for 5 min to ensure even distribution of the MKA particles throughout the paste.

Eight compositions were created in order to assess the effects of AM, SPs, and their combinations on fresh PC paste setting behavior parameters (see Table 2). The content of air-entraining additives was chosen according to the review [27], in which an analysis of the behavior of air-entraining additives in fresh concrete and its effects on hardened concrete.

The compositions in Table 2 aim to comprehensively assess the influence of various superplasticizers (SPs) and an air-entraining admixture (AM) on Portland cement (PC) paste properties. By including pure PC paste as a baseline, the study establishes a reference point for comparison. Each composition then systematically introduces different SPs (PA, PCX, LS) and evaluates their individual effects on workability and setting behavior. Additionally, combinations of SPs with AM are explored to understand potential synergies in enhancing mechanical properties while maintaining workability. This approach allows for a thorough investigation of how different additives interact and impact paste characteristics, guiding the development of optimized lightweight aggregate concrete mixes. Through controlled experimentation, the study aims to identify formulations that strike a balance between improved workability, setting time, and mechanical strength, thereby advancing the understanding and application of sustainable concrete materials.

Fig. 1.

Dynamic viscosity of fresh PC pastes with different MKA to PC ratios: (a) without AM; (b) with AM.

To evaluate the effects of the MKA to PC ratio and AM on the paste's dynamic viscosity fluctuations, eight compositions were made (see Table 3). The content of MKA was chosen according to the results of the study [13]. The quantities of PA and MKA in the forming mixes were fixed and derived from the combined amounts of PC and MKA in percent.

Table 4 lists the compositions of the developing mixes of the porous LWAC samples. In the forming mixes, the water to PC and MKA ratio ranged from 0 to 0.44.

The freshly prepared paste was carefully placed into the steel molds immediately after mixing to prevent segregation or loss of workability. To preserve the structural integrity of the aggregates, the mixture was gently compacted using a wooden board. The mixture was then subjected to further compaction for 1 minute on a vibrating table to eliminate air pockets and ensure a dense and uniform sample.

Fresh PC Pastes that Have Been Altered with Various Chemical Admixtures

Initial and Final Setting Times

Tests on the beginning and final setting timings of fresh PC pastes were carried out in order to choose the optimal SP that (in interaction with AM) assures the minimum setting time delay (see Table 5). With just a 6.4% difference between it and the reference sample, AM has the least impact on the initial setup time, according to the analysis of the data. The new PC paste's first setup time was 9.8% longer due to the composition of PA. It rises by 13.4 to 26.2% for LS and PCX applications. The study demonstrates that the first setup time varies based on the kind of SP.

The initial setup time changed noticeably when AM and SPs were used in conjunction. The initial setup time is longer with PA, LS, and PCX than with the reference samples. The tested samples with LS and PCX started setting almost two times as slowly. In comparison to the reference samples, the final setup time is longer for the AM, PA, LS, and PCX samples. These findings demonstrate that the final setup times for LS and PCX are substantially longer than those for PA. When utilized alone or in conjunction with AM, PA often causes the smallest initial setup and final setup delays.

Fig. 2.

Density of fresh PC with different MKA to PC ratios with or without AM.

Fig. 3.

Apparent porosity of hardened PC pastes with different MKA to PC ratios with or without AM.

Different MKA to PC Ratios in Fresh PC Pastes

Lightweight Aggregate Concrete (LWAC)