Introduction

Plaster (G) is widely used in civil construction for wall coating. Even though the large Brazilian availability of gypsum by the Pólo Gesseiro do Araripe-PE, the production gypsum is challenging with uncontrolled exploitation, indiscriminate firewood usage for calcination, and wastewater management costs. Hence, alternative plastering solutions have commonly being investigated (Geraldo et al., 2017), including phosphogypsum (PG).

Dehydrated phosphogypsum (PG) is a byproduct of the phosphorus fertilizer industry (Cao et al., 2022; Fornés et al., 2021; Mohammed et al., 2018; Pu; Zhu; Huo, 2021) composed of 90-95% CaSO₄⋅2H₂O (Jia; Wang; Luo, 2021). It is a promising candidate to be used as hemihydrate or anhydrous pos-treatment (Cao et al., 2022; Geraldo et al., 2020). Brazil produces about 6 million tons/year of phosphogypsum (Borges, 2011), being the ninth largest producer worldwide.

The gypsum industry has recycled no more than 15% of the world’s PG. The remaining is disposed indiscriminately, polluting the biosphere (Tayibi et al., 2009). It has been used as cement retarder (Andrade Neto et al., 2021; Cai et al., 2021; Tayibi et al., 2009), road filling materials (Jiang et al., 2018), stabilized-soil (Degirmenci, 2008; Mashifana; Okonta; Ntuli, 2018), and gypsum building materials (Cao et al., 2022; Kuzmanović et al., 2021; Ma et al., 2020; Wang; Cui; Xue, 2020). However, its use in plaster building materials is limited due to its possible harmful impurities (P₂O₅ and F impurities, organic matters, and alkali metal salts (Geraldo et al., 2020; Ma et al., 2018; Nizevičienė et al., 2018; Rashad, 2017) that can interfere with its hydration (Cao et al., 2022; Geraldo et al., 2020; Jin et al., 2020; Pundir; Garg; Singh, 2015; Singh, 2003).

One of the ways to eliminate the soluble and co-crystalline impurities while having more stable plasters is by high-temperature calcination (550 ^oC - 1000 ^oC) (Cao et al., 2022; Cesniene, 2007; Liu; Ouyang; Ren, 2020; Ren et al., 2012; Schaefer; Cheriaf; Rocha, 2017; Singh; Garg, 2005) for at least three hours (Cao et al., 2022). Plasters using anhydrite from PG calcinated at 800 ^oC reached 42 MPa in Cesniene (2007) and 48.6 MPa in Cao et al. (2022). On the other hand, plasters using β-hemihydrate PG (β-CaSO₄.1/2H₂O) calcinated at low temperatures (120 ^oC - 400 ^oC) achieved similar compressive strength or water-resistance of plaster in other studies (Bumanis et al., 2018; Değirmenci, 2008; Garg; Pundir; Singh, 2016; Geraldo et al., 2020; Pundir; Garg; Singh, 2015).

In this context, the present study addresses the partial and total replacement (0, 25, 50, 75, and 100%, by weight) of plaster by untreated phosphogypsum (PG) for internal coatings. Recycling phosphogypsum for building materials manufacturing is a safe reuse alternative to clean PG storage areas and preserve natural resources (Mazzilli et al., 2020).

Materials and methods

Materials

Specific plaster for the internal coating of wall and ceiling (G) from Araripina/PE was purchased from a local market. Phosphogypsum (PG, CaSO₄.2H₂O) originated from a phosphate fertilizer industry (Vale Fertilizantes, Uberaba, MG, Brazil). As no significant harmful impurity content was observed in the PG used in the present study and no high mechanical performance is required for use in internal coatings of partition walls, the authors have chosen not to calcine the PG. Therefore, no extra cost related to calcination or other pre-treatment other than drying at (105 ± 5) °C boosts the reuse of raw PG in coating systems consuming low energy (Wei et al., 2023). Untreated phosphogypsum was studied as a fine aggregate for magnesium oxysulfate cement in Wei et al. (2023). Cao et al. (2022) and Pereira et al. (2021) calcinated the PG around 150 °C, which the authors classified as a low-temperature treatment. Hence, PG was oven-dried at (105 ± 5) °C for 24 h. Typically, crystalline-bound water is released from plaster at 120 °C - 180 °C. PG was beige and ranged from fine particles to some particle clusters (ø < 25 mm), while G was finer and white. The moisture contents of G and PG were 5% and 48%, respectively.

Materials characterization

PG and G were analyzed using the following:

1. specific mass (ABNT, 2017a), 25 °C, RH = 63%), considering the average value of six samples and the maximum relative deviation of 2%;

2. unitary mass (ABNT, 2019a), 25 °C, RH = 63%), considering the average value of six samples and the maximum relative deviation of 2%;

3. x-ray fluorescence (XRF) experiments were performed using a Shimadzu spectrometer (model EDX-720) with a tube tension of 50 W (Na to U) using powdered samples (ϕ ≤ 75 µm, previously dried at (105 ± 5) °C by 24 h) under vacuum, with 10 mm collimator;

4. x-ray diffraction (XRD) using a Shimadzu diffractometer (model XRD-7000), using CuKα radiation (40 kV / 30 mA, λ = 1.54056 Å), 2°/min in the interval of 5º < 2θ < 80°. Crystalline phases of previously dried powdered samples (ϕ ≤ 75 µm, 105 ± 5 °C for 24 h) were identified based on the ICDD (International Centre for Diffraction Data);

5. blaine-specific surface including BET surface area and BJH parameters (total pore volume and average pore diameter) were determined (NovaWin2, version 2.2, Quantachrome) at 77.35 °K, with pretreatment of the samples (0.5 g) at 60 ºC at a pressure of 100 μm Hg for 24 h. The samples were pre-treated at 200 °C for two hours for moisture removal; and

6. the particle size distributions of the PG and G were determined by a laser particle size analyzer (Mastersizer 2000, Malvern Instruments, UK, CILAS 920L particle size analyzer) using isopropanol as a dispersant solvent. PG was previously crushed and sieved, while G was not pre-treated. The granulometric range of the equipment was 0.30 µm - 400.00 µm, and the data was processed by the CILAS program (version 2.56).

PG/G Plasters Design and Production

PG and G were previously sieved (ABNT No 10, #2 mm) and then oven-dried to a constant weight at (105 ± 5) °C for 24 h to remove the free water. Plasters were prepared according to NBR 12128-2 (ABNT, 2019c), where each batch mixing used 3,000 g of PG/G (wt.%, Table 1) and distilled water. Mix-ID G100 indicates 100% of G, and G75PG25 means 75% of G and 25% of PG. The water requirement for normal consistency (NC) of plasters was measured using the Modified Vicat Apparatus (ABNT, 2019b), T = 24 ± 4 °C, RH = 65 ± 5%). Although the NBR 12128-2 (ABNT, 2019c) considers a penetration of (30.0 ± 2.0) mm as a satisfactory index for normal consistency, (30.0 ± 0.5) mm was adopted for the sake of accuracy of the water/binder ratios. The required water contents for normal consistency of PG/G plasters are further discussed.

Test procedures

Setting time

Plaster setting times were measured using the Modified Vicat Apparatus, according to NBR 12128-2 (ABNT, 2019c) at T = 24 ± 4 °C, and RH = 65 ± 5%. The result for each composition consisted of the average of the measurements obtained in triplicate (which do not differ by more than 10% from each other) for each mixture.

Compressive and flexural strength

Compressive strength (5 x 5 x 5 cm³) and three-point bending flexural strength (4 x 4 x 16 cm³) of PG/G plaster samples were tested at 7d and 28d, according to NBR 13207-3 (ABNT, 2023), and NBR 13279 (ABNT, 2005), respectively. The strengths were based on the three closest values to the average of six specimens. Each value should not exceed 10% of the initial average. Samples were de-molded after 24 h, cured at room-controlled conditions (T = 24 ± 4 °C, RH = 65 ± 5 %) until the day of the test (7d and 28d), and dried to a constant weight at (40 ± 2) ^oC immediately before the tests. The 28-day test checked for any adverse effects on the compressive strength over time due to phosphogypsum use. Mechanical tests were performed using an I-3025-B Contenco machine (maximum load of 100 tons) with loading speeds of 0.6 kN/S and 50 N/s from compressive and flexural strengths, respectively. The increments of loads applied up to rupture were close to those established by NBR 13207-3 (ABNT, 2023), being around 230 N/s for PG100 and 600 N/s for G100 during compressive strength, as well as 30 N/s for PG100 and 50 N/s for G100 for flexural ruptures. Some PG100 specimens broke in shear.

Tensile bond strength

Tensile bond strengths were achieved at 28d and 56d, according to NBR 13528-2 (ABNT, 2019b). Three concrete slabs (25 x 50 x 2 cm³) reinforced with galvanized iron (2.5 mm mesh every 5 cm) were used as standard substrates, NBR 14081-2 (ABNT, 2015). The substrate surfaces were mechanically brushed throughout the curing process to simulate the roughness of cement masonry blocks. The substrates were then slightly moistened before applying the plasters (Carvalho Junior, 2005). The thickness of the plasters was around 17.5 mm since most plaster coatings are used between 15 mm and 20 mm. Eight metal cylinders were glued to plaster substrates 48 h before tests using a low-viscosity glue. Although the standard requires 12 specimens, 8 were taken per slab, resulting in 24 samples in total. Finally, the tests were performed using CONTROLS set up, with maximum capacity of 500 kgf, and tensile loading increment rates of 19.4 N/s for 0.21 MPa and 64.4 N/s for 0.85 MPa. The applied tensile stresses were between 21 s and 64 s. The average adhesion results were followed by the possible types of rupture described in NBR 13528-2 (ABNT, 2019b), namely Type A – a rupture in the substrate; Type B – a rupture in the substrate/coating interface; Type C – a rupture in the coating; Type D – a rupture in the coating/glue interface; and, Type E – a rupture in the glue/metal cylinder interface.

Capillary water absorption

Capillary water absorptions were monitored at 14d and 28d, following the Pipe Test prescribed by the Centre Scientifique et Techinique de la Construction - CSTC, Hidrofuges de surface: choix et mise en oeuvre (1982). The procedure simulates loads caused by rain with wind acting on the wall surface represented by the water level in the glass pipe (4 cm³, graduated in tenths of a milliliter), which amounts to a static pressure of 140 km/h ≅ manometer column height of 92 mm (Grossi, 2013). Using a caulking compound, the pipe was fixed to the cylindrical specimen (Ø 20 cm x 4 cm). Finally, a digital video camera captured water absorption profiles (three samples for each composition) every 1 min until 15 min or until the volume of 4 cm³ was reached.

Non-standardized complementary tests

Total water absorption by immersion

After capillary absorption tests, wet areas were identified by color difference. The wetted surface diameter was measured with a pachymeter, and its depth was measured five minutes later.

Water loss, density, and color of the hardened plasters

The moisture loss profiles during hardening included weight measurement (until weight stabilization) immediately after molding, followed by 1, 2, 3, 6, 12, and 24 h. The water content of the plasters was determined as the difference between their initial and final weight. The density of the hardened plasters was the ratio between the post-drying weight and the volume of the specimens. Finally, the final color and texture of the hardened coatings were tactile and visually analyzed.

Results and discussion

Discussions about phosphogypsum (PG) and plaster (G) characterization

As can be seen in Table 2, PG (CaO + SO₃ = 93.1%) presented similar chemical composition to G (CaO + SO₃ = 97.6%) without deleterious impurities to the plasters. Although high P₂O₅ could be expected in PG (Geraldo et al., 2020; Ma et al., 2018; Nizevičienė et al., 2018; Rashad, 2017), similar values were found at about 2.21% for G and 3.47% for PG. Mineralogically, the X-ray diffractogram indicated the presence of calcium sulfate hydrate [Ca(SO₄).0.5(H₂O), ICDD 98-003-3984] and magnesium calcite [Mg_0.1Ca_0.9CO₃, ICDD 01-071-1663] in G (Figure 1a). Magnesian calcite is a possible contaminant of gypsum and can precipitate in the voids between the grains of carbonate sediments. However, it is unlikely to have any impact on the results of the investigation. On the other hand, only Ca(SO₄).0.5(H₂O) was detected in PG (Figure 1b).

Average diameters of 17.41 µm and 7.26 µm were found for PG and G, respectively (Figure 2). The PG particles are coarser than those of G above 1.2 µm, being a maximum diameter of 90 µm to PG and 25 µm to G. However, PG has finer particles than G below 1.2 µm, 10% of them smaller than 0.75 µm, while 10% of the G particles were smaller than 0.90 µm. The minimum diameter was 0.13 µm for PG and 0.22 µm for G.

Similar specific weights (PG = 2.668 g/cm³ and G = 2.665 g/cm³) and unit weights (PG = 551 kgf/m³ and G = 600 kgf/m³) meet the maximum of 700 kgf/m³ for unit weights recommended by the NBR 13207 (ABNT, 2017b). However, PG presented a BET surface area approximately twice as large as G (PG = 8.470 m²/g and G = 3.981 m²/g). BJH parameters (total pore volume and average pore diameter) were also measured. Although the superficial pores showed similar average diameters, i.e., 36.177 Å for PG and 36.082 Å for G, PG particles showed three times more pore volume than the G particles (PG = 0.012 cm³/g and G = 0.004 cm³/g). Hence, all this suggests a higher water/PG factor than water/G.

The water requirement for normal consistency of PG/G plasters

Concerning the water required to achieve the normal consistency of PG/G plasters (Figure 3), the water/(PG+G) increased from 0.55 to 1.00 when PG completely replaced G. Despite similar specific weights of PG and P during their replacement by weight, and the higher average particle size of PG (17.41 µm) when compared to G (7.26 µm), PG particles presented a higher specific surface area of 8.47 m²/g versus 3.98 m²/g of plaster. In addition, the total pore volume was 0.012 cm²/g for PG and 0.004 cm²/g for G. There was a good correlation (R² = 0.9994) between the water required for normal consistency as PG gradually replaced G in the plasters ruled by the equation shown in Figure 3.

Comparatively, the water requirement for normal consistency was 0.71 when 100% of calcinated PG at 150 ^oC (β-hemihydrate) was used by Cao et al. (2022). Here the water demand was increased because the higher calcination temperature used by Cao et al. reduced the molar volume of CaSO₄ (fewer water molecules), increasing the packing density of the plasters (Geraldo et al., 2020).

The setting time of PG/G plasters

Plasters’ final setting and open times increased with the PG/G ratio due to higher water content, as seen in Figure 4. Open time is the time difference between the final and initial setting times. According to Antunes, John and Andrade (1999), the productive working period is also called the useful time of the plaster. The dashed red bars indicate the maximum final and minimum initial setting times limited by NBR 13207 (ABNT, 2017b). A higher percentual of P₂O₅ is also reported as delaying the hydration process of plasters (Cai et al., 2021). The open time increased from 8:27 min to 20:26 min when 100% of G was replaced by PG.

Comparatively, 3 min (initial setting time), 8 min (final setting time), and 5 min (open time) were found by Cao et al. (2022) in plasters made with 100% PG calcined at 150 ^oC. Furthermore, initial setting times (5 min to 15 min), and final setting times (18 min to 33 min), which means open times (13 min to 18 min) were achieved in Geraldo et al. (2020) using PG calcinated in a stationary kiln in three different residence times (1 h, 2 h, and 5 h) at 150 ^oC. Other authors (Bumanis et al., 2018, 2019) reported initial and final setting times from 2 min 30 s and 6 min 30 s for pastes made with PG recycled at 160 ^oC or 180 ^oC for 4 h and with a water/PG ratio = 0.80.

The density of PG/G plasters in the hardened state

The voids have risen as the system demanded more water as the content of PG increased. Increased water demand is also associated with a lower crystallization quality of the hardened matrix (Bumanis et al., 2018; Rossetto et al., 2016; Geraldo et al., 2017, 2020; Nizevičienė et al., 2016). Hence, less dense systems were obtained as PG content increased (Figure 5). Furthermore, there was a good linear correlation (R² = 0.9951) between the reduction in the density as PG gradually replaced G in the plasters governed by the equation shown in the graph in Figure 5.

Non-standardized complementary tests. Part A: weight stability of the plasters

The first-24 h water losses after curing are shown in Figure 6. The water loss profiles until the sample’s weight stabilized can be seen in Figure 7, including their constitutional water as the water lost alongside the plaster hydration. The term “water of constitution” refers to the water used to mix the paste. This water can either evaporate or take part in the crystallization of the chemical components of the mass. This process takes some time and directly affects the hardening of aerial binders.

Less water was lost as the PG content increased, including the first 24 h. Generally, plasters with 100% phosphogypsum (PG100) stabilize their weight between 11 and 13 days, and plaster with 100% plaster (G100) between 7 and 9 days. The weight stabilized between 10 and 12 days for G25PG75, 9 to 11 days for G50PG50, and 8 to 10 days for G75PG25. According to normal consistency tests, these facts are due to the higher water/(P+PG) ratio for higher PG contents.

Compressive strength of PG/G plasters

As expected, reduced compressive strengths (Figure 8) were achieved increasing the contents of PG due to more porosity and lower crystallization quality (Bumanis et al., 2018; Rossetto et al., 2016; Geraldo et al., 2017, 2020; Nizevičienė et al., 2016). The average 7d compressive strength reduced from 21.43 MPa to 6.21 MPa when 100% of G was replaced by PG, and from 22.29 MPa to 7.79 MPa at 28d. The dashed red bars indicate the minimum compressive strength limited by NBR 13207 (ABNT, 2017b).

No deleterious impact of using PG in compressive strength was observed over time. Conversely, the level of resistance either remained constant or showed a slight increase at 28 days. As PG and G are non-hydraulic binders, their microstructure as a function of their crystallization stabilizes upon dehydration which took 8d to 12d, as verified by weight stabilization by water loss.

There were good quadratic correlations (7d — R² = 0.9998, and 28d — R² = 0.9980) between the compressive strength as PG gradually replaced G in the plasters ruled by the equations shown in Figure 8. Compressive strengths around 20 MPa were found by Ferreira and Carneiro (2019) using a 100% plaster (w/g = 0.48). Compressive strengths at 28d are consistent with Cao et al. (2022), 10 MPa for plasters using a PG calcinated at 150 ^oC. PG pastes presented compressive strength around 6 MPa — 8 MPa at 28 days in Geraldo et al. (2020) using PG calcinated in a stationary kiln at three holding times (1 h, 2 h, and 5 h) at 150 ^oC.

Flexural tensile strength of PG/G plasters

The flexural tensile strength slightly increased between 7 and 28 days for the same reason observed in the compressive strength behavior (Figure 9). Furthermore, no deleterious impact of using PG in flexural strength was observed over time from 7 days to 28 days. In the same way, the increase in the water/(G+PG) ratio as PG increases resulted in a higher number of voids and lower crystallization quality of the hardened matrix (Bumanis et al., 2018; Rossetto et al., 2016; Geraldo et al., 2017, 2020; Nizevičienė et al., 2016), reducing the flexural tensile strengths as the content of PG increases. The average flexural strength at 7d reduced from 13.64 MPa to 4.44 MPa when 100% of G was replaced by PG, and from 14.10 MPa to 5.98 MPa at 28d. The dashed red bars indicate the minimum flexural strength limited by NBR 13279 (ABNT, 2013).

As to the response of the flexural strength, there were good quadratic correlations (7d — R² = 0.9889, and 28d — R² = 0.9852) between the flexural strength as PG gradually replaced G in the plasters ruled by the equations shown in Figure 9. Comparatively, Cao et al. (2022) observed flexural strengths around 3 MPa at 28d using a PG calcinated at 150 ^oC.

Capillary water absorption of PG/G plasters

Given the rapid water absorption by plaster, capillary water absorptions were measured by the unit ratio of absorbed volume (4 ml) as a function of absorption time at 14d (Figure 10) and 28d (Figure 11) after curing. However, the results were similar at both ages since the water loss stabilized at 13d for all samples. The absorption times decreased with increasing PG in the mixtures. Furthermore, the water absorption occurred linearly over the time from 0 ml to 4 ml for all plasters (Figure 10 and Figure 11).

The water absorption rates (relation of water absorption volume versus absorption time) were quite similar between 14d and 28d for all compositions (Figure 12). They increase as PG content increased, due to the larger pore volume of PG relative to G, as determined by the BJH technique. In addition, the water/binder ratio also increased with increasing PG in the system. The water/PG factor was 1.00, and the water/G factor was 0.55 for normal consistency plasters, proving the higher porosity in the phosphogypsum-based hardened plasters. Comparing to the reference paste (G100), the water absorption rate increased from 1.74 ml/min to 3.33 ml/min at 14d, and from 1.49 ml/min to 3.58 ml/min at 28d when 75% of G was replaced by untreated PG (in weight). On the other hand, 6.15 ml/min was reached for PG100.

There were good polynomial correlations (14d — R² = 0.9967, and 28d — R² = 0.9961) between the capillary water absorption as PG gradually replaced G in the plasters ruled by the equations shown in Figure 10.

Tensile bond strength of PG/G plasters

The tensile bond strengths at 28d and 56d and the failure modes are shown in Figure 13, according to NBR 13528-2 (ABNT, 2019b). The dashed red bars indicate the minimum tensile bond strength limited by NBR 13749 (ABNT, 2013). There were no changes from 28d to 56d, as expected due to the stabilization of water loss between 8d and 12d. As observed in the compressive and tensile strengths, increased porosity by increasing PG also reduced the tensile bond strength. The higher percentages of incorporated PG evidenced the harmful effects of higher w/(G+PG) ratios, thus reducing the adherence capacity of plasters to the substrate, being more critical after the replacement of 75% (by weight) of G by PG (G25PG75).

The rupture forms in the specimens took place predominantly at the substrate/coating interface, and the ruptures in PG100 and G25PG75 were entirely of this type. On the other hand, the rupture in G100 also occurred on the substrate. This behavior was also observed in Ferreira and Carneiro (2019) where most of the ruptures occurred at the substrate/coating interface in plaster coatings. No rupture occurred at the glue/metal cylinder interface.

To access the tensile strength in the region of the PG100 and G25PG75 plasters, 24 specimens of each were reattached with epoxy resin to the standard substrate (disregarding the age of the substrate). New pull-out tests were then performed after 24 h. Tensile bond strengths from 0.32 MPa to 0.55 MPa (average 0.44 MPa) were measured for PG100 plaster. In the case of plaster G25PG75, on the other hand, the strengths ranged from 0.44 MPa to 0.76 MPa (average 0.61 MPa). Hence, the tensile strengths in the PG100 and G25PG75 plasters regions were much higher than those at the substrate/coating interface, indicating the influence of porosity at the substrate/coating interface when increasing PG in mixtures.

Non-standardized complementary tests: part B, total water capillary absorption by immersion

The total water capillary absorption for all compositions occurred superficially (uniform circular ring, Figure 14a) and in-depth (regular conical bulb shape, Figure 14b) about the central point where the pipe was placed. Absorption occurred predominantly by capillarity unaffected by gravity, as slightly higher absorption was observed at the bottom of the bulb. Thus, it is expected that the same situation takes place in G/PG coatings.

The diameter of the circular ring was uniform for all plasters ranging from 51 to 56 mm (53 mm on average) in the first 30 sec, and 57 to 66 mm (62 mm on average) after 5 minutes. The absorption depth was between 17 and 21 mm (18 mm on average) in the first 30 sec followed by 19 to 24 mm (averaged 20 mm) after 5 min.

Practical aspects

A phosphogypsum-based plastering (75 - 95%), ash calcium powder (Sierozem powder, 1 - 3%), retarder (citric acid or sodium tripolyphosphate STPP, 0.1 - 0.5%), composite water retention agent (methylcellulose ether or hydroxypropyl methyl cellulose ether, 1.1 - 5.5%), ethylene-vinyl acetate copolymerized latex powder (0.3 - 1%), and filler (talcum powder or quartz sand, 0 - 20%) has been patented by Chongqing University to be used for a plastering layer of the building wall under the number CN101497519A (Chongqing University, 2009).

Positively, plasters containing 75% uncalcined PG, 25% G, water, and no additives were developed here, satisfying all the requirements of the NBR 13207 (ABNT, 2017b) – “Gypsum for buildings - Requirements” – for coating plasters in the civil construction, and NBR 13279 (ABNT, 2013) – “Mortars applied on walls and ceilings - Determination of the flexural and the compressive strength in the hardened stage” – for internal coatings of partition walls. Although there was a slight difference in color between PG and G in dry powders, all hardened plasters were equally white after curing. Initial setting time = 10:16 min (> 10 min), final setting time = 24:56 min (< 45 min), lower compressive strength at 7d or 28 d = 8.51 MPa (> 8.40 MPa), lower flexural tensile strength at 7d or 28 d = 6.36 MPa (> 3.00 MPa), lower tensile bond strengths at 28d and 56d = 0.3 MPa (> 0.20 MPa).

Adding untreated PG up to 100% required additional water to achieve the normal consistency of the plasters without affecting their physical properties. Water/(G+PG) varied from 0.55 (G100) up to 1 (PG100). Once similar results to those reported in the literature using low calcinated PG were achieved, the untreated PG results were optimistic concerning the setting times. PG setting times, both initial and final, were reported to increase as the calcination time increased in Geraldo et al. (2020). Incorporating PG up to 100% maintained the parameters for plasters, NBR 13207 (ABNT, 2017b), with initial setting times over 10 min and final ones up to 45 min, varying from 10:16 min to 31:31 min. This extended application time of PG-based plasters is positive in a practical aspect as they reduce the content of retarders.

Incorporating untreated PG up to 100% decreased the density of the hardened systems approaching it to those of lighter construction systems. The final density of the hardened plasters was 0.77 g/cm³ (PG100) versus 1.21 g/cm³ (G100). In a practical way, the use of lighter hardened plasters reduces the structural loads leading to lower-cost constructions. Furthermore, water retention based on less water lost as PG content increased by the gradual replacement of G in the plasters, even during the first 24 h. This behavior is positive to reduce the chance of plastic cracking and superficial efflorescence as Na₂SO₄ (Cao et al., 2022).

Except for the plaster with 100% PG (PG100), all plasters achieved the minimum compressive strength of 8.40 MPa (Figure 6) established by NBR 13207 (ABNT, 2017b) for coating plasters in civil construction. Even so, the plaster with 100% PG reached the average value of 7.79 MPa, 93% of the value recommended by the standard. All the evaluated plasters achieved the minimum flexural tensile strength of 3.00 MPa (Figure 7) required by NBR 13279 (ABNT, 2005) for internal coatings of partition walls.

Twice water capillary absorption rates were achieved when untreated PG replaced G up to 75% in weight. PG100 reached around 3.5 times the absorption rate of G100 for both 14d and 28d. However, hydrophobic coatings can be used to protect large plaster products depending on the application.

Finally, incorporating untreated PG up to 100% gradually decreased the tensile bond strength required for internal coatings of partition walls at 28d and 56d. Although the impact was more pronounced using 75% PG and 100% PG, all formulations achieved the minimum 0.20 MPa required by the NBR 13749 (ABNT, 2013). Average tensile bond strengths of 0.44 MPa (PG100) and 0.61 MPa (G25PG75) were found inner the plasters regions, which are much higher than those at the substrate/coating interface, indicating the influence of porosity at the substrate/coating interface when increasing PG in mixtures. Good bonding performance is fundamental for coating plaster since shear and tensile are typical in wall coatings.

Conclusions

Plaster samples containing from 25%, to 100% (by weight) of untreated phosphogypsum were evaluated for internal coatings of partition walls. All plasters showed a similar and uniform white color when hardened. The following conclusions were drawn from the study:

1. plastering comprising 75% untreated PG, 25% G, water, and no additives (water/(G+PG) = 0.81) satisfied all the standards requirements for internal coatings of partition walls;

2. incorporating untreated PG up to 100% demanded more water to achieve the normal consistency of the plasters without compromising the setting times;

3. incorporating untreated PG up to 100% gradually increased the time demanded for weight stability and decreased the density of the hardened systems. Although twice water capillary absorption rates were achieved to G25PG75, hydrophobic coatings can be used; and

4. incorporating untreated PG up to 100% gradually decreased the compressive (7d, and 28d), flexural tensile (7d, and 28 d), and tensile bond strengths (28d, and 56d). However, except for compressive strength of PG100, all the mechanical responses satisfied the standards for internal coatings of partition walls. The substrate/coating porosity of the hardened plasters affected the bond when increasing PG in mixtures.

Acknowledgments

The authors express their gratitude to the Materials Science and Engineering Post-graduate Program for providing partial financial support for this research.

References

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