Effect of calcium and sodium-activated bentonites as waterproofing additives in polymer-modified cement mortars

Abstract

Cement mortar is typically formulated by blending fine aggregate and water, yielding a matrix characterized by heterogeneity and porosity. However, these inherent characteristics can exacerbate structural leakage, allowing water infiltration in buildings. Bentonite is a promising waterproofing additive for various mortar types, owing to its remarkable water swelling properties and recognized attributes of being non-toxic and cost-effective. Hence, this study examined calcium or sodium-activated bentonites as cost-effective waterproofing additives in polymer-modified cement mortars. A commercial mortar with varying amounts (0-2wt.%) of these bentonites was prepared. The research focused on characterizing the additives and assessing mortar performance regarding fluidity, water absorption, mechanical strength, resistance to water penetration under negative and positive pressure, and microstructural features. Results showed sodium bentonite had a smaller particle size, higher reactivity, and swelled three times more than calcium bentonite. These additives altered the fresh mortars rheological properties due to flocculation. Incorporating 2 wt.% of sodium bentonite into the mortar did not significantly change the water absorption or crushing strength of samples cured at room temperature for 28 days. However, it led to an 18.6% reduction in water penetration under positive pressure and prevented liquid percolation to the upper surface of the samples during tests under negative pressure.

Keywords:
Mortar; Bentonite; Waterproofing; Swelling; Hydration

INTRODUCTION

All building structures are vulnerable to weathering, which can cause deterioration over time. The primary catalyst for this degradation is the continual exposure of construction materials to water. This exposure not only induces physical degradation of the structures but also facilitates the transportation of corrosive agents to the substrate, thereby initiating chemical transformations ^1),(2. Improper execution of waterproofing measures can exacerbate the occurrence of infiltrations, which are often difficult to detect. Consequently, rectifying these issues can incur significant costs, potentially requiring demolition and rework, ultimately shortening the building’s lifespan ³. Therefore, ensuring proper execution of waterproofing procedures is of utmost importance. The selection of the appropriate type of surface waterproofing depends on whether it is intended for internal or external areas and the specific type of moisture (see Table I) to which the waterproofing product must be resistant.

Table I
Types of moisture and affected surfaces ^1),(4.

Moisture type	Description	Affected surfaces
Percolation	Characterized by free flow without hydrostatic pressure	Terraces, roofs, gables, and facades
Condensation	Condensation of water on specific surfaces under certain temperature and pressure conditions	Bathroom, kitchen, saunas, refrigerators, etc.
Capillarity	Water naturally present in the subsoil that is absorbed by the substrate through capillarity	Subfloors, masonry, and concrete
By fluid under unilateral or bilateral pressure	The water exerts positive or negative hydrostatic pressure greater than 1 KPa on the waterproofing	Subfloors, water tanks, pools, etc.

In general, waterproofing products are categorized as either rigid or flexible ³:

• Rigid products: These consist of polymer mortars or thermoplastic resins lacking flexibility, making them suitable for indoor environments with minimal exposure to weather conditions.

• Flexible products: These include polymer mortar or asphalt membranes designed for application on external surfaces. They contain polymers that impart flexibility, enabling them to accommodate the natural expansion and contraction of concrete exposed to weather conditions.

Conventional cement mortar is commonly formulated through the precise combination of fine aggregate and water, resulting in a matrix marked by heterogeneity and porosity ⁵. Nonetheless, these inherent characteristics can exacerbate building structural leakage, facilitating water infiltration. In efforts to mitigate mortar permeability and enhance its overall effectiveness, waterproofing products, often referred to as polymer-modified mortars, have garnered significant attention and have undergone thorough scrutiny in recent years. ^6),(7. Such products are primarily composed of cement, inert mineral aggregates, acrylic polymers, and additives. Their waterproofing performance is derived from the presence of hydrophobic additives ^1),(5, permeability-reducing, and miscellaneous admixtures ⁷. These mortars are commonly applied to surfaces such as water tanks, reservoirs, pools, and elevator shafts, requiring resistance to water absorption by capillarity (soil moisture), hydrostatic pressure, and percolation moisture. Additionally, they can be classified as either two-component (comprising solid and liquid reagents) or one-component (where all reagents are solid and mixed with water during preparation) ⁸.

Bentonite is currently widely used to improve the performance of cement mortars and concrete ^{2),(5),(8)-(13}. This clay mineral possesses significant properties including cation exchange capacity, nanoscale particle size, refractoriness, viscosity enhancement when dispersed in water, thixotropic rheological behavior, and others ^{10),(11),(14)-(16}. Due to its exceptional water swelling properties and recognized attributes of being non-toxic, cost-effective, and harmless, bentonite emerges as a viable additive to different types of mortars. By incorporating this material into mortar, it fills the pores within the structure, thereby diminishing water migration and providing excellent waterproofing and impermeability ^11),(17),(18.

Yang et al. ² reported that incorporating bentonite (up to 8 wt.%) into cementitious mortars resulted in substantial increases in crushing strength (61.48%), flexural strength (42.09%), and impermeability (76.47%) of the samples. Additionally, Liu et al. ⁵ conducted a comparative evaluation of different bentonites in cementitious mortars, highlighting the effectiveness of sodium and magnesium-containing ones in enhancing impermeability under pressure. However, achieving the optimal dosage of these additives requires careful formulation adjustments to ensure proper rheology and applicability of the mixture ^10),(19)-(21. Some researchers have developed a humidity-regulating mortar by introducing organobentonite into the polymer-containing mortar formulations, aimed at facilitating automatic adjustment of indoor humidity levels ²². Additionally, other studies have investigated the incorporation of bentonite into mortar and concrete, partially replacing cement. The compressive strength of the modified samples remained comparable to the reference compositions (without bentonite), while demonstrating enhanced resistance to sulfate attack, suggesting the viability of this additive as a cost-effective auxiliary material ^14),(23)-(25.

The protective waterproofing mechanism attributed to bentonites differs from traditional methods and involves several steps ^2),(15),(26: (i) acting as a matrix component with cement, bentonite undergoes pozzolanic reactions with generated calcium hydroxide during cement hydration; (ii) these reactions yield CSH-type hydrates (where C = CaO, S = SiO₂ and H = H₂O) with varying stoichiometries, filling spaces and reducing pore size, with hydration products possibly forming on the bentonite surface; and (iii) when water capillarity absorbs into the mortar, it triggers swelling of bentonite, forming an impermeable film that prevents water passage and significantly reduces further water absorption.

Nonetheless, as emphasized by certain authors ^2),(5, the diverse chemical compositions of bentonites can impact mortar performance differently, posing challenges in comparing results and facilitating practical applications. Within this context, this study aimed to evaluate the impact of incorporating different quantities of calcic or sodium-activated Brazilian bentonites into a polymer-modified mortar containing Portland cement. The study involved characterizing the selected bentonites and analyzing mortar behavior in terms of fluidity, water absorption, crushing strength, and resistance to water penetration under positive and negative pressure.

EXPERIMENTAL

In this study, two distinct types of bentonites, namely Brasgel NT 25 (calcic) and Brasgel T (sodium-activated) were employed, both supplied by Buntech (Brazil). Table II points out that these materials predominantly exhibited particle diameters smaller than 50 μm, with the calcic bentonite (CB) having an average particle size of 11.22 μm, while the sodium-activated bentonite (SAB) showed finer particles with a d₅₀ of 1.06 μm. These characteristics can significantly impact the reactivity of these powders. Consequently, the finer bentonite exhibited over three times the swelling value (25 ml/2g) compared to the CB (8 ml/2g). Based on these results, it can be concluded that the activation process of the sodium bentonite was effective, rendering it more reactive.

Table II
Characteristics of the evaluated bentonites.

Properties	Calcium bentonite (CB)	Sodium-activated bentonite (SAB)
Particle size distribution
d10 (mm)	1.33	0.28
d50 (mm)	11.22	1.06
d90 (mm)	42.17	7.94
Swelling (ml/2g)	8	25
Chemical composition (%)
Na2O	0.2	1.2
MgO	2.0	2.8
Al2O3	21.3	31.1
SiO2	62.1	55.1
K2O	0.7	1.1
CaO	1.5	1.1
Fe2O3	11.1	6.5
TiO2	1.1	1.1
Identified phases	Saponite [Ca0.2Mg3(Si,Al)4O10(OH)2.4H2O], Montmorillonite [Na0.3(Al,Mg)2Si4O10(OH)2.4H2O] and α-quartz [SiO2].	Saponite [Ca0.2Mg3(Si,Al)4O10(OH)2.4H2O], Montmorillonite [Na0.3(Al,Mg)2Si4O10(OH)2.4H2O], kaolinite [Al2Si2O5(OH)4], α-quartz [SiO2] and antigorite [Mg3Si2O5(OH)4].

The bentonites also displayed variations in their chemical compositions, notably with a higher content of Fe₂O₃ and SiO₂ identified in the calcic one (Table II). This elevated presence of silica was also confirmed through X-ray diffraction tests, which revealed more intense peaks of the α-quartz phase (diffractograms not shown here) in the CB. Generally, the primary phases found in the two studied additives were saponite, montmorillonite, and α-quartz (Table II). On the other hand, the high Al₂O₃ content in the SAB may be linked to the presence of the kaolinite phase (Al₂Si₂O₅(OH)₄), exclusively found in this material. As anticipated, the sodium-activated bentonite also contained both Na₂O and CaO in its composition (Table II).

Additionally, a commercial two-component semi-flexible waterproof mortar, Sikatop® 100 (comprised of a mixture of fine aggregates, Portland cement, additives, and acrylic resin, Sika, Brazil) was chosen to incorporate the bentonite additions and its behavior was subject to further detailed analysis. The chemical composition of the solid components of this mortar corresponded to 62.3% CaO, 20.1% SiO₂, 5.3% Al₂O₃, 5.2% MgO, 2.7% Fe₂O₃, and 1.4% K₂O. Moreover, its density was equivalent to 1,3 g/cm³ and particle size lower than 300 mm.

Initially, a comprehensive characterization of the selected materials (bentonites and mortar) was conducted through a series of tests:

• Particle Size Distribution: Micromeritics Sedigraph 5100 sedimentation equipment was employed, adhering to the procedure outlined in ASTM B761-17 ²⁷. Three aliquots were collected and analyzed, with the reported values representing the average measurements.

• Swelling Analysis: The hydration and delamination capacity of the bentonites were assessed in accordance with the protocol outlined in “Cemp 058 - Swelling Determination” ²⁸. This involved the addition of 2 g of dry bentonite to 100 ml of distilled water in a graduated beaker. Following the addition, the beaker was allowed to rest, and readings were taken 24 hours later, expressing the result in ml/2g of bentonite.

• X-ray Fluorescence: The Shimadzu EDX-720 equipment was employed to identify the chemical composition (in terms of oxides) of the selected materials.

• X-ray Diffraction: Measurements to determine the crystalline phases present in the bentonites were conducted using a Bruker D8 Focus apparatus. This process utilized Cu Kα radiation, with the operation set at 30 mA and 40kV, and the 2θ scanning range was established from 5-80º.

Mortar preparation was conducted, incorporating bentonite contents ranging from 0 to 2 wt.%. For this purpose, a mixture of solid components (mortar powders + bentonite) and polymer emulsion (supplied with Sikatop 100 mortar) was prepared in a 3.5:1 weight ratio using a mechanical stirrer. Subsequently, these mortar blends were applied in the form of three cross-applied coats, with a minimum interval of 3 hours between applications, either onto a concrete substrate (for water penetration resistance tests) or onto a plastic-covered surface (for other tests). The samples were then maintained at room temperature (~25°C) for 28 days before being subjected to performance characterization.

A waterproof mortar must effectively resist moisture through percolation, capillarity, and exposure to both positive and negative pressures. Furthermore, the mortar requires sufficient mechanical strength and a level of flexibility to endure the natural expansion and contraction of concrete when subjected to varying weather conditions. Desirable properties include good compatibility, workability, or a flow time of at least 60 minutes. These attributes were evaluated through the tests highlighted in Table III.

Table III
Performance tests of the prepared polymeric mortars.

Test	Unit	Parameter	Test method
Flow variation after 60 min	%	< 35	ABNT NBR 11905 29 and ASTM C230/230 30
Water absorption	%	Max. 12	ASTM D 570 31
Crushing resistance	MPa	Comparative test among formulations
Water penetration under negative pressure	MPa	Pressure = 0.10	ABNT NBR 11905:2015 29
Water penetration under positive pressure	MPa	Pressure = 0.10
		Pressure = 0.25
Microstructural analysis (SEM and XRD)	Aiming to identify resulting porosity and phase transformations resulting from the interaction between the materials.

The preparation of 1.5 kg of mortar was undertaken for flowability testing. In these experiments, a plastic sheet was affixed to the workbench surface, and the mortar was poured into a conical mold meeting ASTM C230/230 standard ³⁰. Upon filling the mold, it was raised, and the final diameter was measured at three distinct positions to determine the average flow value. Following each test, the mortars were collected and placed in beakers for subsequent flowability assessments conducted at 20, 40, 60, 90, and 180 minutes from the initiation of mortar preparation. Moreover, measurements were discontinued when the spread reached a value equal to or less than 130 mm. The variation in mortar flowability was computed using Equation A, to achieve values equal to or less than 35% within the analyzed time frame.

$FV\left(\%\right)=\frac{\left(IF-Ft\right)}{IF}\times 100$ (A)

where: FV (%) = flow variation; IF (mm) = initial flow; Ft (mm) = flow at time t.

For the water absorption tests, a plastic sheet was secured onto a workbench, and three successive coats of waterproof mortar were applied in a crisscross pattern, with a 3-hour interval between coats ³³. Following a 14-day curing period at room temperature, the cementitious membrane was detached from the plastic, flipped, and left for an additional 14 days (resulting in a total curing time of 28 days). Three specimens, each measuring 100 mm x 100 mm, were extracted from every tested composition. Subsequently, these specimens were immersed in potable water, and water absorption was estimated after 7 days (Equation 2). As per ABNT NBR 15885, the obtained values should be below 12%.

$A\left(\%\right)=\frac{\left(M_2-M_1\right)}{M_2}\times 100$ (B)

where: A (%) = water absorption; M2(g) = sample mass after 7 days; M1(g) = initial sample mass.

Crushing strength measurements were carried out using cylindrical specimens (40 mm x 40 mm) obtained after 28 days of curing at room temperature. The experiments were conducted on an EMIC DL 10000 apparatus, applying a loading rate of 1.3 mm/min. Microstructural characterization of the samples, obtained post-fracture, was performed through X-ray diffraction, and scanning electron microscopy analyses using TESCAN MIRA equipment.

Figure 1 illustrates the tests employed to determine resistance to water penetration under positive and negative pressure. In the negative pressure test, the face in contact with hydraulic pressure remains unsealed, while waterproofing mortar is applied to all other faces of the concrete block. Conversely, in the positive pressure test, the waterproof mortar is applied to the face in contact with hydraulic pressure, while waterproofing mortar is applied to all other faces of the concrete block. Mortar curing lasted 28 days, and measurements were conducted at 0.10 or 0.25 MPa, following ABNT NBR 11905:2015 ¹⁸. These analyses were performed at Falcão Bauer (São Paulo), a laboratory specializing in standardized tests in the field of civil construction.

Figure 1:
Image of measurements carried out for the determination of resistance to water penetration under negative and positive pressure.

RESULTS AND DISCUSSION

Initially, the impact of adding varying quantities of calcium and sodium bentonites (0, 0.5, 1, and 2 wt.%) to the selected mortar was assessed to ascertain potential changes in the rheological properties of the mixtures. Overall, a decline in flowability was detected with increasing bentonite content (Figures 2a and 2c), likely attributable to potential flocculation of the mortars.

Figure 2:
Variation of mortar flowability prepared with different bentonite contents (0-2 wt.%) over time. (a and c) Calcium bentonite and (b and d) sodium-activated bentonite.

According to Shiroma ³², bentonites can form a colloidal suspension when dispersed in water. Each colloidal particle comprises 10³ to 10⁹ atoms, leading to spherical shapes (spherocolloids) or asymmetric forms such as fibers (like cellulose) or plates (like most clay minerals). Moreover, bentonites demonstrate a bipolar nature in colloidal suspension, with negative charges on the faces and positive charges on the edges, resulting in three interaction types between colloidal particles ^19),(20),(32: (i) face-to-face (FF), (ii) edge-to-edge (EE), and (iii) face-to-edge (FE). If the particles are predominantly asymmetric, as in a colloidal suspension of sodium bentonite, they may experience flocculation due to interactions between EE or EF, resulting in the formation of a rigid and elastic gel (gelation) ³². In such cases, the gel structure resembles a house card, thereby altering the fluid’s rheology, as depicted in Figure 2.

Despite notable fluctuations in the flow levels of mortars containing bentonites (Figures 2c and 2d), none of the tested samples exhibited a spread variation exceeding 35% within the 60-minute interval, even with the addition of 2 wt.% of these additives. This characteristic aligns with the requirements outlined in ABNT NBR 11905 ²⁹. Although the incorporation of higher bentonite contents into the mortar was initially explored in this study, it was deemed impractical due to the resulting reduced flowability. Consequently, this study focused on investigating bentonite contents of up to 2 wt.%.

A comparison of the obtained results (Figure 2) with those presented by Liu et al. ⁵ reveals a consistent trend of reduced flow with the addition of bentonite, particularly pronounced in the case of the finer and more reactive sodium-activated bentonite.

Figure 3 illustrates the results of water absorption and crushing strength for the examined mortars. No significant alterations in water absorption were discerned among the samples, despite variations in the type and increasing quantities of bentonite (Figure 3a). Notably, mortars containing CB exhibited higher water absorption values compared to those containing SAB, potentially due to the former’s lower swelling capacity as indicated in Table II. The unexpected increase in water absorption suggests that the addition of 2 wt.% of calcium bentonite may have been insufficient to achieve detectable waterproofing effects.

Figure 3:
(a) Water absorption and (b) crushing strength of the prepared mortar containing distinct amounts of calcium bentonite (CB) or sodium-activated bentonite (SAB). All samples were cured at room temperature (~25?C) for 28 days.

Another critical aspect is the impact of bentonite on the crushing strength of mortars after a 28-day curing period. As shown in Figure 3b, the inclusion of the selected additives led to a slight decrease in the mechanical strength of the specimens. To elucidate this phenomenon, the phase transformations within the microstructure of the cured samples were examined through X-ray diffraction tests. Figure 4 illustrates the obtained diffraction patterns, revealing that certain initial phases of the commercial mortar (before polymeric emulsion mixing) persisted even after the curing process, including α-quartz, hatrurite, and periclase. Furthermore, following a 28-day curing period, the generation of hydrated phases such as Ca(OH)₂ and Ca₅Si₆O₁₆(OH)₂ was noted.

Figure 4:
XRD profiles of the mortars prepared with 0 or 2 wt.% of bentonite and after curing at room temperature for 28 days. Q = α-quartz (SiO₂, 77-1060), S = Hatrurite (Ca₃SiO₅, 86-402), M = Periclase (MgO, 87-651), H = Portlandite (Ca(OH)₂, 78-315) and C = Riversideite-9A (Ca₅Si₆O₁₆(OH)₂, 23-329).

The microstructure of samples containing 0 or 2 wt.% of bentonites underwent further analysis using scanning electron microscopy to examine the distribution of phases and pores in the solidified mortars, as well as the presence of hydrated phase crystals. Although not depicted here, images at a lower magnification (100x) revealed numerous pores along the fractured surface of the samples, with voids measuring up to 244 μm. Pores around 200 μm in size may be linked to air entrapment during processing, as noted by ³³. Consequently, the use of defoamers is commonly recommended to mitigate such defects.

Additionally, hydrated calcium silicate crystals resulting from cement hydration were identified in the microstructure of the investigated compositions (see Figure 5). Owing to the reduced amount of bentonite added (2 wt.%), isolated particles of this additive were not discernible, hindering the confirmation of the interaction mechanism between bentonite and cement as proposed by Yang et al. ². Nevertheless, some researchers associate bentonite with facilitating the micro-filling of concrete and mortar pores with hydrated phase crystals ^11),(26, a phenomenon potentially corroborated by this study.

Figure 5:
Micrographs highlighting the presence of crystals from the hydrated phases contained in the mortars: (a - b) without the addition of bentonite (0 wt.%) or containing 2wt.% of (c - d) sodium-activated bentonite or (e - f) calcium bentonite. All samples were analyzed after 28 days of curing at room temperature (~25°C).

In the water penetration resistance tests, the data collected indicated that all examined compositions fell below the minimum requirements outlined in ABNT NBR 11905:2015 (where samples should remain impermeable with no liquid percolation), both for positive pressure resistance (0.25 MPa pressure for 24 hours) and negative pressure (0.1 MPa pressure for 48 hours) ²⁹. Despite this, the sample containing the application of the mortar containing 2 wt.% sodium bentonite exhibited an 18.6% decay in water penetration in the positive pressure resistance test (Figure 6). Moreover, in measurements conducted under negative pressure (Figure 7), all samples displayed lateral water percolation, yet a significant reduction in water penetration was observed, particularly for the mortar containing sodium bentonite, where no moisture was detected on the upper surface of the specimens. Hence, in line with ², adding bentonite to mortars appears to enhance their resistance to water penetration and percolation.

Figure 6:
Images depicting samples (150 mm x 150 mm) undergoing tests for the determination of the resistance to water penetration under positive pressure (0.25 MPa pressure for 24 hours). The red arrows denote the liquid percolation height, while the numbers below the photos emphasize the average value of the measured heights. The concrete specimens received applications of the reference mortar (0 wt.% of bentonite), or mortars containing 2 wt.% of calcium or sodium-activated bentonites, on their surfaces.

Figure 7:
Images of samples (250 mm x 500 mm) subjected to negative pressure resistance tests. The concrete cylinders received applications of the (a) reference mortar (0 wt.% of bentonite), or mortars containing 2 wt.% of (b) calcium bentonite, or (c) sodium-activated bentonite, on their surfaces.

CONCLUSIONS

The addition of bentonite led to alterations in the rheological properties of fresh mortars (formulated at a ratio of 3.5 (solid part):1 (liquid part)) due to the flocculating effect of this additive, restricting the incorporation of the selected additives to a maximum of 2 wt.% to facilitate their potential application. Conversely, sodium-activated bentonite induced more significant flocculation of the mortars, which was attributed to its improved reactivity and swelling capacity when compared to the calcium-based additive. Adding 2 wt.% of bentonites to the selected mortar did not yield substantial changes in water absorption values or crushing strength of the samples cured at room temperature (~25°C) for 28 days. Hence, these contents did not compromise the performance of the analyzed compositions concerning these properties. Applying the prepared mortars onto the surface of concrete samples and subsequently assessing them for resistance to water penetration under positive or negative pressure highlighted the potential benefits of utilizing sodium bentonite as a waterproofing additive. The addition of 2 wt.% of this bentonite to the mortar resulted in an 18.6% decrease in water penetration under positive pressure and prevented liquid percolation to the upper surface of the samples during tests under negative pressure. To enable the incorporation of larger quantities of bentonite into these monolithic materials, adjustments to the formulation of the prepared mortar are necessary. Additional strategies, such as the use of defoamers (to minimize air entrapment and reduce the number and size of pores in the microstructure), deflocculant (to regulate flocculation and enhance flowability and workability of the mixtures), and reduced cellulose thickener (for improved control of rheology and processing), may also be required.

ACKNOWLEDGMENTS

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001. The authors would like to thank Buntech (Brazil) for supplying the bentonites used in this study.

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Edited by

• (AE: R. Salomão)

Publication Dates

• Publication in this collection
  06 Dec 2024
• Date of issue
  2024

History

• Received
  14 Feb 2024
• Reviewed
  01 May 2024
• Accepted
  24 Sept 2024