Development of Self-Healing Cement Slurry Through the Incorporation of Dual-Encapsulated Polyacrylamide for the Prevention of Water Ingress in Oil Well

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Article Development of Self-Healing Cement Slurry through the Incorporation of Dual-Encapsulated Polyacrylamide for the Prevention of Water Ingress in Oil Well

G. Richhariya 1, D.T.K. Dora 1,* , K.R. Parmar 2, K.K. Pant 2, N. Singhal 3, K. Lal 4 and P.P. Kundu 5

1 Department of Petroleum & Energy Studies, DIT University, Dehradun 248009, India; [email protected] 2 Department Chemical Engineering, Indian Institute of Technology, Delhi 110016, India; [email protected] (K.R.P.); [email protected] (K.K.P.) 3 Department of Chemistry, DIT University, Dehradun, 248009, India; [email protected] 4 Institute of Drilling Technology, ONGC, Dehradun 248003, India; [email protected] 5 Department of Chemical Engineering, Indian Institute of Technology, Roorkee 247667, India; [email protected] * Correspondence: [email protected]; Tel.: +91-94387-25976

Received: 25 May 2020; Accepted: 26 June 2020; Published: 29 June 2020

Abstract: In the present work, a novel cross-linked polymer was synthesized though the anionic polymerization of cyanoacrylate with moisture as an initiator, methylene-bis-acrylamide as a cross-linker, and linseed oil as a spacer. Two layers of the synthesized polymer was coated over polyacrylamide for its homogenous impregnation in Class-G cement slurry for the synthesis of cement core. Fourier Transformation Infrared spectroscopy and X-Ray diffraction spectrum of the synthesized polymer and cement core were obtained to investigate the presence of different functional groups and phases. Moreover, the morphologies of the dual-encapsulated polyacrylamide was observed through scanning electron microscopy. Furthermore, the water-absorption capacity of the synthesized dual-encapsulated polyacrylamide in normal and saline conditions were tested. A cement core impregnated with 16% of dosage of dual-encapsulated polyacrylamide possesses an effective self-healing capability during the water-flow test. Moreover, the maximum linear expansion of the cement core was observed to be 26%. Thus, the impregnation of dual-encapsulated polyacrylamide in cement slurry can exhibit a superior self-healing behavior upon water absorption in an oil well.

Keywords: DUAL coated Polyacrylamide (PAM); self-healing cement; water absorption; expansion; microcrack

1. Introduction In oil well-cementing operations, ordinary Portland cement (API standard) is mixed with water and other additives such as dispersant, ﬂuid loss additives, and friction loss additives to form cement slurry. The so-formed slurry is pumped downhole around the casing in an oil well. The cement sheath formed around the casing serves various functions such as supporting the weight of casing string, isolation of various zones around the casing and preventing the casing from oxidation. Often, cracks are developed in the cement sheath owing to shrinkage, de-bonding of the cement sheath from the casing or well-bore [1]. The generation and propagation of cracks usually depend upon the downhole stresses, temperature, and chemical reactions [2–4]. The crack generation in the

Materials 2020, 13, 2921; doi:10.3390/ma13132921 www.mdpi.com/journal/materials Materials 2020, 13, 2921 2 of 22 cement sheath causes the migration of the formation fluids through the micro-fractures, thereby resulting in sustained casing pressure [5–7]. Upon crack development, the formation fluid starts migrating through the cement sheath into the well-bore, resulting in excessive water production or sometimes causes contamination of aquifers [8,9]. Therefore, the service life of the cementing can be increased either through crack-repairing by squeeze-cementing or application of self-healing cement. The squeeze-cementing technique was one of the most common methods for repairing the cracks in the cement sheath. However, the low success rate of 50%, excessive time consumption, and expensive nature of squeeze-cementing hiders its application now a days [10]. Moreover, the production needs to be stopped during the squeeze-cementing technique. Thus, self-healing cement can be an effective solution to overcome the issues related to squeeze-cementing for the prevention of water production in the oil well. The autogenous self-healing technique is the inherent capability of the cementitious composite attributed to further hydration and carbonation [11–13]. The mechanism of autogenous self-mechanism is of two-fold such as hydration of unhydrated cement [14] and carbonation owing to precipitation of calcium carbonate [15]. During hydration, the tricalcium silicate (C3S) and dicalcium silicate (C2S) present in cement converts to C–S–H gel (calcium silicate hydrate phases) and portlandite (Ca(OH)2), thereby healing the crack [16]. However, Lapech et al. [17] investigated that the cracked cement concrete specimen did not possess any feature of autogenous healing of cracks during two weeks of saturation with water. On the other hand, the mechanism of carbonation is owing to the attack of the CO2 dissolved in water on the cement, thereby decomposing C–S–H phases into CaCO3 and silica gel, resulting in decrease in the cement strength [16]. Moreover, this intrinsic process may repair limited crack width up to 100 µm [18]. Therefore, chemical and biological amendment of the cementitious composite may result in better healing capacity. The self-healing capacity can be achieved through the incorporation of bacterial species into the cementitious matrix, as investigated by various researchers [18–21]. The water penetration through the cracks, activate the dormant microorganisms through precipitation of sufficient amount of the microbial calcite, in turn heals the cracks and thereby hinders the further ingress of water [18,22]. However, the bacterial species loses its healing capacity owing to higher pH of the concrete. Moreover, the spores of the bacterial species may get damaged because of the shear forces generated, shrinkage during mixing and drying of the concrete [23]. Thus, researchers have used diatomaceous earth as an efficient carrier for the transportation of bacteria. Diatomaceous earth immobilizes bacteria by precipitation of calcium carbonate though decomposing large amount of urea [24]. However, the water absorption by diatom during transportation limits its application. Thus, alginate hydrogel being an excellent immobilizer for bacteria though precipitating calcium carbonate was used by Belie [25]. The alginate hydrogel demonstrated good bacterial preservation along with good water absorption and moisture retaining capacity. However, the low compressive strength owing to the alginate hydrogel curb its use. Still various researchers have investigated the effect of bacterial incorporation in the cement matrix; however, the biomass viability test is yet to be explored [26]. Thus, the incorporation of abundantly available, chemically inert, durable, low toxic super absorbent polymers (SAP) in the cement matrix was emerged in the field of self-healing cementing applications. The inherent hydrophilic groups of SAP are responsible for enhancing the interaction with the water molecules through hydrogen bond and the produced charged groups open the cross-linking of SAP, which in turn becomes the driving force of water absorption in terms of hundred times of its own mass [27]. Upon mixing with concrete, SAP forms a swollen hydrogel by absorbing and retaining water, thereby reducing the autogenous contraction and possible crack development [28,29]. Moreover, during the formation of microcracks in the cement sheath, SAP makes it water impermeable by virtue of its water-absorption capacity followed by swelling characteristics. The application of SAP for self-healing of cracks in the cement matrix was further explored by various researchers such as Lee et al. [30], and Snoeck et al. [31,32]. The impregnation of SAP by 5% (weight basis) in cement slurry resulted in a reduction of water flow through the 0.1 mm crack by 90% in 3 h [30]. The increased Materials 2020, 13, 2921 3 of 22 re-swelling ratio, particle size (500 µm) and reduction in initial swelling of SAP contribute in the improvement of self-healing cement [33]. However, the added SAP will swell by absorbing water in the cement slurry, thereby increases the viscosity, which in turn decreases the pumpability [28]. Moreover, if the dosage of SAP is less in the cement slurry, the plugging may not be effective. From the current literature review, the SAP in the cement slurry swells by absorbing water during its transportation into the oil well. Moreover, after placement of cement around the casing in the oil well, the SAP releases the water and shrinks, thereby leaving behind pores and thus decreasing the strength of the cementing. Thus, encapsulation of SAP can be an alternative method to isolate, protect, and control during its transportation along the cement slurry into the oil well. Therefore, Liu et al. [28] have encapsulated the SAP with multiple coatings of gypsum and chitosan before mixing with cement. However, the outer gypsum coating may erupt in basic solution, thereby exposing the chitosan layer, which may scrap away during pumping of the cement slurry. Polyacrylamide (PAM) is high molecular weight water swellable, biocompatible polymers synthesized from acrylamide and its derivatives, thus widely used as a thickening agent, cross-linker, lubricant, and oil recovery agent. N–N methylene bis-acrylamide (MBIS) is a cross-linking agent used for polymer synthesis. Cyanoacrylate is a monomer of cyanoacrylate ester and the acryl group rapidly undergo chain-growth polymerization. Linseed oil being a spacer, controls the rate of reaction of cyanoacrylate during sudden exposure to moisture. Considering the fascinating characteristics of PAM, MBIS, Cyanoacrylate, and linseed oil in the present work, a dual-encapsulated of polyacrylamide was synthesized. The first layer of encapsulation comprises of methylene bis-acrylamide cross-linked with linseed oil and cyanoacrylate. Furthermore, another layer of fine silica, methylene bis-acrylamide cross-linked with linseed oil and cyanoacrylate was encapsulated. Moreover, the various dosage of dual-encapsulated PAM was homogeneously impregnated in cement slurry to prepare the cement core for its characterization. The water-absorption capacity of the synthesized polymers was investigated both in distilled water and saline water. Furthermore, the as-prepared cement core was split and performed for water-flow test to investigate its self-healing capacity. Moreover, the rheological behavior of the cement slurry with varying dosage of the dual-encapsulated polyacrylamide, were investigated to judge the pumpability through viscosity measurement. Simultaneously, the linear expansion against water and compressive strength of the synthesized cement core were measured, compared with the neat cement core to judge its applicability in the oil well-cementing.

2. Materials and Methods

2.1. Materials Class-G oil well cement (API, High sulfate resistant), dispersant DO-65, and silica fumes were kindly provided by Institute of Drilling Technology, ONGC, Dehradun, India. DO-65 as a dispersant improves the rheological properties of the cement slurries. Moreover, it reduces friction pressure drop ﬂow during pumping operations, thereby resulting in an improved placement which yields perfect zonal isolation across the zone of interest [34]. PAM (C3H5NO)n was purchased from Sigma Aldrich Ltd., India. MBIS (C7H10N2O2) was purchased from Spectrochem India Pvt. Ltd. Cyanoacrylate was purchased from Rebecca polymers, India. Linseed oil (C57H98O6) was purchased from the local market. All the chemicals were analytical grade and used as received.

2.2. Synthesis of Dual Coated SAP and Cement Core The schematic diagram for the synthesis of dual coated PAM and cement core is presented as Figure1. Briefly, a weighed amount of MBIS was poured in linseed oil and then stirred in a magnetic stirrer at 300 rpm for 10 min to form a MBIS solution of 1:2 w/v ratio. A weighed amount of PAM (250 µm) was added to the MBIS solution followed by the addition of cyanoacrylate during stirring to obtain 1:3 w/v ratio. The formed viscous solution was stirred for 2 min to attain the first encapsulation of PAM. The obtained single coated PAM (SPAM) was dried in a hot air oven at 40 ◦C for 24 h. For the MaterialsMaterials 20202020,, 1313,, 2921 2921 44 of of 21 22 magnetic stirrer, with the further addition of silica fumes (SiO2) during stirring to obtain dual coated second encapsulation, SPAM, MBIS solution, and cyanoacrylate was mixed in a magnetic stirrer, PAM (DPAM). The addition of silica fumes in the outer layer during DPAM synthesis helps in with the further addition of silica fumes (SiO ) during stirring to obtain dual coated PAM (DPAM). improvement of the bonding of PAM in the 2cement matrix along with a reduction in macropore The addition of silica fumes in the outer layer during DPAM synthesis helps in improvement of the formation [35]. The DPAM was again dried in the hot air oven (ACM 22064-I, AcMas Technologies bonding of PAM in the cement matrix along with a reduction in macropore formation [35]. The DPAM Pvt. Ltd., Mumbai, India) for 24 h. Prior to the preparation of cement slurry, Class-G cement (Table was again dried in the hot air oven (ACM 22064-I, AcMas Technologies Pvt. Ltd., Mumbai, India) for 1) was sieved through 300-micron screen. Different weight percentages (4, 8, 12, 16) of DPAM, DO- 24 h. Prior to the preparation of cement slurry, Class-G cement (Table1) was sieved through 300-micron 65, and water were added to cement for the preparation of different samples of cement core (Table screen. Different weight percentages (4, 8, 12, 16) of DPAM, DO-65, and water were added to cement 2). for the preparation of different samples of cement core (Table2).

Figure 1. Schematic diagram for the synthesis of DPAM and cement core. Figure 1. Schematic diagram for the synthesis of DPAM and cement core. Table 1. Composition of Class-G oil well cement. Table 1. Composition of Class-G oil well cement. Components SiO2 CaO Fe2O3 Al2O3 SO3 MgO K2O Na2O Other Components SiO2 CaO Fe2O3 Al2O3 SO3 MgO K2O Na2O Other (wt.%) 22.84 63.72 5.02 3.46 1.69 1.30 0.46 0.89 22.84 (wt.%) 22.84 63.72 5.02 3.46 1.69 1.30 0.46 0.89 22.84 Table 2. Mixing proportion of the cement paste. Table 2. Mixing proportion of the cement paste. Weight of Sl. No. Specimen No. DPAM (wt.%) DO 65 (wt.%) Water (wt.%) Sl. WeightCement of Cement (g) DPAM DO 65 Water Specimen No. No. 1 S1(g) 500 (wt.%) 4 (wt.%) 0.2 (wt.%) 44 2 S2 500 8 0.2 44 1 3S1 S3 500 500 12 4 0.2 0.2 4444 4 S4 500 16 0.2 44 2 S2 500 8 0.2 44 5 Neat Cement (NC) 500 0 0.2 44 3 S3 500 12 0.2 44 The mixture was blended in a warring blender at 12,000 rpm to obtain the cement slurry. 4 S4 500 16 0.2 44 The cement slurry was further poured in a cylindrical steel mold (37.3 38.5 mm2) and allowed to set × for 48 h for theNeat synthesis Cement of cement core. The snapshots of the diﬀerent samples of cement core are 5 500 0 0.2 44 presented as Figure(NC)2a–d. During the synthesis of cement core, it was observed that beyond 16% dose of DPAM, the cement core loses its structural integrity.

Materials 2020, 13, 2921 5 of 22 Materials 2020, 13, 2921 5 of 21

Figure 2. SnapshotsSnapshots of of the the synthesized synthesized cement cement core core with with (a ()a 4%) 4% (b ()b 8%) 8% (c) ( c12%) 12% and and (d) ( d16%) 16% dosage dosage of DPAM.of DPAM. 2.3. Water-Absorption Test The mixture was blended in a warring blender at 12,000 rpm to obtain the cement slurry. The cementThe slurry salinity was of further produced poured water in a incylindrical an oil well steel varies mold from(37.3 × fresh 38.5 watermm2) and to highallowed saline to set water for 48(300,000 h for the ppm) synthesis with respect of cement to depth core. ofThe the snapshot oil wells [of36 ,the37]. different Furthermore, samples Anrey of cement et al. [core37] hadare presentedinvestigated as thatFigure the 2a–d. produced During water the synthesis of Gulf of of Guiena cement contains core, it saltswas ofobserved sodium that and beyond potassium. 16% Thus,dose ofin DPAM, this work, the water-absorption cement core loses tests its structural [38] of PAM, integrity. SPAM, and DPAM were investigated in distillated water as well as in varied saline water of sodium and potassium. For this, 1 g of DPAM was treated 2.3.with Water-Absorption distilled water andTest different concentration of potassium chloride, sodium chloride solutions separatelyThe salinity for 24 hof at produced 1000 rpm. water The samplesin an oil werewell varies intermittently from fresh removed water to at high every saline 1 h, vacuum water (300,000 filtered, ppm)and then with weighed respect beforeto depth carrying of the oil out well the further[36,37]. water-absorptionFurthermore, Anrey test. et The al. [37] water-absorption had investigated capacity that thewas produced calculated water as per of Equation Gulf of Guiena (1). contains salts of sodium and potassium. Thus, in this work, mt mi water-absorption tests [38] of PAM, SPAM, andw = DPAM− were investigated in distillated water as well(1) mi as in varied saline water of sodium and potassium. For this, 1 g of DPAM was treated with distilled where w is the water-absorption capacity (g/g), m (g) and m (g) are the weight of the DPAM at t = 0 water and different concentration of potassium chlorii de, sodiumt chloride solutions separately for 24 and time t, respectively. h at 1000 rpm. The samples were intermittently removed at every 1h, vacuum filtered, and then weighed2.4. Characterization before carrying out the further water-absorption test. The water-absorption capacity was calculated as per Equation (1). Scanning electron microscopy (SEM) (FEI Quanta 200, ThermoFisher Scientific, New Delhi, India) − was used to characterize the morphologies= of the synthesized PAM, SPAM, and DPAM. The PAM,(1) SPAM, and DPAM particles were spread over the carbon tape mounted on a SEM plate. The samples on wherethe SEM w plateis the was water-absorption then coated with capacity 2–5 nm (g/g), of gold mi to(g) make and m themt (g) conductive,are the weight before of the obtaining DPAM the at SEMt = 0 andmorphologies. time t, respectively. The Fourier Transformation Infrared (FTIR) spectroscopy (NICOLET iS50, ThermoFisher Scientific, New Delhi, India) and X-Ray diffraction (XRD) (X’PERT PRO, PANanalytical, Mumbai, 2.4.India) Characterization spectrum of the synthesized polymer and cement core were obtained. FTIR measurements 1 wereScanning conducted electron in transmission microscopy model (SEM) in (FEI which Quanta the scan 200, resolution ThermoFisher was 4Scientific, cm− and New there Delhi, were India)64 scans was averaged used to characterize for each measurement. the morphologies The XRD of the spectrum synthesized were PAM, obtained SPAM, within and theDPAM. range The of PAM,well-known SPAM, cement and DPAM material particles 2◦ to 90were◦ at aspread grade over of 0.02 the◦ incrementscarbon tape of mounted Cu Kα radiation, on a SEM whereas plate. The for samplesPAM, SPAM, on the and SEM DPAM plate the was 2θ valuesthen coated were keptwith in2–5 the nm range of gold of 2◦ toto make 80◦. Goniometer them conductive, (Model before 90 Pro obtainingSeries, Rame-hart the SEM Instrument morphologies. Co., Succasunna, The Fourier NJ, USA)Transformation was used to measureInfrared the(FTIR) contact spectroscopy angle of the (NICOLETcement core. iS50, ThermoFisher Scientific, New Delhi, India) and X-Ray diffraction (XRD) (X’PERT PRO, PANanalytical, Mumbai, India) spectrum of the synthesized polymer and cement core were obtained. FTIR measurements were conducted in transmission model in which the scan resolution was 4 cm−1 and there were 64 scans averaged for each measurement. The XRD spectrum were obtained within the range of well-known cement material 2° to 90° at a grade of 0.02° increments of

Materials 2020, 13, 2921 6 of 21

Cu Kα radiation, whereas for PAM, SPAM, and DPAM the 2θ values were kept in the range of 2° to 80°. Goniometer (Model 90 Pro Series, Rame-hart Instrument Co., Succasunna, NJ, USA) was used to measure the contact angle of the cement core.

2.5. Water-Flow Test through the Microcracked Specimen For microcrack generation, first small groves of 1/8” depth on both sides of the sample were made exactly at the mid span along the diameter by tensile splitting using a loading device [33]. Sharp edged thin metallic plates were placed on each side of the grove and then pressure was applied at an increasing rate to induce a single through crack as shown in Figure 3b. The snapshot of obtained split samples is presented as Figure 3c. The morphology at lower magnification (4X) were obtained at both top and bottom part of the split samples using image analyzer (BX53M, Olympus, Hamburg, Germany)Materials 2020 for, 13 measuring, 2921 the crack width and presented as Figure 4. The crack width was measured6 of 22 by using image J software (1.53b, National Institute of Health, Maryland, MD, USA) at different positions for each sample. The average values of the crack width obtained are 73.07, 64.42, 62.65 and 70.242.5. Water-Flowµm for the Testsamples through S1, theS2, Microcracked S3, and S4, Specimenrespectively. Thus, owing to the similar average crack width,For these microcrack four samples generation, were chosen first smallfor further groves analysis. of 1/8” depth on both sides of the sample were madeFor exactly water-flow at the test, mid cement span alongpermeameter the diameter (CGP-9 by0, tensileOFI Testing splitting Equipment using a loadingInc., Houston, device TX, [33 ]. USA)Sharp was edged used. thin The metallic water plates was wereallowed placed to flow on each through side of the the specimen grove and (Figure then pressure 3f) under was varied applied differentialat an increasing pressure rate ranging to induce from a single 0.02 to through 0.12 MPa. crack Inas this shown study, in a Figure 5 HP 3airb. compressor The snapshot was of obtainedused to generatesplit samples the differential is presented pressure as Figure across3c. Thethe specimen. morphology Briefly, at lower the magnificationsplit specimens (4X) were were assembled obtained andat bothfastened top andin a bottombrass specimen part of the holder split with samples a rubber using sleeve image arrangement analyzer (BX53M, (Figure Olympus, 3d,e) to avoid Hamburg, any liquidGermany) leakage. for measuringWith the system the crack filled width with and water, presented the desired as Figure test4. The pressure crack widthwas applied was measured to water by reservoirusing image and Jthe software initial reading (1.53b, National of the gauge-glass Institute of was Health, recorded. Maryland, Simultaneously, MD, USA) at a di cleanfferent collection positions bottlefor each was sample. weighed The and average placed values in position of the crack to co widthllect the obtained water arepercolating 73.07, 64.42, through 62.65 andthe 70.24specimen.µm for Afterthe samplesevery 4 h S1, of S2, continuous S3, and S4, water respectively. flow the Thus,weight owing of the to bottle the similar was measured average crackto calculate width, the these mass four flowsamples rate through were chosen the specimen. for further analysis.

FigureFigure 3. 3. SchematicSchematic diagramdiagram of of the the cement cement core withcore awith single a penetratingsingle penetrating crack for thecrack ﬂow for measurement the flow measurement(a) pre-notched (a specimen) pre-notched subjected specimen to crack subjected generation to (b )crack crack generationgeneration (c()b snapshot) crack ofgeneration split specimen (c) snapshot(d,e) core of holdersplit specimen (f) assembled (d–e) corecore withholder artiﬁcial (f) assembled crack. core with artificial crack.

For water-flow test, cement permeameter (CGP-90, OFI Testing Equipment Inc., Houston, TX, USA) was used. The water was allowed to flow through the specimen (Figure3f) under varied differential pressure ranging from 0.02 to 0.12 MPa. In this study, a 5 HP air compressor was used to generate the differential pressure across the specimen. Briefly, the split specimens were assembled and fastened in a brass specimen holder with a rubber sleeve arrangement (Figure3d,e) to avoid any liquid leakage. With the system filled with water, the desired test pressure was applied to water reservoir and the initial reading of the gauge-glass was recorded. Simultaneously, a clean collection bottle was weighed and placed in position to collect the water percolating through the specimen. After every 4 h of continuous water flow the weight of the bottle was measured to calculate the mass flow rate through the specimen. Materials 2020, 13, 2921 7 of 22 Materials 2020, 13, 2921 7 of 21

FigureFigure 4. Stereomicrograph 4. Stereomicrograph at at the the crackcrack for the samples samples (a ()a S1) S1 (b) ( bS2) S2(c) (S3c) ( S3d) S4. (d) S4.

2.6. Investigation2.6. Investigation on Swelling,on Swelling, Rheological, Rheological, and andMechanical Mechanical Properties Properties The swellingThe swelling properties properties of the of prepared the prepared cement ceme coresnt werecores investigatedwere investigated using using an annular an annular expansion moldexpansion [39]. Briefly, mold the [39]. cement Briefly, slurry the cement with additives slurry with was additives poured was into poured the annulus into the between annulus thebetween two rings the two rings and allowed to set for 48 h. Then the mold was put in a water bath for 8 h. Upon and allowed to set for 48 h. Then the mold was put in a water bath for 8 h. Upon absorbing water, absorbing water, the cement core swells thereby increasing the distance between the two slits in the the cement core swells thereby increasing the distance between the two slits in the outer periphery. outer periphery. The distance between the two slits was measured before and after swelling using a The distancedigital screw between gauge, the then two the slits linear was expa measurednsion was before calculated and after by Equation swelling (2). using a digital screw gauge, then the linear expansion was calculated by Equation (2). − = (2) (Mt Mi) Linearexpansion = − (2) Where Mi and Mt are the micrometer measurements atM initiali and at time “t” respectively. The rheological parameters viz. viscosity and shear stress of the cement slurry with a varying wheredosageMi and of MDPAMt are thewas micrometerinvestigated measurementsusing a rheometer at initial(MCR-72, and Anton at time Par, “t” Graz, respectively. Austria). Briefly, Thethe as-prepared rheological DPAM parameters impregnated viz. viscosity cement and slurry shear was stress placed of thein the cement coaxial slurry cylinder with of a varyingthe dosagerheometer, of DPAM then was the investigatedtemperature was using adjusted a rheometer to 40 °C. (MCR-72,The sample Anton was then Par, subjected Graz, Austria). to a stepped Briefly, the as-preparedramp of 5 s−1 DPAMand the impregnatedviscosity measurements cement slurry were wasconducted placed at in 20 the different coaxial shear cylinder rates ranging of the rheometer, from 5 to 400 s−1. For the investigation of compressive strength, different dosages of DPAM were mixed in 1 then the temperature was adjusted to 40 ◦C. The sample was then subjected to a stepped ramp of 5 s− the cement slurry as mentioned in section 2.2, poured in a cubical shape mold (2 × 2 × 2) and then 1 and the viscosity measurements were conducted at 20 different shear rates ranging from 5 to 400 s− . cured at 50 °C for 24 h. The compressive strength of the prepared cubical specimen was measured For the investigation of compressive strength, different dosages of DPAM were mixed in the cement using Compression Testing Machine (AIM 320-AN CTM, Aimil Ltd., New Delhi, India). slurry as mentioned in Section 2.2, poured in a cubical shape mold (2 2 2) and then cured at 50 C × × ◦ for 243. h. Results The compressive and Discussion strength of the prepared cubical specimen was measured using Compression Testing Machine (AIM 320-AN CTM, Aimil Ltd., New Delhi, India). 3.1. Polymer Synthesis Mechanism 3. Results and Discussion The plausible mechanism for the synthesis of the novel cross-linked polymer is presented as 3.1. PolymerScheme Synthesis1–3. In the Mechanismfirst step, the moisture through lone pair of oxygen attacks the double bond of the cyanoacrylate monomer to generate a carbanion (Scheme 1A). Furthermore, during propagation, the The plausible mechanism for the synthesis of the novel cross-linked polymer is presented as Schemes1–3. In the first step, the moisture through lone pair of oxygen attacks the double bond of the cyanoacrylate monomer to generate a carbanion (Scheme1A). Furthermore, during propagation, Materials 2020,, 13,, 29212921 8 of 2221 Materials 2020, 13, 2921 8 of 21 initiated carbanion monomer reacted with other cyanoacrylate monomers in rapid succession to initiatedthegenerate initiated carbanioncyanoacrylate carbanion monomer monomerpolymeric reacted reactedcarbanion with with other (Schem other cyanoacrylatee cyanoacrylate1B). The polymer monomers monomers with in active rapid in rapid carbanionsuccession succession was to generatetothen generate reacted cyanoacrylate cyanoacrylate with the cross-linkerpolymeric polymeric carbanion MBIS carbanion resulting (Schem (Scheme ine 1B).shifting1 B).TheThe polymerof carbanion polymer with withto active the active MBIScarbanion carbanion molecule was thenwas(Scheme thenreacted 2). reacted Furthermore,with with the thecross-linker cross-linkerMBIS is MBIShelping MBIS resulting in resulting cross-linking in inshifting shifting of ofcyanoacrylateof carbanion carbanion to topolymers the the MBIS MBIS by molecule adding (Scheme(Schemeanother 2propagating2).). Furthermore, cyanoacrylate MBIS is helpinghelping polymer inin cross-linkingcross-linkingto the other ofofacrylamide cyanoacrylatecyanoacrylate of MBIS. polymerspolymers The bybygenerated addingadding anotheranothercarbanion propagatingpropagating on the MBIS cyanoacrylate cyanoacrylate molecule polymercan polymer be shifted to theto othertothe the other acrylamide abundantly acrylamide of MBIS.available of The MBIS. linseed generated The oil generated carbanionmolecule carbanionon(Scheme the MBIS 3). on The moleculethe termination MBIS canmolecule be of shiftedthe can cross-linking tobe theshifted abundantly reaction to the abundantlyoccurred available by linseed availablethe transfer oil moleculelinseed of the oil anion (Scheme molecule to the3). (SchemeThelinseed termination oil, 3). andThe thentermination of the neutralized cross-linking of the by cross-linking the reaction addition occurred reaction of H+ generated byoccurred the transfer in by the the initiation oftransfer the anion ofstep. the to The anion the addition linseed to the + linseedoil,of the and cross-linker oil, then and neutralized then increases neutralized by the additionsolidityby the addition in of the H sygenerated ofnthesized H+ generated in polymer. the initiationin the The initiation linseed step. The step.oil additionalso The acted addition of as the a ofcross-linkerspacer the cross-linker resulting increases in increasesthe thereduction solidity the solidity of in rate the of synthesizedin reactionthe synthesized of polymer. cyanoacrylate polymer. The linseed as The it does linseed oil alsonot oilallow acted also moisture asacted a spacer as toa spacerresultingpenetrate resulting in and the thereby reduction in the controllingreduction of rate ofof the reaction rate initiation of reaction of cyanoacrylate step. of cyanoacrylate as it does as not it does allow not moisture allow moisture to penetrate to penetrateand thereby and controlling thereby controlling the initiation the step.initiation step.

Scheme 1. ((A)) Initiation Initiation ( B) Propagation reaction of polymer synthesis. Scheme 1. (A) Initiation (B) Propagation reaction of polymer synthesis.

Scheme 2. Cross-linking reaction of cyanoacrylate and MBIS. Scheme 2. Cross-linking reaction of cyanoacrylate and MBIS.

Materials 2020, 13, 2921 9 of 22 Materials 2020, 13, 2921 9 of 21

Scheme 3. Termination by solvent (linseed oil) transfer. Scheme 3. Termination by solvent (linseed oil) transfer. 3.2. Characterization of PAM, SPAM, and DPAM 3.2. Characterization of PAM, SPAM, and DPAM 3.2.1. FTIR Analysis 3.2.1. FTIR Analysis The FTIR spectrum of the PAM, SPAM, and DPAM (Figure5) conﬁrm the presence of various transmittanceThe FTIR bandsspectrum and of functional the PAM, groups. SPAM, The and transmittance DPAM (Figure bands 5) confirm attributed the to presence various functionalof various groupstransmittance in PAM bands was similar and functional to the literature groups. [The40,41 tran]. Thesmittance NH groups bands appear attributed at the to transmittance various functional band 1 ofgroups 3461 cmin PAM− in PAM.was similar However, to the during literature the synthesis [40,41]. The of SPAMNH groups and DPAM, appear the at the transmittance transmittance band band for −1 1 1 NHof 3461 groups cm was in PAM. observed However, to shift fromduring 3461 the cm synthesis− to 3307cm of SPAM− . This and is DPAM, due to the the cross-linking transmittance reaction band −1 −1 1 offor cyanoacrylate NH groups was and observed MBIS (Scheme to shift2). Infrom PAM, 3461 the cm band toat 3307cm 2928 cm. −Thiswas is attributeddue to the to cross-linking asymmetric −1 CHreaction2 groups, of cyanoacrylate which were alsoand observedMBIS (Scheme for SPAM 2). In and PAM, DPAM. the band Distinctive at 2928 peaks cm werewas observedattributedfor to 2 1 PAM,asymmetric SPAM, CH and groups, DPAM atwhich a transmittance were also observed band of 2858for SPAM cm− , and which DPAM. was assigned Distinctive to the peaks –N–CH were2 1 −1 bonds.observed The for intense PAM, SPAM, transmittance and DPAM bands at ata transmittance 1688 and 1552 band cm− ofwere 2858 observedcm , which in thewassynthesized assigned to polymersthe –N–CH assigned2 bonds. to The C=O intense owing totransmittance the termination bands of theat polymer1688 and synthesis 1552 cm reaction−1 were observed through linseedin the oilsynthesized (Scheme3 polymers), the inherent assigned NH bondsto C=O of owing the amidic to the group. termination The transmittance of the polymer bands synthesis between reaction 1400 1 andthrough 1000 linseed cm− were oil (Scheme attributed 3),to the the inherent presence NH of bo –C–N-linksnds of the and amidic CH 2group.groups. The The transmittance polycyanoacrylate bands −1 1 2 (Schemebetween1 )1400 in SPAM and and1000 DPAM cm were causes attributed transmittance to the bands presence at 1625 of and–C–N-links 1410 cm −and, which CH wasgroups. assigned The −1 topolycyanoacrylate C=N groups [42 ].(Scheme In SPAM 1) andin SPAM DPAM, and a DPAM distinctive causes transmittance transmittance band bands also at observed 1625 and at 1410 725 cmcm− ,, which waswas attributedassigned to to C=N the –CH groups2 wagging [42]. In owing SPAM to and the coatingDPAM, of a thedistinctive linseed oiltransmittance (Scheme3). Inband DPAM, also −1 1 2 theobserved bands at 937725 andcm 1051, which cm− wasconﬁrms attributed the presenceto the –CH of Si–OH wagging stretching owing andto the Si–OH coating rocking, of the owing linseed to 1 −1 theoil (Scheme coating of3). silica. In DPAM, The band the bands at around at 937 788 and cm −1051is allocatedcm confirms to Si–O–Si the presence cross-linking of Si–OH bonds stretching [43]. and Si–OH rocking, owing to the coating of silica. The band at around 788 cm−1 is allocated to Si–O– Si cross-linking bonds [43].

Materials 2020, 13, 2921 10 of 21 Materials 2020, 13, 2921 10 of 22 Materials 2020, 13, 2921 10 of 21

Figure 5. FTIR spectrum of PAM, SPAM, and DPAM. Figure 5. FTIRFTIR spectrum spectrum of PAM, SPAM, andand DPAM.DPAM. 3.2.2.3.2.2. XRDXRD AnalysisAnalysis 3.2.2. XRD Analysis XRDXRD analysisanalysis waswas conductedconducted toto investigateinvestigate thethe crystallinecrystalline structurestructure andand phases present in PAM, XRD analysis was conducted to investigate the crystalline structure and phases present in PAM, SPAM,SPAM, DPAMDPAM (Figure(Figure6 6).). InIn FigureFigure6 6,, thethe XRDXRD patternpattern ofof PAM showed intense peaks peaks at at 2 2θ θvaluesvalues of SPAM,11.6°, 19.7°, DPAM 19.9°, (Figure 24.5°, 6). 29.5°,In Figure 29.8° 6, theand XR 30.4°.D pattern These of intense PAM showed peaks reveal intense that peaks the at PAM 2θ values used ofis of 11.6◦, 19.7◦, 19.9◦, 24.5◦, 29.5◦, 29.8◦ and 30.4◦. These intense peaks reveal that the PAM used is 11.6°, 19.7°, 19.9°, 24.5°, 29.5°, 29.8° and 30.4°. These intense peaks reveal that the PAM used is crystallinecrystalline amorphousamorphous in nature [44]. [44]. For For SPAM SPAM similar similar intense intense peaks peaks were were observed observed at at2θ 2 θvaluesvalues of crystalline23.86° and amorphous41.09° owing in to nature the encapsulation [44]. For SPAM of MBIS. similar Furthermore, intense peaks for were DPAM observed along with at 2θ the values intense of of 23.86◦ and 41.09◦ owing to the encapsulation of MBIS. Furthermore, for DPAM along with the 23.86°peaks andof PAM 41.09° and owing SPAM, to the other encapsulation intense peaks of MBIS. at 2θ Furthermore, values of 49.6°, for DPAM59.9° and along 68.2° with owing the intense to the intense peaks of PAM and SPAM, other intense peaks at 2θ values of 49.6◦, 59.9◦ and 68.2◦ owing to peaks of PAM and SPAM, other intense peaks at 2θ values of 49.6°, 59.9° and 68.2° owing to the theencapsulation encapsulation of silica of silica particles. particles. The The decrease decrease in inthe the intensity intensity of of the the peaks peaks for for SPAM SPAM and DPAM isis encapsulation of silica particles. The decrease in the intensity of the peaks for SPAM and DPAM is duedue toto thethe disturbeddisturbed eveneven outlineoutline ofofthe theatoms atoms during during encapsulation. encapsulation. due to the disturbed even outline of the atoms during encapsulation.

Figure 6. XRD pattern of PAM, SPAM, and DPAM. Figure 6. XRD pattern of PAM, SPAM, and DPAM. 3.2.3. SEM Analysis Figure 6. XRD pattern of PAM, SPAM, and DPAM.

The SEM micrographs for PAM at diﬀerent magniﬁcations are presented as Figure7a–c. The average particle size of PAM chosen for dual encapsulation was found to be 224 µm (Figure7a,b).

Materials 2020, 13, 2921 11 of 21

3.2.3. SEM Analysis Materials 2020, 13, 2921 11 of 22 The SEM micrographs for PAM at different magnifications are presented as Figure 7a–c. The average particle size of PAM chosen for dual encapsulation was found to be 224 µm (Figure 7a,b). Upon further magnification, magnification, cracks cracks and and crevasses crevasses we werere observed observed in in the the PAM PAM particle (Figure (Figure 77b,c).b,c). The irregular shape and the observed cracks of thethe particles imparts an affirmative affirmative effect effect on the quality of of encapsulation. encapsulation. The The SEM SEM micrographs micrographs for for SPAM SPAM are are presented presented as Figure as Figure 7d–f.7d–f. After After the first the firstlayerlayer of encapsulation of encapsulation by cross-linked by cross-linked polymer, polymer, the size the of sizethe PAM of the particles PAM particles was doubled, was doubled, i.e., 542 i.e.,µm 542(Figureµm 7d,e). (Figure 7d,e).

FigureFigure 7. 7. SEMSEM micrograph micrograph of of the the synthesized synthesized ( (aa––cc)) PAM PAM ( d–f) SPAM (g–i) DPAM.DPAM.

Upon further further magnification magnification (Figure (Figure 7f),7f), a alayer layer of ofsynthesized synthesized polymer polymer was was observed observed at the at outer the outerlayer of layer the ofSPAM the SPAM particles. particles. Simultaneously, Simultaneously, the SEM the micrographs SEM micrographs for DPAM for DPAMis presented is presented as Figure as Figure7g–i. The7g–i. size The of size the of particle the particle after after DPAM DPAM synthe synthesissis increased increased to to762 762 µmµm (Figure (Figure 7g)7g) owing owing to thethe encapsulation ofof thethe second second layer layer of of cross-linked cross-linke polymerd polymer embedded embedded with with silica silica fumes. fumes. The outerThe outer layer oflayer the of DPAM the DPAM particles particles was observed was observed to be flooded to be withflooded silica with particles silica (Figureparticles7h,i). (Figure The observed7h,i). The cracksobserved in PAM cracks particles in PAM (Figure particles7b,c) (Figure were padded 7b,c) we upre through padded the up migration through ofthe the migration cross-linked of the polymer cross- andlinked silica polymer particles and intosilica the particles cracks into during the encapsulation.cracks during encapsulation. Furthermore, Furthermore, no cracks were no observedcracks were in SPAMobserved and in DPAM SPAM particles and DPAM (Figure particles7f,i), confirming (Figure the7f,i), attainment confirming of the the better attainment encapsulation. of the better encapsulation. 3.3. Characterization of Cement and Cement Core 3.3. Characterization of Cement and Cement Core 3.3.1. FTIR Analysis 3.3.1.Figure FTIR Analysis8 shows the FTIR spectrum of Class-G cement and the synthesized cement core. 1 A very weak peak at 3465 cm− assigned to symmetric and asymmetric O–H stretching. In both, Figure 8 shows the FTIR spectrum of Class-G cement and the1 synthesized cement core. A very the spectrum, the transmittance−1 bands at 1445, 871 and 712 cm− were assigned to v3, v2, and v4 weak2 peak at 3465 cm assigned to symmetric and1 asymmetric O–H stretching. In both, the spectrum, CO3 −, respectively [45]. The band at 871 cm− in the unhydrated Class-G cement was attributed to the transmittance bands at 1445, 871 and 712 cm−1 were assigned to v3, v2, and v4 CO32−, respectively the stretching mode Si–O. The addition of DPAM in Class-G cement for the synthesis of the cement [45]. The band at 871 cm−1 in the unhydrated Class-G cement was attributed to the stretching mode core caused the introduction of various functional groups in the cement core as discussed in Figure5.

Materials 2020, 13, 2921 12 of 21

Si–O. The addition of DPAM in Class-G cement for the synthesis of the cement core caused the Materials 2020, 13, 2921 12 of 22 introduction of various functional groups in the cement core as discussed in Figure 5.

FigureFigure 8.8. FTIR spectrum of cement and cement core.core. 3.3.2. XRD Analysis 3.3.2. XRD Analysis Similarly, the XRD pattern of Class-G cement and cement core is presented as Figure9. For Class-G Similarly, the XRD pattern of Class-G cement and cement core is presented as Figure 9. For Class- cement the intense peaks at 2θ values of 29.48◦, 32.24◦ and 32.42◦ indicate the presence of phases of G cement the intense peaks at 2θ values of 29.48°, 32.24° and 32.42° indicate the presence of phases tricalcium and dicalcium silicate. Furthermore, some overlapped peaks at 32.24◦ and 34.42◦ were of tricalcium and dicalcium silicate. Furthermore, some overlapped peaks at 32.24° and 34.42° were observed owing to the presence of C2S (Ca2SiO4). Moreover, cement upon hydration produces hydrated observed owing to the presence of C2S (Ca2SiO4). Moreover, cement upon hydration produces calcium silicate (Ca2SiO5H2) which has an intense peak at 2θ = 29.48◦ and Ca(OH)2 at 2θ = 41.6◦ [46]. hydrated calcium silicate (Ca2SiO5H2) which has an intense peak at 2θ = 29.48° and Ca(OH)2 at 2θ = Simultaneously, in the XRD pattern of cement core, other intense peaks in the range of 15◦–25◦ were 41.6° [46]. Simultaneously, in the XRD pattern of cement core, other intense peaks in the range of 15°– observed owing to the mixing of DPAM in cement paste. This causes an introduction of functional 25° were observed owing to the mixing of DPAM in cement paste. This causes an introduction of groups of the organic matter of DPAM (Figure9) and e ﬀects the cementitious properties of the cement functional groups of the organic matter of DPAM (Figure 9) and effects the cementitious properties in the mixture. The scattered DPAM in the cement paste yields a crystalline cement core owing to the of the cement in the mixture. The scattered DPAM in the cement paste yields a crystalline cement chemical interaction of the various cement phase viz. C3S, C2S, and C3S with the polar functional core owing to the chemical interaction of the various cement phase viz. C3S, C2S, and C3S with the groupsMaterials of2020 the, 13 organic, 2921 polymer of DPAM. 13 of 21 polar functional groups of the organic polymer of DPAM.

FigureFigure 9.9. XRD pattern of cement and cement core.

3.3.3. Contact Angle Measurement The snapshots of water contact angle measurement of the sample S4 is presented as Figure 10. It is observed that the water contact angle of 119.3° at the left and 116.1° at the right (Figure 10a), reveals that initially the sample was hydrophobic [47]. The change in contact angle between right and left side may be due to the surface irregularities of the cement core. After 1h of exposure with water, the average water contact angle is observed to decrease to 108.3° (Figure 10b). Further elapse of time at 6 h of exposure, the average water contact angle further reduced to 48.8° (Figure 10c), thereby revealing that the characteristic of the material changes to hydrophilic. This change in characteristics of the cement core sample suggests that the degradation of outer hydrophobic coatings had occurred within 6 h of exposure, thereby exposing the PAM to the water and in turn absorbing water.

Figure 10. Water contact angle measurement of sample S4 at time of exposure of (a) t = 0 h (b) t = 1h and (c) t = 5 h.

3.4. Investigation of Water Absorption of PAM, SPAM, and DPAM The water-absorption capacity of PAM, SPAM, and DPAM are presented as Figure 11a. It is observed that the water-absorption capacity of PAM is more than that of SPAM and DPAM. Moreover, the water-absorption capacity of PAM reaches its equilibrium value within less span of time, whereas for the SPAM and DPAM, the water-absorption capacity increases slowly with respect to time. The encapsulation of the MBIS and further silica over PAM hinders the direct exposure of PAM to water, thus decreasing both the water-absorption capacity and its rate. Furthermore, the detachment of outer layers over PAM over a period of time of 6h causes the migration of water into SPAM and DPAM, thereby increasing the rate of absorption and water-absorption capacity with

Materials 2020, 13, 2921 13 of 21

Materials 2020, 13, 2921 13 of 22 Figure 9. XRD pattern of cement and cement core.

3.3.3. Contact Contact Angle Angle Measurement The snapshots of of water water contact contact angle angle measurement measurement of of the the sample sample S4 S4 is ispresented presented as asFigure Figure 10. 10 It. isIt observed is observed that that the thewater water contact contact angle angle of 119.3° of 119.3 at the◦ at left the and left 116.1° and 116.1at the◦ rightat the (Figure right (Figure10a), reveals 10a), thatreveals initially that initiallythe sample the was sample hydrophobic was hydrophobic [47]. The [47 change]. The in change contact in angle contact between angle betweenright and right left sideand leftmay side be due may to be the due surface to the irregularities surface irregularities of the cement of the core. cement After core. 1h of After exposure 1h of with exposure water, with the water,average the water average contact water angle contact is observed angle is to observed decrease to to decrease 108.3° (Figure to 108.3 10b).◦ (Figure Further 10b). elapse Further of elapsetime at of6 htime of exposure, at 6 h of exposure, the average the water average contact water angle contact furthe angler reduced further to reduced 48.8° (Figure to 48.8 10c),◦ (Figure thereby 10c), revealing thereby thatrevealing the characteristic that the characteristic of the material of the materialchanges changesto hydrophilic. to hydrophilic. This change This changein characteristics in characteristics of the cementof the cement core sample core sample suggests suggests that the that degradation the degradation of outer of hydrophobic outer hydrophobic coatings coatings had occurred had occurred within within6 h of exposure, 6 h of exposure, thereby thereby exposing exposing the PAM the to PAM the water to the and water in andturn in absorbing turn absorbing water. water.

Figure 10. Water contact angle measurement of sample S4 at time of exposure of (a) t = 0 h (b) t = 1 h Figure 10. Water contact angle measurement of sample S4 at time of exposure of (a) t = 0 h (b) t = 1h and (c) t = 5 h. and (c) t = 5 h. 3.4. Investigation of Water Absorption of PAM, SPAM, and DPAM 3.4. Investigation of Water Absorption of PAM, SPAM, and DPAM The water-absorption capacity of PAM, SPAM, and DPAM are presented as Figure 11a. It is observedThe water-absorption that the water-absorption capacity capacity of PAM, of SPAM, PAM is and more DPAM than that are of presented SPAM and as DPAM. Figure Moreover,11a. It is observedthe water-absorption that the water-absorption capacity of PAM capacity reaches itsof equilibrium PAM is more value than within that less of span SPAM of time,and whereasDPAM. Moreover,for the SPAM the water-absorption and DPAM, the water-absorptioncapacity of PAM reaches capacity its increases equilibrium slowly value with within respect less tospan time. of time,The encapsulation whereas for the of SPAM the MBIS and and DPAM, further the silicawater-abso over PAMrption hinders capacity the increases direct exposure slowly with of PAMrespect to water,to time. thus The decreasing encapsulation both of the the water-absorption MBIS and further capacity silica andover itsPAM rate. hinders Furthermore, the direct the exposure detachment of PAMof outer to layerswater, overthus PAMdecreasing over a bo periodth the of water-absorption time of 6h causes capacity the migration and its of rate. water Furthermore, into SPAM andthe detachmentDPAM, thereby of outer increasing layers the over rate PA ofM absorption over a period and water-absorptionof time of 6h causes capacity the migration with respect of water to time into of SPAMexposure. and Simultaneously, DPAM, thereby the increasing DPAM particles the rate took of aabsorption little longer and period water-absorption of time to swell capacity as compared with to SPAM owing to the hydrophobic characteristics of the outer encapsulated silica layer. The DPAM particles behave as SPAM after the detachment of the silica layer during stirring and swell further by absorbing the water. Furthermore, DPAM was exposed to various concentrations (30,000 to 270,000 PPM) of KCl and NaCl solutions to investigate the water-absorption capacity in saline conditions as represented in Figure 11b,c, respectively. When DPAM was exposed to distilled and saline water, the water-absorption capacity remained constant over a period of 6 h, then the particles start absorbing water. It is observed from the ﬁgure that the water-absorption capacity decreases with increase in the concentration of KCl and NaCl solution. The increase in solution concentration causes in decrease in the polymer-solvent interaction, owing to the formation of electric double layers around the polar groups viz. C=O, C–N + + with the Na or K and Cl− ions, thereby inhibiting the migration of water into the particles. Moreover, after prolonged use, the salt content wrench water from the particles owing to the osmosis eﬀect, and thus the water-absorption capacity decreases. Materials 2020, 13, 2921 14 of 21

respect to time of exposure. Simultaneously, the DPAM particles took a little longer period of time to swell as compared to SPAM owing to the hydrophobic characteristics of the outer encapsulated silica layer. The DPAM particles behave as SPAM after the detachment of the silica layer during stirring and swell further by absorbing the water. Furthermore, DPAM was exposed to various concentrations (30,000 to 270,000 PPM) of KCl and NaCl solutions to investigate the water-absorption capacity in saline conditions as represented in Figures 11b,c, respectively. When DPAM was exposed to distilled and saline water, the water- absorption capacity remained constant over a period of 6 h, then the particles start absorbing water. It is observed from the figure that the water-absorption capacity decreases with increase in the concentration of KCl and NaCl solution. The increase in solution concentration causes in decrease in the polymer-solvent interaction, owing to the formation of electric double layers around the polar groups viz. C=O, C–N with the Na+ or K+ and Cl− ions, thereby inhibiting the migration of water into Materials 2020the, 13 particles., 2921 Moreover, after prolonged use, the salt content wrench water from the particles owing 14 of 22 to the osmosis effect, and thus the water-absorption capacity decreases.

Figure 11.FigureWater-absorption 11. Water-absorption capacity capacity of of (a ()a) synthesized synthesized polymers polymers in distilled in distilled water (b water) DPAM (b in) DPAMKCl in KCl solution (csolution) DPAM (c) DPAM in NaCl in NaCl solution. solution.

3.5. Linear3.5. Expansion Linear Expansion The linear expansion of the cement core doped with a various dosage of DPAM was The linearcontinuously expansion measured ofthe as acement function coreof time doped over awith period a of various 20 h and dosage presented of DPAMas Figure was 12. It continuously is measuredobserved as a function that all of the time specimens over a perioddid not ofexpand 20 h andin the presented initial hour, as then Figure the 12expansion. It is observed increases that all the specimensexponentially did not expand with respect in the to initial time. The hour, stagnant then ph thease expansion is owing to increasesthe dual encapsulation exponentially of silica with respect to time. Theand stagnantMBIS, after phase degradation is owing of the to outer the dual coated encapsulation layers the PAM of get silica exposed and and MBIS, the cement after degradationcore of starts expanding by absorbing water. As the dosage of DPAM increases, the stagnation time the outer coateddecreases. layers This may the PAMbe because get exposedwith an increase and the incement dosage of core DPAM, starts the expanding force imparted by absorbingby the water. As the dosageexposed of DPAMPAM particles increases, becomes the more stagnation in 16% dosage time as decreases. compared to This 4% maydosage. be It because is also observed with an increase in dosagethat of DPAM, as percentage the force of dosage imparted of DPAM by the in exposedthe cement PAM core particlesincreases the becomes linear expansion more in 16%also dosage as comparedincreases. to 4% dosage. The increase It is alsoin the observed concentration that of as DPAM percentage causes ofmore dosage absorption of DPAM water, in thereby the cement core Materials 2020, 13, 2921 15 of 21 increases the linear expansion also increases. The increase in the concentration of DPAM causes more absorptionincreasing water, thereby the linear increasing expansion. For the 16% linear DPAM expansion. dosage, the For cement 16% slurry DPAM could dosage, attain a maximum the cement slurry could attainlinear a maximum expansion of linear 0.26 mm/mm. expansion of 0.26 mm/mm.

Figure 12.FigureLinear 12. Linear expansion expansion of of cement cement core with with different diﬀerent dosage dosage of DPAM. of DPAM.

Figure 13. Water flow through single crack in the cement core for (a) various dosage of DPAM at 0.4 atm differential pressure (b) various differential pressures for 16% dosage DPAM cement core.

Materials 2020, 13, 2921 15 of 21 increasing the linear expansion. For 16% DPAM dosage, the cement slurry could attain a maximum linear expansion of 0.26 mm/mm.

Figure 12. Linear expansion of cement core with different dosage of DPAM.

3.6. Water-Flow Measurement Figure 13a represents the dependence of water-flow rate on time of exposure at a differential pressure of 0.4 atm, through the synthesized cracked cement sheath with various dosage of DPAM. MaterialsAt the 2020beginning, 13, 2921 the water-flow rate is maximum and as the time of exposure increases,15 ofthe 22 degradation of the outer layer occurs, thereby exposing the PAM particles. The exposed PAM particles absorb water and thus heals the crack, which in turn decreases the water-flow rate. 3.6. Water-Flow Measurement It is also revealed from Figure 13a that with an increase in the dosage of DPAM, the water-flow rate throughFigure 13 thea representsspecimen decreases. the dependence It is observed of water-flow that the rate specimen on time with of exposure16% of DPAM at a di ceasesfferential the pressurewater flow of completely 0.4 atm, through after 12 the h synthesizedof exposure owin crackedg to cementthe self-healing sheath with of PAM. various Thus, dosage specimen of DPAM. with At16% the of beginning DPAM theis chosen water-flow for ratethe iswater-flow maximum andmeasurement as the time offor exposure various increases,differential the degradationpressure as ofrepresented the outerlayer as Figure occurs, 13b. thereby As the exposing pressure the is PAMincreased particles. the flow The exposedrate also PAM increases particles according absorb to water the andHagen thus Poiseuille heals the law crack, [48]. which in turn decreases the water-flow rate.

Figure 13. Water flow through single crack in the cement core for (a) various dosage of DPAM at Figure 13. Water flow through single crack in the cement core for (a) various dosage of DPAM at 0.4 0.4 atm differential pressure (b) various differential pressures for 16% dosage DPAM cement core. atm differential pressure (b) various differential pressures for 16% dosage DPAM cement core. It is also revealed from Figure 13a that with an increase in the dosage of DPAM, the water-flow rate through the specimen decreases. It is observed that the specimen with 16% of DPAM ceases the water flow completely after 12 h of exposure owing to the self-healing of PAM. Thus, specimen with 16% of DPAM is chosen for the water-flow measurement for various differential pressure as representedMaterials 2020, as13, Figure2921 13b. As the pressure is increased the flow rate also increases according to16 of the 21 Hagen Poiseuille law [48]. FigureFigure 1414 representsrepresents thethe snapshotssnapshots ofof thethe cementcement corecore beforebefore andand afterafter absorptionabsorption ofof water.water. ItIt isis observedobserved thatthat thethe polymerpolymer inin thethe cementcement corecore swelledswelled byby absorbingabsorbing waterwater andand thusthus healsheals thethe crackscracks generatedgenerated inin thethe cementcement core.core.

FigureFigure 14.14. Snapshots of split cement core ((aa)) beforebefore ((bb,–cc)) after after water water absorption. absorption.

FigureFigure 1515 representsrepresents thethe snapshotssnapshots ofof thethe cementcement corecore (S4)(S4) beforebefore andand afterafter water-ﬂowwater-flow test.test. ItIt isis observedobserved thatthat thethe generatedgenerated crackcrack waswas pluggedplugged afterafter 1212 hh ofof continuous continuous water water ﬂow. flow.

Figure 15. Snapshots of sample S4 (a) before (b) after water-flow test.

3.7. Rheological Behavior The rheological behavior of the cement slurry with 16% dosage of DPAM is represented as Figure 16. From the shear stress vs. shear rate results, it is clear that the cement slurries behave like a Bingham plastic. Moreover, the variation of viscosity with respect to the shear rate is also presented in Figure 16. At the onset of the rheological study, the cement slurry tends to possess higher viscosity owing to its gelation characteristics. At the lower shear rate, the yield point of gelation of cement slurry breaks quickly and thus the viscosity drops drastically. Beyond yield shear stress, the viscosity tends to decrease at a lower rate owing to the deformation of the cement slurry. Further increase in the shear rate the viscosity tends to increase because of the further hydration of cement slurry to form a paste. Furthermore, there is not much variation in shear stress and viscosity found with respect to change in dosage of DPAM in the cement slurry.

Materials 2020, 13, 2921 16 of 21

Figure 14 represents the snapshots of the cement core before and after absorption of water. It is observed that the polymer in the cement core swelled by absorbing water and thus heals the cracks generated in the cement core.

Figure 14. Snapshots of split cement core (a) before (b–c) after water absorption.

MaterialsFigure2020, 1513, 2921represents the snapshots of the cement core (S4) before and after water-flow test.16 ofIt 22is observed that the generated crack was plugged after 12 h of continuous water flow.

Figure 15. Snapshots of sample S4 ( a) before ( b) after water-flow water-ﬂow test.

3.7. Rheological Behavior The rheological behavior of the cement slurry with 16% dosage of DPAM is represented as Figure 1616.. From the shear stress vs. shear rate results, it is clear that the cement slurries behave like a Bingham plastic.plastic. Moreover, thethe variationvariation of of viscosity viscosity with with respect respect to to the the shear shear rate rate is is also also presented presented in Figurein Figure 16 .16. At At the the onset onset of of the the rheological rheological study, study, the the cement cement slurry slurry tends tends toto possesspossess higherhigher viscosity owing to its gelation characteristics.characteristics. At At the the lower shear rate, the yield point of gelation of cement slurry breaks quickly and thus the viscosity drops dr drastically.astically. Beyond yield shear stress, the viscosity tends toto decreasedecrease atat a a lower lower rate rate owing owing to to the the deformation deformation of of the the cement cement slurry. slurry. Further Further increase increase in the in shearthe shear rate rate the the viscosity viscosity tends tends to increaseto increase because because of theof the further further hydration hydration of of cement cement slurry slurry to to form form a paste.a paste. Furthermore, Furthermore, there there is is not not much much variation variation ininshear shear stressstress andand viscosityviscosity foundfound withwith respect to Materialschange 2020 in dosage, 13, 2921 of DPAM inin thethe cementcement slurry.slurry. 17 of 21

FigureFigure 16. 16. RheologicalRheological behavior behavior of of the the cement cement paste. paste. 3.8. Compressive Strength 3.8. Compressive Strength The compressive strength of cubical shaped cement core was measured using the compression test The compressive strength of cubical shaped cement core was measured using the compression machine. The measured compressive strength of the specimens for various curing time is presented in test machine. The measured compressive strength of the specimens for various curing time is Figure 17. It is observed that the compressive strength of the specimen decreases in a little margin with presented in Figure 17. It is observed that the compressive strength of the specimen decreases in a an increase in dosage of DPAM. The maximum compressive strength of 21.6 MPa was observed for the little margin with an increase in dosage of DPAM. The maximum compressive strength of 21.6 MPa sample S1 after a curing time of 48 h and for the 16% DPAM cement core (S4) possess a compressive was observed for the sample S1 after a curing time of 48 h and for the 16% DPAM cement core (S4) strength of 18.8 MPa. possess a compressive strength of 18.8 MPa.

Figure 17. Compressive strength of cement core.

Furthermore, a comparison of the linear expansion, water-absorption capacity and compressive strength of the as-prepared cement cores in this work with the other synthesized core reported in the literature is represented as Table 3. The difference in water-absorption capacity of polymer, linear expansion and compressive strength of the cement core are owing to the different types of polymer and material encapsulation chosen for impregnation. Furthermore, the water-absorption capacity owing to impregnation of SAP alone in the cement core is found to be maximum [49]; however such

Materials 2020, 13, 2921 17 of 21

Figure 16. Rheological behavior of the cement paste.

3.8. Compressive Strength The compressive strength of cubical shaped cement core was measured using the compression test machine. The measured compressive strength of the specimens for various curing time is presented in Figure 17. It is observed that the compressive strength of the specimen decreases in a little margin with an increase in dosage of DPAM. The maximum compressive strength of 21.6 MPa was observed for the sample S1 after a curing time of 48 h and for the 16% DPAM cement core (S4) Materials 2020, 13, 2921 17 of 22 possess a compressive strength of 18.8 MPa.

FigureFigure 17.17. CompressiveCompressive strengthstrength ofof cementcement core.core.

Furthermore,Furthermore, aa comparisoncomparison ofof thethe linearlinear expansion,expansion, water-absorptionwater-absorption capacitycapacity andand compressivecompressive strengthstrength ofof thethe as-preparedas-prepared cementcement corescores inin thisthis workwork withwith thethe otherother synthesizedsynthesized corecore reportedreported inin thethe literatureliterature isis representedrepresented asas TableTable3 .3. The The di differenceﬀerencein inwate water-absorption r-absorption capacitycapacity ofof polymer,polymer, linearlinear expansionexpansion andand compressivecompressive strengthstrength ofof thethe cementcement corecore areare owingowing toto thethe didifferentﬀerent typestypes ofof polymerpolymer andand materialmaterial encapsulationencapsulation chosenchosen forfor impregnation.impregnation. Furthermore,Furthermore, thethe water-absorptionwater-absorption capacitycapacity owingowing toto impregnationimpregnation ofof SAPSAP alonealone inin thethe cementcement corecore isis foundfound toto bebe maximummaximum [[49];49]; howeverhowever suchsuch high absorption during the preparation of cement slurry before transportation into the oil well is undesirable. In the present work, the compressive strength of the cement core is in the concurrence. Materials 2020, 13, 2921 18 of 22

Table 3. Comparison of the results with the earlier reported studies.

Water-Absorption Capacity Linear Expansion of Sl. No. Type of Cement and Additives Compressive Strength (MPa) Reference of Polymer (G/g) Cement (µm/m) 1 Class-G Oil well cement, SAP, Calcium carbonate 12.5 (within 35 min) Not studied 24.4 Liu et al. [23] 2 Cement, SAP 163 100–200 Not studied Snoeck et al. [49] 3 Cement with ﬂy ash, SAP 2.59 20 10.6 Lee et al. [33] 4 Cement, Fly ash, SAP 8 (after 2 h) Not studied 15 Snoeck et al. [50] 5 SAP, poly-carboxylate ether, ordinary Portland cement 27.5 (within 15 min) 100 54 Baloch et al. [35] 6 Cement, chitosen, PAM, Class-G oil well cement 4.8 (within 5 h) Not studied 38–41 Liu et al. [44] 7 Portland Cement, 5%SAP 0.08 Not studied 28.74 Mpa Rai and Singh [46] 8 Class F Fly ash, CAC, sodium hexametaphosphate Not studied 180 25 Sugama and Pyatina [51] 9 Epoxy acrylate, OPC, Cellulose Not studied 900 7 (Flexural strength) Lv et al. [52] 10 Class-G oil-well cement, DPAM 2.5 (after 26 h) 260 21.6 Present work Materials 2020, 13, 2921 19 of 22

4. Conclusions To hinder the permeability of water and improve the stability of the hardened cement paste a dual coated water swellable polymer was synthesized and then impregnated in cement paste to enhance crack self-healing characteristics elicited by water leakage through the crack. Based on the experimental results the following conclusions may be drawn.

The synthesized DPAM start absorbing water after 6 h of exposure, thereby increases its stability • during cement slurry transportation into the oil well, thus justiﬁes its impregnation in the cement slurry. The DPAM absorbs water about 250% of its own weight after 26 h time of exposure. Moreover, • it can also absorb water about 40% of its own weight under a maximum oil well salinity of 270,000 ppm. The synthesized cement core with 16% dosage of DPAM possessed better self-healing capability • within 12 h. With increase in dosage of DPAM in the cement core the linear expansion increases with a • maximum of 0.26 mm/mm. The cement core with 4% dosage of DPAM has shown a maximum compressive strength • of 21.6 MPa.

Thus, the novel dual polymer encapsulation prevents water migration into PAM during its transportation with the cement slurry into the oil well. Furthermore, upon crack generation in the cement sheath, the PAM plug the crack owing to water absorption instigating self-healing. Therefore, the cement slurry with 16% dosage of DPAM can be a potential cementitious material for preventing the produced water ﬂow in the oil well with a little compromise with the compressive strength.

Author Contributions: G.R. and D.T.K.D. have equally contributed towards experimentation, synthesis of polymer and samples. K.R.P. has contributed towards the characterization of the different samples. Further G.R., D.T.K.D., and K.R.P. have contributed in manuscript writing. K.K.P., N.S., K.L. and P.P.K. have given their suggestions and accordingly manuscript was modified. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors acknowledge their work to IDT, ONGC for providing the materials and DIT, University for providing facility for experimentation. Conflicts of Interest: The authors declare no conflict of interest.

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