Long-Term Compressive Strength of Polymer Concrete-Like Composites with Various Fillers

materials

Article Long-Term Compressive Strength of Polymer Concrete-like Composites with Various Fillers

Joanna Julia Sokołowska Department of Building Materials, Faculty of Civil Engineering, Warsaw University of Technology, 00-637 Warsaw, Poland; [email protected]; Tel.: +48-22-2346482

Received: 18 February 2020; Accepted: 4 March 2020; Published: 7 March 2020

Abstract: The durability of building composites with polymer matrix, such as polymer concretes, is considered high or excellent. However, very few studies are available that show the properties of such composites tested long after the specimens’ preparation, especially composites with fillers other than traditional rock aggregates. The paper presents the long-term compressive strength of polymer concrete containing common and alternative fine fillers, including quartz powder (ground sand) and by-products of the combustion of Polish fossil fuels (coal and lignite), tested nine or 9.5 years after preparation. The results were compiled with the data for respective specimens tested after 14 days, as well as 1.5 and 7 years. Data analysis confirmed the excellent durability of concrete-like composites with various fillers in terms of compressive strength. Density measurements of selected composites showed that the increase in strength was accompanied by an increase in volumetric density. This showed that the opinion that the development of the strength of composites with polymer matrices taking place within a few to several days was not always justified. In the case of a group of tested concrete-like composites with vinyl-ester matrices saturated with fly ashes of various origins, there was a further significant increase in strength over time.

Keywords: long-term compressive strength; durability; polymer composites; polymer concrete; mineral ﬁllers; fossil fuel combustion by-products; ﬂy ash

1. Introduction Polymer concrete-like composites are similar to ordinary concretes, but with completely cement-free polymer matrices or matrices of two co-binders, i.e., mineral cement and a significant amount of polymer. The examples of such composites are polymer concretes (PC) and mortars with fillers of various sizes and polymer-cement concretes (PCC) with complementing polymer-cement mortars and pastes [1–4]. Each time, presenting the concept of polymer concrete-like composites and their advantages to researchers familiar only with traditional concrete (that is often slightly modified with polymers, e.g., through the use of polymer-based admixtures), at some point, high durability [1–4] was indicated. The high or excellent durability of these composites is mainly the consequence of the presence of a significant amount of polymer, which allows obtaining tighter matrices. Therefore, such matrices are more resistant to external environmental onslaughts, including destructive chemical processes (acid and alkaline corrosion, leaching of easily soluble compounds, etc.) [5–8], as well as physical processes (freezing and thawing of penetrating water, thermal shock or, in the case of PCC, destruction resulting from increasing the volume of crystallization products, etc.) [2,9–11]. At the same time, the matrices present higher adhesion to fillers. As a result, composites with such matrices are mechanically stronger [2,6]. However, not much of the published data concerned polymer concrete-like composites tested at a later age, confirming their ability to maintain mechanical properties over time, i.e., confirming their durability due to this criterion. The issue of durability requires even more clarification when

Materials 2020, 13, 1207; doi:10.3390/ma13051207 www.mdpi.com/journal/materials Materials 2020, 13, 1207 2 of 12 wastes or by-products are applied to the aforementioned composites as the substitutes of the regular components. Such composites have been recently eagerly designed and produced, following the assumptions of the sustainable development concept, i.e., a concept strongly promoted for the past few years. Composites containing such wastes and/or by-products (e.g., fly ash, perlite powder, recycled glass, aggregate leftover mineral dust, etc.) are subjected to various tests [5–7,9–13], but usually only at the early stage of their service-life. The level of mechanical properties of such composites after a long period of use remains unclear. The aim of the paper is to discuss the “long-term compressive strength” of polymer concretes with traditional quartz fillers (including coarse and fine aggregates, the same as those used in ordinary Portland cement concrete and very fine quartz powders) and sustainable secondary materials, namely the by-products of the combustion of two kinds of fossil fuels, i.e., coal (or so-called “hard coal”) and lignite (or “brown coal”), mined in deposits located in Poland. The combustion processes are intended to produce electricity or heat. Due to differences in the construction of furnaces and associated combustion installations, the remaining fly ashes differ in terms of morphology and granulation, so they differently affect the consistency of PC mix [13] which, among others, influences the microstructure and, therefore, the properties of hardened composites containing such fly ashes. The tested composites are cement-free (the binder consists solely of vinyl-ester resin). There is also no water in the composition of these composites. Water disturbs or prevents setting of the vinyl-ester resin (to avoid the negative effect of moisture from the aggregate on the binder setting, aggregates are usually dried before dispensing them into the mix). In the case of this type of concrete, there is no hydration reaction, and the fly ashes present in the composite, despite the fact that they contain amorphous silica, do not participate in the pozzolanic reaction. Therefore, fly ashes in PC are only a very fine filler/aggregate fraction. The influence of the presence of siliceous fly ash (the by-product of conventional coal combustion) on the properties of polymer mortars and concretes is relatively well recognized. Such composites may present better mechanical properties and workability [5,6,14,15], which is partly due to the almost perfectly spherical shape of the siliceous fly ash particles. The effect of the presence of by-products of coal FBC (fluidized bed combustion), their granulation, and morphology on the properties of PC is less recognized. The fly ash remaining after hard coal fluidized combustion consists of much finer irregular particles, and its chemical composition contains more calcium compounds (up to over 20% by mass). The fly ash remaining after lignite fluidized combustion is a mixture of spherical and irregular particles and contains less calcium compounds (but still more than siliceous fly ash). It can be found that the use of microfillers of an increased content of calcium compounds as substitutes for quartz fillers may lead to a greater increase in the compressive and flexural strength of polymer composites [16].

2. Genesis of Research The results presented in the paper were obtained by the continuation of research conducted in 2010–2011 [7,17,18] that was focused on assessing the possibility of using various fly ashes as substitutes for regular microfillers (i.e., fillers of a size usually up to 120 µm [19]) in polymer composites, and later research on the durability of such composites conducted in 2018 [20]. In the framework of the second study, the author carried out a series of mechanical tests of vinyl-ester concretes 1.5 and seven years after production. The results of tests carried out on 1.5-year-old specimens showed a clear increase in compressive strength in comparison to the strength of 14-day-old specimens. The increase ranged from 8.6% to 36.2% (on average, by 18.4%). The tests carried out on seven-year-old specimens also showed a noticeable increase in compressive strength. The upward trend was observed in the case of polymer composites containing both types of fly ash: siliceous fly ash (the by-product of conventional coal combustion) and FBC fly ash. In the case of the composite with fluidized fly ash, strength gain values were even higher (a maximum of 60.4%, 22% on average). The conclusion from that research was that for polymer concretes containing fly ash being a coal combustion product (CCP), the development of compressive strength occurs for a long time [20]. Materials 2020, 13, x FOR PEER REVIEW 3 of 12

MaterialsSuch2020 a, 13conclusion, 1207 contradicts the opinion that the development of the strength of composites3 of 12 with polymer matrices takes place within a relatively short time, or at least indicates that it is not the correctSuch opinion a conclusion in every contradicts case. Figure the opinion 1a presents that the developmentthe general ofconcept the strength of the of development composites with of polymercompressive matrices strength takes in place time within of two a relativelyconcrete-like short composites, time, or at leasti.e., polymer-cement indicates that it is concrete not thecorrect (PCC) opinion(of a matrix in every that case.is partly Figure cementitious1a presents and the generalpartly polymeric) concept of theand developmentpolymer concrete of compressive (PC) (only strengthpolymer inmatrix). time of The two concept concrete-like assumes composites, that the i.e.,development polymer-cement of polymer concrete concrete (PCC) compressive (of a matrix thatstrength is partly is very cementitious intense and and short, partly i.e., polymeric) PC develops and 60% polymer of the concrete maximum (PC) strength (only polymer within one matrix). day Theand conceptca. 80% assumesof the maximum that the development strength within of polymer three da concreteys [2]. This compressive is related strengthto the dynamics is very intense of the andsetting short, and i.e., curing PC develops of the 60%polyme of ther maximumbinder, usually strength the within thermosetting one day and resin, ca. 80%which of thequickly maximum goes strengththrough withinthe phase three of days gelation [2]. Thisto the is relatedmore and to themore dynamics intense ofpolymerization the setting and and curing creation of the of polymer the 3D binder,cross-linked usually structure the thermosetting of the polymer resin, matrix. which Thermose quickly goestting through resins tend the phase to develop of gelation the vast tothe majority more andof the more mechanical intense polymerizationstrength within andfirst creationdays (e.g., of theepoxy 3D resins cross-linked develop structure about 80% of theof the polymer vast majority matrix. Thermosettingof the strength resinswithin tend the first to develop 1–2 days) the [21]. vast This majority is why of thepolymer mechanical concretes strength tend to within develop first utility days (e.g.,efficiency epoxy after resins developseveral aboutdays 80%(which of the depend vast majoritys on humidity of the strength and withinambient the firsttemperature; 1–2 days) [21the]. Thisabovementioned is why polymer model concretes assumed tend temperature to develop utilityno lower effi ciencythan 15°C). after severalGenerally, days after (which 14 days, depends further on humiditysignificant and development ambient temperature; of mechanical the strength abovementioned is not expected. model assumed temperature no lower than Figure 1b presents a trial of recalculation of the model using the natural logarithm function that 15◦C). Generally, after 14 days, further significant development of mechanical strength is not expected. allowed indicating the moment of achieving potentially full (100%) compressive strength as 52 days.

(a) (b)

FigureFigure 1.1. DevelopmentDevelopment ofof thethe compressivecompressive strengthstrength ofof polymerpolymer compositescomposites expressedexpressed asas thethe relativerelative increaseincrease in in time: time: ( a(a)) model model of of polymer polymer concrete concrete (PC) (PC) and and polymer-cement polymer-cement concrete concrete (PCC) (PCC) developed developed by Czarneckiby Czarnecki [2]; [2]; (b) recalculation(b) recalculation of the of ﬁrstthe first model model using using the naturalthe natural logarithm logarithm function. function.

FigureThe empirical1b presents data a trial obtained of recalculation in the framework of the model of usingthe previously the natural mentioned logarithm functionresearch that[20] allowedcompared indicating to the theabovementioned moment of achieving model potentially(determined full for (100%) polymer compressive concrete strength with an as assumed 52 days. maximumThe empirical compressive data strength obtained of 100 in the MPa) framework with the time of the variable previously expressed mentioned on a logarithmic research scale [20] comparedare given toin theFigure abovementioned 2. In the case model of a group (determined of vinyl-ester for polymer composites concrete cont withaining an assumed various maximum fly ashes, compressivethere was a clear strength tendency of 100 for MPa) further with development the time variable of compressive expressed on strength. a logarithmic scale are given in FigureThe2. In newly the case obtained of a group data of presented vinyl-ester in composites this pape containingr, i.e., compressive various fly strength ashes, there of 9.5-year-old was a clear tendencyconcretes, for partly further complemented development earlier of compressive findings, strength.as concretes identical in terms of quantitative and qualitativeThe newly composition obtained were data tested presented at a inlater this date paper, to see i.e., if compressivethere was even strength a further of 9.5-year-old increase in concretes,strength. What partly is complemented more, the base earlier of the findings, composit ases concretes analyzed identical in terms in of terms long-term of quantitative compressive and qualitativestrength (considered composition as a werekind of tested measure at a of later dura datebility) to was see supplemented if there was even with a vinyl-ester further increase concretes in strength.containing What fly ashes is more, from the lignite base of combustion—tested the composites analyzed 14 days in terms and nine of long-term years after compressive production. strength (considered as a kind of measure of durability) was supplemented with vinyl-ester concretes containing fly ashes from lignite combustion—tested 14 days and nine years after production.

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FigureFigure 2. Development 2. Development of of the the strength strength of polymer concre concretestes with with various various mass mass contents contents (0.4%–20.1%) (0.4–20.1%) of Figure 2. Development of the strength of polymer concretes with various mass contents (0.4%–20.1%) fly ashof fly in aash microfiller in a microfiller fraction fraction in time; in FAtime;F, flyFA ashF, fly from ash from fluidized fluidized bed combustionbed combustion (FBC); (FBC); FA SFA, siliceousS, of fly ash in a microfiller fraction in time; FAF, fly ash from fluidized bed combustion (FBC); FAS, fly ashsiliceous from fly conventional ash from conventional combustion combustion (chart based (chart on based data on published data published by the by author the author in [20 in] compared[20] siliceous fly ash from conventional combustion (chart based on data published by the author in [20] to thecompared model calculatedto the model on calculated the basis on of the [2]). basis of [2]). compared to the model calculated on the basis of [2]). 3. Materials3. Materials and and Methods Methods 3. Materials and Methods TheThe polymer polymer used used to to prepare prepare all all concretesconcretes presented presented in inthe the study study was was synthetic synthetic vinyl-ester vinyl-ester resin resin of low viscosity (350 ± 50 mPa·s at 25 °C) and high flexural strength and tensile strength (declared by of lowThe viscosity polymer (350 used to50 prepare mPa s at all 25 concretes◦C) and presented high flexural in the strength study andwas synthetic tensile strength vinyl-ester (declared resin the producer as respectively± 110· MPa and 75 MPa). Therefore, concretes made from this resin should ofby low the viscosity producer (350 as respectively ± 50 mPa·s at 110 25 °C) MPa and and high 75 MPa).flexural Therefore, strength and concretes tensile madestrength from (declared this resin by retain the high mechanical strength in long-term exploitation, even when exposed to aggressive theshould producer retain as the respectively high mechanical 110 MPa strength and 75 in long-termMPa). Therefore, exploitation, concretes even made when from exposed this toresin aggressive should media. The chemical formula of vinyl-ester (the polyester modified by introducing the fragments of retain the high mechanical strength in long-term exploitation, even when exposed to aggressive media.the corresponding The chemical bisphenol formula ofepoxy vinyl-ester resin to the (the struct polyesterure of the modified molecule) by is introducing presented in the Figure fragments 3. of media.the corresponding The chemical bisphenol formula epoxyof vinyl-ester resin to the(the structure polyester of modified the molecule) by introducing is presented the in fragments Figure3. of the corresponding bisphenol epoxy resin to the structure of the molecule) is presented in Figure 3.

Figure 3. Vinyl-ester resin before cross-linking [22].

The fillers included coarse and fine aggregates, the same as those used in ordinary Portland cement concrete, two commerciallyFigure 3. Vinyl-esterVinyl-ester available resinquartz before powders cross-linking (produced [[22].22 by]. mechanical milling of quartz sand), and three by-products of the combustion of two fossil fuels—hard coal and lignite. The fillersThe fillersfillersincluded included in particular: coarse coarse and and fine fine aggregates, the same as those used in ordinary Portland cement concrete, two commercially available quartz powders (produced by mechanical milling of • conventional coarse aggregate: natural gravel of a fraction of 4/8 mm (marked G), quartz sand), sand), and and three three by-products by-products of of the the combustion combustion of two of two fossil fossil fuels—hard fuels—hard coal coaland andlignite. lignite. The • conventional fine aggregate: standard sand (acc. to the EN-196-1 standard) or river sand of a Thefillers fillers includedfraction included inof 0/2particular: in mm particular: (marked S), • • conventional conventional coarse commercial aggregate: microfiller: natural two grav quartzel of powders a fraction (marked of 4/8 Qmm1 and (marked Q2), G), •conventional coarse aggregate: natural gravel of a fraction of 4/8 mm (marked G), • conventionalsustainable fine microfiller aggregate: substitu standardte: siliceous sand fly(acc. ash to (conventio the EN-196-1nal hard standard) coal combustion or river sandby- of a conventionalproduct, marked fine aggregate: FAS) and two standard fluidized sand fly ashes (acc. (hard to the coal EN-196-1 FBC by-product, standard) marked or river FAF, sandand of a • fraction of 0/2 mm (marked S), fractionlignite of FBC 0/2 mmby-product, (marked marked S), FAF2). • conventional commercial microfiller: two quartz powders (marked Q1 and Q2), conventional commercial microfiller: two quartz powders (marked Q1 and Q2), • sustainable microfiller substitute: siliceous fly ash (conventional hard coal combustion by- sustainable microfiller substitute: siliceous fly ash (conventional hard coal combustion by-product, • product, marked FAS) and two fluidized fly ashes (hard coal FBC by-product, marked FAF, and marked FAS) and two fluidized fly ashes (hard coal FBC by-product, marked FAF, and lignite FBC lignite FBC by-product, marked FAF2). by-product, marked FAF2).

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Figure 4 presents the particle size distribution plots and SEM micrographs of all microfillers listed above. The particle size distribution measurements were done by the laser scattering method using the laser analyzer Horiba LA-300 (Kyoto, Japan). The method based on the Mie theory [23] involved passing laser beams through a 0.2% sodium polymetaphosphate solution containing microfiller particles (additionally dispersed by ultrasounds) and determining the particle size (in the range of 0.01–600 µm). The SEM micrographs were made with a Hitachi TM-1000 Scanning Electron Microscope (Tokyo, Japan) using a back scattered electrons (BSE) detector. The values of statistical parameters describing the fillers’ particle size distributions, as well as specific surface area (calculated from the distribution, making an assumption about the spherical shape of the particles) are given in Table 1. The microfillers were characterized by different particle morphologies and grading. The lowest values of maximum particle size, Dmax (67.52 µm and 58.95 µm), were registered in the case of quartz powders (Q1 and Q2). On the contrary, siliceous fly ash (FAS) contained the largest particles (size up to 200 µm), and its particle size distribution showed the highest values of the mode and median (4.96 µm and 25.30 µm, respectively). In the case of fly ashes from fluidized combustion (FAF and FAF2), the mode was 0.25 µm, which was 20 times lower compared to siliceous fly ash, while the median Materials 2020, 13, 1207 5 of 12 was 0.26 µm or 0.27 µm—values two orders of magnitude lower than in the case of siliceous fly ash. It is worth noting that the grading of both fluidized fly ashes was described by a bimodal distribution. TheFigure abovementioned4 presents the mode particle value size applied distribution to the incr plotseased and number SEM micrographsof particles smaller of all than microfillers 1 µm. A listed similar mode value (0.28 µm) was observed in the case of quartz powder Q1, which was why fly ashes above. The particle size distribution measurements were done by the laser scattering method using from FBC seemed to be more similar to this conventional quartz filler in terms of grading. The fly ash the laser analyzer Horiba LA-300 (Kyoto, Japan). The method based on the Mie theory [23] involved originating from lignite fluidized bed combustion (FAF2) was slightly thicker and had a smaller passingspecific laser surface beams area through than the a 0.2% fly ash sodium from hard polymetaphosphate coal fluidized bed solution combustion containing (FAF). Both, microfiller however, particles (additionallyhad a more dispersed developed by ultrasounds) specific surface and determiningarea than quartz the particle powders. size Meanwhile, (in the range in ofterms 0.01–600 of µm). Themorphology, SEM micrographs siliceous were fly ash made containing with spherical a Hitachi grains TM-1000 was more Scanning similar Electronto quartz powders, Microscope which (Tokyo, Japan)contained using aangular back scattered grains resulting electrons from (BSE) mechanical detector. milling The of values quartz. of statistical parameters describing the fillers’The particle density size was distributions, also determined as for well all as microfillers. specific surface The test area was (calculated performed fromin a Le the Chatelier distribution, makingvolumenometer an assumption (consisting about of the a flat spherical-bottomed shape flask) of according the particles) to the are procedure given in described Table1. in the EN 1936 standard. The results of density are given in Table 1.

Figure 4. Particle size distribution plots and SEM micrographs (magnification: 6000 ) of the considered Figure 4. Particle size distribution plots and SEM micrographs (magnification: ×6000×) of the fillersconsidered (Q1,Q2, fillers quartz (Q powders;1, Q2, quartz FA powders;S, siliceous FA flyS, siliceous ash; FA flyF, fluidizedash; FAF, fluidized fly ash from fly ash hard from coal hard combustion; coal FAF2combustion;, fluidized flyFAF2 ash, fluidized from lignite fly ash combustion). from lignite combustion).

Table 1. Statistical parameters describing the particle size distribution and speciﬁc surface area (SSA)

of fillers; Q1,Q2, quartz powders; FAS, siliceous fly ash; FAF, FAF2, fluidized fly ashes.

Raw Material Quartz Sand Quartz Sand Coal Coal Lignite Conventional FBC FBC Production Grinding Grinding Combustion Combustion Combustion

Parameter Filler Q1 [20] Filler Q2 Filler FAS [20] Filler FAF [20] Filler FAF2

D min, µm 0.26 1.32 0.17 0.12 0.17 Mode, µm 0.28 2.45 4.96 0.25 0.25 Median, µm 2.44 7.18 25.30 0.27 0.26 D max, µm 67.52 58.95 200.00 77.34 101.46 SSA 1, m2/m3 8 701 5 857 18 294 14 825 12 215 Density, kg/m3 2650 2650 2110 2440 2550 1 SSA was calculated from the particle size distribution, making an assumption about the spherical shape.

The microfillers were characterized by different particle morphologies and grading. The lowest values of maximum particle size, Dmax (67.52 µm and 58.95 µm), were registered in the case of quartz powders (Q1 and Q2). On the contrary, siliceous fly ash (FAS) contained the largest particles (size up to 200 µm), and its particle size distribution showed the highest values of the mode and median (4.96 µm and 25.30 µm, respectively). In the case of fly ashes from fluidized combustion (FAF and FAF2), the mode was 0.25 µm, which was 20 times lower compared to siliceous fly ash, while the median Materials 2020, 13, 1207 6 of 12

Materials 2020, 13, x FOR PEER REVIEW 6 of 12 was 0.26 µm or 0.27 µm—values two orders of magnitude lower than in the case of siliceous fly ash. It is worthTable noting 1. Statistical that the parameters grading of describi bothng fluidized the particle fly size ashes distributi wason described and specific by surface a bimodal area distribution. The abovementioned(SSA) of fillers; mode Q1, Q2 value, quartz applied powders; toFA theS, siliceous increased fly ash; number FAF, FAF2, of fluidized particles fly ashes. smaller than 1 µm. A similarRaw Material mode value Quartz (0.28 Sandµm) was Quartz observed Sand in the case Coal of quartz powder Coal Q1, which Lignite was why fly ashes from FBC seemed to be more similar to this conventionalConventional quartz fillerFBC in terms of grading.FBC The fly Production Grinding Grinding ash originating from lignite fluidized bed combustionCombustion (FAF2) was Combustion slightly thicker Combustion and had a smaller specific surface area thanFiller the fly ash fromFiller hard coal fluidizedFiller bed combustionFiller (FA ).Filler Both, however, Parameter F had a more developedQ specific1 [20] surface areaQ2 than quartzFA powders.S [20] Meanwhile,FAF [20] in terms ofFA morphology,F2 siliceousD min, fly µm ash containing0.26 spherical grains1.32 was more0.17 similar to quartz0.12 powders, which0.17 contained angularMode, grains µm resulting0.28 from mechanical2.45 milling of quartz.4.96 0.25 0.25 Median, µm 2.44 7.18 25.30 0.27 0.26 The density was also determined for all microfillers. The test was performed in a Le Chatelier D max, µm 67.52 58.95 200.00 77.34 101.46 volumenometer (consisting of a flat-bottomed flask) according to the procedure described in the EN SSA 1, m2/m3 8 701 5 857 18 294 14 825 12 215 1936Density, standard. kg/m The3 results2650 of density are2650 given in Table21101. 2440 2550 Polymer1 SSA wasconcrete-like calculated fromcomposites the particle size prepared distributi foron, making testing an contained assumption vinyl-ester about the spherical binder, shape. conventional fine (S) and coarse (G) aggregate, and a mix of quartz powder (Q1 or Q2) and chosen fly ash (FAS, FAF, or Polymer concrete-like composites prepared for testing contained vinyl-ester binder, conventional FAF2). The quantitative compositions of tested composites are presented on Figure5. The compositions fine (S) and coarse (G) aggregate, and a mix of quartz powder (Q1 or Q2) and chosen fly ash (FAS, FAF, were determined according to the statistical Box design (variant of the CCD design [24]) assuming or FAF2). The quantitative compositions of tested composites are presented on Figure 5. The threecompositions factors that were were determined expressed according as the mass to the ratios statis oftical the Box components. design (variant The of firstthe CCD variable design was [24]) the ratio of theassuming amount three of factors binder that and were the expressed amount as of the basic mass aggregate ratios of the (i.e., components. gravel and The sand), first variable B/(G + wasS), in the rangethe 6.0–10.0.ratio of the The amount second of binder variable and the was amount the ratio of basic of theaggregate amount (i.e., of gravel binder and and sand), the B/(G total + S), amount in of microfillerthe range fraction 6.0–10.0. (including The second quartz variable powder was the and ratio fly of ash), the amount B/(FA + ofQ), binder in the and range the total of 0.4–0.6. amount Theof third variablemicrofiller was fraction the ratio (including of the amount quartz powder of fly ashand fly and ash), the B/(FA amount + Q), of in microfillerthe range of fraction,0.4–0.6. The FA third/(FA + Q), in thevariable range was of 0.0–1.0.the ratio Theof the use amount of such of fly a design ash and allowed the amount obtaining of microfiller composites fraction, of FA/(FA various + quantitativeQ), in the range of 0.0–1.0. The use of such a design allowed obtaining composites of various quantitative composition, which were predestined for statistical analysis. The quantitative compositions were composition, which were predestined for statistical analysis. The quantitative compositions were identical to the compositions of concretes tested in previously mentioned studies, so it was possible to identical to the compositions of concretes tested in previously mentioned studies, so it was possible to comparecompare the the results. results.

FigureFigure 5. 5.Vinyl-ester Vinyl-ester concretes compositions compositions (% (%by bymass) mass) based based on the on statistical the statistical Box design Box design (experimental(experimental CCD CCD designdesign of of three three factors factors expressed expressed as the as mass the ratios mass of ratios components: of components: B/(G + S) in B/(G + S) inrange range of 6.0–10.0 of 6.0–10.0, , B/(FA B/ (FA+ Q) +in Q)the inrange the of range 0.4–0.6, of 0.4–0.6,and FA/(FA and + FAQ) /in(FA the+ rangeQ) in of the 0.0–1.0). range of 0.0–1.0).

SpecimensSpecimens tested tested in inthe the presentedpresented research research were were the the halves halves of prisms of prisms with dimensions with dimensions of 40 × 40 of × 40 40 × 160160 mm mm33 remainingremaining after after the the flexural flexural test test (there-poi (there-pointnt bending bending test). test). The Thepresented presented compressive compressive × strengthstrength values values were were the the average average (of(of 4 or or 2 2 results). results). Specimens Specimens were were stored stored in the in laboratory the laboratory conditions conditions for periods of 14 days (time of curing of the polymer concretes recommended in the EN 1542 standard), for periods of 14 days (time of curing of the polymer concretes recommended in the EN 1542 standard), 9 years in the case of concretes with fly ashes remaining from hard coal combustion (FAF and FAS), or 9.5 years in the case of concretes with fly ash remaining from lignite combustion (FAF2). Materials 2020, 13, 1207 7 of 12 Materials 2020, 13, x FOR PEER REVIEW 7 of 12

9 yearsThe changein the case in theof concretes compressive with fly strength ashes remaining in time from was hard considered coal combustion the measure (FAF and of theFAS), long-term or performance.9.5 years in the It was case legitimateof concretes to with speak fly ash of “change”remaining from in compressive lignite combustion strength, (FAF2 because). the specimens The change in the compressive strength in time was considered the measure of the long-term originated from the same prisms, and the results obtained for the halves of the same prisms were performance. It was legitimate to speak of “change” in compressive strength, because the specimens compared. Each time, one half of a prism was compressed after 14 days, and the other half was stored originated from the same prisms, and the results obtained for the halves of the same prisms were andcompared. destroyed Each after time, 9 or one 9.5 half years. of a prism was compressed after 14 days, and the other half was stored andBefore destroyed the destructive after 9 or 9.5 testsyears. were carried out, the specimens were also examined in terms of the volumetricBefore density. the destructive The density tests waswere determined carried out, accordingthe specimens to thewere method also ex describedamined in terms in the of EN the 12390-7 standard.volumetric The density. density The was density calculated was determined on the basisaccording of measurements to the method described of mass in and the volumeEN 12390-7 obtained by waterstandard. displacement The density (i.e.,was calculated the method on the for basis determining of measurements the density of mass of irregularly-shaped and volume obtained specimens; by in thiswater particular displacement case, (i.e., the the abovementioned method for determining halves ofthe prisms density remaining of irregularl aftery-shaped flexural specimens; test). in this particular case, the abovementioned halves of prisms remaining after flexural test). 4. Results and discussion 4. Results and discussion 4.1. Compressive Strength of Vinyl-ester Concretes with Fly Ash from Lignite Combustion 4.1. Compressive Strength of Vinyl-ester Concretes with Fly Ash from Lignite Combustion The results of compressive strength tests of concretes containing various contents of vinyl-ester The results of compressive strength tests of concretes containing various contents of vinyl-ester binder and particular fillers, including quartz powder Q2 and fly ash of lignite FAF2 (in amounts binder and particular fillers, including quartz powder Q2 and fly ash of lignite FAF2 (in amounts as as shownshown in the Figure Figure 5),5), are are given given in in the the Figure Figure 6.6 For. For an an easier easier discussion, discussion, the composites the composites were were groupedgrouped in in terms terms of of the the share share of fly fly ash ash in in the the microf microfilleriller (values (values in the in round the round brackets) brackets) and, generally, and, generally, thethe amount amount of of fly fly ash ash in in the the composite.composite.

Figure 6. Compressive strength of vinyl-ester concretes of various mass contents of ﬂy ash remaining Figure 6. Compressive strength of vinyl-ester concretes of various mass contents of fly ash remaining fromfrom lignite lignite FBC FBC obtained obtained afterafter 14 days days and and 9.5 9.5 years; years; FA, FA, fly ﬂyash; ash; PC, PC,polymer polymer concrete. concrete.

AsAs in in the the case case of of vinyl-ester vinyl-ester concretes concretes containing containing fly flyashes ashes remaining remaining from fromhard coal hard combustion, coal combustion, alsoalso in thein casethe case of analogous of analogous composites composites containing containing identical identical amounts amounts of fly of ash fly from ash lignitefrom lignite combustion, andcombustion, in the case and of a numberin the case of compositesof a number with of co evenmposites more with diverse even quantitative more diverse compositions, quantitative it was confirmedcompositions, that theit was compressive confirmed strengththat the ofcompressive such concrete-like strength of polymer such concrete-like composites polymer significantly increasedcomposites with significantly time. increased with time. TheThe increase increase in in strength strength of the tested tested composit compositeses ranged ranged from from 9.5 MPa 9.5 MPato even to even27.5 MPa 27.5 (on MPa (on averageaverage by by 16.6 16.6 MPa), MPa), whichwhich corresponded corresponded to topercentage percentage changes changes in the in range the rangeof 9.1%–28.8%. of 9.1–28.8%. In the In the casecase of initiallyof initially very very weak weak concreteconcrete containing containing a asmall small amount amount of vinyl-este of vinyl-esterr resin resin(10% by (10% composite by composite mass) and almost 80% of fluidized fly ash in the microfiller (Composition ID No. 8), which after 14 mass) and almost 80% of fluidized fly ash in the microfiller (Composition ID No. 8), which after 14 days of curing as characterized with compressive strength of 14.8 MPa, after 9.5 years, it turned out to be twice as strong (31.9 MPa). There was no clear explanation as to how the development of compressive strength was progressing for all the composites of various quantitative compositions. However, analyzing the obtained data, Materials 2020, 13, x FOR PEER REVIEW 8 of 12 days of curing as characterized with compressive strength of 14.8 MPa, after 9.5 years, it turned out toMaterials be twice2020 ,as13 ,strong 1207 (31.9 MPa). 8 of 12 There was no clear explanation as to how the development of compressive strength was progressingone could recognize for all the that composites vinyl-ester concretesof various of quantitative the same range compositions. of substitution However, quartz microfiller analyzing with the obtainedfly ash (FA data,/M =one21% could or FA recognize/M = 50%) that showed vinyl-ester similar concretes tendencies of the (with same the range test of probability substitutionp-value quartz= microfiller0.95, α = 0.05) with with fly aash good (FA/M correlation = 21% (Ror >FA/M0.8): = the 50%) more showed fly ash similar in the composite,tendencies the(with smaller the test the probabilityincrease in strengthp-value = over 0.95, time α = 0.05) (Figure with7). a good correlation (R > 0.8): the more fly ash in the composite, the smaller the increase in strength over time (Figure 7).

Figure 7. TheThe increase increase in in compressive compressive strength strength of vinyl-ester vinyl-ester concretes with various content of fly fly ash of lignite FBC registered after 9.5 years in relation to flyfly ash mass content; the tendencies (statistical significancesignificance levellevelα α= 0.05)= 0.05) were were determined determined separately separately for concretes for concretes where 21%where of the21% quartz of the microfiller quartz microfillerwas substituted was substituted by fly ash by (FA fly/M ash= (FA/M21%) and= 21%) concretes and concretes where where half of half the of quartz the quartz microfiller microfiller was wassubstituted substituted by fly by ash fly (FAash/ M(FA/M= 50%). = 50%).

This effect, however, could be easily explained. Fly ash FAF2 was much finer than quartz powder This effect, however, could be easily explained. Fly ash FAF2 was much finer than quartz powder Q2, which it replaced (compare the appropriate median and mode values listed in Table1), and therefore, Q2, which it replaced (compare the appropriate median and mode values listed in Table 1), and the fly ash was a component with the finest grains. Thus, the more the fly ash was in the composite, therefore, the fly ash was a component with the finest grains. Thus, the more the fly ash was in the the easier it filled the inter-grain voids of the other fillers’ fractions. This meant that the microstructure composite, the easier it filled the inter-grain voids of the other fillers’ fractions. This meant that the of concrete with a higher content of fly ash was tighter at the start and left less possibility for its physical microstructure of concrete with a higher content of fly ash was tighter at the start and left less densification of the structure in later times, so the increase in compressive strength over time should possibility for its physical densification of the structure in later times, so the increase in compressive also be smaller. strength over time should also be smaller. Figure8 presents the micrographs of two vinyl-ester concretes with di fferent fluidized fly ash Figure 8 presents the micrographs of two vinyl-ester concretes with different fluidized fly ash contents (21% and 79%) in microfiller fraction taken shortly after the specimens’ preparation (in 2010). contents (21% and 79%) in microfiller fraction taken shortly after the specimens’ preparation (in 2010). The SEM micrographs were made with a Hitachi TM-1000 Scanning Electron Microscope using a The SEM micrographs were made with a Hitachi TM-1000 Scanning Electron Microscope using a BSE BSE detector. Before making the micrographs, the specimens were impregnated in epoxy resin (in a detector. Before making the micrographs, the specimens were impregnated in epoxy resin (in a vacuum chamber) and prepared by multi-stage grinding (in the presence of diamond suspensions) vacuum chamber) and prepared by multi-stage grinding (in the presence of diamond suspensions) and polishing. Surface preparation was carried out using the Struers TegraPol-21 grinding-polishing and polishing. Surface preparation was carried out using the Struers TegraPol-21 grinding-polishing machine and the Struers TegraDoser-5 diamond suspension dispensing kit. machine and the Struers TegraDoser-5 diamond suspension dispensing kit. The SEM micrographs showed that the microstructure of vinyl-ester concrete with a higher content The SEM micrographs showed that the microstructure of vinyl-ester concrete with a higher contentof fly ash of andfly ash lower and content lower content of quartz of powderquartz powder in the microfiller in the microfiller fraction fraction (FA/M =(FA/M79%, Q= /79%,M = Q/M21%) =was 21%) well was compacted, well compacted, and very fineand particlesvery fine of particles fly ash were of fly regularly ash were distributed regularly in thedistributed polymer in phase. the polymerIn the case phase. of vinyl-ester In the case concrete of vinyl-ester in which concre quartzte powder in which predominated quartz powder in the predominated microfiller (FA in/M the= microfiller21%, Q/M =(FA/M79%), = one 21%, could Q/M see = 79%), that betweenone could the see large that angularbetween grains the large of quartz, angular there grains were of quartz, spaces therefilled were with aspaces polymer filled less with saturated a polymer with less mineral saturated filler. with mineral filler.

Materials 2020, 13, 1207 9 of 12

Materials 2020, 13, x FOR PEER REVIEW 9 of 12

(a) (b)

FigureFigure 8. SEM 8. SEM micrographs micrographs (magnification: (magnification: 6000×) 6000 )of of vinyl- vinyl-esterester composites composites with with various various contents contents of of fly ash FAF: (a) composite with 21% content of fly× ash in the microfiller (ID: No. 3 acc. to Figure 5); (b) fly ash FAF:(a) composite with 21% content of fly ash in the microfiller (ID: No. 3 acc. to Figure5); (b) compositecomposite with 79% 79% content content of offly fly ash ash in microfiller in microfiller (ID: (ID:Np. 7 Np. acc. 7to acc. Figure to Figure5) 5)

4.2. Development4.2. Development of Compressiveof Compressive Strength Strength ofof Vinyl-ester Concretes Concretes with with Various Various Fly FlyAshes Ashes TheThe mechanical mechanical tests tests results results obtained obtained for concrete concrete with with fly fly ash ash of lignite of lignite FBC FBC (Figure (Figure 6) clearly6) clearly showed that in the case of such a filler, the increase of the compressive strength of polymer concrete showed that in the case of such a filler, the increase of the compressive strength of polymer concrete proceeded for a longer time. However, these results did not answer the question about when there proceededwas a progressive for a longer increase time. However, in the compressive these results strength did notof vinyl-ester answer the conc questionretes with about fly whenashes thereand was a progressivewhether it increaseended before in the the compressive test date (i.e., strength earlier than ofvinyl-ester 9.5 years after concretes the composites with fly were ashes made). and whetherTo it endedexplore before this theissue, test it dateis worth (i.e., earlierlooking thanat the 9.5 additional years after results the composites recently obtained were made). for identical To explore thiscomposites issue, it is (in worth qualitative looking and at quantitative the additional compos resultsition) recently as discussed obtained in the for article identical cited compositesin the (in qualitativeIntroduction. and For quantitative vinyl-ester conc composition)retes with asfly discussedashes originatin in theg articlefrom hard cited coal in thecombustion Introduction. For vinyl-ester(specifically, concretes Specimen with ID No. fly 3 ashes and No. originating 7 accordin fromg to hardFigure coal 5), combustionnew compressive (specifically, strength tests Specimen ID No.were 3 andcarried No. out 7 nine according years after to Figure the specimens’5), new compressive preparation, and strength the results tests were compared carried out with nine the years afterdata the specimens’obtained after preparation, 14 days and and after the seven results years were (data compared publishedwith in [20]). the Table data obtained2 contains after such14 a days data sheet. Moreover, the table contains data obtained for concrete with the same quantitative and after seven years (data published in [20]). Table2 contains such a data sheet. Moreover, the table composition, but with fly ash remaining from lignite combustion. contains data obtained for concrete with the same quantitative composition, but with fly ash remaining from ligniteTable combustion. 2. Compressive strength of chosen vinyl-ester composites (ID: No. 3 and No. 7 acc. to Figure 5)

with various types of fly ash (FAS, siliceous fly ash; FAF, fly ash from hard coal FBC; FAF2, fly ash from Tablelignite 2. Compressive FBC) obtained strength after 14 ofdays, chosen 7 years, vinyl-ester and 9 years composites or after 14 (ID:daysNo. and 39.5 and years. No. 7 acc. to Figure5) with various types of fly ash (FA , siliceous fly ash; FA , fly ash from hard coal FBC; FA , fly ash from S Age: F Age: Age: F2 lignite FBC) obtained after 14 days, 7 years, and 9 years or after 14 days and 9.5 years. Fly FA/M, FA/PC, 14 Days 7 Years 9 or 9.5 Years Δ7/Δ9/9.5, ID Ash % kg/kg fc, Age:CV, fc, Age:CV, fc, Age:CV, % MPa 14 Days% MPa 7 Years% MPa9 or 9.5 Years% FA/PC, Fly Ash ID FA/M, % ∆7/∆9/9.5,% FAS kg/kg 94.2fc , 3.3CV, 103.2fc, 0.0CV, 120.0fc, 3.3CV, 9.6/27.4 FAF № 3 21 5.3 101.0MPa 1.2% 104.4MPa 0.5% 115.3MPa 7.1% 3.4/14.2 1 FAFAF2S 104.394.2 8.4 3.3 103.2- 0.0- 122.4 120.0 1.4 3.3 - 9.6/17.4/27.4 FAFAFS № 3 21 5.3 96.6101.0 3.6 1.2 104.4 - 0.5- 105.0 115.3 4.7 7.1 - 3.4 /8.7/14.2 FA 104.3 8.4 - - 122.4 1 1.4 - /17.4 FAF2F № 7 79 20.1 90.7 4.8 110.8 0.0 107.5 1.9 22.2/18.5 FAFAF2S 94.096.6 3.5 3.6 - -- -105.2 105.0 1 9.7 4.7 - /11.9 - /8.7 FAF № 7 791 Specimens 20.1 with fly90.7 ash from 4.8lignite FBC 110.8 tested after 0.0 9.5 years. 107.5 1.9 22.2/18.5 1 FAF2 94.0 3.5 - - 105.2 9.7 - /11.9 In the case of the 1firstSpecimens series withof composites fly ash from (ID: lignite No. FBC 3 acc. tested to afterFigure 9.5 years.5) of identical quantitative composition differing only in the type of fly ash, a greater increase in strength (both after seven and nine years), was found when using siliceous fly ash FAS (Δ7 = 9.6%, Δ9 = 27.4%) than in case of fly ash In the case of the first series of composites (ID: No. 3 acc. to Figure5) of identical quantitative from fluidized hard coal combustion, FAF (Δ7 = 3.4%, Δ9 = 14.2%). However, it should be noted that composition differing only in the type of fly ash, a greater increase in strength (both after seven and nine years), was found when using siliceous fly ash FAS (∆7 = 9.6%, ∆9 = 27.4%) than in case of fly ash from fluidized hard coal combustion, FAF (∆7 = 3.4%, ∆9 = 14.2%). However, it should be noted Materials 2020, 13, 1207 10 of 12 that the strength values of composites determined after the same longer time were almost identical, both after seven years (103.2 MPa and 104.4 MPa) and after nine years (120.0 MPa and 115.3 MPa). An analogous concrete with FA2 fly ash from lignite combustion tested after 9.5 years was characterized with practically imperceptibly higher strength of 122.4 MPa. In case of the second series of composites (Specimen ID No. 7 acc. to Figure5), the compressive strength determined after nine or 9.5 years for composites with various fly ashes also adopted very close values (differences up to 2.5 MPa). This may indicate that a period of nine years was sufficient to obtain a full range of strength. When it came to the period of seven years, it was difficult to formulate one conclusion. Again, the amount of the finest fraction—the fly ash—should be taken into account. For a series of composites containing less fly ash, i.e., 21% fly ash in the general microfiller fraction (5.3% of the total composite mass, Specimen ID No. 3), the difference between strength tested after seven and nine years was significant. Meanwhile, for concrete containing much fluidized fly ash, i.e., 79% of microfiller fraction (20.1% of the composite mass, Specimen ID No. 7), no increase in strength between seven and nine years was noted (the values were very close—110.8 MPa and 107.5 MPa). Thus, in the last case, seven years seemed to be sufficient time to achieve a full strength. For each polymer concrete compressive strength test, the coefficient of variation (within-batch variation) was calculated. The CV ranged from 1.4–7.1% for composites containing less fly ash and 1.9–9.7% for concretes richer in fly ash. Such values could be considered satisfactory if they were assessed in the context of ordinary concrete with a similar strength, whereas given other composite material and specimens sizes, there are no particular guidelines for such an assessment [25,26]. One can expect that the numerous grains of the finest fillers, thus the smallest particles of microfiller fraction, could be the physical obstacles to the free formation of structures of polymerized chains of the binder; thus, the polymerization proceeded slower, and therefore, the polymer matrix strengthened for a longer time before the composite obtained its final properties. Moreover, the additional cross-linking of the polymer could have been initialized by calcium compounds, which were present in significant amounts in the by-products of fluidized combustion. This would explain why, in the case of a composite richer in fluidized fly ash, compressive strength stabilized faster.

4.3. Volumetric Density Conclusions regarding the potential densification of the microstructure deduced on the basis of the compressive strength gain in time were confirmed by the results of the measurement of the density of composites. Table3 contains data regarding the change in volumetric density over time of the abovementioned composites with fly ashes remaining after coal combustion (FAS and FAF) at the age of 14 days and nine years. One can observe that in the case of composites in which more quartz powder (with a density of 2650 kg/m3) was replaced by lighter siliceous fly ash (with density of 2110 kg/m3) or fluidized fly ash (with a density of 2440 kg/m3), i.e., composites with a lower initial volumetric density, the increase in density was greater.

Table 3. Density of composites with various fly ashes (FAS, siliceous fly ash; FAF, fluidized fly ash) determined after 14 days and 9 years.

Age: Age: Gain 14 Days 9 Years FA/PC, Fly Ash ID FA/M, % kg/kg D, CV, D, CV, ∆D, ∆D, kg/m3 % kg/m3 % kg/m3 % FA 2149 0.7 2201 3.7 52 2.4 S № 3 21.0 5.3 FAF 2162 0.24 2185 0.5 23 1.1 FA 2112 2.3 2201 2.2 89 4.2 S № 7 79.0 20.1 FAF 2102 1.4 2188 0.8 86 4.1 Materials 2020, 13, 1207 11 of 12

5. Conclusions The following conclusions emerged from the investigations of the long-term compressive strength of vinyl-ester concretes with various microﬁllers:

1. The results confirmed that in the case of tested polymer concretes prepared with the use of commercially available vinyl-ester resin as a binder, a significant improvement of compressive strength after a period of several years was noted. This was evidence that vinyl-ester concretes were durable polymer composites when analyzed in terms of mechanical strength criterion. 2. The increase in compressive strength was noted regardless of the characteristic or origin of the microfiller used, i.e., in the case of commonly used commercial quartz powders and when using by-products remaining after combustion of various fossil fuels in various installations (including fly ashes from FBC, whose impact on the long-term properties of polymer composites had not been recognized until this point). 3. As shown, the increase in compressive strength continued for several years; however, the values of strength stabilized for different composites after different times, depending on the amount of microfiller the vinyl-ester binder was saturated with: the less fly ash, the longer the time until strength stabilization. Nonetheless, the obtained results indicated that the period of nine years was sufficient to obtain a full range of strength of tested PC. Moreover, in cases of composite rich in fluidized fly ash, the shorter period of seven years seemed to be sufficient. 4. The morphology, particles size distribution, and density of the microfillers used affected microstructure and, as a consequence, the volumetric density of hardened composites: the more fly ash in the composite, the lower the initial volumetric density of PC, but the greater the increase in density with time. After nine years, the density of all tested concretes adopted had very similar values of ca. 2200 kg/m3.

To explain the phenomena, the physical eﬀects of which were the subject of this paper, the author plans additional long-term research focused on changes in the internal structure and microstructure of polymer concretes for the various stages of their service-life.

Funding: This research was supported by the Faculty of Civil Engineering of Warsaw University of Technology. Conﬂicts of Interest: The author declares no conﬂict of interest.

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