Effect of Microwave Radiation on the Compressive Strength of Solid Ceramic Brick

Ci a ion: P ˚uša, D.; Šuhajda, K.; Žajdlík, T.; S obodo á, K.; Š ’as ník, S.; Hobzo a, K.; Venk bec, V. E ec o Mic owa e Radia ion on he Comp essi e S eng h o Solid Ce amic B ick. Buildings 2023,13, 1018. h ps://doi.o g/10.3390/ buildings13041018 Academic Edi o : Tomasz Sadowski Recei ed: 7 Ma ch 2023 Re ised: 30 Ma ch 2023 Accep ed: 11 Ap il 2023 Published: 13 Ap il 2023 Copy igh : © 2023 by he au ho s. Licensee MDPI, Basel, Swi ze land. This a icle is an open access a icle dis ibu ed unde he e ms and condi ions o he C ea i e Commons A ibu ion (CC BY) license (h ps:// c ea i ecommons.o g/licenses/by/ 4.0/). buildings A icle E ec o Mic owa e Radia ion on he Comp essi e S eng h o Solid Ce amic B ick Da id P ˚uša 1,* , Ka el Šuhajda 1, Tomáš Žajdlík1, Ka eˇ ina S obodo á1, S anisla Š ’as ník1, Kla a Hobzo a 1 and Vacla Venk bec 2 1Ins i u e o Building S uc u es, Facul y o Ci il Enginee ing, B no Uni e si y o Technology, Ve eˇ í331/95, 602 00 B no, Czech Republic; [email p o ec ed].cz (K.Š.); [email p o ec ed].cz (T.Ž.); 234215@ u b .cz (K.S.); [email p o ec ed].cz (S.Š.); 157465@ u b .cz (K.H.) 2Depa men o Building, Ene gy and Ma e ial Technology, Facul y o Enginee ing Sciences & Technology, UiT The A c ic Uni e si y o No way, Lod e Langesga e 2, N-8514 Na ik, No way; acla [email p o ec ed] *Co espondence: da id.p usa@ u b .cz; Tel.: +420-737-459-492 Abs ac : Mic owa e adia ion is widely u ilized in cons uc ion p ac ice, especially o d ying building ma e ials, emedia ing damp mason y, o s e iliza ion o bio ic pes s ha ha e in es ed building s uc u es. The a ailable scien i ic and echnical li e a u e epo s ha ce ain ma e ials exposed o mic owa e adia ion do no change hei physical and mechanical p ope ies, al hough his has no ye been adequa ely e i ied. This pape builds on many yea s o esea ch in he a ea, adding o and e ining exis ing in o ma ion, p o iding new insigh s in o he mechanical and physical p ope ies o commonly a ailable building ma e ials ha ha e been exposed o con olled mic owa e adia ion, and compa ing hem wi h e e ence alues. The expe imen al esea ch on a se o clay b ick samples is ca ied ou using es s acco ding o Eu opean s anda ds, and i ocuses on he e ec o mic owa e adia ion on comp essi e s eng h and wa e abso p ion. The expe imen al samples we e compa ed o e e ence samples ha had no been subjec ed o p olonged exposu e o mois u e and we e subsequen ly ea ed wi h mic owa e adia ion o dehumidi ica ion. Keywo ds: ce amic; b ick; mic owa e exposu e; adia ion; ma e ial d ying; non-des uc i e es ing; ul asound; mechanical p ope ies; comp essi e s eng h; mason y es o a ion; mois u e elimina ion; s e iliza ion 1. In oduc ion Add essing mois u e in building s uc u es is one o he mos ele an opics in he cons uc ion indus y. The e is a ange o me hods o p e en ing mois u e ing ess in o s uc u es and o he ac ual emo al o mois u e om building ma e ials. These me hods can be oday conside ed ai ly e ec i e, bu i is necessa y o de elop hem u he and o come up wi h mo e economical and, in pa icula , mo e en i onmen ally iendly solu ions. Mois u e p oblems in building ma e ials a e pa icula ly se ious because he wa e con ained in hem g ea ly a ec s hei physical and mechanical p ope ies. Cu en ly, he e a e mul iple me hods ha a e used o emo ing mois u e om building ma e ials in he Czech Republic and Eu ope in gene al, which mee mos o he equi emen s o oday’s building p ac ice. The basic c i e ia o hei use a e he speed and e iciency o d ying, he economic demands o he p ocess, he cos o necessa y equipmen , and he o e all en i onmen al iendliness o he me hod. Commonly used me hods o d ying building ma e ials a e e icien , bu he d ying ime can be a he long, and hei applicabili y and sui abili y depend on he ype o building ma e ial. One app oach o d ying building ma e ials is h ough he use o mic owa e adia ion. This me hod has been known since abou 1945, bu in cons uc ion p ac ice, i is cu en ly s ill no widely used. This is due o he highe pu chase cos o mic owa e gene a o s and Buildings 2023,13, 1018. h ps://doi.o g/10.3390/buildings13041018 h ps://www.mdpi.com/jou nal/buildings Buildings 2023,13, 1018 2 o 11 he lack o knowledge o he basic p inciples o mic owa e d ying, which is also ela ed o heal h conce ns abou exposu e o his ype o adia ion. A combina ion o hese easons, oge he wi h low awa eness abou he me hod, esul s in limi ed in e es in he use o mic owa e adia ion in cons uc ion p ac ice. 1.1. Li e a u e Re iew The o igins o using mic owa e adia ion da e back o he i s hal o he 20 h cen u y, al hough i s exis ence was p edic ed as ea ly as 1865 by James Cle k Maxwell [ 1 ]. Augus Žáˇcek, a p o esso a he Facul y o Science o Cha les Uni e si y in P ague, was among he i s o desc ibe he p inciple o magne on oscilla ion in 1924; in a o eign jou nal, his disco e y was published in 1928, and since hen, he has been conside ed he in en o o magne on [ 2 ]. I was no un il he end o he wa and a e wa d ha he subjec o mic owa e hea ing came up in ea nes , and one o he mos impo an people in ol ed was Pe cy L. Spence , who was in e es ed in consume and comme cial mic owa e o ens. He iled his i s pa en on he mic owa e hea ing o ood in Oc obe 1945. His associa es desc ibe he disco e y as a g adual ial-and-e o p ocess, wi h expe imen s such as making co n pop o eggs explode. Pe cy Spence wo ked o Ray heon, whe e he main ocus was on he in en ion o he mic owa e o en, bu o he companies also esea ched mic owa e adia ion by jus concen a ing mo e on indus ial applica ions; o example, in 1947, a pape was published ha add essed p ocess accele a ion h ough mic owa es [ 3 ]. La e , in he 1960s and 70s, he de elopmen o mic owa e echnology ad anced so much ha he i s household mic owa e o ens began o appea . Today almos e e y home has a mic owa e, and he p inciple o mic owa e hea ing is used in many o he indus ies ou side he ood sec o [ 4 ]. I has been widely used, o example, in medicine [ 5 ], he a o emen ioned ood indus y [ 6 ], he mili a y [ 7 ], as well as wi hin he cons uc ion indus y [ 8 ]. Wi hin his indus y, a numbe o s udies ha e al eady been ca ied ou con i ming he po en ial use o mic owa es o d ying building ma e ials [9–12]. 1.1.1. Mic owa es Mic owa es a e a pa o elec omagne ic adia ion wi h a equency be ween 300 MHz and 300 GHz, co esponding o wa eleng hs be ween 1 m and 1 mm. Fo indus ial pu poses, mo e equencies a e pe mi ed, bu in cons uc ion, he globally used equency was 2.45 GHz wi h a co esponding wa eleng h o 12.2 cm. Mic owa es belong o a b oad spec um o elec omagne ic wa es, which also co e , o example, he isible ligh ange. Thei beha io is desc ibed by Maxwell’s equa ions. I holds ha a any gi en poin and any gi en ins an , he ec o s cha ac e izing magne ic and elec ic ields a e pe pendicula o each o he , and bo h a e pe pendicula o he di ec ion o mo ion o he wa e [8,13]. Mic owa e adia ion causes hea ing, and he molecules become o ien ed acco ding o hei pola i y in he elec ic ield. When mic owa e adia ion comes in o con ac wi h wa e molecules, he elec omagne ic ene gy ans o ms, and hea ing occu s. This is ollowed by he hea ing o he cons uc ion ma e ials [8]. Mic owa es all in he ange o elec omagne ic wa es wi h a lowe equency han sola adia ion, so hey do no lea e any esidual adia ion ha is ha m ul o heal h. The use o mic owa e de ices is comple ely sa e, and any damage o heal h can only occu om di ec exposu e om a ew cm o a leas se e al minu es, ei he made in en ionally o by he imp ope handling o he de ice [8]. Excessi ely s ong mic owa e adia ion poses he highes isk o human heal h. The use o mic owa e equipmen wi h adia o s emi ing such elec omagne ic adia ion ou side poses a se ious haza d o people in he a ea o high mic owa e ene gy densi y. The pe mi ed le els o elec omagne ic ield s eng h in he ange o 2.45 GHz a e es ablished by EU egula ions, including Di ec i e 2004/40/EC and Recommenda ion 1999/510/EC, as well as by coun y-speci ic egula ions. The egula ions ypically se he pe missible elec omagne ic ield s eng h ( om 7 V/m o 61 V/m) o wa densi y ( om 0.1 W/m 2 o 10 W/m2). Buildings 2023,13, 1018 3 o 11 1.1.2. Theo y o Elec omagne ic Field The undamen al heo y o he elec omagne ic ield was based on a numbe o physical laws d awn om expe imen s and indings linked o names such as Coulomb, Sa a , Ampe e, and Fa aday. Fa aday’s wo k, pa icula ly he disco e y o elec omagne ic induc ion, and he wo k o Maxwell, who de eloped a model o he elec omagne ic ield ollowing he disco e y o he concep o a displacemen cu en , we e o majo impo ance o he de elopmen o he heo y [14]. The gene al elec omagne ic ield, na u al o man-made, is non-s a iona y ( he e is cons an ime a iabili y). The a iabili y o he ield can o en be conside ed ma ginal o slow. Following his simpli ica ion, he ield can be classi ied in o ou ypes: (a) a s a ic ield, whe e all cha ges a e conside ed o be a es ; (b) a s a iona y ield gene a ed by cha ges ha o m s a iona y cu en s; (c) a quasi-s a iona y ield, which is a simpli ica ion o he gene al non-s a iona y ield because he so-called displacemen cu en agains he ee elec on cu en s is dis ega ded he e; and (d) a non-s a iona y ield, i.e., a gene al elec omagne ic ield [14]. 1.1.3. Applica ions in he Cons uc ion Indus y The e a e mul iple applica ions o mic owa e ene gy wi hin he cons uc ion indus y. I is used o d ying building ma e ials and s uc u es [ 15 ], accele a ing he solidi ica ion o mix u es [ 16 ], o he s e iliza ion o bio ic pes s [ 17 ], o e en o mois u e measu e- men s [18]. The ma e ial o be hea ed is subjec ed o a high- equency elec omagne ic ield, causing he pola iza ion o molecules, conduc ion, and magne ic p ocesses. The in e nal ene gy o he molecules g adually inc eases, which causes he ma e ial o be hea ed. Du ing pe ec mic owa e hea ing, he dis ibu ion o he in ensi y o he mic owa e ield is in he a ea comple ely e en, and hea is gene a ed uni o mly ac oss he en i e olume o he ma e ial, bu in a eal si ua ion, his is no he case. The p esence o wa es depends on he design o he equipmen used and he ype o ma e ial inside he hea ing chambe . The amoun o ene gy abso bed a ies wi h he size, shape, dielec ic cons an , and pe mi i i y o he ma e ial. In addi ion o hese p ope ies o he hea ed objec i sel , pa icula ly impo an a e he equency and in ensi y o he elec omagne ic ield. The hea gene a ed in he objec s sp eads o he su oundings h ough hea ans e . I mic owa e ene gy is applied o he ma e ial o oo long, i can lead o o e hea ing and damage o he objec [ 16 ]. One o he ad an ages o mic owa e adia ion is selec i e hea ing. Componen s we e hea ed acco ding o hei abili y o abso b elec omagne ic ene gy; he e o e, mainly, he mos abso ben one was hea ed, which is usually loosely bound wa e . This componen hen hea s he o he ma e ial componen s, which esul s in a mo e in-dep h hea ing o he objec . Mic owa e ene gy can hea up he ma e ial mo e e ec i ely compa ed o egula hea ing om he su ace. I was used o he d ying o a ious ma e ials and he accele a ion o he ha dening o mix u es. Disad an ages o his me hod include highe ene gy consump ion, possible changes in he mechanical p ope ies o he objec , and possible local o e hea ing due o he inconsis ency o he mic owa e ield and he inhomogenei y o he hea ed ma e ial. Las bu no leas , he e is also he equi emen o p o iciency while wo king wi h EMW adia ion [16,19]. The mic owa e d ying/ha dening p ocess can be di ided in o ou s ages. The i s one is he ac ual hea ing o he wa e molecules in conjunc ion wi h he seconda y hea ing o he ma e ial. The second s age consis s o he e apo a ion o wa e con ained in he su ace laye o he ma e ial. Du ing he hi d s age, he olume o wa e inc eases due o he hea ing o he wa e . This causes an inc ease in p essu e, which sp eads in all di ec ions, including o he su ace o he ma e ial, o cing he hea ed wa e o be expelled o he su ace. A g adual cooling p ocess is he las s age. The wa e on he su ace o he ma e ial is con inuously e apo a ed. The di e ence in mois u e con en a he su ace and in he dep h o he ma e ial esul s in a di e ence in he pa ial p essu e, which enables he anspo o mois u e o he su ace. The e apo a ion o mois u e om he su ace o he Buildings 2023,13, 1018 4 o 11 d ying ma e ial equi es a conside able amoun o hea . Consequen ly, he su ace o he ma e ial and he su ounding ai a e cooled [16]. The ollowing ac o s in luence he a e o d ying: empe a u e g adien , mois u e con en o he su ace laye , ela i e humidi y o he su oundings o he d ying ma e ial, he abili y o he ma e ial o dis ibu e wa e om i s co e o he su ace, he su ace inish o he d ied ma e ial, and he mal conduc i i y o he d ying ma e ial [16,19]. 2. Me hods and Ma e ials The expe imen al de e mina ion o he basic cha ac e is ics o he s udied ma e ial is p esen ed in he ollowing chap e . I should be no ed ha all he expe imen s he ein p esen ed we e ca ied ou on a ce amic cons uc ion ma e ial, namely solid bu n clay b icks. The pu pose o his was o expe imen ally e i y he in luence o b ick d ying by mic owa e adia ion on he mechanical p ope ies o he b ick. Basic cha ac e is ics: b ick dimensions we e 290 × 140 × 65 mm; b icks we e o med by p essing; he e we e 10 b ick samples in he expe imen ; bulk densi y was 1718-kg · m −3 ; comp essi e s eng h was 20 MPa as s a ed by he manu ac u e . Be o e measu emen , he samples we e d ied o a cons an weigh . The ac ual expe imen is based on he ea men o 10 ully wa e -soaked ce amic b icks wi h mic owa e adia ion and a ying exposu e imes o he b icks. The i s in e al o exposu e o EMW adia ion was 60 min, he second in e al was 120 min, and he hi d in e al was 240 min. The indi idual expe imen s we e ca ied ou in wo a ian s; in he i s one, he EMW adia ion was applied only o one cycle, and in he second a ian o en cycles. Subsequen ly, an ul asonic wa e was used, and i s ansi ime was measu ed. F om hese da a, a calcula ed es ima e o he comp essi e s eng h o each b ick was de e mined. The measu emen s o he obse ed pa ame e s we e always aken be o e and a e exposu e o mic owa e adia ion. In Figu e 1we can see he layou o he b ick samples and he EMW gene a o . Buildings 2023, 13, x FOR PEER REVIEW 4 o 12 o he ma e ial. The second s age consis s o he e apo a ion o wa e con ained in he su ace laye o he ma e ial. Du ing he hi d s age, he olume o wa e inc eases due o he hea ing o he wa e . This causes an inc ease in p essu e, which sp eads in all di ec- ions, including o he su ace o he ma e ial, o cing he hea ed wa e o be expelled o he su ace. A g adual cooling p ocess is he las s age. The wa e on he su ace o he ma e ial is con inuously e apo a ed. The diffe ence in mois u e con en a he su ace and in he dep h o he ma e ial esul s in a diffe ence in he pa ial p essu e, which enables he anspo o mois u e o he su ace. The e apo a ion o mois u e om he su ace o he d ying ma e ial equi es a conside able amoun o hea . Consequen ly, he su ace o he ma e ial and he su ounding ai a e cooled [16]. The ollowing ac o s in luence he a e o d ying: empe a u e g adien , mois u e con en o he su ace laye , ela i e humidi y o he su oundings o he d ying ma e ial, he abili y o he ma e ial o dis ibu e wa e om i s co e o he su ace, he su ace inish o he d ied ma e ial, and he mal conduc i i y o he d ying ma e ial [16,19]. 2. Me hods and Ma e ials The expe imen al de e mina ion o he basic cha ac e is ics o he s udied ma e ial is p esen ed in he ollowing chap e . I should be no ed ha all he expe imen s he ein p e- sen ed we e ca ied ou on a ce amic cons uc ion ma e ial, namely solid bu n clay b icks. The pu pose o his was o expe imen ally e i y he in luence o b ick d ying by mic o- wa e adia ion on he mechanical p ope ies o he b ick. Basic cha ac e is ics: b ick dimensions we e 290 × 140 × 65 mm; b icks we e o med by p essing; he e we e 10 b ick samples in he expe imen ; bulk densi y was 1718-kg·m −3 ; comp essi e s eng h was 20 MPa as s a ed by he manu ac u e . Be o e measu emen , he samples we e d ied o a cons an weigh . The ac ual expe imen is based on he ea men o 10 ully wa e -soaked ce amic b icks wi h mic owa e adia ion and a ying exposu e imes o he b icks. The i s in- e al o exposu e o EMW adia ion was 60 min, he second in e al was 120 min, and he hi d in e al was 240 min. The indi idual expe imen s we e ca ied ou in wo a ian s; in he i s one, he EMW adia ion was applied only o one cycle, and in he second a ian o en cycles. Subsequen ly, an ul asonic wa e was used, and i s ansi ime was measu ed. F om hese da a, a calcula ed es ima e o he comp essi e s eng h o each b ick was de e - mined. The measu emen s o he obse ed pa ame e s we e always aken be o e and a e exposu e o mic owa e adia ion. In Figu e 1 we can see he layou o he b ick samples and he EMW gene a o . Figu e 1. Scheme o he mic owa e gene a o assembly o ce amic b ick exposu e. Figu e 1. Scheme o he mic owa e gene a o assembly o ce amic b ick exposu e. 2.1. Ma e ial Solid bu n clay b icks a e sui able o s anda d load-bea ing and in ill plas e ed mason y. The ce amic ma e ial is a he po ous, b i le, ha d, wa e -insoluble, chemically esis an , and hea esis an . I is a poo he mal conduc o and a good elec ical insula o . I is p oduced by i ing a na u al clay-based aw ma e ial (oxide ce amics), which esul s in a e y igid ma e ial wi h good esis ance o high empe a u es. The physical p ope ies o ce amic b icks we e speci ied by echnical s anda ds. The equi ed po osi y was a leas 10%, and he comp essi e s eng h alue o he b icks was he key pa ame e o de e mining he load-bea ing capaci y o he mason y. Buildings 2023,13, 1018 5 o 11 Raw ma e ials o he p oduc ion o b icks consis o pa icles o wea he ed ocks, especially g ani e, gneiss, and po phy y. The p oduc s o ock wea he ing a e he clay, sil , sand, and a ious impu i ies. The mos impo an componen is clay, speci ically clay mine als such as illi e, mon mo illoni e, and kaolini e. In e ms o chemis y, hese a e hyd a ed aluminum silica es wi h a cha ac e is ic laye ed s uc u e. 2.2. Equipmen In cons uc ion p ac ice, he e a e se e al ypes o EMW adia ion cu en ly used o d y damp building s uc u es. Many de ices d y he mason y only by a aching an EMW gene a o wi h a magne on bu using EMW an ennas makes d ying mo e e icien . The basic d ying me hods in cons uc ion p ac ice use a od-shaped o unnel-shaped EMW an enna [15]. In his expe imen , only he unnel-shaped an enna was used. In he d ying p ocess, a de ice equipped wi h a unnel-shaped an enna was a ached o he mason y a a dis ance o 0 o 50 mm and le o ope a e o a ce ain pe iod o ime a any gi en loca ion. The d ying o he whole a ea was achie ed by he successi e cyclical eposi ioning o he an enna. I is impo an o d y he loca ions mo e han once, as o he wise, he o ma ion o subsu ace pocke s du ing apid d ying can esul in only he su ace o he mason y being d ied and mois u e emaining in he dep h o he ma e ial. Cyclic d ying p e en s he o ma ion o pocke s and ensu es hea ing in he dep h o he ma e ial. This expe imen used a po able mic owa e gene a o Romill, G1/2011, o i adia e he b icks. The used ol age was 230 V, 50 Hz, he powe inpu was 1.5 kW, he mic owa e equency was 2450 MHz, and he maximum mic owa e powe was 1 kW. 2.3. Expe imen al D ying o he B icks A o al o 10 samples we e p epa ed o he expe imen al measu emen s. The samples we e s o ed in labo a o y condi ions whe e he indoo ai empe a u e was con olled and anged om 19.9 ◦C o 22.3 ◦C. The p ocedu e o conduc ing he es can be summa ized in a sequence o labo a o y ac i i ies. Fi s , he samples we e d ied in a d ying chambe o hei cons an weigh a a empe a u e o 100 ◦ C. A e wa d, hey we e weighed, and hei basic mechanical and physical p ope ies we e de e mined and compa ed wi h he manu ac u e ’s decla ed alues. Nex , he samples we e soaked in a wa e ank o 24 h. Then, hey we e weighed again, and hei alues o wa e abso p ion and appa en po osi y we e calcula ed. In he nex s ep, he samples we e d ied using mic owa e adia ion in wo se s ( i e samples we e exposed o mic owa e adia ion only once, and he o he i e samples we e exposed o i o 10 cycles). In bo h cases, 1 sample was i adia ed o 1 h, 2 samples o 2 h, and he las 2 samples o 4 h. A me al-shielded chambe called a Fa aday cage was c ea ed a ound he samples o limi he sp ead o mic owa e adia ion o he su ounding a ea. A e exposu e, he samples we e weighed and hen d ied in he d ying chambe o hei cons an weigh a he empe a u e o 100 ◦ C. Subsequen ly, he p opaga ion speed o he ul asonic pulse was measu ed, and he calcula ed comp essi e s eng h o he b ick was de e mined. The measu emen s esul s can be seen in Table 1. 2.4. Physical and Mechanical P ope ies o B icks The bulk densi y o he b icks was de e mined acco ding o s anda d ˇ CSN 72 2603 [ 20 ], as he samples we e geome ically egula . The moni o ed p ope y o he b ick samples was e alua ed in a so-called ai -d y s a e in he labo a o y en i onmen [21]. The mois u e abso p ion was de e mined acco ding o he na ional s anda d ˇ CSN 72 2603 [ 20 ]. In conjunc ion wi h he de e mina ion o bulk densi y, he mois u e abso p ion and he appa en po osi y o he samples we e de e mined [21]. The co ela ion be ween he eloci y o ul asonic wa e p opaga ion and he eloci y o sound p opaga ion in he ma e ial was used o quan i y he comp essi e s eng h o he b icks. Buildings 2023,13, 1018 6 o 11 Table 1. Summa y o he inpu pa ame e s o he solid bu n b ick samples P20. Sample Numbe Bulk Densi y [kg·m−3] Mois u e Abso p ion [%] Comp essi e S eng h [Mpa] Appa en Po osi y [%] 1 1.737 14.72 32.2 25.58 2 1.418 14.62 33.6 25.12 3 1.755 14.17 44.3 24.86 4 1.769 13.28 31.1 23.49 5 1.720 15.01 34.5 25.96 6 1.726 14.25 43.3 24.59 7 1.719 13.65 40.1 23.46 8 1.739 13.20 42.9 22.96 9 1.736 13.66 31.9 23.72 10 1.726 14.16 32.8 24.44 2.5. Ul asonic Pulse Veloci y Me hod The expe imen was pe o med by measu ing he wa e passage h ough he samples, and i was ca ied ou by he di ec sounding o he samples. The ollowing p ocedu e was used o measu e he ime o passage o he ul asonic pulse: along he leng h o he b ick was he measu emen pe o med in h ee measu ing poin s, which we e e enly dis ibu ed o e he measu ed a ea. The ac ual equency o he p obes was chosen as 54 kHz in o de o sa is y he condi ion a ≥ 1.25 λ (whe e ais he minimum dimension o he sample a he measu emen poin and λ is he wa eleng h), and he bonding agen was Sonogel, commonly used in he medical indus y. A each measu ing poin , h ee measu emen s o he ul asonic pulse passage ime we e aken. Ul asound is de ined as he mechanical ib a ion o en i onmen al pa icles a e- quencies g ea e han 20 kHz. Fo es ing building ma e ials, p obes wi h a equency anging om 40 kHz o 150 kHz a e ypically used. When an ul asonic wa e p opaga es h ough a medium, i s pa icles ib a e in di e en di ec ions ela i e o he di ec ion o he wa e’s p opaga ion. Acco dingly, ul asonic wa es can be dis inguished in o ou ypes: longi udinal (pa icles o he medium ib a e pa allel o he di ec ion o he wa e’s p opaga ion), ans e se (pa icles o he medium ib a e pe pendicula o he di ec ion o p opaga ion o he wa e), su ace ( hey a el along he su ace o hick solids and do no in luence he bulk o he medium below), and pla e wa es ( hese wa es a e p oduced in hin me al, whose hickness is simila o he wa eleng h) [22]. The concep o he me hod consis s o ansmi ing epea ed ul asonic pulses in o he ma e ial by he ansmi ance ansduce and hen de ec ing he pulses passed h ough he examined ma e ial by he ecei ing ansduce , i.e., measu ing he ime in e al ha elapses du ing he pulse passage h ough he measu ing base. Measu emen s can be made by di ec , indi ec , and semi-di ec sounding. Measu emen s made by he ul asonic pulse eloci y me hod a e in luenced by se e al ac o s, namely, mois u e, de ec s in he s uc u e o he es ed ma e ial, he dimensions and shape o he sample, he na u al equency o he p obes, as well as he means o acous ic coupling be ween he p obes and he es ed ma e ial. I is necessa y o ake hese ac o s in o accoun when de eloping a measu emen me hodology o speci ic ma e ials [22,23]. A co ela ion exis s o p edic ing he comp essi e s eng h o b icks om pa ame e s measu ed by he ul asonic pulse eloci y me hod. Based on he esul s o he expe imen al measu emen s, an es ima e o he comp essi e s eng h o he b icks was de e mined acco ding o he o mula [22]: c=10.754V−0.5367V2+6.1707 [mPa]   =0.698, (1) whe e cis he comp essi e s eng h o he b ick in mPa, Vis he ul asonic pulse p opaga ion eloci y in m·s−1, is he co ela ion coe icien . Buildings 2023,13, 1018 7 o 11 3. Resul s Physical P ope ies o B icks P io o ac ual exposu e o mic owa e adia ion, he basic physical p ope ies o he es ed samples we e de e mined; in pa icula , his included he bulk densi y, mois u e abso p ion, appa en po osi y, and comp essi e s eng h o he indi idual b icks. These alues we e compa ed wi h he in o ma ion decla ed by he manu ac u e . The alues can be seen in Table 2. Table 2. O e iew o he physical p ope ies o he ce amic solid bu n b ick samples. Quan i y Uni Values Decla ed by he Manu ac u e A e age o he Measu ed Values Bulk densi y [kg·m−3]1600 1.705 ±0.204 Mois u e abso p ion [%] 14.1 14.07 ±1.23 Appa en po osi y [%] - 24.42 ±1.98 Comp essi e s eng h [mPa] 20.0 36.7 ±10.7 4. Discussion o Resul s The analysis o he ob ained esul s p o ided insigh s in o he sui abili y o using mic owa e adia ion o d ying b ick componen s. The expe imen s pe o med con i med he insigni ican e ec o mic owa e d ying on he comp essi e s eng h o he po ous ce amic ma e ial. The expe imen al esul s can be obse ed in Tables 3and 4and he esul ing ul asonic wa e p opaga ion imes a e shown in Table 5. G aphical ep esen a ion o he esul s can be seen in Figu es 2and 3. Table 3. Weigh summa y o each sample a e d ying, soaking, and exposu e o one cycle o mic owa e adia ion. Sample Numbe Exposu e Time [h] Cons an Weigh [g] Weigh o he Soaked Sample [g] Weigh o Sample a e he Exposu e o Mic owa e Radia ion [g] 4 2 4.670 5.290 4.830 7 2 4.536 5.155 4.896 8 4 4.590 5.196 4.657 9 1 4.582 5.208 5.102 10 4 4.554 5.199 4.586 Table 4. Weigh summa y o each sample a e d ying, soaking, and exposu e o mic owa e adia ion; hese samples we e exposed o 10 adia ion cycles. Sample Numbe Exposu e Time [h] Cons an Weigh [g] Weigh o he Soaked Sample [g] Weigh o Sample a e he Exposu e o Mic owa e Radia ion [g] 1 2 4.583 5.258 4.844 2 1 4.535 5.198 5.042 3 2 4.631 5.287 4.660 5 4 4.540 5.225 4.544 6 4 4.555 5.204 4.559 Buildings 2023,13, 1018 8 o 11 Table 5. O e iew o he a e age passage ime o he ul asonic wa e. Sample Numbe A e age Passage Time o he Ul asound h ough he B icks (D ied S a e; Cons an Weigh ) a = 54 kHz [µs] A e age Passage Time o he Ul asound h ough he B icks (A e Exposu e o Mic owa e Radia ion) a = 54 kHz [µs] 1 160.59 157.00 2 154.41 151.80 3 116.34 114.70 4 165.17 169.70 5 151.05 143.40 Buildings 2023, 13, x FOR PEER REVIEW 8 o 12 Table 4. Weigh summa y o each sample a e d ying, soaking, and exposu e o mic owa e adia- ion; hese samples we e exposed o 10 adia ion cycles. Sample Numbe Exposu e Time [h] Cons an Weigh [g] Weigh o he Soaked Sample [g] Weigh o Sample a e he Exposu e o Mic owa e Radia ion [g] 1 2 4.583 5.258 4.844 2 1 4.535 5.198 5.042 3 2 4.631 5.287 4.660 5 4 4.540 5.225 4.544 6 4 4.555 5.204 4.559 Table 5. O e iew o he a e age passage ime o he ul asonic wa e. Sample Numbe A e age Passage Time o he Ul asound h ough he B icks (D ied S a e; Cons an Weigh ) a = 54 kHz [µs] A e age Passage Time o he Ul asound h ough he B icks (A e Exposu e o Mic owa e Radia ion) a = 54 kHz [µs] 1 160.59 157.00 2 154.41 151.80 3 116.34 114.70 4 165.17 169.70 5 151.05 143.40 Figu e 2. O e iew o he passage ime o he ul asonic wa e h ough he indi idual ce amic b ick samples. 0 20 40 60 80 100 120 140 160 180 200 12345678910 Passage ime o he ul asound [μs] Sample Passage ime o he ul asound h ough he b icks in d ied s a e a = 54 kHz Passage ime o he ul asound h ough he b icks a e exposu e o mic owa e adia ion a = 54 kHz Figu e 2. O e iew o he passage ime o he ul asonic wa e h ough he indi idual ce amic b ick samples. The comp essi e s eng h da a o he es ed ce amic b icks we e calcula ed es ima es based on he ul asonic pulse me hod, which is di ec ly a ec ed by he homogenei y o he ma e ial on he ansmission pa h be ween he ansmi e and he ecei e , pa icula ly in he ma e ial s uc u e. I was also ecognized ha he mois u e con en o ma e ials a ec s he p opaga ion speed o he ul asonic pulse [ 24 , 25 ]. The p esence o ma e ial mois u e, which appea s as a liquid phase o wa e in he po ous s uc u e o he ma e ials and ills he ee po e space, inc eases he eloci y o he ul asound p opaga ion in he ma e ial ( he ul asound eloci y in wa e is abou 3.5 imes highe han in ai ) [ 22 ]. The e o e, i is necessa y o de e mine he mois u e s a e o he samples du ing he measu emen s o ensu e he ep oducibili y o he measu emen esul s. To elimina e he in luence o mois u e on he esul s, he ce amic b icks we e d ied o hei cons an weigh be o e he expe imen . As al eady men ioned, he esul s de i ed om he ul asonic pulse eloci y me hod a e in luenced by he shape and dimension o he b ick uni s, he mine alogical componen s o he b icks, hei po osi y, and any de ec s in hei s uc u e. These ac o s a e also e lec ed, o a ying deg ees, in he calib a ion equa ions de eloped o de e mining comp essi e s eng h om he esul s o non-des uc i e es s. The e alua ion o calib a ion equa ions was based on he alue o he co ela ion coe icien , which assessed he s eng h o he ela ionship be ween he measu emen esul s and he ac ual comp essi e s eng h alues. Calib a ion equa ions wi h a co ela ion coe icien o ≥ 0.85 we e conside ed o be Buildings 2023,13, 1018 9 o 11 easonably usable, al hough calib a ion equa ions wi h ≥ 0.9 we e mo e app op ia e [ 22 ]. The ela ionship ha was used o he compu a ional es ima ion o he comp essi e s eng h o ce amic b icks was = 0.85. Buildings 2023, 13, x FOR PEER REVIEW 9 o 12 Figu e 3. O e iew o calcula ed comp essi e s eng h es ima es o indi idual samples a e d ying and a e exposu e o mic owa e adia ion. The comp essi e s eng h da a o he es ed ce amic b icks we e calcula ed es ima es based on he ul asonic pulse me hod, which is di ec ly affec ed by he homogenei y o he ma e ial on he ansmission pa h be ween he ansmi e and he ecei e , pa icu- la ly in he ma e ial s uc u e. I was also ecognized ha he mois u e con en o ma e ials affec s he p opaga ion speed o he ul asonic pulse [24,25]. The p esence o ma e ial mois u e, which appea s as a liquid phase o wa e in he po ous s uc u e o he ma e ials and ills he ee po e space, inc eases he eloci y o he ul asound p opaga ion in he ma e ial ( he ul asound eloci y in wa e is abou 3.5 imes highe han in ai ) [22]. The e- o e, i is necessa y o de e mine he mois u e s a e o he samples du ing he measu e- men s o ensu e he ep oducibili y o he measu emen esul s. To elimina e he in luence o mois u e on he esul s, he ce amic b icks we e d ied o hei cons an weigh be o e he expe imen . As al eady men ioned, he esul s de i ed om he ul asonic pulse eloci y me hod a e in luenced by he shape and dimension o he b ick uni s, he mine alogical compo- nen s o he b icks, hei po osi y, and any de ec s in hei s uc u e. These ac o s a e also e lec ed, o a ying deg ees, in he calib a ion equa ions de eloped o de e mining com- p essi e s eng h om he esul s o non-des uc i e es s. The e alua ion o calib a ion equa ions was based on he alue o he co ela ion coefficien , which assessed he s eng h o he ela ionship be ween he measu emen esul s and he ac ual comp essi e s eng h alues. Calib a ion equa ions wi h a co ela ion coefficien o ≥ 0.85 we e con- side ed o be easonably usable, al hough calib a ion equa ions wi h ≥ 0.9 we e mo e app op ia e [22]. The ela ionship ha was used o he compu a ional es ima ion o he comp essi e s eng h o ce amic b icks was = 0.85. Fo bu n clay b icks, in gene al, a mino c ack in he b ick is no a de ec unless i ad e sely affec s he decla ed comp essi e s eng h. In e ms o he assessmen o he comp essi e s eng h o solid bu n b icks by he ul asonic me hod, he in luence o dis- o de ly de ec s in he mic os uc u e o he b ick samples is e y p onounced, and he e- o e, he obse ed da a a e mo e o an es ima e. I was echnically no possible o ca y ou objec i ely conclusi e des uc i e comp essi e s eng h es s on he b icks examined, and he e o e, a calcula ed es ima e o comp essi e s eng h was used—as seen in he esul s 0 5 10 15 20 25 30 35 12345678910 Comp essi e s eng h o he samples [MPa] Sample Es ima ion o comp essi e s eng h o d ied samples Es ima ion o comp essi e s eng h a e exposu e o mic owa e adia ion Figu e 3. O e iew o calcula ed comp essi e s eng h es ima es o indi idual samples a e d ying and a e exposu e o mic owa e adia ion. Fo bu n clay b icks, in gene al, a mino c ack in he b ick is no a de ec unless i ad e sely a ec s he decla ed comp essi e s eng h. In e ms o he assessmen o he comp essi e s eng h o solid bu n b icks by he ul asonic me hod, he in luence o diso de ly de ec s in he mic os uc u e o he b ick samples is e y p onounced, and he e o e, he obse ed da a a e mo e o an es ima e. I was echnically no possible o ca y ou objec i ely conclusi e des uc i e comp essi e s eng h es s on he b icks examined, and he e o e, a calcula ed es ima e o comp essi e s eng h was used—as seen in he esul s abo e. This es ima ion has i s limi a ions in he o m o s a is ical e o . Despi e he abo e, his comp essi e s eng h es ima ion p ocedu e can be ecommended o u he esea ch [22]. 5. Conclusions Based on he expe imen s ca ied ou and hei subsequen e alua ion, i was possible o con i m he e ec o d ying by mic owa e adia ion on he comp essi e s eng h o po ous ce amic ma e ial. The da a p o e ha mic owa e adia ion can be a sui able me hod o d ying solid bu n b icks as i does no a ec he comp essi e s eng h o he b icks. The expe imen was conduc ed in wo se s, whe e he solid bu n b icks we e d ied by mic owa e adia ion o di e en pe iods o ime, and e en a e en cycles o exposu e, he e we e no changes in he measu ed s eng h alues. The po osi y o ce amics is usually 15–25%, so i is unde s ood ha he e was in e nal po e space a ailable o he emo al o wa e apo in o he su ounding en i onmen .