The Effect of Specimen Size on Acoustic Emission Parameters and Approximate Position of Defects Obtained during Destructive Testing of Cementitious and Alkali-Activated Degraded Fine-Grained Materials

Ci a ion: Topolᡠ, L.; Kocáb, D.; H ubý, P.; Jakubka, L.; Hoduláko á, M.; Halamo á, R. The E ec o Specimen Size on Acous ic Emission Pa ame e s and App oxima e Posi ion o De ec s Ob ained du ing Des uc i e Tes ing o Cemen i ious and Alkali-Ac i a ed Deg aded Fine-G ained Ma e ials. Ma e ials 2023,16, 3527. h ps://doi.o g/ 10.3390/ma16093527 Academic Edi o s: Mikołaj Mi´skiewicz and Danu a Ba na -Hunek Recei ed: 3 Ap il 2023 Re ised: 24 Ap il 2023 Accep ed: 2 May 2023 Published: 4 May 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/). ma e ials A icle The E ec o Specimen Size on Acous ic Emission Pa ame e s and App oxima e Posi ion o De ec s Ob ained du ing Des uc i e Tes ing o Cemen i ious and Alkali-Ac i a ed Deg aded Fine-G ained Ma e ials Libo Topolᡠ1,* , Dalibo Kocáb1, Pe H ubý2, Luboš Jakubka 1, Michaela Hoduláko á1 and Romana Halamo á1 1Facul y o Ci il Enginee ing, B no Uni e si y o Technology, Ve eˇ í331/95, 60200 B no, Czech Republic; dalibo .kocab@ u b .cz (D.K.); lubos.jakubka@ u b .cz (L.J.); [email p o ec ed].cz (M.H.); omana.halamo a@ u b .cz (R.H.) 2Facul y o Chemis y, B no Uni e si y o Technology, Pu kyˇno a 464, 61200 B no, Czech Republic; [email p o ec ed] *Co espondence: libo . opola @ u b .cz; Tel.: +420-541-147-664 Abs ac : Two sizes o es samples we e selec ed o in es iga e he e ec o size on he le el o deg ada ion. The smalle es specimens had dimensions o 40 × 40 × 160 mm, and he la ge ones had dimensions o 100 × 100 × 400 mm. Bo h sizes o es specimens we e always made o he same mo a . In one case, Blas Fu nace Cemen was chosen as he binde . In he o he case, i was an alkali-ac i a ed ma e ial as a possibly mo e en i onmen ally economical subs i u e. Bo h ypes o ma e ial we e deposi ed in h ee deg ading solu ions: magnesium sulpha e, ammonium ni a e and ace ic acid. The e e ence se was s o ed in a wa e ba h. A e six mon hs in he deg ada ion solu ions, a s a ic elas ic modulus was de e mined o he specimens du ing his es , and he acous ic emission was measu ed. Acous ic emission pa ame e s we e e alua ed: he numbe o hi s, he ampli ude magni ude and a slope om he ampli ude magni ude e sus ime ( his slope should co espond o he Kaise e ec ). Fo mos o he pa ame e s s udied, he size e ec was mo e e iden o he mo e deg aded specimens, i.e., hose placed in agg essi e solu ions. The app oxima e loca ion o eme ging de ec s was also de e mined using linea localisa ion o smalle specimens whe e he deg ada ion e ec was mo e signi ican . In mo e agg essi e en i onmen s (ace ic acid, ammonium ni a e), he highe esis ance o ma e ials based on alkaline-ac i a ed slag was mo e e iden , e en in he case o la ge es bodies. The expe imen s show ha he acous ic emission esul s ag ee wi h he esul s o he s a ic modulus o elas ici y. Keywo ds: size e ec ; deg ada ion; acous ic emission me hod; s a ic modulus o elas ici y; Kaise e ec ; blas u nace slag; blas u nace cemen ; linea localisa ion 1. In oduc ion Thanks o i s e sa ili y, a ailabili y, qui e su icien du abili y, ela i ely low p ice and easy handling, conc e e has been and will con inue o be he mos common cons uc ion ma e ial a ound he wo ld. Howe e , conc e e p oduc ion is associa ed wi h a la ge consump ion o na u al esou ces and ene gy, as well as wi h signi ican CO 2 emissions, pa icula ly due o he p esence o cemen as essen ial conc e e componen . I is es ima ed ha up o one on o CO 2 is eleased du ing he p oduc ion o one on o Po land cemen [ 1 ], and acco ding o S a is a, Inc. (Be lin, Ge many) [ 2 ] 4.4 billion me ic ons o cemen we e p oduced in 2021. Gene ally, cemen p oduc ion co esponds o 5 o 8% o global CO 2 emissions [ 3 ]. The e o e, e o s o eplace cemen pa ially o ully by indus ial by- p oduc s [ 4 – 6 ] and was e alumino-silica e sou ces such as slags and ly ashes ha e been inc easing. Alongside o he al e na i e binde s such as calcium sul oalumina e cemen s o Ma e ials 2023,16, 3527. h ps://doi.o g/10.3390/ma16093527 h ps://www.mdpi.com/jou nal/ma e ials Ma e ials 2023,16, 3527 2 o 17 supe sul a ed cemen s [ 7 ], alkali-ac i a ed cemen less binde s ha e a ac ed inc easing scien i ic a en ion du ing he las decades. Mos building s uc u es a e subjec ed o comp essi e, ensile, momen loads and shea loads du ing hei se ice li e—and o he ex e nal deg ading in luences. Fo example, huge amoun s o conc e e based on o dina y Po land cemen ha e been used as cons uc ion ma e ials o sewe s and was ewa e ea men plan s. Exposu e o an agg essi e, acid- ich en i onmen leads o se e e deg ada ion [ 8 , 9 ]. Fo hese easons, i is necessa y o look o ma e ials ha esis deg ada ion be e han ma e ials based on he o dina y Po land cemen . Addi ionally, while cemen is s ill he mos commonly used binde , al e na i es in he o m o alkali-ac i a ed ma e ials a e eme ging [10]. Alkali-ac i a ed ma e ials a e based on sui able aluminosilica e p ecu so s wi h a su icien ly high con en o amo phous phase, such as g anula ed blas u nace slag, ly ash and me akaolin. In sho , a e he dissolu ion o he amo phous phase, which is acili a ed by he p esence o an alkaline ac i a o , usually sodium wa e glass o sodium hyd oxide solu ion, polycondensa ion eac ions ake place and hyd a ion p oduc s wi h binding p ope ies a e o med. The binde s uc u e and composi ion depend on he chemis y o bo h aluminosilica e p ecu so s and ac i a o s, as well as on he cu ing condi ions and o he pa ame e s. I can be summa ised ha he main hyd a ion p oduc o blas u nace slag alkaline ac i a ion is calcium–aluminium–silica e–hyd a e, as a de i a e o he C-S-H known om hyd a ion Po land cemen sys ems, while he alkaline ac i a ion o ly ash and me akaolin o ms a mo e c oss-linked h ee-dimensional s uc u e o sodium–aluminium– silica e–hyd a e [ 11 ]. To each su icien ly high ea ly age s eng h, me akaolin and ly ash usually equi e a high amoun o ac i a o and hea cu ing, which means addi ional economic and echnological demands. Tha is why alkali-ac i a ed ma e ials based on he Blas Fu nace Slag, alkali-ac i a ed using he mix u e o sodium wa e glass and sodium hyd oxide, we e used in his s udy. Mos o he ecen s udies dealing wi h he chemical deg ada ion o cemen -based ma e ial a e limi ed o one size o es specimens, e.g., [ 12 – 16 ]. E en s udies on he chemical deg ada ion o alkali-ac i a ed ma e ials a e usually pe o med on small specimens o he same size wi hin a gi en piece o esea ch, e.g., [ 17 – 19 ]. In ou case, we ocused on he e ec o deg ada ion on di e en specimen sizes o wo mo a s on he base o CEM III/A and alkali-ac i a ed ma e ial. Those ma e ials we e chosen due o hei simila indus ial use and compa able p ope ies. Size e ec is widely discussed o commonly used conc e es ( ein o ced o un ein- o ced) [ 20 , 21 ] in e ms o ac u e mechanics [ 22 , 23 ] o heo e ical calcula ions o mod- els [ 24 , 25 ]. In o he wo ks, o example [ 26 ], esea che s ha e es ed di e en sizes o conc e e block specimens o comp essi e s eng h unde dynamic uniaxial loads. Ve y ew s udies ha e e alua ed he e ec o sample size on deg aded specimens using simul aneous non-des uc i e me hods. Howe e , in he case o labo a o y s udies using small specimens (in ci il enginee ing, di e en ypes o pas es), esea che s can make signi ican inaccu acies in hei conclusions. Using la ge specimens inc eases he de ec isk, allowing he deg adan o pene a e mo e easily. Thus, usually, be e esul s (i.e., comp essi e s eng h, e c.) a e ob ained hanks o he use o small specimens wi h dimensions such as 20 × 20 × 100 mm. Ou esea ch also in ol es using he same mix u es o small and la ge es specimens, which is no commonly ound in he li e a u e. This way, he ad e se e ec s o using di e en ypes and amoun s o agg ega e in he mix u e we e elimina ed. Ano he aspec ha signi ican ly dis inguishes his pape om many o he s is he use o he acous ic emission me hod o e eal he e ec o he size o he deg aded specimens on he pa ame e s ob ained. The acous ic emission me hod was applied du ing he s a ic modulus o elas ici y es . This pape , he e o e, looks a he e ec o he size o he es specimens on he pa ame e s ob ained du ing des uc i e es ing. Ma e ials 2023,16, 3527 3 o 17 2. Ma e ials Alkali-ac i a ed blas u nace slag binde sys ems we e p epa ed using slag wi h a Blaine ineness o 400 m 2· kg −1 (A celo Mi al Os a a, s. .o., Czech Republic). The slag’s phase composi ion was de e mined by X- ay powde di ac ion (XRD) using he Rie eld me hod wi h in e nal s anda d (CaF 2 ). The amo phous con en o slag was abou 70.5%. The main c ys alline phases we e ake mani e (19.8%), calci e (6.7%), qua z (2.7%), and me wini e (0.4%). Liquid sodium wa e glass—silica e modulus (Ms) = 0.5 (VodníSklo, a.s., Czech Republic)—was used as an alkaline ac i a o in he 6% Na 2 O- ela ed dose pe he slag weigh . The compa a i e cemen i ious binde was p epa ed using he CEM III/A 32.5 R (Ho néS nie cemen plan , Cemmac s. .o., Slo akia). The chemical composi ions o he Blas Fu nace Slag and Blas Fu nace Cemen we e de e mined using X- ay luo escence analysis (XRF), as shown in Table 1. Table 1. Chemical composi ions (%) o Blas Fu nace Slag (BFS) and Blas Fu nace Cemen (BFC = CEM III/A) as de e mined by X- ay luo escence. CaO SiO2MgO Al2O3SO3TiO2K2O MnO Na2O Fe2O3LOI BFS 37.0 39.4 8.6 8.1 1.4 0.3 1.2 0.9 0.4 0.7 2 BFC 45.6 31.6 5.8 7.4 3.3 0.4 0.8 0.6 0.3 1.4 2.8 P epa a ion and Composi ion o Tes ing Specimens—Mo a s The es ing specimens we e p epa ed as p esc ibed in he Eu opean s anda d EN 196-1; he composi ion o each mix u e is in Table 2. The mixing ook 3 min, and he ac i a o and he slag (o CEM III/A) we e mixed using a low speed du ing he ini ial 30 s, and hen he s anda dised sand was added and mixed o ano he 30 s. This was ollowed by high-speed mixing o 30 s and con inued by wiping he walls o he con aine o 30 s. A inal mixing using high speed o 60 s was implemen ed a he end. The labo a o y mixe (Ki chenAid Robo A isan 175) was used o he mo a p epa a ion. The p epa ed mix u e was cas in o he moulds and compac ed on he compac ing able o 30 s o elimina e he ai en apped. The es ing specimens we e demoulded a e 24 h and pu in o wa e s o age, whe e hey we e s o ed o 28 days a 25 ◦ C ( he specimens we e no s o ed in a empe ed oom; he empe a u e o 25 ◦ C was he long- e m a e age o he pe iod unde s udy). Then, he deg ada ion es ing s a ed wi h he imme sion o he es ing specimens in he speci ic solu ions (magnesium sulpha e (50 g/L), ammonium ni a e (6 mol/dm 3 ) and ace ic acid (pH = 3), ap wa e as a e e ence). The sand- o-binde mass a io in mo a s was 3:1. The wa e - o-binde a io (w/b) was he same (0.45) o bo h es ed binde s. Cemen (CEM III/A 32.5R) was chosen as he binde because i was blas - u naced acco ding o he ma king and had minimum admix u es. Blas - u nace slag is he mos commonly used cemen eplacemen in he Czech Republic’s cemen - ee mixes. Table 2. Composi ion o indi idual mo a mixes (weigh in g ams). Ma e ial Blas Fu nace Slag CEM III/A Sand Sodium Wa e Glass (Ms = 0.5) Wa e AAS mo a * 22.0 – 66.1 4.6 7.2 CEM mo a ** – 22.5 67.4 – 10.1 * AAS mo a is made based on alkali-ac i a ed blas u nace slag (AAS) binde . ** CEM mo a is made based on blas u nace cemen (CEM) binde . 3. Tes ing Me hods 3.1. Desc ip ion o Deg ada ion En i onmen s Deg ada ion en i onmen s we e designed o simula e a ious deg ada ion mecha- nisms and p o ide e e ence specimens as a blank. Wa e embedding o specimens was chosen as a e e ence. A solu ion o ace ic acid o pH ≈ 3 was selec ed o simula e be- ha iou unde he acidic condi ions, hus he decalci ica ion mechanism along wi h he Ma e ials 2023,16, 3527 4 o 17 pa ial dissolu ion o he silicon-con aining gels. Nex , decalci ica ion wi hou he aligned dissolu ion o silica e uni s was s udied using he 6 mol/dm 3 ammonium ni a e solu ion. Sulpha e co osion was hen simula ed using he 50 g/L solu ions o magnesium sulpha e. Deg ada ion solu ions we e ully eplaced e e y 28 days o s imula e he deg ada ion p ocess. The pH o he ace ic acid en i onmen was measu ed and kep a he alue o app oxima ely pH ≈3 by adding an ex a dose o ace ic acid h ee imes pe week. The ongoing p ocesses we e obse ed by measu ing he pH (Me le Toledo Se en- Compac ™ Duo S213 using an InLab Rou ine elec ode wi h ange o pH 0–14) o all he solu ions be o e eplacemen wi h esh ones. The p og ess in ime can be seen in Table 3. I can be seen om he able ha e en hough one day he pH was educed o a alue close o 3 ( o ace ic acid), jus be o e he exchange he alue was highe again. Table 3. pH alue o solu ions measu ed jus be o e exchange o esh solu ions. Wa e Ace ic Acid Magnesium Sulpha e Ammonium Ni a e Days AAS CEM AAS CEM AAS CEM AAS CEM 0 7.45 3 8.1 6.24 28 12.48 10.78 4.99 5.1 9.25 9.67 9.37 9.64 56 12.65 10.57 5.37 5.49 9.39 9.53 9.57 9.71 84 9.45 7.73 4.52 4.92 8.89 9.47 9.02 9.71 112 9.66 8.53 4.57 4.89 8.87 9.3 9.11 9.13 140 9.03 8.33 4.5 4.64 8.9 9.06 8.69 8.13 168 8.35 8.05 4.24 4.17 8.58 8.91 8.58 8.66 Each binde ype’s specimens (3 pcs o 100 × 100 × 400 mm and 21 pcs o 40 × 40 × 160 mm) we e placed in o one con aine o a speci ic solu ion ( app ox. 50 L ). The olume a io be ween he specimens and he deg ada ion solu ion was 1:3 du ing he es ing pe iod. Con aine s we e kep a labo a o y condi ions (20 ◦ C) o he whole es ing pe iod. The e ec o deg ada ion media was in ensi ied using he ai s eam bubbling. 3.2. S a ic Modulus o Elas ici y and Comp essi e S eng h A e emo ing he es specimens om he indi idual deg ada ion solu ions, he s a ic modulus o elas ici y and hen he p isma ic comp essi e s eng h we e de e mined o all specimens. The modulus o elas ici y es was pe o med acco ding o ISO 1920-10. The loading p og ess o he es specimens is shown in Figu e 1. The basic load s ess was always 0.5 MPa ( he applied o ce was 1 kN o smalle specimens and 5 kN o bigge specimens), and he uppe load s ess was 1/3 o he expec ed comp essi e s eng h. The comp essi e s eng h was es ima ed based on he esul s o pa allel es s pe o med on o he specimens. S ill, hese es s a e no desc ibed in his pape ( hese we e he lexu al ensile s eng h, he comp essi e s eng h on agmen s and he dynamic modulus o elas ici y). The loading o smalle specimens was ca ied ou in he DELTA 3-600 (FORM+TEST) es p ess and he loading o bigge specimens in he ALPHA 3-3000S (FORM+TEST) es p ess, in bo h cases using he P o eus so wa e. The de o ma ions o he es specimens we e measu ed du ing es ing using wo elec onic s ain ansduce s LD-DD1-2 (FORM+TEST). Using he Spide 8 da a logge (HBM), a eco d o he applied o ce and de o ma ion o he es specimens was ob ained h oughou he es . A e he modulus o elas ici y es , comp essi e s eng h was de e mined on all es specimens acco ding o EN 12390-3. Ma e ials 2023,16, 3527 5 o 17 Ma e ials 2023, 16, x FOR PEER REVIEW 5 o 18 Figu e 1. The loading p og ess o he es specimens. 3.3. Acous ic Emission Me hod An acous ic emission me hod is a ool o he non-des uc i e moni o ing o ac i e dynamic changes in a s essed ma e ial [27]. The basis o his me hod is he con inuous moni o ing o he acous ic esponse caused by he ini ia ion and p opaga ion o damage unde s ess (mechanical, chemical o he mal) [28–30]. A ansien elas ic wa e is gene a ed locally due o he apid elease o ene gy as he s essed ma e ial p opaga es h ough he ma e ial un il i eaches he su ace. The su ace wa es a e hen cap u ed by piezoelec ic senso s and subsequen ly con e ed o elec ical signals [31]. The elec ical signals a e ampli ied, p ocessed and inally s o ed in a eco ding de ice. Unlike o he non- des uc i e de ec ion me hods, acous ic emission signals can only be de ec ed when damage occu s wi hin he ma e ial [32]. Commonly used acous ic emission signal cha ac e is ics a e shown in hi o m in Figu e 2, including ampli ude, ene gy, numbe o ings, ise ime and du a ion. The acous ic emission ampli ude co esponds o he poin o maximum alue, exp essed on a decibel scale. The ene gy o he wa e o m is he a ea abo e he h eshold and below he en elope cu e. Any oscilla ion o he elec ical signal exceeding a h eshold alue is conside ed a ing coun . The ing coun can e lec he signal s eng h and equency and is widely used o e alua e acous ic emission ac i i ies. The ene gy and ampli ude o acous ic emission can be used o cha ac e ise he magni ude o damage [33,34]. The ime be ween he s a ime o a single acous ic emission signal and he peak ampli ude is called he ise ime, and he ime be ween he s a ime o a single acous ic emission signal and he decay ime is called he du a ion [35]. In his s udy, only wo o he abo e pa ame e s we e used: he numbe o eco ded hi s and he AE ampli ude. Figu e 1. The loading p og ess o he es specimens. 3.3. Acous ic Emission Me hod An acous ic emission me hod is a ool o he non-des uc i e moni o ing o ac i e dynamic changes in a s essed ma e ial [ 27 ]. The basis o his me hod is he con inuous moni o ing o he acous ic esponse caused by he ini ia ion and p opaga ion o damage unde s ess (mechanical, chemical o he mal) [ 28 – 30 ]. A ansien elas ic wa e is gene a ed locally due o he apid elease o ene gy as he s essed ma e ial p opaga es h ough he ma e ial un il i eaches he su ace. The su ace wa es a e hen cap u ed by piezoelec ic senso s and subsequen ly con e ed o elec ical signals [ 31 ]. The elec ical signals a e ampli ied, p ocessed and inally s o ed in a eco ding de ice. Unlike o he non-des uc i e de ec ion me hods, acous ic emission signals can only be de ec ed when damage occu s wi hin he ma e ial [32]. Commonly used acous ic emission signal cha ac e is ics a e shown in hi o m in Figu e 2, including ampli ude, ene gy, numbe o ings, ise ime and du a ion. The acous ic emission ampli ude co esponds o he poin o maximum alue, exp essed on a decibel scale. The ene gy o he wa e o m is he a ea abo e he h eshold and below he en elope cu e. Any oscilla ion o he elec ical signal exceeding a h eshold alue is conside ed a ing coun . The ing coun can e lec he signal s eng h and equency and is widely used o e alua e acous ic emission ac i i ies. The ene gy and ampli ude o acous ic emission can be used o cha ac e ise he magni ude o damage [ 33 , 34 ]. The ime be ween he s a ime o a single acous ic emission signal and he peak ampli ude is called he ise ime, and he ime be ween he s a ime o a single acous ic emission signal and he decay ime is called he du a ion [ 35 ]. In his s udy, only wo o he abo e pa ame e s we e used: he numbe o eco ded hi s and he AE ampli ude. The Kaise e ec p inciple appea ed du ing he specimens’ cyclic loading (see Figu e 3 ). The AE signals we e insigni ican unless he s ess exceeded he p e iously applied max- imum p essu e. F om poin A o B, he AE signals we e eleased con inuously, bu a e unloading up o poin C and eloading un il poin B, no signal could be obse ed unless i exceeded poin B. This phenomenon is called i e e sibili y and is known as he Kaise e ec . As long as he loading con inued, he AE signals we e emi ed. When he loading cycles eached highe s ess in poin D, he ma e ial en e ed an uns able phase in which he p e ious mic oc acks expanded signi ican ly and se ious damage occu ed. AE signals could be seen e en be o e eaching poin D, which means ha he Kaise e ec a he highe s ess le el ends o dec ease [37,38]. Ma e ials 2023,16, 3527 6 o 17 Ma e ials 2023, 16, x FOR PEER REVIEW 6 o 18 Figu e 2. Schema ic diag am o AE pa ame e s [36]. The Kaise e ec p inciple appea ed du ing he specimens’ cyclic loading (see Figu e 3). The AE signals we e insigni ican unless he s ess exceeded he p e iously applied maximum p essu e. F om poin A o B, he AE signals we e eleased con inuously, bu a e unloading up o poin C and eloading un il poin B, no signal could be obse ed unless i exceeded poin B. This phenomenon is called i e e sibili y and is known as he Kaise e ec . As long as he loading con inued, he AE signals we e emi ed. When he loading cycles eached highe s ess in poin D, he ma e ial en e ed an uns able phase in which he p e ious mic oc acks expanded signi ican ly and se ious damage occu ed. AE signals could be seen e en be o e eaching poin D, which means ha he Kaise e ec a he highe s ess le el ends o dec ease [37,38]. Figu e 3. P inciple o Kaise e ec . Linea (one-dimensional) localisa ion o acous ic sou ces was pe o med conside ing he wa e p opaga ion eloci y o each ma e ial ype sepa a ely (de e mined om ul asonic wa es); see Figu e 4. Localised AE e en s we e eco ded along he line be ween he ansduce s and used o subsequen AE analysis o exclude any o he ecei ed signals [39]. Figu e 2. Schema ic diag am o AE pa ame e s [36]. Ma e ials 2023, 16, x FOR PEER REVIEW 6 o 18 Figu e 2. Schema ic diag am o AE pa ame e s [36]. The Kaise e ec p inciple appea ed du ing he specimens’ cyclic loading (see Figu e 3). The AE signals we e insigni ican unless he s ess exceeded he p e iously applied maximum p essu e. F om poin A o B, he AE signals we e eleased con inuously, bu a e unloading up o poin C and eloading un il poin B, no signal could be obse ed unless i exceeded poin B. This phenomenon is called i e e sibili y and is known as he Kaise e ec . As long as he loading con inued, he AE signals we e emi ed. When he loading cycles eached highe s ess in poin D, he ma e ial en e ed an uns able phase in which he p e ious mic oc acks expanded signi ican ly and se ious damage occu ed. AE signals could be seen e en be o e eaching poin D, which means ha he Kaise e ec a he highe s ess le el ends o dec ease [37,38]. Figu e 3. P inciple o Kaise e ec . Linea (one-dimensional) localisa ion o acous ic sou ces was pe o med conside ing he wa e p opaga ion eloci y o each ma e ial ype sepa a ely (de e mined om ul asonic wa es); see Figu e 4. Localised AE e en s we e eco ded along he line be ween he ansduce s and used o subsequen AE analysis o exclude any o he ecei ed signals [39]. Figu e 3. P inciple o Kaise e ec . Linea (one-dimensional) localisa ion o acous ic sou ces was pe o med conside ing he wa e p opaga ion eloci y o each ma e ial ype sepa a ely (de e mined om ul a- sonic wa es); see Figu e 4. Localised AE e en s we e eco ded along he line be ween he ansduce s and used o subsequen AE analysis o exclude any o he ecei ed signals [ 39 ]. Ma e ials 2023, 16, x FOR PEER REVIEW 7 o 18 Figu e 4. Schema ic diag am o AE linea localisa ion. In he expe imen s p esen ed he e, he moni o ing o AE ac i i y was pe o med wi h a dual-channel uni DAKEL ZEDO (ZD Rpe y, P ague, Czech Republic) wi h he ollowing inpu pa ame e s o hi -de ec o : • The h eshold alue o indi idual AE hi s was se a 200% abo e he noise le el; • A sampling o AE hi s was se o 5 MHz; • The cu -o equency o he low-pass il e was se o 500 kHz. The o al gain was 34 dB, bu only by p e-ampli ie . The AE senso s we e a ached o he specimens wi h beeswax in a hin laye ; see Figu e 5. The loca ion o he acous ic emission senso s on he specimen was p opo ional o he size o he es specimen. Figu e 5. Loca ion o acous ic emission senso s on a smalle (le ) and bigge ( igh ) specimen. 4. The Ongoing Chemical P ocesses The deg ada ion o bo h es ed binde sys ems due o he imme sion in ammonium ni a e and ace ic acid was ela ed o decalci ica ion. Decalci ica ion ep esen s a p ocess whe e he simul aneous leaching o Ca2+ om he binde gel con empo aneously wi h he o ma ion o co esponding calcium sal s occu s. This is ela ed o he simul aneous educ ion o he C/S a io in he binde gel, which can esul in he esidual S-H gel wi h Figu e 4. Schema ic diag am o AE linea localisa ion. Ma e ials 2023,16, 3527 7 o 17 In he expe imen s p esen ed he e, he moni o ing o AE ac i i y was pe o med wi h a dual-channel uni DAKEL ZEDO (ZD Rpe y, P ague, Czech Republic) wi h he ollowing inpu pa ame e s o hi -de ec o : •The h eshold alue o indi idual AE hi s was se a 200% abo e he noise le el; •A sampling o AE hi s was se o 5 MHz; •The cu -o equency o he low-pass il e was se o 500 kHz. The o al gain was 34 dB, bu only by p e-ampli ie . The AE senso s we e a ached o he specimens wi h beeswax in a hin laye ; see Figu e 5. The loca ion o he acous ic emission senso s on he specimen was p opo ional o he size o he es specimen. Ma e ials 2023, 16, x FOR PEER REVIEW 7 o 18 Figu e 4. Schema ic diag am o AE linea localisa ion. In he expe imen s p esen ed he e, he moni o ing o AE ac i i y was pe o med wi h a dual-channel uni DAKEL ZEDO (ZD Rpe y, P ague, Czech Republic) wi h he ollowing inpu pa ame e s o hi -de ec o : • The h eshold alue o indi idual AE hi s was se a 200% abo e he noise le el; • A sampling o AE hi s was se o 5 MHz; • The cu -o equency o he low-pass il e was se o 500 kHz. The o al gain was 34 dB, bu only by p e-ampli ie . The AE senso s we e a ached o he specimens wi h beeswax in a hin laye ; see Figu e 5. The loca ion o he acous ic emission senso s on he specimen was p opo ional o he size o he es specimen. Figu e 5. Loca ion o acous ic emission senso s on a smalle (le ) and bigge ( igh ) specimen. 4. The Ongoing Chemical P ocesses The deg ada ion o bo h es ed binde sys ems due o he imme sion in ammonium ni a e and ace ic acid was ela ed o decalci ica ion. Decalci ica ion ep esen s a p ocess whe e he simul aneous leaching o Ca2+ om he binde gel con empo aneously wi h he o ma ion o co esponding calcium sal s occu s. This is ela ed o he simul aneous educ ion o he C/S a io in he binde gel, which can esul in he esidual S-H gel wi h Figu e 5. Loca ion o acous ic emission senso s on a smalle (le ) and bigge ( igh ) specimen. 4. The Ongoing Chemical P ocesses The deg ada ion o bo h es ed binde sys ems due o he imme sion in ammonium ni a e and ace ic acid was ela ed o decalci ica ion. Decalci ica ion ep esen s a p ocess whe e he simul aneous leaching o Ca 2+ om he binde gel con empo aneously wi h he o ma ion o co esponding calcium sal s occu s. This is ela ed o he simul aneous educ ion o he C/S a io in he binde gel, which can esul in he esidual S-H gel wi h limi ed u ili y alue ins ead o he C-S-H o he C-A-S-H. The alkali-ac i a ed blas u nace slag has highe esis ance agains decalci ica ion compa ed o O dina y Po land Cemen , as s a ed in s udies such as [ 40 – 42 ]. The alkali-ac i a ed blas u nace slag binde gel has mo e in ensely c oss-linked silica e chains, since alumina uni s a e implemen ed in he binde gel s uc u e (C-A-S-H) compa ed o he main hyd a ion p oduc o cemen hyd a ion (C-S-H gel). Mo eo e , he A-S-H can o m a s able passi a ion laye . Nex , he pH o he po e solu ion o he alkali-ac i a ed slag is highe han he one in cemen i ious sys ems. Decalci ica ion due o he e ec o he ammonium ni a e akes place only when he pH o he solu ion is lowe han 9.25 [ 43 ], so since he ini ial pH o he alkali-ac i a ed sys em was highe , i helped o p e en deg ada ion. Ma e ials 2023,16, 3527 8 o 17 Finally, he e was no po landi e in alkali-ac i a ed slag. Po landi e p esen in a hyd a ed cemen i ious sys em can be easily leached ou om he ma ix. This p o ec s he C-S-H gel om decalci ica ion, bu simul aneously, once he po landi e is leached, he po es o he ma ix a e opened and he a e o deg ada ion is inc eased, since he pe meabili y o he ma ix inc eases. The bigge he specimen, he lowe de e io a ion o p ope ies was obse ed. This can be explained ia he mass a io be ween he deg ada ion medium and he specimen size. Bigge specimens con ain mo e Ca 2+ in p opo ion o ele an anions in he deg ada ion solu ion; hus, deg ada ion does no each such a a e ( olume and mass a io be ween he specimen/liquid di e s o bigge and smalle specimens). Then, agg essi e solu ions do no pene a e o such a dep h, and he esidual e ec i e non-a ec ed c oss-sec ion o he specimen is mo e signi ican . This e ec was mo e se e e o he cemen i ious sys em. No deg ada ion e ec o imme sion on he ap wa e was obse ed, ega dless o he specimen size and he ype o he binde . The imme sion in he 50 g/L solu ion o magnesium sulpha e did no esul in any signi ican changes in he p ope ies o he cemen i ious binde , no ma e he size o he specimen. The deg ada ion o he calcium–silica e-based binde s due o he e ec o magnesium sulpha e solu ion can be explained by he o ma ion o he expansi e p oduc as a p oduc o he eac ion be ween he calcium ions in he ma ix and sulphu anions o he pene a ing solu ion. These eac ion p oduc s can be gypsum, e ingi e and b uci e. The decalci ica ion can be obse ed along wi h hose p ocesses, lea ing he pa ially decalci ied binde gel behind [44]. The small es ing specimens p epa ed om alkali-ac i a ed slag mo a showed wo se esis ance agains sulpha e co osion compa ed o he cemen i ious one. The e is a dis- c epancy in he s udies compa ing deg ada ion esis ance be ween o dina y cemen and alkali-ac i a ed slag. Bel ame e al. and Mi hun e al. s a e a lowe esis ance o alkali-ac i a ed binde s compa ed o cemen i ious ones; on he o he hand, o he s such as Allah edi e al. and Bakha e e al. p esen ed a highe esis ance. The la ge specimens o alkali-ac i a ed mo a did no show such apid de e io a ion o es ed p ope ies as he smalle ones. This could, again, be ela ed o he a ec ed a ea o he specimen and he o e all a io be ween he specimen and he agg essi e media [44–47]. 5. Resul s and Discussion 5.1. Size E ec The g aphs in Figu e 6show wo ep esen a i e s ess load cu es o de e mining he s a ic modulus o elas ici y ( ed cu e). The plo s also show he hi eco d o he acous ic emission in he o m o ampli ude (blue poin s). These poin s a e in e laced wi h a line (linea eg ession) whose slope de e mines he in luence o he Kaise e ec [ 48 – 50 ] on he acous ic emission signals. The highe slope o his line e lec s he mo e signi ican demons a ion o he Kaise e ec . In he ma e ial ha exhibi s an ini ial ailu e unde a speci ic load, he Kaise e ec desc ibes he absence o acous ic emission un il ha load is exceeded. The Kaise e ec esul s om discon inui ies c ea ed in he ma e ial du ing p e ious s eps ha do no mo e, expand, o p opaga e un il he o me s ess is exceeded. The g aphs a e jus a demons a ion o he wo selec ed specimens. In he o he pa s o he esul s, he indi idual se s will be ep esen ed by he a e ages o he alues and he coe icien o a ia ion, see Tables 4and 5. Ma e ials 2023,16, 3527 9 o 17 Ma e ials 2023, 16, x FOR PEER REVIEW 9 o 18 exceeded. The Kaise e ec esul s om discon inui ies c ea ed in he ma e ial du ing p e ious s eps ha do no mo e, expand, o p opaga e un il he o me s ess is exceeded. Figu e 6. Example o he loading specimens du ing s a ic modulus o elas ici y wi h he eco ding o acous ic emission (ampli ude o hi s) o a smalle ( op g aph) and a bigge (bo om g aph) specimen made o CEM III/A imme sed in he ace ic acid solu ion. The g aphs a e jus a demons a ion o he wo selec ed specimens. In he o he pa s o he esul s, he indi idual se s will be ep esen ed by he a e ages o he alues and he coe icien o a ia ion, see Tables 4 and 5. Figu e 6. Example o he loading specimens du ing s a ic modulus o elas ici y wi h he eco ding o acous ic emission (ampli ude o hi s) o a smalle ( op g aph ) and a bigge ( bo om g aph ) specimen made o CEM III/A imme sed in he ace ic acid solu ion. Table 4. Tes esul s o CEM III/A-based mo a ( alues a e he a e age o h ee specimens wi h coe icien o a ia ion). Specimen Ec(GPa) c,p ism (Mpa) Numbe o Hi s (-) A e age AE Ampli ude (dB) Slope (dB/s) Smalle Wa e 40.56 (0.009) 61.85 (0.103) 1443 (0.010) 41.9 (0.153) −0.027 (0.295) Magnesium sulpha e 39.68 (0.010) 65.15 (0.069) 1213 (0.113) 46.8 (0.044) −0.037 (0.104) Ammonium ni a e 9.44 (0.081) 10.78 (0.112) 262 (0.257) 39.4 (0.047) −0.012 (0.212) Ace ic acid 18.17 (0.052) 12.37 (0.185) 263 (0.250) 36.4 (0.074) −0.005 (0.163) Bigge Wa e 40.99 (0.010) 66.55 (0.037) 318 (0.204) 46.1 (0.024) −0.030 (0.157) Magnesium sulpha e 40.40 (0.010) 64.89 (0.017) 310 (0.299) 47.4 (0.019) −0.036 (0.078) Ammonium ni a e 25.26 (0.024) 37.21 (0.126) 157 (0.441) 44.9 (0.056) −0.028 (0.374) Ace ic acid 30.54 (0.014) 39.96 (0.017) 291 (0.231) 49.1 (0.106) −0.031 (0.158) Ma e ials 2023,16, 3527 16 o 17 11. P o is, J. In oduc ion and Scope. In Alkali Ac i a ed Ma e ials; RILEM S a e-o - he-A Repo s; Sp inge : Do d ech , The Ne he lands, 2014; Volume 13, pp. 1–9. ISBN 978-94-007-7671-5. 12. 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[C ossRe ] Disclaime /Publishe ’s No e: The s a emen s, opinions and da a con ained in all publica ions a e solely hose o he indi idual au ho (s) and con ibu o (s) and no o MDPI and/o he edi o (s). MDPI and/o he edi o (s) disclaim esponsibili y o any inju y o people o p ope y esul ing om any ideas, me hods, ins uc ions o p oduc s e e ed o in he con en .