Drying-Induced Atomic Structural Rearrangements in Sodium-Based Calcium-Alumino-Silicate-Hydrate Gel and the Mitigating Effects of Zr!" #anoparticles Kengran Yang, V. Ongun Özçelik, Nishant Garg, Kai Gong and Claire E. White* Department o" Civil & Environmental Engineering and Andlinger Center for Energy and the Environment, Princeton University, Princeton, U*% Abstract Con#entional drying of (olloidal !aterials and gels (including (e!ent, (an lead to detri!ental effe(ts due to the buildup of internal stresses as /ater e#aporates from the nano/!i(ros(opi( pores. 1o/e#er, the underlying nanos(opi( alterations in these gel !aterials that are, in part, responsible "or !a(ros(opi(all&2!easured strain #alues, espe(iall& at lo/ relati#e humidit&, re!ain a topi( of open debate in the literature. In this study, sodium2based (al(ium2alumino- sili(ate2hydrate (C-(N,2%2*21, gel, the !a4or binding phase of sili(ate2a(ti#ated blast furna(e slag (one type of lo/2CO5 (e!ent), is in#estigated from a drying perspe(ti#e, since it is kno/n to suffer extensi#el& from drying-induced !i(rocra(king. 7& e!ploying in situ synchrotron 8-ra& total s(attering !easure!ents and pair distribution function +'D9, anal&sis /e sho/ that the signi:(ant (ontributing "a(tor to the strain de#elop!ent in this !aterial at extre!el& lo/ relati#e humidit& +;<, is the local atomi( structural rearrange!ent of the C-(N,2%2*21 gel, including (ollapse of interla&er spa(ing and slight disintegration of the gel. =oreo#er, anal&sis of the !edium range (1.0 ? 2.2 nm, ordering in the 'D9 data re#eals that the 'D9-deri#ed strain #alues are in !uch (loser agree!ent (sa!e order of !agnitude, /ith the !a(ros(opi(all& !easured strain data, (ompared to previous results based on re(iprocal spa(e 8-ra& diffra(tion data. 9rom a !itigation standpoint, /e sho/ that s!all a!ounts of @rO5 nanoparti(les are able to a(ti#el& reinforce the structure of sili(ate2a(ti#ated slag during drying, pre#enting atomi( le#el strains from de#eloping. =e(hanisti(all&, these nanoparti(les induce gro/th of a sili(a2ri(h gel during drying, /hi(h, via densit& functional theor& (al(ulations, /e sho/ is attributed to the high sur"a(e rea(tivit& of tetragonal ZrO5. 1 Introduction Drying is a (om!on (and potentiall& detri!ental, processing te(hniAue used in a #ariet& of industries, including sol-gel synthesis and (era!i(s. Bhe re!o#al of Cuid (usuall& /ater) from a porous !aterial (an lead to a buildup of internal stresses.> Bypi(all& the body /ill not dr& uni"or!l&, and there"ore a drying front /ill de#elop in the !aterial /here the dried outer (omponent /ants to shrink, /hereas the !oist interior restrains shrinkage. Bhis !is!at(h in strain leads to tensile stresses de#eloping on the sur"a(e of the !aterial, and i" these stresses e6(eed the tensile strength, the !aterial /ill (ra(k. =itigation strategies include (areful (ontrol of !aterial thi(kness,5 use of sur"a(e (oatings to pre#ent (ra(king during e#aporationD and (ontrolled drying.E One (lass of !aterial that is used in signi:(ant Auantities around the /orld that is prone to suffer from drying, and there"ore (ra(king, is (oncrete. %lthough there are se#eral viable !ediating te(hnologies a#ailable "or 'ortland (e!ent2based (oncrete to pre#ent drying- induced !i(rocra(king, the underlying !e(hanis!s responsible "or the !a(ros(opi(all&2 !easured strains re!ain some/hat unclear, espe(iall& those oc(urring at lo/ relati#e humidit& +F1, G 40%). =oreo#er, the de#elopment of lo/2CO5 (e!ent alternati#es, such as alkali2 a(ti#ated !aterials +%%=s), has led to a ree6a!ination of (e!ent degradation phenomena, including drying-induced mi(rocra(king. Gi#en that ordinar& 'ortland (e!ent +O'C) production a((ounts "or H ? I< of anthropogeni( H CO5 e!issions, there is a pressing need to de#elop and i!ple!ent sustainable alternati#es. %%=s are one of the !ost (ompetiti#e alternati#es, and ha#e been sho/n to e!it less CO5 +J E; ? I;<, (ompared /ith O'C.K2I %%=s utilize aluminosili(ate2ri(h pre(ursor !aterials, including industrial b&-products such as ground granulated blast2furna(e slag, Cy ash from coal2:red po/er plants and (al(ined (la&s (e.g., !etakaolin), /hi(h "or! !e(hani(all& hard binders (gels, /hen a(ti#ated by alkaline solutions (or solids, as is the case for 1-part mi6es9,>;).K,>> Further!ore, gi#en (orre(t !i6 designs, %%=s ha#e (omparable !e(hani(al per"or!an(e>5 and (ostM to O'C, and (an be tuned to ha#e superior properties via spe(i:( che!i(al (ompositions, such as high ther!al per"or!ance>D and lo/ per!ea.ilit&.>E Ne#ertheless, Auestions re!ain regarding the long-ter! durabilit& of %%=s, /ith the underlying degradation !e(hanis!s often "ounded at the atomi(0nanos(ale, such as (arbonation-induced >H2>I (he!i(al rea(tions (from at!ospheri( and a((elerated CO5 (onditions, and sul"ate atta(k of the binder gel.>L,5; 'rogress is being !ade to elucidate the !e(hanis!s responsible "or (he!i(al degradation of different types of %%=s, /ith the ai! to pinpoint /hi(h !i6 designs are !ost resistant to different "or!s of degradation.>K,5>,55 1o/e#er, !i(rocra(king re!ains an outstanding durabilit& and aestheti( issue "or sili(ate2a(ti#ated slag-based %%=s, and since sili(ate2a(ti#ated slag possesses superior setting ti!es,5D strength de#elopment,5D and per!eabilit&>E (ompared /ith other %%=s, having a funda!ental s(ienti:( understanding of the underlying (ause(s, of !i(rocra(king is needed together with potential solutions for mini!izing/!itigating this issue. 2 Bhe sus(eptibilit& of sili(ate2a(ti#ated slag to !i(rocra(king has been kno/n "or de(ades,5E25M /here the extent of !i(rocra(king in sili(ate2a(ti#ated slag paste is (onsiderabl& larger (ompared /ith O'C paste.5M Collins and *anja&an proposed that the difference in drying shrinkage bet/een sili(ate2a(ti#ated slag and O'C is due to their different pore size distributions, i!posing different le#els of (apillar& "orce on the O'C/sili(ate2a(ti#ated slag paste as /ater is re!o#ed from the pores.5M 1o/e#er, it is kno/n that the pretreat!ent process (i.e., re!o#al of pore liAuid) reAuired .& the t/o !a4or pore (hara(terization te(hniAues (i.e., nitrogen sorption and !ercur& intrusion porosi!etr&, !a& alter the pore structure of (e!entitious !aterials to a non-negligible extent.5I2DD =ore re(entl&, based on the per!eabilit& results from the bea!2bending te(hniAue, *(herer et al. esti!ated the pore dia!eters of O'C paste to be J 1.5 ? H nm, 5I /hi(h /as also obser#ed .& @eng et al. using nitrogen sorption /ith freeze2drying as the !ethod of pretreat!ent.DE Fe(ent nitrogen sorption !easure!ents on sili(ate2a(ti#ated slag per"or!ed .& Blyth et al.>E /ith pretreat!ent via isopropanol sol#ent e6(hange sho/ed that the !a4or pore size obtained from the desorption (ur#e of the isother! is J D ? E nm, /hi(h is (lose to that of O'C. 1o/e#er, it /as also sho/n .& Blyth et al. that the per!eabilit& of sili(ate2a(ti#ated slag is lo/er than that of O'C paste (0.0001 nm5 (ompared /ith 0.005 nm5 at M da&s "or /ater/pre(ursor ratio of 0.5 +/0( ratio "or O'C)).>E %s reported .& *(herer, the per!eabilit& has an in#erse relationshi to the stresses that de#elop during drying,DH and there"ore, this difference in per!eabilit& bet/een sili(ate2a(ti#ated slag and O'C paste !a& be responsible in part "or the sus(eptibilit& of sili(ate2 a(ti#ated slag paste to mi(rocra(k. During drying of (e!entitious !aterials, it is generall& a((epted that there are no signi:(ant (hanges oc(urring to the atomi( structure of the paste. Internal stresses lead to shrinkage of the body via (onsolidation of the parti(les (i.e., rearrange!ent of the (e!ent grains). Onl& /hen the F1 rea(hes ver& lo/ le#els +G 20%) does /ater e#aporate from the interla&er spa(ing of cal(ium2 sili(ate2hydrate (C-S21, gel, the !a4or binding phase of O'C paste, leading to a (ollapse of this interla&er spa(ing and an associated additional shrinkage at the !a(ros(opi( le#el.DK 1o/e#er, a "e/ arti(les ha#e pointed to additional (hanges in the C-S21 gel structure due to drying, spe(i:(all& intra2 and inter2granular (ohesion of C-S21,DM (hanges in the pol&!erization state of the sili(ate spe(iesDI and a de(rease in atomi( ordering (as !easured via the /idth of 5L*i nuclear !agneti( resonance (N=F, peaks, /ith the proposed "or!ation of ne/ Ca-O2*i bonds as /ater is re!o#ed from the interla&er).DL %dditional research is reAuired to substantiate these results, and to unco#er the link bet/een these proposed (hanges and the !a(ros(opi(all& !easured drying shrinkage.