Enhanced Metakaolin Reactivity in Blended Cement with Additional Calcium Hydroxide.

Kira Weise,Eduardus Koenders,Aaron Duncan,Neven Ukrainczyk

doi:10.3390/ma15010367

Abstract

This study aims to increase the pozzolanic reactivity of metakaolin (MK) in Portland cement (PC) blends by adding additional calcium hydroxide (CH_add) to the initial mixture. Cement paste samples were prepared with PC, MK and water with a water-to-binder ratio of 0.6. Cement replacement ratios were chosen from 5 to 40 wt.% MK. For higher replacement ratios, i.e., 20, 30 and 40 wt.% MK, CH_add was included in the mixture. CH_add-to-MK ratios of 0.1, 0.25 and 0.5 were investigated. Thermogravimetric analysis (TGA) was carried out to study the pozzolanic reactivity after 1, 7, 28 and 56 days of hydration. A modified mass balance approach was used to normalize thermogravimetric data and to calculate the calcium hydroxide (CH) consumption of samples with CH_add. Results showed that, without CH_add, a replacement ratio of 30 wt.% or higher results in the complete consumption of CH after 28 days at the latest. In these samples, the pozzolanic reaction of MK turned out to be restricted by the amount of CH available from the cement hydration. The increased amount of CH in the samples with CH_add resulted in an enhanced pozzolanic reaction of MK as confirmed by CH consumption measurements from TGA.

Highlights

Concrete, with cement as binder, is the most used construction material in the world [1]
The change to renewable energy causes the global amount of fly ash [5] and ground granulated blast-furnace slag to significantly decline [6], which underlines the importance for research on supplementary cementitious materials (SCMs) that are available for replacement of Portland cement (PC) in large amounts
In recent years, calcined clay has turned out to be a serious alternative for a pozzolanic SCM that can be used in mortars and concrete [7,8]

Summary

Introduction

With cement as binder, is the most used construction material in the world [1]. Briki et al [28] indicated that the addition of extra calcium hydroxide in a blended binder paste (CH_add/MK~0.36) results in a higher degree of reaction of MK, but only up to seven days of curing, followed by a stagnation at later ages. This experiment was performed to prove the hypothesis that a lack of CH may most likely not be the only reason for the declining MK reaction at later ages, consisting of a limestone–calcinedclay–cement blend (the so-called LC3) system. Different from the LC3‐system considered by Briki et al [32], when focusing on relatively low replacement ratios of PC cboynMsidKer(e~d22b.3y wBrti.%ki)e, tthael. p[3r2e]s,ewnthpenapfeorcurespinogrtosnprreellaimtivinealyrylofwindreinpglascoenmtehnet rreaaticotsivoitfyPoCf bmyeMtakKao(~li2n2‐.t3o‐wcet.m%e)n, tthbeinpdreesresnwt ipthapheirgrhepreoprtlsacpermeleimntinraatriyosfi. nTdhienghsyopnotthheesirseatoctibveitcyoonf‐ mfiremtaekdaoislinth-taot-,cfeomr ehnigt bhinredpelrascwemithenhtigrhatrieopsliancecmemenetnrta‐mtioest.aTkhaoelhinypsoytshteemsiss,tothbee acmonofiurnmteodf icsatlhciautm, fohryhdirgohxridepelgaecnemereantetdrabtyiotshienacleitme eantd-mbeltiatekaroealicntisoynsstemmasy, tbheecoameoudnetciosfivcealicniuthme hpyodzrzooxlaidneicgreenaecrtaitoend, bays CthHe aislitaecarnitdicbaellriteearcetancttio(endsumcat)y, abnecdotmhuesdmecaisyivaelsion tdheefipnoezazololawneicr rteharcetsihoonl,dasfoCrHthies raecprliaticceaml reenatctraantito(e[2d8u]c.tT),haenrdeftohrue,stmheayaiamlsoofdtehfiins estauldoywiesrttohrinesvheostldigfaoter tthhee rpeopzlzaocelamneicnrteraacttioiv[i2ty8]o. fTMheKreffoorrhe,igthereapimlacoefmtheinstsrtautdioysiisntcoeimnveenstt–imgaetteatkhaeolpinozszyosltaenmics r(e2a0c, t3i0viatnydo4f 0MwKt.%fo)rbhyigahddreinpglaecxetmraecnatlrcaiutimoshiyndcreomxiednet–(CmHet_aakdado)litno tshyestienmitisal(2p0a,s3te0manixd‐ 4tu0rwe.tT.%he) bpyroapdodsiendgaepxptraoaccahlcciouumldhpyodsrsoixbildyecl(aCrHify_athdedr)etdoutchteioinnitniaclopmapstreesmsiivxetusrtere. nTghteh pforor phoigshedceamppernotarcehpclaocuelmd epnotsrsaibtiloyscolafrMifyKthanedresdhuocwtiopnosinsibcolemimprpersosivveemsetrnetnsgftohr fdourrhaibgihl‐ citeym, ewnht ererepltahceemadendtitrioatniaols CofHMmKaayndassshuorwe spuofsfisciibelnetimalpkraolivneimtyeinntsaforreidnuforarcbeildityc,ownchreertee tnheeedadediftoiornaanl eCfHfecmtivaey paassusirveastuiofnficoifenstteaelk. alinity in a reinforced concrete needed for an effective passivation of steel

Materials and Methods

Sample Preparation

Findings

Measurement Methods