materials
Article A New, Carbon-Negative Precipitated Calcium Carbonate Admixture (PCC-A) for Low Carbon Portland Cements
Lewis McDonald 1, Fredrik P. Glasser 2 and Mohammed S. Imbabi 1,* 1 School of Engineering, University of Aberdeen, Aberdeen AB24 3UE, UK; [email protected] 2 Carbon Capture Machine (UK) Limited, Aberdeen AB10 1YL, UK; [email protected] * Correspondence: [email protected]; Tel.: +44-1224-272506
Received: 10 January 2019; Accepted: 11 February 2019; Published: 13 February 2019
Abstract: The production of Portland cement accounts for approximately 7% of global anthropogenic CO2 emissions. Carbon CAPture and CONversion (CAPCON) technology under development by the authors allows for new methods to be developed to offset these emissions. Carbon-negative Precipitated Calcium Carbonate (PCC), produced from CO2 emissions, can be used as a means of offsetting the carbon footprint of cement production while potentially providing benefits to cement hydration, workability, durability and strength. In this paper, we present preliminary test results obtained for the mechanical and chemical properties of a new class of PCC blended Portland cements. These initial findings have shown that these cements behave differently from commonly used Portland cement and Portland limestone cement, which have been well documented to improve workability and the rate of hydration. The strength of blended Portland cements incorporating carbon-negative PCC Admixture (PCC-A) has been found to exceed that of the reference baseline—Ordinary Portland Cement (OPC). The reduction of the cement clinker factor, when using carbon-negative PCC-A, and the observed increase in compressive strength and the associated reduction in member size can reduce the carbon footprint of blended Portland cements by more than 25%.
Keywords: CO2 emissions; carbon CAPture and CONversion (CAPCON); Precipitated Calcium Carbonate (PCC); limestone; strength; rheology; XRD analysis; Life Cycle Assessment
1. Introduction
1.1. Background Estimates vary, but the cement industry currently produces at least 2.8 billion tonnes of cement per annum, accounting for ~7% of global anthropogenic CO2 emissions [1–4]. Cement production is expected to increase yearly to more than 4 billion tonnes per annum by 2040. In 2009, the International Energy Agency (IEA) proposed the use of carbon capture and storage to reduce these emissions [5]. Since the IEA’s proposal to reduce the carbon emissions of the cement industry, new technologies have been developed to either capture and utilise post-combustion CO2 or convert it into useful products. Some of these technologies are discussed in Section 1.2. Calcium carbonate, in the form of limestone, has seen an increase in its use as a cement additive to reduce the carbon footprint of cement clinker, which is due to the relatively low CO2 output of the process in comparison to Portland cement manufacture [6]. Most codes of practice allow a maximum of 5% ground limestone, containing >70% calcium carbonate, to be added to create Portland limestone cement (PLC) blends as distinct from Ordinary Portland Cement (OPC). Ground limestone has also been demonstrated to increase workability without impacting strength if used in quantities of less than or equal to the maximum allowable 5% in accredited PLC blends [7–9].
Materials 2019, 12, 554; doi:10.3390/ma12040554 www.mdpi.com/journal/materials Materials 2019, 12 FOR PEER REVIEW 2 Materials 2019, 12, 554 2 of 14 been demonstrated to increase workability without impacting strength if used in quantities of less than or equal to the maximum allowable 5% in accredited PLC blends [7–9]. We present the first results of tests on the use of carbon-negative Precipitated Calcium Carbonate We present the first results of tests on the use of carbon-negative Precipitated Calcium Carbonate Admixture (PCC-A), a carbon CAPture and CONversion (CAPCON) product of the Carbon Capture Admixture (PCC-A), a carbon CAPture and CONversion (CAPCON) product of the Carbon Capture Machine (CCM) (see http://www.ccmuk.com for more information), to reduce the carbon footprint of Machine (CCM) (see http://www.ccmuk.com for more information), to reduce the carbon footprint blended cements. Unlike ground limestone, which is carbon positive, PCC-A from the CCM CAPCON of blended cements. Unlike ground limestone, which is carbon positive, PCC-A from the CCM process is carbon negative and sequesters between 100 and 350 kgCO2 per tonne of PCC-A produced, CAPCON process is carbon negative and sequesters between 100 and 350 kgCO2 per tonne of PCC- depending on the process inputs. Our proposal for a more sustainable future, where cement production A produced, depending on the process inputs. Our proposal for a more sustainable future, where is decarbonised at source, is illustrated in Figure1. cement production is decarbonised at source, is illustrated in Figure 1.
CO2
e
n i
Br CCM
PMC
PXC
PCC Figure 1. Proposal for the production of a new generation of enhanced performance, low carbon PrecipitatedFigure 1. Proposal Calcium for Carbonate the production Admixture of a (PCC-A) new genera blendedtionPortland of enhanced cements performance, can be achieved low carbon in situ usingPrecipitated the Carbon Calcium Capture Carbonate Machine Admixture (CCM). (PCC-A) blended Portland cements can be achieved in situ using the Carbon Capture Machine (CCM). The role of calcium carbonate in blended Portland cements spans chemical and physical properties. The chemicalThe role roleof calcium is through carbonate the formation in blended of (hemi- Portland and mono-)cements carboaluminatespans chemical phases and physical during hydrationproperties. [10 The]. Physically,chemical role it is is predominantly through the format an inertion filler of (hemi- [8,11– 13and]. Unpublishedmono-) carboaluminate data suggests phases that theduring control hydration of grain [10]. size Physically, and the morphology it is predominantly of the PCC-A an inert in the filler CCM’s [8,11–13]. CAPCON Unpublished process can data be beneficiallysuggests that used the to control produce of blendedgrain size Portland and th cementse morphology that do of not the compromise PCC-A in strength,the CCM’s durability CAPCON or passivationprocess can while be beneficially offering performance used to produce benefits bl thatended match Portland or surpass cements OPC. that do not compromise strength,The testdurability methods or reportedpassivation in thiswhile paper offering were performance used to evaluate, benefits for that the match first time, or surpass the effects OPC. of addingThe carbon-negative test methods reported PCC-A to in OPC, this paper potentially were leadingused to to evaluate, the development for the first of newtime, classes the effects of high of performance,adding carbon-negative low carbon PCC-A Portland to OPC, cement potentially blends. leading The methods to the development were used to of determine new classes strength, of high rheology,performance, passivation low carbon potential Portland and the cement reduction blends in embodied. The methods CO2 content.were used It has to beendetermine shown strength, that the userheology, of PCC-A passivation in blended potential cements and can the achieve reduction an increased in embodied strength CO2 content. at contents It has well been beyond shown the that 5% limitthe use for of ground PCC-A limestone, in blended while cements concurrently can achieve reducing an increased the embodied strength CO at2 contentscontent bywell 25% beyond or more. the 5% limit for ground limestone, while concurrently reducing the embodied CO2 content by 25% or 1.2.more. Carbon Capture and Utilisation in Cement Production Portland cement is the most abundantly manufactured material globally and is also one of the 1.2. Carbon Capture and Utilisation in Cement Production largest sources of CO2 emissions [1–4]; it is thus a prime candidate for the emerging carbon capture and utilisationPortland cement industry is tothe prioritise. most abundantly A recent report manufactured published material by Chatham globally House and [ 14is] also estimates one of that, the tolargest effectively sources control of CO the2 emissions impact of[1–4]; the cementit is thus industry a prime oncandidate the environment, for the emerging the clinker carbon factor capture (i.e., activeand utilisation fraction) inindustry cements to has prioritise. to be reduced A recent to 60%report of thepublished present by value Chatham by 2050. House Once [14] this estimates has been achieved,that, to effectively other methods control have the toimpact be developed of the cement that rely industry on carbon on the capture environment, and utilisation. the clinker factor (i.e., Anactive alternative fraction) toin carboncements capture has to be outlined reduced in to the 60% Chatham of the present House value report by is 2050. the useOnce of this “novel has cements”—lowbeen achieved, other carbon methods alternatives have to to be traditional develope Portlandd that rely cement on carbon blends. capture These and cements utilisation. include allegedlyAn alternative carbon-free to magnesium carbon capture oxide outlined derived fromin the magnesium Chatham silicatesHouse report (MOMS) is the cements, use of calcium “novel sulfoaluminatecements”—low (CSA)carbon cementsalternatives and to carbonated traditional calcium Portland silicate cement (CCSC) blends. cements These cements amongst include others. However,allegedly carbon-free novel cements magnesium have seen oxide little marketderived penetration from magnesium due to theirsilicates high (MOMS) production cements, costs (MOMScalcium
Materials 2019, 12 FOR PEER REVIEW 3 sulfoaluminate (CSA) cements and carbonated calcium silicate (CCSC) cements amongst others. Materials 2019, 12, 554 3 of 14 However, novel cements have seen little market penetration due to their high production costs (MOMS cements), specialty usage (CSA cements), scalability issues (CCSC cements) and resistance tocements), change by specialty codes of usage practice, (CSA consumer cements), groups scalability and issuesstandards (CCSC institutes, cements) compared and resistance to tried, to changetested andby codestrusted of Portland practice, consumercement alternatives. groups and The standards Chatham institutes, House compared report claims to tried, that tested carbon and capture trusted methodsPortland could cement reduce alternatives. CO2 emissions The Chatham by 95–100%. House Similarly, report claims a study that of carbonseveral captureactive carbon methods capture could plantsreduce conducted CO2 emissions by David by 95–100%. and Herzog Similarly, [15] a study found of the several typical active efficiency carbon capture to be plants90% for conducted direct emissions.by David andIn both Herzog cases, [15 the] found conclusions the typical relate efficiency to high cost, to be non-viable 90% for direct carbon emissions. capture Inand both storage cases, routesthe conclusions and are therefore relate to of high limited cost, non-viablevalue. The carbonestimates capture also ignore and storage the contributions routes and are of thereforesecondary of emissionslimited value. from Thequarrying, estimates transport also ignore and grinding, the contributions where the of secondaryCO2 cannot emissions be easily fromcaptured—see quarrying, Figuretransport 2. and grinding, where the CO2 cannot be easily captured—see Figure2.
Figure 2. Breakdown of CO2 emissions of Portland cement production. Data taken from the Chatham FigureHouse 2. report Breakdown [14]. of CO2 emissions of Portland cement production. Data taken from the Chatham House report [14]. Worldwide, CO2 capture and utilisation methods for the cement industry currently include the productionWorldwide, of carbon-negative CO2 capture and admixtures utilisation through methods mineralisation for the cement (Carbon industry Capture currently Machine) include [6] the and productioncarbonation of curedcarbon-negative cements (Solidia, admixtures CarbonCure, through COmineralisation2NCRETE) [(Carbon16–18]. Capture Machine) [6] and carbonationCarbonation cured cements cured cements (Solidia, were CarbonCure, first proposed CO2NCRETE) in the 1970s, [16–18]. but their development has been stuntedCarbonation due to process cured complexity, cements were cost first and applicationproposed in scope. the 1970s, In the carbonationbut their development curing process, has CObeen2 is stuntedconverted due into to process the various complexity, polymorphs cost ofand calcium applicat carbonate—calciteion scope. In the beingcarbonation the most curing common—through process, CO2 isthe converted interaction into of CSHthe various gel with polymorphs CO2 (gas) proceeding of calcium in carbonate—calcite two stages: initially being the CSH the “sheds” most common— Ca forming throughCaCO3 andthe interaction a low Ca CSH of CSH limit gel is reachedwith CO at2 (gas) Ca/Si proceeding ~1 which, uponin two continued stages: initially carbonation, the CSH yields “sheds” silica Cagel forming and calcium CaCO carbonate3 and a low [14 Ca]. This CSH differs limit fromis reached the normal at Ca/Si hydration ~1 which, route upon where continued calcium carbonation, hydroxide is yieldsformed silica in place gel and of calcium calcium carbonate carbonate [19 ].[14]. Prior This to COdiffers2 exposure, from the the normal free water hydration content route of the where cement calciumis reduced hydroxide to ensure is thatformed the COin place2 can of diffuse calcium efficiently carbonate throughout. [19]. Prior The to cementCO2 exposure, is then exposedthe free water to CO 2 contentat pressures of the upcement to 5 atm.is reduced After exposure,to ensure that the unreactedthe CO2 can cement diffuse may efficiently be hydrated throughout. again to The ensure cement that isunreacted then exposed phases to can CO continue2 at pressures to hydrate up andto 5 provide atm. After further exposure, increases the in strength unreacted [20]. cement The CO may2 content be hydratedof carbonation again to cured ensure cements that unreacted was verified phases by Seocan etcontinue al. [21] to through hydrate XRD and analysis provide and further determined increases to inrange strength from [20]. 6.9 to The 11% CO of2 thecontent cement, of carbonation depending oncured composition. cements was verified by Seo et al. [21] through XRD analysisThe carbonation and determined curing processto range is from claimed 6.9 to to 11% provide of the cement cement, with depending enhanced on durability,composition. as the calciumThe carbonation carbonate precipitation curing process within is claimed the CSH to pr gelovide creates cement a denser, with enhanc less poroused durability, structure as the [22 ]. calciumHowever, carbonate carbonation precipitation reduces thewithin alkalinity the CSH of thegel cementcreates pastea denser, and inless turn porous can lead structure to the [22]. early However,corrosion carbonation of reinforcing reduces steel, especially the alkalinity in the of presence the cement of chloride paste ionsand [in23 ].turn The can combined lead to reliancethe early on corrosionhigh pressure of reinforcing CO2 infusion steel, and especially the accompanying in the presence acidification of chloride of the ions cement [23]. The binder combined with consequent reliance onpassivation high pressure loss limits CO2 theinfusion application and the scope accompanying of such carbonation acidification methods of the to pre-cast,cement binder unreinforced with consequentconcrete products, passivation such loss as bricks,limits blocksthe application and paving scope tiles, of which such carbonation together account methods for no to morepre-cast, than unreinforced20% of all concrete concrete uses products, [24]. such Such as cements, bricks, bl however,ocks and could paving potentially tiles, which be together used with account non-steel for noreinforcement, more than 20% such of asall glass concrete fibres. uses [24]. Such cements, however, could potentially be used with non-steelUnlike reinforcement, carbonation such curing, as glass mineralisation fibres. through carbon capture serves more than one role; the CO2Unlikeis converted carbonation into minerals, curing, suchminera aslisation calcium through carbonate carbon and magnesium capture serves carbonate, more than which one are role; used thein aCO multitude2 is converted of industries. into minerals, The method such as of calcium mineralisation carbonate can and include magnesium the carbonation carbonate, of metalwhich oxide are by-products from mining processes [25] or through precipitating the minerals using CO2 absorption
Materials 2019, 12, 554 4 of 14 in an aqueous alkali solution [26]. In the cement industry, calcium carbonate can be used as a clinker replacement, as described in Section 1.1. Based on theory and experience, the maximum efficiency of CO2 mineralisation has been estimated at 68% [27]. The use of mineralised CO2 in the form of PCC-A (produced through carbon CAPCON technology developed by CCM) as a cement admixture is the focus of this paper, where first results have shown that a significant reduction of CO2 emissions of around 25% can be achieved by the substitution of clinker with carbon-negative PCC-A (see Section 3.4). The PCC-A in cement reacts with water during formation to produce hydrated calcium carboaluminates that develop from the hydration and partial carbonation of C3A phases. In doing so, the interaction between water and C3A is modified, leading to a more rapid formation of ettringite. By stabilising carboaluminates, sulfate is forced to remain in the ettringite and so conversion back to monosulfoaluminate becomes thermodynamically unfavourable [8]. This potentially impacts the role of gypsum, a widely used setting retarder [28]. The reference CO2 content of an off-the-shelf PLC was determined in the course of the tests, using XRD. At 11.4%, it has a comparable CO2 content to carbonation cured cements at 11.0% [21], but with none of the inherent disadvantages or limitations. Further testing of PCC-A blended cements is expected to confirm that a significantly higher embodied CO2 content can be achieved without either detriment to quality or limitation to the range of products made and applications deployed, when using these new generation Portland cements.
2. Materials and Methods
2.1. Materials Two types of cement were used in the tests carried out—a CEM I 52.5N OPC and a CEM II/A-LL 32.5R PLC. Hanson Cement UK supplied both products. The main constituents of the cements were determined through XRD analysis and Rietveld Refinement, as detailed in Sections 2.2 and 3.1. The manufacturer specifies that both cements have a Blaine fineness between 300 and 500 m2/kg. The PCC-A used is a product of the carbon CAPCON process developed by Carbon Capture Machine (UK) Limited. It was prepared from CO2-rich flue gas, where CO2 was selectively dissolved at the point of emission in dilute aqueous alkali (NaOH) solution. The solution was then mixed with calcium-containing brine to produce the PCC-A, which was subsequently filtered, rinsed and dried before use to remove contaminants, such as NaCl.
2.2. X-ray Diffraction (XRD) Analysis Samples of the PCC-A, and both cement types, were tested using a PANalytical X’Pert X-ray powder diffractometer to determine the composition. Powder specimens were analysed at room temperature at a 2θ range from 30◦ to 80◦, Cu K alpha radiation, using a scan period of 10 min.
2.3. Rheological Data The rheological data of the freshly mixed cement pastes were measured using a continually increasing shear rate up to 200 1/s over a period of 100 s before decreasing at the same rate to 0. A 25 mm coaxial cylinder spindle was used in conjunction with a 40 mm sample pot. The cement and PCC-A were weighed to provide a total solids content of 200 g and the desired water/solids ratio was also weighed. The PCC-A was added to the water and stirred continuously to ensure it was evenly dispersed. The PCC-A/water slurry was then combined with the cement in a mixing bowl and mixed continuously for 90 s; this mixing period was considered sufficient to ensure the hydration of the cement paste while reducing the risk of early-set formation. Within 30 s of the mixing ending, the paste was transferred to the sample pot and connected to the rheometer and the test was started. Data were collected for samples of OPC, PLC and PCC-A blended cement ranging from 0 to 20% PCC-A content. As the water/cement ratio decreased, the range of PCC-A cements that were sufficiently workable to test also decreased. Materials 2019, 12, 554 5 of 14
2.4. Compressive Strength Cement pastes were prepared in the same way as for rheological testing, but with a longer mix time in accordance with BS EN 196-1:2016 [29]. A longer mix time, compared with the rheological tests, was used to ensure the PCC-A was well distributed in the paste before casting. The paste was then cast in 50 × 50 × 50 mm cube steel moulds. Samples were maintained at a constant temperature of 25 ◦C for 24 h before being demoulded. Samples were thereafter fully submerged in a water bath at an ambient temperature for 7 or 28 days. It was observed during casting that cement pastes containing a high PCC-A content were less workable, i.e., they were more viscous than OPC and at the highest PCC-A content were difficult to cast. After curing, the samples were wiped dry before being tested. Testing was carried out using a hydraulic ram at a loading rate of 1 mm/min. The applied load was increased until the sample fractured. Data were collected electronically with a measurement of displacement and force recorded 6 times per second. The compressive strength was calculated as a variant of the equation provided in BS EN 196-1:2016 [29] to account for the different cube size tested:
F R = c (1) c 2500 where Rc is the compressive strength (MPa), Fc is the load recorded at the point of fracture (N) and 2500 is the area (mm2) that the load was applied to, i.e., 50 × 50 mm.
2.5. Scanning Electron Microscope (SEM) Imaging Fragments from the compressive strength testing were collected and used for SEM imaging. Pieces no greater than 10 mm in any dimension were coated using carbon evaporation under vacuum. The device used was a Zeiss GeminiSEM Field Emission Scanning Electron Microscope, which allowed for multiple methods of imaging to be used. Images of the fracture surface were obtained using secondary electron (SE) imaging and electron backscatter diffraction (BSE). SE imaging is the detection of low energy electrons emitted from the surface of the specimen exposed to the high energy electron beam and can be used to determine specimen topography. BSE is the detection of primary electrons that have been diffracted by an angle greater than 90◦. The angle at which the electron has been diffracted is related to the atomic structure of the scattering material and can be used to reveal changes in specimen composition [30].
3. Results and Discussion
3.1. XRD Analysis XRD diffractograms of the OPC and PCC-A blended cement samples tested were analysed using Rietveld Refinement, using Profex/BGMN [31] in accompaniment with structure data from ICSD [32]. Tables1 and2 provide, in standard notation, the phases identified in the cement clinker and the oxides present in those phases. The PLC used for comparison interestingly had a calcium carbonate content of 18.27%, which was more than the maximum amount of PCC-A blended with OPC that was tested (15%).
Table 1. Quantitative analysis of Ordinary Portland Cement (OPC) used in tests and oxides present.
Mineralogical Composition Phase Wt. % Chemical Composition Oxide Wt. %
C3S 59.65 SiO2 20.28 C2S 15.24 Al2O3 4.71 C3A 11.81 Fe2O3 3.27 C4AF 8.65 CaO 67.13 CSH2 4.65 SO3 2.54 MgO 0.67 K2O 1.40 Materials 2019, 12, 554 6 of 14
Table 2. Quantitative analysis of Portland limestone cement (PLC) and oxides present.
Mineralogical Composition Phase Wt. % Chemical Composition Oxide Wt. %
C3S 60.11 SiO2 15.23 C2S 7.66 Al2O3 3.33 C3A 9.42 Fe2O3 2.75 C4AF 1.03 CaO 64.40 CSH2 3.51 SO3 1.41 CaCO3 18.27 MgO 0.64 K2O 0.83 CO2 11.41
The chemical composition of the PCC-A used in the blended cements is summarised in Table3. This PCC-A was comprised of calcite with a very small, residual salt content remaining from the production process. No other CaCO3 polymorphs were identified using Rietveld Refinement.
Table 3. Quantitative analysis of PCC-A used in cement blends.
Compound Wt. % Calcite 99.8 Aragonite 0.0 Vaterite 0.0 Halite, NaCl 0.2
The NaCl content attributed to the PCC-A dropped to 0.02% when a 10% blended PCC-A cement was produced. This is the same as the NaCl content in the fresh tap water used to cure the test samples. Although the presence of NaCl can lead to an increased compressive strength, at this very low level it would be limited to no more than ~0.3%. A 10+% relative increase in the 28-day compressive strength of PCC-A blended cements, therefore, cannot be attributed to the presence of NaCl. The logical conclusion is that it is attributed to the use of PCC-A. XRD analysis provides a powerful tool for identifying the purity of PCC-A samples and will be of great benefit in future when different PCC-A polymorphs and granularities are used.
3.2. Determination of Strength After 7 days of curing, the PCC-A cement blends showed a reduction in strength with the carbon-negative Precipitated Calcium Carbonate (PCC) content above 4%. However, after 28 days, an increase in strength was observed up to 10% PCC. This increase in strength was accompanied by a decrease in the workability of the cement paste, tentatively attributable to the formation of carboaluminate phases, which act as a strong binder between the cement paste, unreacted PCC-A and, in the case of concrete, aggregates [13]. The complete set of results from the compressive strength testing is presented in Figure3. It can be seen that an increase in strength is gained after 7 days of curing for PCC-A content between 1 and 4%. Beyond 4% PCC-A, the strength of the cement starts to decrease as the PCC-A content increases. After 28 days, the strength increase resulting from the inclusion of PCC-A continues up to and beyond 15%, reaching a maximum strength of 54.8 MPa at 10% PCC-A content. This represents an increase of approximately 20% compared to the OPC reference. A PLC sample was separately tested and found to have a compressive strength of 18.1 MPa and 22.3 MPa at 7 and 28 days, respectively. MaterialsMaterials 2019 2019, ,12 12 FOR FOR PEER PEER REVIEW REVIEW 77 increase of approximately 20% compared to the OPC reference. A PLC sample was separately tested Materialsincrease2019 of, 12approximately, 554 20% compared to the OPC reference. A PLC sample was separately tested7 of 14 andand found found to to have have a a compressive compressive strength strength of of 18.1 18.1 MPa MPa and and 22.3 22.3 MPa MPa at at 7 7 and and 28 28 days, days, respectively. respectively.
FigureFigure 3. 3. Compressive Compressive strength strength of of cement cementscementss containing containing PCC-A PCC-A after afterafter 7 7 and and 28 28 day day of of curing, curing, with with an an increaseincrease in in strength strength of of up up to to 4%4% 4% contentcontent content inin in thethe the formerformer former andand and 10%10% 10% contentcontent content inin in thethe the latter,latter, latter, respectively.respectively. respectively.
SinceSince all all the the tests teststests were werewere carried carriedcarried out outout on onon cement cementcement pa pastes,pastes,stes, the thethe expected expectedexpected 52.5 52.552.5 MPa MPaMPa compressive compressivecompressive strength strengthstrength ofof the the OPC OPC was was not not reached—this reached—this value value is is normally normally attributed attributed to to mortar mortar strength.strength. strength. FutureFuture workwork onon strengthstrength strength willwill will requirerequire require testingtesting testing samplessamples samples atat at lowerlower lower water/cement water/cement water/cement (w/c)(w/c) (w/c) ratios ratios and and increasingincreasing the the curing curing time time up up to to 365 365 days. days. OPC, OPC, PL PLCPLCC andand 10%10% PCC-A PCC-A willwill alsoalso be be tested tested at at a a similar similar workability,workability, asas as isis is shownshown shown inin in Section Section 3.33.3. 3.3.. The The use use of of PCC-A PCC-A in in mortars mortars will will be be studied, studied, as as the the formationformation ofof carboaluminate carboaluminatecarboaluminate phases phasesphases has hashas been beenbeen documented documenteddocumented to have totostrong havehave bonding strongstrong bonding propertiesbonding propertiesproperties between aggregates betweenbetween aggregatesandaggregates cement and and paste cement cement [13]. paste paste [13]. [13]. 3.3. Rheological Analysis 3.3.3.3. Rheological Rheological Analysis Analysis Cement pastes with water/cement ratios of 0.5, 0.4 and 0.3 were tested using the method CementCement pastespastes withwith water/cementwater/cement ratiosratios ofof 0.0.5,5, 0.40.4 andand 0.30.3 werewere testedtested usingusing thethe methodmethod described in Section 2.3. Data were recorded using a Brookfield R/S+ rheometer in conjunction describeddescribed in in Section Section 2.3. 2.3. Data Data were were recorded recorded usin usingg a a Brookfield Brookfield R/S+ R/S+ rheometer rheometer in in conjunction conjunction with with with the Rheo3000 software package. The addition of PCC-A to cement created a more viscous paste thethe Rheo3000Rheo3000 softwaresoftware package.package. TheThe additionaddition ofof PCC-APCC-A toto cementcement createdcreated aa moremore viscousviscous pastepaste (Figures4–7), whereas the PLC tested for comparison was less viscous (Figures4,6 and7). (Figures(Figures 4–7), 4–7), whereas whereas the the PLC PLC tested tested for for comparison comparison was was less less viscous viscous (Figures (Figures 4, 4, 6 6 and and 7). 7).
FigureFigureFigure 4. 4. Shear4. Shear Shear stress stress stress measured measured measured in in in cement cement cement samples samples samples withwith with water/cementwater/cement water/cement (w/c) (w/c)(w/c) ratio ratio ratio = = =0.5 0.5 0.5 as as asrate rate rate of of of shear increases. shearshear increases. increases.
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Figure 5. Shear stress measured in cement samples containing high PCC-A content with w/c = 0.5 as shear rate increases. Figure 5. Shear stress measured in cement samples containing high PCC-A content with w/c = 0.5 as shearAs the rate w/c increases. ratio of the cement pastes decreases, the PCC-A cement samples over 10% became too viscous for the rheometer to start and are not included in Figures 6 and 7. This is likely due to the largeAs shear the stressw/c ratio that ofcan the be cementobserved pastes in the decrease initial parts, the of PCC-Athe tests, cement where samplessetting of over the cement10% became paste toostarts viscous early. forHowever, the rheometer the pastes to start then and break are down not includ quicklyed andin Figures become 6 andfluid 7. and This workable is likely again.due to Thethe largeovert shearrise instress viscosity that can is betermed observed “false in theset” initial and partcan ofsometimes the tests, wherebe overcome setting ofby the using cement admixed paste startsplasticiser. early. The However, false-setting the pastes compositions, then break assuming down quickly that normaland become strength fluid gain and occurs workable at later again. stages, The overtmight riseactually in viscosity be useful is intermed developing “false sprayedset” and co canncretes—exploring sometimes be overcome this is a taskby using for the admixed future. plasticiser.However, these The false-setting compositions compositions, would not be assuming useful in that conventional normal strength construction. gain occurs at later stages, might actually be useful in developing sprayed concretes—exploring this is a task for the future. Figure 5. Shear stress measured in cement samples containing high PCC-A content with w/c = 0.5 as However,Figure these 5. Shear compositions stress measured would in cement not be samples useful incontaining conventional high PCC-A construction. content with w/c = 0.5 as shear rate increases. shear rate increases.
As the w/c ratio of the cement pastes decreases, the PCC-A cement samples over 10% became too viscous for the rheometer to start and are not included in Figures 6 and 7. This is likely due to the large shear stress that can be observed in the initial part of the tests, where setting of the cement paste starts early. However, the pastes then break down quickly and become fluid and workable again. The overt rise in viscosity is termed “false set” and can sometimes be overcome by using admixed plasticiser. The false-setting compositions, assuming that normal strength gain occurs at later stages, might actually be useful in developing sprayed concretes—exploring this is a task for the future. However, these compositions would not be useful in conventional construction.
Figure 6. Shear stress in cement samples at w/c = 0.4. The 15 and 20% PCC-A samples became Figureunworkable 6. Shear at this stress w/c inwithout cement the samples use of plasticiser at w/c = and 0.4. have The 15been and omitted. 20% PCC-A samples became Figure 6. Shear stress in cement samples at w/c = 0.4. The 15 and 20% PCC-A samples became unworkable at this w/c without the use of plasticiser and have been omitted. unworkable at this w/c without the use of plasticiser and have been omitted.
Figure 6. Shear stress in cement samples at w/c = 0.4. The 15 and 20% PCC-A samples became unworkable at this w/c without the use of plasticiser and have been omitted.
Figure 7. Shear stress in cement samples containing w/c = 0.3. The 10% PCC-A became unworkable at
this w/c and are excluded.
As the w/c ratio of the cement pastes decreases, the PCC-A cement samples over 10% became too viscous for the rheometer to start and are not included in Figures6 and7. This is likely due to the large shear stress that can be observed in the initial part of the tests, where setting of the cement paste starts early. However, the pastes then break down quickly and become fluid and workable again. The overt rise in viscosity is termed “false set” and can sometimes be overcome by using admixed plasticiser. The false-setting compositions, assuming that normal strength gain occurs at later stages, might actually be useful in developing sprayed concretes—exploring this is a task for the future. However, these compositions would not be useful in conventional construction. As the 10% PCC-A cement samples showed the greatest compressive strength of those tested (see Figure3), cement pastes of OPC and PLC that provided a similar workability by adjusting the w/c
Materials 2019, 12 FOR PEER REVIEW 9
Figure 7. Shear stress in cement samples containing w/c = 0.3. The 10% PCC-A became unworkable at this w/c and are excluded. Materials 2019, 12, 554 9 of 14 As the 10% PCC-A cement samples showed the greatest compressive strength of those tested ratio(see Figure were found. 3), cement These pastes w/c ratiosof OPC are and shown PLC inthat Figure provided8. The a volumesimilar workability of water required by adjusting for a 10% the PCC-Aw/c ratio to were obtain found. a similar These workability w/c ratios are to OPCshown is in 25% Figure higher. 8. The For volume PLC, the of requiredwater required volume for is a 2.5% 10% lower.PCC-A Further to obtain work a similar is clearly workability needed to to support OPC is the25% wide higher. use ofFor PCC-A PLC, the cement required blends volume in future. is 2.5% lower. Further work is clearly needed to support the wide use of PCC-A cement blends in future.
FigureFigure 8.8. WorkabilityWorkability ofof threethree differentdifferent blendsblends toto determinedetermine thethe w/cw/c ratio for whichwhich OPC,OPC, PLCPLC andand 10%10% PCC-APCC-A samplessamples exhibitexhibit similarsimilar rheologicalrheological properties.properties.
FurtherFurther workwork isis plannedplanned andand willwill bebe carriedcarried outout onon thethe rheologyrheology ofof PCC-APCC-A cementcement blendsblends toto definitivelydefinitively establishestablish whywhy theythey behavebehave inin aa mannermanner dissimilardissimilar toto whatwhat isis knownknown forfor PLC.PLC.
3.4.3.4. Scope forfor EmissionEmission ReductionReduction ofof PCC-APCC-A CementCement BlendsBlends The reduction in CO emissions through the use of PCC-A can be calculated as follows: The reduction in CO22 emissions through the use of PCC-A can be calculated as follows:
CO reduction, C = w · m + w · m , (2) CO22 reduction, C = wopcopc ∙ mopcopc + wpccpcc ∙ mpccpcc, (2) wherewhere wwopcopc and w wpcc areare the the mass mass fractions fractions of of OPC OPC and and PCC-A PCC-A respectively, respectively, and and mopc mopc andand mpcc m ispcc theis themass, mass, in grams, in grams, of ofthe the CO CO2 emitted2 emitted in inthe the production production of of 1 1kg kg of of the the respective respective product. The finalfinal potentialpotential reductionreduction inin cementcement isis thenthen calculatedcalculated usingusing thethe reductionreduction inin clinkerclinker requiredrequired toto achieveachieve thethe samesame compressivecompressive strengthstrength asas OPC.OPC. TheThe finalfinal reductionreduction inin COCO22 emissionsemissions isis thenthen foundfound as:as:
%% reduction reduction CO CO2 =2 = (1 (1− −(C (Ci i· ∙ (R(Rc-opcc-opc/R/Rc-ic-i))/C))/Copc))opc ∙ ))100,· 100, (3) (3) wherewhere CCii isis thethe COCO22 reduction ofof aa PCC-APCC-A withwith ii == 1–151−15 wt.%wt.% PCC-A,PCC-A, CCopcopc isis thethe COCO22 emittedemitted fromfrom OPCOPC production,production, RRc-opcc-opc isis the the mean compressive strength of OPCOPC andand RRc-ic-i is thethe meanmean compressivecompressive strengthstrength ofof thethe PCC-APCC-A blendedblended cementcement withwith ii == 1–15%.1−15%. For example, thethe reductionreduction inin COCO22 for the 10% PCC-APCC-A cementcement isis calculatedcalculated asas follows:follows:
CO2 reduction, C10% = 0.9 ∙ 951.5 gCO2/kg + 0.1∙ (−100) gCO2/kg, (4) CO2 reduction, C10% = 0.9 · 951.5 gCO2/kg + 0.1 · (−100) gCO2/kg, (4)
CC10%10% == 846.35 846.35 gCO gCO22/kg,/kg, (5)(5)
% reduction CO2 = (1 − (846.35 gCO2/kg · (44.8 MPa/54.8 MPa))/ 951.5 gCO2/kg)) · 100, (6) % reduction CO2 =(1 − (846.35 gCO2/kg ∙ (44.8 MPa/54.8 MPa))/ 951.5 gCO2/kg))∙ 100, (6) % reduction CO2 = 27.3%. (7)
The reduction in CO2 emission% attributed reduction toCO2 the = use27.3%. of PCC-A as a carbon-negative clinker (7) substitute, excluding any embodied CO2 reduction that may be attributed to decarbonising the cement manufacturingThe reduction process in CO (as2suggested emission inattributed Figure1 ),to is the summarised use of PCC-A in Figure as a9 .carbon-negative The values for clinker clinker substitutionsubstitute, excluding alone are displayedany embodied as dashed CO2 lines;reduction the total that reduction may be values,attributed incorporating to decarbonising the effect the of thecement strength manufacturing increase of PCC-Aprocess cement(as suggested blends in and Figure the associated 1), is summarised reduction in in Figure the structural 9. The values member for size/volume, are represented as solid lines. Materials 2019, 12 FOR PEER REVIEW 10 Materials 2019, 12 FOR PEER REVIEW 10 clinker substitution alone are displayed as dashed lines; the total reduction values, incorporating the effect of the strength increase of PCC-A cement blends and the associated reduction in the structural Materialsclinker2019 substitution, 12, 554 alone are displayed as dashed lines; the total reduction values, incorporating10 of 14the effectmember of thesize/volume, strength increase are represented of PCC-A as cement solid lines. blends and the associated reduction in the structural memberTraditionally, size/volume, the are production represented of one as solid kilogram lines. of OPC clinker emits on average ~950 gCO2 [33]. TheTraditionally, productionTraditionally, of the onethe production productionkilogram ofof of onePCC-A one kilogram kilogram sequesters of of OPC OPC~100–350 clinker clinker gCO emits emits2, depending on on average average on ~950 ~950 the gCO alkaligCO2 [2 33 used[33].]. TheThein productionthe production capture ofprocess. of one one kilogram kilogramTo date, of ofthe PCC-A PCC-A carbon sequesters sequesters mitigation ~100–350 ~100–350 of two gCO alkalis gCO2,2 depending, dependinghas been onexamined—sodium on the the alkali alkali used used ininhydroxide the the capture capture and, process. process. to a limited To To date, date, extent, the the carbon ammonia.carbon mitigation miti Thegation production of of two two alkalis alkalis of sodium has has been been hydroxide examined—sodium examined—sodium has a carbon hydroxidehydroxideintensity (i.e., and, and, the to to aweight limiteda limited ratio extent, extent, of CO ammonia.2 ammonia.emitted in The Theproduction production production per ofunit of sodium sodium weight hydroxide ofhydroxide product has made)has a a carbon ofcarbon ~870 intensityintensitygCO2/kg (i.e., (i.e.,[34] the andthe weight weightammonia ratioratio has ofof a CO carbon2 emittedemitted intensity in in production production of ~1,600 per gCO per unit unit2/kg weight [35]. weight However, of ofproduct product there made) made) is ofa body ~870 of ~870gCOof evidence gCO2/kg2 /kg[34] to [and 34suggest] andammonia ammoniathat 80%has a hasof carbon ammonia a carbon intensity used intensity ofin ~1,600the of reaction ~1,600 gCO2 gCO /kgcould [35].2/kg be However, [regenerated35]. However, there and thereis reused,a body is a bodyofand evidence so of the evidence carbon to suggest intensity to suggest that could 80% that of be 80% ammonia reduced of ammonia to used 667 gCOin used the2/kg inreaction thedue reaction to could its lower couldbe regeneratedmaterial be regenerated demand and reused, during and reused,andthe CO so and the2 capture socarbon the process. carbon intensity intensity For could further couldbe inreducedformation, be reduced to 667 see to gCO the 667 2Technology gCO/kg due2/kg to due its Ce lower tontre its Mongstad lowermaterial material demand (TCM) demand Chilledduring duringtheAmmonia CO the2 capture CO Process2 capture process. (CAP), process. For which further For is furtherbased information, on information, the chemistrysee the see Technology theof the Technology NH Ce3-COntre2 Centre- MongstadNH3 system Mongstad (TCM) [36]. (TCM) Chilled ChilledAmmoniaThe Ammonia final Process CO Process2 (CAP),sequestered (CAP), which using which is based either is basedon sodiumthe on chemistry the hydroxide chemistry of the or ofNH ammonia the3-CO NH23- -CONH is then23 -system NH 1003 system gCO[36]. 2 or [36 350]. gCOThe2The, respectively. final final CO CO2 2sequestered sequestered The PCC-A using usingused in either the testsodium results hydroxide hydroxide that have or or been ammonia ammonia reported is is then was then 100produced 100 gCO gCO2 or 2using or350 350gCOsodium gCO2, 2respectively., hydroxide, respectively. andThe The thePCC-A PCC-A values used used in inFigure in the the test test9 reflect results results that. that Figure havehave beenbeen10 shows reported reported the potential was was produced produced reduction using using in sodiumsodiumemissions hydroxide, hydroxide, if recovered and and the ammonia the values values inis inused Figure Figure as9 thereflect 9 reflect alkali that. that.in Figurethe Figure PCC-A 10 10shows pr showsoduction the the potential potentialprocess. reduction Alkalisreduction from in in emissionsemissionswaste could if if recovered recoveredpotentially ammonia ammonia offer equally is is used used low as as thecarbon the alkali alkali intensity in in the the PCC-A alternatives. PCC-A production production process. process. Alkalis Alkalis from from wastewaste could could potentially potentially offer offer equally equally low low carbon carbon intensity intensity alternatives. alternatives.
(a) (b)
Figure 9. Clinker substitution(a) with P CC-A produced using NaOH: (b) (a ) CO2 emissions of PCC-A
FigureFigurecement 9. Clinker9.blends Clinker substitutionwith substitution increasing with with PCC-APCC-A PCC-A produced content produced usingand strength NaOH:using NaOH: (aconsiderations,) CO2 (emissionsa) CO2 emissionsand of PCC-A (b) percentageof cement PCC-A blendscementreduction with blends in increasing CO with2 emissions PCC-Aincreasing of content PCC-A PCC-A andcement co strengthntent blends and considerations, through strength clinker considerations, and substitution (b) percentage and and (strength reductionb) percentage gain. in
COreduction2 emissions in CO of PCC-A2 emissions cement of PCC-A blends cement through blends clinker through substitution clinker and substitution strength gain. and strength gain.
(a) (b)
Figure 10. Clinker substitution(a) with PCC-A produced using NH :( a ) CO (b)emissions of PCC-A cement Figure 10. Clinker substitution with PCC-A produced using NH3 3: (a) CO2 2 emissions of PCC-A cement blends with increasing PCC-A content and strength considerations, and (b) percentage reduction in Figureblends 10.with Clinker increasing substitution PCC-A withconten PCC-At and producedstrength considerations, using NH3: (a) COand2 emissions(b) percentage of PCC-A reduction cement in CO2 emissions of PCC-A cement blends through clinker substitution and increase in strength. blendsCO2 emissions with increasing of PCC-A PCC-A cement conten blendst and through strength clin kerconsiderations, substitution and (inbcrease) percentage in strength. reduction in
CO2 emissions of PCC-A cement blends through clinker substitution and increase in strength. The introduction of PCC-A blended cements could have a significant impact on the carbon footprint of the cement industry (and with that the built environment), due to the reduction in CO 2 Materials 2019, 12 FOR PEER REVIEW 11 Materials 2019, 12 FOR PEER REVIEW 11
Materials 2019, 12, 554 11 of 14 The introduction of PCC-A blended cements could have a significant impact on the carbon footprint of the cement industry (and with that the built environment), due to the reduction in CO2 footprint of the cement industry (and with that the built environment), due to the reduction in CO2 emissions in the PCC-A production process and the subsequent substitution of clinker using PCC-A. emissions in the PCC-A production process and the the subsequent subsequent substitution substitution of of clinker clinker using using PCC-A. PCC-A. As shown in Figure 9, the 15% PCC-A blended cement made using a NaOH CAPCON process As shownshown in in Figure Figure9, the9, the 15% 15% PCC-A PCC-A blended blende cementd cement made usingmade a using NaOH a CAPCON NaOH CAPCON process provided process provided a reduction in emissions of 27% compared to OPC. A PCC-A made using an ammonia provideda reduction a inreduction emissions in ofemissions 27% compared of 27% to compared OPC. A PCC-A to OPC. made A PCC-A using an made ammonia using process an ammonia would process would deliver a 30% reduction. Future work on the use of PCC-A will involve a detailed from processdeliver awould 30% reduction. deliver a 30% Future reduction. work on Future the use work of PCC-A on the will use involve of PCC-A a detailed will involve from a cradle-to-grave detailed from cradle-to-grave Life Cycle Assessment (LCA) of PCC-A cement blends, and it will study how they cradle-to-graveLife Cycle Assessment Life Cycle (LCA) Assessment of PCC-A (LCA) cement of blends,PCC-A andcement it will blends, study and how it they will comparestudy how against they compare against other cements and cement blends in terms of overall performance, resistance to compareother cements against and other cement cements blends and in terms cement of overallblends performance, in terms of overall resistance performance, to deterioration resistance and cost. to deterioration and cost. 3.5. SEM Imaging 3.5. SEM Imaging SEM micrographs were obtained using a secondary electron detector. Figure 11 is of the non-optimisedSEM micrographs PCC-A thatwere was obtained used to us produceing a secondary the samples electron and testdetector. results Figure reported 11 is in of this the paper, non- optimisedat progressively PCC-A higher that magnifications.was used to produce Figure the12 presents samples and and the test backscatter results reported electron in detector this paper, images at progressivelyof the fracture higher surface magnifications. of the 15% PCC-A Figure cement 12 presen blend.ts and Both the large backscatter and small electron PCC-A detector crystals images were ofidentified the fracture on the surface fracture of the surface 15% ofPCC-A the sample cement using blend. SEM Both imaging, large and the small latter PCC-A possibly crystals due to were the identifiedincreased surfaceon the areafracture for reactionsurface requiredof the sample for the using formation SEM ofimaging, carboaluminate the latter phases. possibly Figure due 13to isthe a increased surface area for reaction required for the formation of carboaluminate phases. Figure 13 is increasedbackscatter surface detector area image for reaction of the fracture required surface for the of formation the 10% and of carboaluminate 15% PCC-A cement phases. blends. Figure 13 is a backscatter detector image of the fracture surface of the 10% and 15% PCC-A cement blends.
(a) 200× (b) 4000× (a) 200× (b) 4000× Figure 11. SEM micrographs of the PCC-A that was used in cement blends using secondary electron Figure 11. SEM micrographsmicrographs of of the the PCC-A PCC-A that that was was used used in cementin cement blends blends using using secondary secondary electron electron (SE) (SE) detection. Grain size was estimated to range from less than 100 nm to 20 µm. (a) 200× (SE)detection. detection. Grain Grain size was size estimated was estimated to range fromto range less thanfrom 100 less nm than to 20 µ100m. (nma) 200 to× 20magnification µm. (a) 200× of magnification of PCC-A and (b) 4000× of the same PCC-A sample. magnificationPCC-A and (b )of 4000 PCC-A× of and the same(b) 4000× PCC-A of the sample. same PCC-A sample.
(a) 4000× (b) 10,000× (a) 4000× (b) 10,000× Figure 12. SE images showing fracture surface topograp topographyhy in a sample of the 15% PCC-A cement Figure 12. SE images showing fracture surface topography in a sample of the 15% PCC-A cement blend at different magnifications.magnifications. (a)) 40004000×× magnificationmagnification and and ( (bb)) 10,000× 10,000× magnification.magnification. blend at different magnifications. (a) 4000× magnification and (b) 10,000× magnification.
Materials 2019, 12, 554 12 of 14 Materials 2019, 12 FOR PEER REVIEW 12
(a) 4000×, 10% PCC-A (b) 4000×, 15% PCC-A
Figure 13. ElectronElectron backscatter backscatter diffraction diffraction (BSE) (BSE) images images displaying displaying various various phases phases on on fracture fracture surface. surface. Each phase present is displayed by different intensitiesintensities of black and white, with black representing unreacted PCC-A fragments. ( a) 40004000×× magnificationmagnification of of 10% PCC-A cement blend andand ((bb)) 40004000×× magnificationmagnification of 15% PCC-A cement.
SEM imaging is a useful tool in determining the presence of PCC-A in cement blends and for SEM imaging is a useful tool in determining the presence of PCC-A in cement blends and for grain size determination. Further work to determine the presence of carboaluminate phases and the grain size determination. Further work to determine the presence of carboaluminate phases and the progression of phase composition is planned for samples cured from 1, 7, 28 and 365 days. progression of phase composition is planned for samples cured from 1, 7, 28 and 365 days. 4. Conclusions 4. Conclusions PCC-A, made using flue gas CO2 in CCM’s CAPCON process, used as a Portland cement PCC-A, made using flue gas CO2 in CCM’s CAPCON process, used as a Portland cement admixture, leads to an increased compressive strength that peaks at 10 wt.% PCC-A content. admixture, leads to an increased compressive strength that peaks at 10 wt.% PCC-A content. Combined with the carbon-negative nature of PCC-A, the resulting reduction in the clinker factor Combined with the carbon-negative nature of PCC-A, the resulting reduction in the clinker factor and the cumulative carbon content, the carbon footprint of a PCC-A blended cement containing 15% and the cumulative carbon content, the carbon footprint of a PCC-A blended cement containing 15% PCC-A is equivalent to ~73% of that of the OPC average. PCC-A is equivalent to ~73% of that of the OPC average. The strengths of PCC-A cement blends increase with PCC-A content, but the workability reduces, The strengths of PCC-A cement blends increase with PCC-A content, but the workability and, speculation notwithstanding, a full and detailed understanding of the processes that are involved reduces, and, speculation notwithstanding, a full and detailed understanding of the processes that is needed. are involved is needed. To achieve the same level of workability as an OPC paste, a PCC-A cement paste requires 25% To achieve the same level of workability as an OPC paste, a PCC-A cement paste requires 25% more water, which is not how a PLC cement behaves (requiring 2–5% less water). more water, which is not how a PLC cement behaves (requiring 2–5% less water). Future work to be undertaken on the effects of PCC-A in blended cements will include: Future work to be undertaken on the effects of PCC-A in blended cements will include: • The testing of PCC-A for use in mortars to determine the binding effects on aggregates; • The testing of PCC-A for use in mortars to determine the binding effects on aggregates; • • Examination of of PCC-A PCC-A cements cements after long after curing long times curing to study times the to evolution study theof carboaluminate evolution of phases;carboaluminate phases; • Investigation of the inverse effects on rheology comparedcompared withwith PLC;PLC; • Full LCA of PCC-A blended cements using more alkali-efficientalkali-efficient mineralisationmineralisation processes;processes; • Preparation of PCC-A blended cements containing polymorphs of PCC-A to determine if crystal structure plays a role in reactivity duringduring hydration.hydration.
Author Contributions: Contributions:The The named named authors—L.M., authors—L.M., F.P.G. andF.P.G. M.S.I.—all and M. contributedS.I.—all contributed to the writing to ofthe this writing article. of this article. Funding: The first named author L.M., a PhD scholar at the University of Aberdeen working under the supervision Funding:of M.S.I. and The F.P.G., first is named sponsored author through L.M., a fullya PhD funded scholar studentship at the University by CCM (UK) of Aberdeen Ltd. working under Acknowledgments:the supervision of M.S.I.The cementsand F.P.G., used is sponsored in this work through were a kindly fully funded provided studentship by Hanson byCement CCM (UK) UK. Ltd.Electron Microscopy was performed in the ACEMAC Facility at the University of Aberdeen. Conflicts of Interest: The authors declare no conflict of interest. Acknowledgments: The cements used in this work were kindly provided by Hanson Cement UK. Electron Microscopy was performed in the ACEMAC Facility at the University of Aberdeen Conflicts of Interest: The authors declare no conflict of interest.
Materials 2019, 12, 554 13 of 14
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