A discussion of the papers “ Impact of hydrated magnesium carbonate additives on the carbonation of reactive MgO cements ” and “ Enhancing the carbonation of MgO cement porous blocks through improved curing conditions ”, by C. Unluer & A. Al-Tabbaa

Abstract

8This paper is a discussion of two recent papers by Unluer & Al-Tabbaa [1, 2] which analysed accelerated 9 carbonation of reactive MgO blocks. We suggest that the authors have incorrectly analysed key data, leading 10 to overstated claims of MgO carbonation. Based on reassignment of their X-ray diffraction data, it is proposed 11 that little MgO carbonation occurred in the samples discussed in those papers, with CaCO3 instead forming 12 during accelerated carbonation. We also draw attention to the thermodynamic instability of nesquehonite 13 under ambient conditions, which calls into question the long-term stability of these binders. 14 Discussion 15Cements containing reactive magnesia are of great interest as alternative binders, as they have been 16 proclaimed to embody potentially lower CO2 emissions during manufacture and service. Two recently 17 published papers by Unluer and Al-Tabbaa [1, 2] have added to the body of literature on these cements, 18 studying the effect of hydrated magnesium carbonate (HMC) addition and curing conditions, respectively, on 19 the properties and structure of porous reactive MgO cement blocks exposed to accelerated carbonation 20 conditions. We will focus the discussion here on the first of these two papers, as the results presented in the 21 second are largely an extension of the first, and contain similar points requiring re-analysis. 22In these papers the authors claim to carbonate MgO to form a range of magnesium carbonates which 23 constitute their binding phases; this is a key aspect of the 'green' credentials proposed for these alternative 24 cements. Unfortunately, we are unable to reach the same conclusions made by the authors, based on our 25 own analysis of the data presented in their papers. In our opinion, the scientific discussion in these two 26 papers is based upon poorly-assigned X-ray diffraction patterns, which have led to incorrect interpretations 27 of thermal analysis data, and consequently erroneous claims of high levels of carbonation. 28In the paper "Impact of hydrated magnesium carbonate additives on the carbonation of reactive MgO 29 cements" [1], the authors produce blocks containing natural aggregates, pulverised fuel ash (PFA) as filler, 30and MgO as the key anhydrous precursor, with hydrated magnesium carbonates (HMCs) added to some of 31 the mixes. The combination of hydration and carbonation is proposed to lead to the formation of additional 32 hydrated magnesium carbonates as binding phases, when cured under either natural or accelerated 33 carbonation conditions. The authors achieved some interesting strength data, exceeding 20 MPa in 34 compression in some instances, which shows that their methodology is of some interest. 35However, there are several apparent discrepancies in the peak assignments in the two XRD patterns used by 36 the authors to identify hydration products after accelerated carbonation (Fig. 7 in [1]). This graphic is 37 reproduced here as Figure 1, with our suggested peak assignments for their first set of XRD patt...

Magnesia-Based Cements: A Journey of 150 Years, and Cements for the Future?

“…Free energy phase diagram for hydrous magnesium carbonates per mole of Mg, adapted from ref , under conditions of P CO 2 = 400 ppm and P H 2 O = 32 mbar (saturation vapor pressure at 298 K), along with the densities per mole of Mg atoms of stable magnesium phases.…”

Accelerated carbonation of reactive MgO and Portland cement blends under flowing CO2 gas

“…CO2 curing has been suggested to accelerate the carbonation of cement-based products and facilitate carbon sequestration in solid mineral phases [16,17]. During CO2 curing, dissolved and ionised CO2 induces the carbonation of Ca 2+ /Mg 2+ ions from the cement matrix, which then precipitates in the voids of the matrix as carbonates (anhydrous and hydrated) in a short period of time, boosting setting and hardening process, forming a dense, strong and potentially stable structure [18][19][20]. MC usually contains more than 85% active MgO, and the CO2 sequestration capacity can reach up to 92.8 wt%, which is higher than the capacity of PC (50.4 wt%) based on theoretical calculation.…”

Microanalysis of Primary Biological Particles from Model Grass over Its Life Cycle

“…91 92 Recent thermodynamic calculations by Chaka and Felmy are in agreement with these observations, indicating that hydromagnesite is more stable at room temperature. 93 This issue was recently discussed in the context of reactive MgO cements, 94 with the conclusion that caution is required in the application of such cementing systems, as the changes in crystal composition, volume, and morphology that would take place during the conversion of nesquehonite into hydromagnesite can be expected to fundamentally weaken the structure of these binders during their service life and also to lead to dimensional instability of the materials as a whole (Figure 3.).…”

“…97 Further refinement of processing conditions led to the production of MgO-based blocks reaching strengths of 22 MPa after only 14 days, 27 which are similar to the strengths of many commercially available masonry units and thus offer a pathway for the uptake of this technology as a replacement for bricks or PC-based masonry blocks. The detailed identification of the phase equilibria, particularly the nature and stability of the magnesium carbonates present as a function of sample curing and conditioning, is still under discussion in some of these systems, 94 These works do, however, confirm that conditions with both elevated CO 2 and elevated relative humidity are required for the production of a useful product, with limited strengths observed under atmospheric curing conditions. This would limit the technology to precasting applications, where curing can be controlled in a factory, rather than application on-site.…”