A New, Carbon-Negative Precipitated Calcium Carbonate Admixture (PCC-A) for Low Carbon Portland Cements

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; Life Cycle Assessment; Precipitated Calcium Carbonate (PCC); XRD analysis; carbon CAPture and CONversion (CAPCON); limestone; rheology; strength.

Figure 1

Proposal for the production of a new generation of enhanced performance, low carbon Precipitated Calcium Carbonate Admixture (PCC-A) blended Portland cements can be achieved in situ using the Carbon Capture Machine (CCM).

Figure 2

Breakdown of CO₂ emissions of Portland cement production. Data taken from the Chatham House report [14].

Figure 3

Compressive strength of cements containing PCC-A after 7 and 28 day of curing, with an increase in strength of up to 4% content in the former and 10% content in the latter, respectively.

Figure 4

Shear stress measured in cement samples with water/cement (w/c) ratio = 0.5 as rate of shear increases.

Figure 5

Shear stress measured in cement samples containing high PCC-A content with w/c = 0.5 as shear rate increases.

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.

Figure 8

Workability of three different blends to determine the w/c ratio for which OPC, PLC and 10% PCC-A samples exhibit similar rheological properties.

Figure 9

Clinker substitution with PCC-A produced using NaOH: (a) CO₂ emissions of PCC-A cement blends with increasing PCC-A content and strength considerations, and (b) percentage reduction in CO₂ emissions of PCC-A cement blends through clinker substitution and strength gain.

Figure 10

Clinker substitution with PCC-A produced using NH₃: (a) CO₂ emissions of PCC-A cement blends with increasing PCC-A content and strength considerations, and (b) percentage reduction in CO₂ emissions of PCC-A cement blends through clinker substitution and increase in strength.

Figure 11

SEM micrographs of the PCC-A that was used in cement blends using secondary electron (SE) detection. Grain size was estimated to range from less than 100 nm to 20 µm. (a) 200× magnification of PCC-A and (b) 4000× of the same PCC-A sample.

Figure 12

SE images showing fracture surface topography in a sample of the 15% PCC-A cement blend at different magnifications. (a) 4000× magnification and (b) 10,000× magnification.

Figure 13

Electron backscatter diffraction (BSE) images displaying various phases on fracture surface. Each phase present is displayed by different intensities of black and white, with black representing unreacted PCC-A fragments. (a) 4000× magnification of 10% PCC-A cement blend and (b) 4000× magnification of 15% PCC-A cement.