Feasibility of Using Calcium Silicate Carbonation to Synthesize High-Performance and Low-Carbon Cements

Carbon dioxide (CO 2 ) pretreatment to concrete at the early curing ages is a feasible technique towards achieving a greener and more sustainable concrete industry. However, the carbonation efficiency of ordinary concrete blocks is greatly limited due to their dense compactness. Here we proposed that the aeration of concrete blended with high-volume industry wastes may bring about improved sustainability of the precast concrete industry by enhancing the carbonation efficiency. We designed two different CO 2 pretreatment courses: that is, the gas pressure of CO 2 = 0.1 atm (0.01 MPa) for 8 h and 1 atm (0.1 MPa) for 4 h. The strength and microstructure of the aerated concrete with and without the CO 2 pretreatments were investigated by macro-and microscopic tests. Results showed that the CO 2 pretreatments greatly altered the minerals and pore structure of the aerated concrete blocks. The higher CO 2 pressure with less curing time raised the carbonation extent, crystal size, and compressive strength at 3 days. Compared with ordinary solid concrete, the aerated concrete showed a higher CO 2 sinking efficiency by more than one order of magnitude. This technique would potentially sink millions of tons of CO 2 from the precast aerated concrete industry.

Section: Results and Discusionsupporting

confidence: 81%

CO₂ Pretreatment to Aerated Concrete with High-Volume Industry Wastes Enables a Sustainable Precast Concrete Industry

Yan

Sun

et al. 2021

“…Lime mortar is typically prepared by mixing hydrated lime powder (Ca(OH) 2 , portlandite) with water and allowing the paste to carbonate in air. While this process is more sustainable than the use of Portland cementsince atmospheric CO 2 is sequestered while the mortar carbonates 51 mortar carbonation is slow (i.e., months to years). Further, incomplete carbonation leaves unreacted Ca(OH) 2 within substrates, which can produce high pH conditions that could be harmful to corals.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

Composite Substrates Reveal Inorganic Material Cues for Coral Larval Settlement

Levenstein

Marhaver

Quinlan

et al. 2022

The widespread loss of stony reef-building coral populations has been compounded by the low settlement and survival of coral juveniles. To rebuild coral communities, restoration practitioners have developed workflows to settle vulnerable coral larvae in the laboratory and outplant settled juveniles back to natural and artificial reefs. These workflows often make use of natural biochemical settlement cues, which are presented to swimming larvae to induce settlement. This paper establishes the potential for inorganic cues to complement these known biochemical effects. Settlement substrates were fabricated from calcium carbonate, a material present naturally on reefs, and modified with additives including sands, glasses, and alkaline earth carbonates. Experiments with larvae of two Caribbean coral species revealed additive-specific settlement preferences that were independent of bulk surface properties such as mean roughness and wettability. Instead, analyses of the substrates suggest that settling coral larvae can detect localized topographical features more than an order of magnitude smaller than their body width and can sense and positively respond to soluble inorganic minerals such as silica (SiO 2 ) and strontianite (SrCO 3 ). These findings open a new area of research in coral reef restoration, in which composite substrates can be designed with a combination of natural organic and inorganic additives to increase larval settlement and perhaps also improve post-settlement growth, mineralization, and defense.

“…Research exploring tobermorite synthesis from calcium silicates and calcium aluminum silicates has demonstrated that alkaline solutions are required for precipitation. ,,, Sodium bases, such as NaOH, have been shown to facilitate the precipitation of tobermorite better than other alkali metals. , Tobermorite precipitation from calcium silicates has also been described as the crystallization of CSH gel. ,− The formation of tobermorite and other CCSH phases from pseudowollastonite has been shown to increase when exposed to both sodium and CO 2 ; CCSH phases have precipitated when the carbon is delivered as a salt, for example, as Na 2 CO 3 or NaHCO 3 or CO 2 gas and NaOH. − However, the effects of CO 2 and sodium concentrations on the amount and phases of precipitated CCSH have not been explored in detail. Prior work in our laboratory suggested that the crystal structure and dissolution characteristics of the parent silicate were a major factor impacting CCSH precipitation .…”

Section: Introductionmentioning

confidence: 99%

“…A large body of literature explores mix designs and curing techniques that incorporate waste slags and ashes to further improve OPC hydration. − The waste products can improve the compressive strength and other properties by increasing the formation of calcium silicate hydrate (CSH) gel, the key reaction product of OPC hydration . These materials are rich in calcium silicates, which are generally not hydraulically activated, like OPC, but they can be reacted to form cementitious materials on their own using alkali or temperature activation. − As one example, the calcium silicate pseudowollastonite (α-CaSiO 3 ) can be carbonated in alkaline conditions and elevated temperatures to produce crystalline calcium silicate hydrate (CCSH) phases alongside calcium carbonates. − …”

Section: Introductionmentioning

confidence: 99%

Coprecipitation of Crystalline Calcium Silicates and Carbonates from the Hydrothermal Reaction of Pseudowollastonite

Tolliver,

Nguyen,

Dela Cerna

et al. 2024

Self Cite

Calcium silicates are abundant but sparingly soluble feedstocks of interest for making low-carbon alternative cements. Under hydrothermal and alkaline conditions, they can form crystalline calcium silicate hydrate (CCSH) products, which are abundant in Roman concrete, or they can form carbonates when CO 2 is present. To understand when coprecipitation of CCSH and carbonate phases is possible, we studied the hydrothermal carbonation of a model calcium silicate, pseudowollastonite (α-CaSiO 3 ), at 150 °C and high pH as a function of CO 2 source [CO 2 (g) or Na 2 CO 3 ] and different concentrations of sodium, alumina, and silica. Our experiments produced a range of CCSH phases including tobermorite−13 Å, rhodesite, and pectolite, as early as 1 day after the start of our experiments. After 7 days of curing in a 2 M NaOH solution, over 10% of the samples had been converted to these CCSH phases. We also observed the formation of CaCO 3 as both aragonite and calcite when carbon was introduced to our experimental system. The carbon source impacted the ratio of the CaCO 3 to CCSH phases in the reaction products. The availability of Na 2 CO 3 produced a balance between the CaCO 3 and CCSH phases, whereas CO 2 (g) produced more CaCO 3 , with samples that were over one-third carbonate by mass. Higher concentrations of Na + increased the precipitation of both the CaCO 3 and CCSH phases. The presence of excess silica, in the form of dissolved borosilicate glass from our reaction vessels under alkaline reaction conditions, also enhanced the formation of CCSH phases formed in some experiments. Supplemental Al 2 O 3 , a common constituent in many silicate feedstocks, also enhanced CCSH formation, likely by forming aluminum-substituted phases under the conditions tested here. These chemical insights can enable the design of formulation and curing guidelines for novel cementitious materials.