Phlego Cement from a New Earth-Inspired Clinker

Abstract

Improved cement for concrete is provided having reduced carbon footprint and improved mechanical properties. A limestone-free process of making the clinker provides a 70% reduction of carbon footprint vs. conventional manufacture of Portland cement. Curing the resulting cement in a temperature range from 80° C. to 100° C. advantageously enhances growth of fibrous minerals in the concrete.

Claims

1. A method of making a clinker for use in cement production, the method comprising: providing raw meal including silica, aluminum oxide, alkali oxides, calcium oxide and sulfur oxide; wherein a weight fraction of silica in the raw meal is between 40% and 60%; wherein a weight fraction of the aluminum oxide in the raw meal is between 10% and 25%; wherein a weight fraction of the alkali metal oxides in the raw meal is between 5% and 15%; wherein a weight fraction of the calcium oxide in the raw meal is between 5% and 15%; wherein a weight fraction of the sulfur oxide in the raw meal is between 0.5% and 2%; wherein a weight fraction of carbon-containing chemical species in the raw meal is less than 1%; firing the raw meal to provide the clinker.

2. The method of claim 1, wherein the firing the raw meal to provide the clinker is performed in a temperature range from 900° C. to 1200° C.

3. A method of forming concrete, the method comprising: performing the method of claim 1 to provide a clinker; slaking the clinker to provide a slurry; curing the slurry in a temperature range from 80° C. to 100° C. to promote the growth of fibrous minerals.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIGS. 1A-B are SEM images of exemplary prior cementitious microstructures.

[0013] FIG. 2 is a table showing exemplary compositions for this work.

[0014] FIG. 3 shows decreased carbon emission according to principles of this work.

[0015] FIGS. 4A-B are SEM images of exemplary growth of fibrous minerals made via hydrothermal synthesis.

DETAILED DESCRIPTION

A) Decarbonizing Cement Manufacturing

[0016] Decarbonizing cement manufacturing is not a trivial task. The fraction of Portland cement being responsible for the emissions (i.e., the CaO from limestones) is also responsible for the formation of the glue of Portland cement (the calcium-silicate hydrates, C-S-H). Therefore, the task requires substituting that fraction with something that promotes the formation of different cementitious phases. The greener glue must also ensure durability and serviceability.

[0017] Current technologies are focusing on alternative processes and/or materials—from capturing CO.sub.2 from calcination for its use in concrete curing technology, to the production of alternative cement blends, to the use of alternative raw materials that promote different cementitious phases.

[0018] Curing carbonation relies on carbon upcycling, and is an accelerated curing process into vessels that capture waste carbon dioxide from calcination, injects supercritical (i.e., Temperature=31° C. and Pressure=72.8 atm) CO.sub.2 into fresh concrete, and traps the CO.sub.2 by mineralizing it into calcite (CaCO.sub.3). While the technology induces the cementation of a concrete that is potentially carbon-neutral, it has shortcomings: it is limited to precast concrete products and forms carbonate minerals. These minerals are notoriously brittle and subject to chemical weathering. Furthermore, the exploitation of this technology is also hindered by the high costs for adapting cement plants to needs.

[0019] Alternative cement blends substitute a fraction of Portland cement (the source of CaO) with a source of Si and Al, namely alumino-silicate materials. The alumino-silicate material can either be an industrial byproduct, cheaper fly ash and slag coming, respectively, from coal-burning plants and steel production, or a raw material of natural origin, namely volcanic ash or metakaolin. Since the amount of CaO content in the Portland clinker strongly controls compressive strength, cementation is induced by adding alkaline solutions to promote the activation of different cementitious phases (e.g., sodium-alumino-silicate hydrates, N-A-S-H). Challenges range from costs to operational safety concerns associated with the causticity of the chemical additives. Furthermore, with coal plants being retired and steel production declining, fly ash and slag are not as plentiful as they once were.

[0020] The alternative raw material presented here is solid and naturally contains both alkaline earth oxides (CaO and MgO) and alkali metals oxides (Na.sub.2O and K.sub.2O)

B) Examples of Prior Art Cementitious Microstructures

[0021] FIGS. 1A and 1B are scanning electron microscope images of Roman marine concrete (MacFarlane et al., 2020) and the concrete-like rock from a caldera (Vanorio and Kanitpanyacharoen, 2015). Both microstructures show the micro-fibrous nature of the cementitious microstructure.

C) Exemplary Clinker Composition of this Work

[0022] An exemplary set of compositions for this work is given in the table of FIG. 2. Here the Leucitophyre column is composition of the input rock (i.e., the igneous alternative to limestone and clay), the raw meal column is composition of the input material for clinker pyroprocessing, and the pozzolanic ash column is the composition of the ash to be mixed with the clinker and slaked to provide the phlego cement.

[0023] The overall process flow is:

1) grind up the input rock to provide the raw meal,
2) fire the raw meal to provide the clinker (E.g., a firing temperature in the range 900-1200 C for a residence time of 60 minutes, similar to the process used to fire conventional raw meal to provide conventional clinker),
3) the user then slakes the clinker,
4) the user then makes a mixture of the slaked clinker and pozzolanic ash and cures to form a mortar with desirable properties as described above.

[0024] The raw meal contains minerals that lack the carbon- or carbonate-ions. As such, the clinker process is carbon neutral. The raw meal can contain both alkaline earth metal (i.e., magnesium, calcium, strontium, and barium) oxides and alkali metal oxides (i.e., sodium and potassium oxides). Once mixed with pozzolanic ash, the clinker provides a hybrid CASH-geopolymer cement—the Phlego cement, that is expected to have superior mechanical properties and serviceability.

[0025] Furthermore, these rock types exhibit a naturally-blended composition leading to a clinker that has the ability to create a crossbred mortar made of:

(1) natural mineral fibers of ettringite (C-S-A-H, a calcium-sulfo-aluminate hydrates),
(2) C-A-S-H phases (calcium-alumino-silicate hydrates) that are present in modern Al-rich pozzolanic cements, and
(3) geopolymers.
Current cement clinkers ensure the presence of some of these phases through the addition of additives.

[0026] The igneous rock is called Leucite-Tephrite or Tephrite-Basanite (or alternatively, Foid-Monzodiorite/Monzogabbro). These terms refer to rocks all sharing the same elemental composition. Differences simply lie in the type of microstructure and level of crystallinity of the rock. This type of igneous rocks is ubiquitous in Nature, and is found in both active and ancient ultra-potassic or calc-alkaline magmatism above subduction margins.

[0027] The composition of this new raw meal is made of:

[0028] (a) silica (40% to 60% by weight);

[0029] (b) aluminum oxide (10% to 25% by weight);

[0030] (c) Sulfur oxide (0.5% to 2% by weight), which together with the presence of CaO and Al.sub.2O.sub.3, has the capability of forming fibrous ettringite (C-S-A-H, calcium-sulfo-aluminate hydrates). This provides a natural fiber reinforced mortar;

[0031] (d) Calcium oxide (5% to 15% by weight), which together with the presence of Al.sub.2O.sub.3, forms Al-tobermorite (C-A-S-H, calcium-alumino-silicate hydrates) when mixed with pozzolanic ash;

[0032] (e) Alkali metal oxides (Na.sub.2O and K.sub.2O, 5% to 15% by weight). Alkalis are important as they serve the dual purpose of (1) speeding up the hydration process of the C-A-S-H and (2) favoring chemical cross-linking, which leads to the formation of polymeric phases. In current clinkers, the presence of alkalis is ensured through the pyroprocessing of clay-rich rocks, which are added to the raw material. The presence of alkali metal oxides within a naturally blended CaO and Al.sub.2O.sub.3 mix also ensures the formation of geopolymers when the clinker is slaked under alkaline conditions and temperature as low as 80° C. By having approximately as much alkali metal oxides (Na.sub.2O and K.sub.2O) as CaO, together with the presence of Al.sub.2O.sub.3, this blend has the capability of producing a clinker that is, by its nature, hydraulic.

[0033] An important aspect of this raw meal is what's not in it—carbon-containing chemical species in the raw meal are less than 1% by weight, and are preferably as close to 0% by weight as possible.

[0034] To produce the Phlego cement one part of this new clinker can be mixed with two parts of pozzolanic ash, either from natural origin or industrial byproducts (e.g. volcanic or fly ashes). Each lump of the clinker phase being dispersed within the pozzolan will then constitute a functional and structural unit of the cement. When slaked, each lump of clinker forms fibrous ettringite (C-S-A-H, calcium-sulfo-aluminate hydrates), creating a cluster of intertwined fibers being dispersed within the pozzolanic ash and branching out to the surrounding areas. This forms a natural fiber reinforced material. Due to the presence of alkali metal oxides (Na.sub.2O and K.sub.2O) and CaO, both in the clinker and pozzolanic ash, fibrous ettringite will result embedded in a crossbred matrix made of a geopolymer and C-A-S-H (C-A-S-H, calcium-alumino-silicate hydrates).

[0035] Besides the environmental benefit of cutting down on CO.sub.2 emissions, the impact of this cement is on expanded durability, improved physico-chemical resilience, and serviceability. The most notable impact is on applications that require absorption of strain energy and/or stability in harsh environments. These applications include (1) concrete that must perform in areas experiencing seismic ground shaking and (2) cement sheath between wellbore casing that is exposed to injection of CO.sub.2, acid fluids injected for fracking, or re-injection of wastewater from fracking, which pose risks to water resources, (3) encapsulation of industrial waste and (4) planetary shelters and habitats of tomorrow.

D) Reduced Carbon Emission

[0036] FIG. 3 shows mass loss from heating at 900° C. (pyroprocessing) both limestone and the new binder precursor. The mass loss from limestone (squares) is stoichiometric (43.9%, the slope of the fitting trend), and corresponds to the mass of CO.sub.2 emitted upon the calcination of CaCO.sub.3. The new binder precursor (circles) is carbon-free compared to limestone, suggesting a huge saving in CO.sub.2 emissions during thermal decomposition.

E) Examples of Growth of Fibers Via Hydrothermal Synthesis

[0037] FIGS. 4A-B are images showing examples of growth of fibers through hydrothermal synthesis (Head et al., 2018). Promoting the growth of fibrous minerals internally to the binder through the use of both an alkali-calcic clinker composition (as described above) in conjunction with a synthesis that mimics natural hydrothermal systems (˜80-100° C.) is uniquely advantageous as it (a) eliminates the problem of adding fibers to a material, which increases the viscosity of the cement and (b) makes fibers to blend-in the mortar, both chemically and mechanically. The former avoids problems related to workability of the slurry; the latter promotes the bonding process at the matrix-fiber interface.