Enhancing the Reactivity of Limestone to Reduce Clinker Factor in Cement Production Janit Niyom, Guillermo Puerta Falla, Gaurav Sant

Department of Civil and Environmental Engineering, UCLA Abstract

As the fourth largest contributor to greenhouse gas emissions worldwide, ordinary portland cement (OPC) production has become an extensive cause of concern for the environment. One method of limiting such emissions involves substituting cement with other materials like limestone, which so far is confined to small replacement levels. To maintain equivalent mechanical properties to traditional Portland cements, this study investigates means to enhance limestone reactivity by addition of suitable alumina sources such as calcium aluminate cements (CACs), alumina silicates, and pure alumina. Thermogravimetric analysis (TGA), isothermal calorimetry, and compressive strength tests quantify the properties of these blends. Results show that aluminosilicates improved systems across all substitution levels while calcium aluminate cements (CACs) did not improve any blends. Systems containing reactive aluminium oxide showed improvements in strength only in the presence of limestone.

Acknowledgments

Introduction/Background

Materials and Methods

Results

Conclusions and Future Research

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Thermogravimetric Analysis (TGA)

It can be seen that portlandite amounts reduce for all blends. In the case of SECAR51 and Almatis 25R systems, this is due to OPC reduction. However, for the particular cases of Metakaolin and Alphabond, the reduction in portlandite is much steeper, indicating portlandite consumption. Metakaolin is widely known for its pozzolanic behavior, the ability to produce additional C-S-H from portlandite, explaining the sharp portlandite reduction. In the case of the Alphabond, portlandite transformation takes place but the products of such a process are not well established at this point.

Systems containing SECAR51, Almatis 25R and Alphabond show behaviors that deviate from that of pure OPC and limestone-OPC as the amount of aluminous material increases. Those systems show different acceleration and deceleration periods, some of them having more than one of each. This behavior is because the aluminous material used is reacting and producing heat additional to the heat of hydration. Systems containing Metakaolin also show behavior deviating from the control systems, but each stage of hydration is easily identifiable with defined acceleration and deceleration periods.

Isothermal Calorimetry

Compressive Strength

A significant reduction of strength in both CAC and Alphabond systems can be seen as greater substitution levels are included in the blend, possibly due to a reduction in OPC reactivity since dissolution rates of CAC and Alphabond are much higher than those of OPC, which makes dissolution of products used to form CSH more difficult. However the Alphabond-limestone systems show a slight recovery, producing values of strength above the pure limestone, possibly due to production of stable phases formed from limestone and alphabond. Metakaolin-limestone systems show the best behavior, somewhat better than Alphabond-limestone systems, but in this case the pozzolanic behavior of Metakaolin is most likely responsible for the strength recovery.

CAC Systems Aluminosilicate Systems Aluminum Oxide Systems

Alumina Sources: Four different alumina sources were used and tested at 5, 10, and 15% replacement

• Almatis 25R (CAC) • Secar51 (CAC)

Limestone: • Manufactured by OMYA • Substituted into the system at 30% replacement

OPC: • Type I/II Manufactured by Lehigh

Thermogravimetric Analysis (TGA) Can measure phase compositions of cement blends • derived TGA mass % curves show temperature

boundary of the cement decomposition reaction • Analysis yields information regarding mass

contents of portlandite and calcium carbonate

Isothermal Calorimetry Tracks hydration process of the cement paste • Cement hydration is an exothermic reaction • Results provide a way to compare paste

properties to heat flow and release

Compressive Strength Testing Determines mechanical strength of cement pastes studied compared to a pure cement control • Cement cubes (50mm x 50mm x 50mm) were

tested under uniaxial compressive stress conditions

• Graphs shown are from 90 day samples, best indicating long term strength

Calcium Silicate Hydrate (C-S-H) A major contributor to both strength and porosity in a cementing system Created by reaction of the silicate phases:

2C3S + 11H → C3S2H8 + 3CH tricalcium silicate water C-S-H calcium hydroxide

(portlandite)

or pozzolanic reaction: CH + S + H C-S-H

To William Herrera and the Dean of HSSEAS Vijay Dhir for hosting the HSSRP. To James Che for his help with my presentations and posters. To Professor Sant, whom emphasized that work is easy, but learning from it is a skill. Finally, much gratitude to my amazing lab supervisors, Tandré Oey and Guillermo Puerta Falla, for the support they gave throughout the program; I wouldn't have had such as amazing an experience without you two!

• Produced large values of heat, but performance was rather poor

• It is plausible that hindrance of OPC hydration is caused by the very fast dissolution rates of CAC

• Provided the best behavior across all levels of replacement

• Larger amounts of C-S-H assumed to form in the system due to pozzolanic reaction greatly enhanced the system

• This is a promising direction towards maintaining strength equivalence in low cement content systems

• In the absence of limestone, alphabond mixtures performed very poorly

• With limestone additions, alphabond systems performed almost as well as the aluminosilicate systems, a possible result of limestone’s enhanced reactivity

Future Work The work ahead will evolve to further clarify the process of cement hydration in the presence of CAC phases. Systems containing Alphabond and limestone should undergo extensive research in order to determine its mechanism of portlandite consumption and if/how limestone is reacting during hydration.

In this investigation, different aluminous sources were used in small amounts to test limestone's reactivity under aluminate excess conditions. The success of the present research would imply a reduction in carbon dioxide emissions (CO2) related to OPC production since good quality cement pastes could be produced with largely reduced quantities of OPC in the future.

The Influence of Limestone • Extremely abundant material on Earth

already used in ordinary portland cement (OPC) production

• May only replace cement clinker by 15% before compromising properties of system due to its inert nature

• May be further introduced into system by additions of aluminous materials

Thermogravimetric Analysis

Isothermal Calorimetry

Compressive Strength

• Alphabond 300 (aluminum oxide) • Metakaolin (aluminosilicate)

A model of an well-crystallized CSH structure in hydrated cement (left) compared to a CSH model that is poorly crystallized (right) showing where water pores would exist. In reality, CSH is poorly crystallized.

Cement Chemist Notation C = CaO S = SiO2 H = H2O A = Al2O3