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Nanotechnology for Greener Concrete By Taijiro Sato
This article reports recent research by NRC-IRC on the use of nano-CaCO3 as a supplementary cementing material in concrete.
The cement industry is seeking ways to reduce the energy and resource consumption of concrete, the most widely used building material. Presently, the manufacture of one tonne of cement requires about 1.5 tonnes of raw materials and releases about 1 tonne of CO2.
It has been known for some time that the use of supplementary cementing materials (SCMs) as a partial replacement of ordinary Portland cement in concrete can reduce its environmental load. The use of substantial volumes of SCMs reduces CO2 emissions and
saves energy and natural resources. However, high volumes of SCMs, including fly ash and slag, delay the early hydration of ordinary Portland cement and slow down the initial strength development of concrete. This can be a major disadvantage in situations where delayed curing adversely affects construction scheduling.
To reduce the environmental effects of concrete without extending curing time, the National Research Council Institute for Research in Construction (NRC-IRC) initiated a study on the use of nano-CaCO3 as a supplementary cementing material.
Current Work
The NRC-IRC study compared the effects of two different types of CaCO3 micro-CaCO3
and nano-CaCO3 — on the hydration of ordinary Portland cement (OPC). Scanning
electron microscope images of both sizes of CaCO3 are shown in Figure 1, with
respective magnifications of 5000× and 50,000×. The average particle size of the micro-CaCO3 is approximately 1000 (Tai, please add correct ratio – readers will have trouble
with the units stated) times larger than that of the nano-CaCO3.
The reaction between ordinary Portland cement and water results in heat development. A conduction calorimeter was used to measure the rate of this heat development as a function of time, indicating how quickly or how slowly hydration takes place.
The rate of heat development of OPC hydration is shown in Figure 2 for OPC Control, OPC with fly ash and OPC with fly ash with the additions of micro- and nano-CaCO3.
The contents of fly ash and both micro- and nano-CaCO3 were 50% and 20%,
respectively, based on the total mass of OPC and fly ash.
The result from the conduction calorimetry indicates that the rate of heat development of hydration of OPC was significantly delayed by high volumes of fly ash. When micro-CaCO3 was added to the OPC/fly ash blend, the rate of heat development was slightly
accelerated above that without micro-CaCO3 addition. When the nano-CaCO3 was added
In order to understand the mechanism of this accelerating effect, the intensive scanning electron microscope study was also conducted. Figure 3 shows the SEM image of the surface of the hydrated OPC with the addition of nano-CaCO3. It clearly shows that the
C-S-H (calcium silicate hydrate, the most important hydration product) grows around the nano-CaCO3 particle, suggesting that the addition of nano-CaCO3 enhances the
nucleation of C-S-H, and therefore accelerates the hydration of OPC.
The current research shows that finely ground limestone (CaCO3) can be used in
combination with SCMs to replace a certain portion of ordinary Portland cement and thereby save energy and natural resources. The addition of CaCO3 has a positive effect on
the hydration of cement paste and consequently the strength development of the mixture.
Future Work
The research confirms previous indications that the addition of ground CaCO3 to
mixtures with supplementary cementing materials improves the rate of hydration and that the finer the CaCO3, the faster the rate of hydration.
Thus far, NRC-IRC has studied the effect of the addition of SCMs and ground CaCO3 on
cement paste. The next stage will extend the research to concrete mixes typically used in construction. In addition to confirming the positive effect of ground CaCO3 on the rate of
hydration, several other practical issues require examination.
One of these is the dispersion of nano-CaCO3 within cement paste. The nano-particles
tend to agglomerate (i.e. stick together) by themselves. If consistent dispersion of nano- CaCO3 can be accomplished, the accelerating effect can be obtained more efficiently with
a minimum amount of nano-CaCO3. Another aspect to investigate is whether the addition
of nano-CaCO3 has a significant affect on workability.
The use of nano-CaCO3 as the accelerating agent has the potential to compensate for the
delay in early hydration and initial strength development of ordinary Portland cement caused by high volumes of supplementary cementing materials. This means that
significant volumes of supplementary cementing materials could be added to concrete to reduce energy and resource inputs without adversely affecting strength development.
Taijiro Sato, PhD, is a research associate in the Building Envelope and Structure program of the NRC Institute for Research in Construction (NRC-IRC). He can be contacted via e-mail at taijiro.sato@nrc-cnrc.gc.ca.
(a) (b)
Figure 1: Scanning electron microscope image of (a) micro-CaCO3 and (b) nano-CaCO3
0 5 10 15 0 1 2 3 R at e of H eat D e ve lop m ent , J/ g/ h Time, d
OPC + Fly Ash (1:1) + Nano-CaCO3[20%]
OPC + Fly Ash (1:1) + Micro-CaCO3 [20%]
OPC + Fly Ash (1:1) OPC Control
Nano-CaCO3Particle
C-S-H Growth
