Page 2 - Open Access - Feb 2020

TECHNICAL PAPER

for cement substitutes. This increase in demand can be Table 1: Summary of the emission factor of different
leveraged by combined substitution of SCMs, which are likely binding materials
to accommodate a wide range of SCMs with various levels
of quality and reactivity potential. The cement sustainability oPtions for Binding emission factor sources
initiative (CSI) predicts that the contribution of limestone materials (kg-co 2 Per kg-
would increase from 8% in 2014 to nearly 18% by 2050 material)
as supplementary binding material, as shown in Figure Clinker + Gypsum 0.77-0.93 [16–19]
1. Additionally, calcined clay is expected to compensate Calcined clay 0.2-0.35 [18]
availability of fly ash and slag as a pozzolanic cement substitute.
Limestone 0.008 [18]
Belite Ye’elimite Ferrite 0.6-0.66 [18]
clinker
Carbonated calcium 0.54 [18]
silicate clinkers
GGBS 0.070 [17]
LC3 binder 0.46-0.61 [16,17,19]

Limestone powder was traditionally considered as an inert
filler. A more rational understanding of the physio-chemical
contribution of limestone in a hydrating cementitious matrix has
been emphasised in literature [20,21] . The chemical interaction of
Figure 1: Contribution of different cementitious and pozzolanic carbonate species has created avenues for judicious limestone
materials to constituent binding materials as suggested in the CSI additions [22,23] . The aluminate-carbonate interaction can alter
report [14] . physical structure development because of the difference in
space-filling characteristics of the additional hydrate phases [24] .
2.1 Addition of Finely Ground Limestone The extent of benefit that could be availed from such interaction
Powder needs to be elucidated with scientific research studies.
Ground limestone is drawing interest as an additive in cement
worldwide. A locally available resource for cement plants and 2.2 Calcined clay as a pozzolanic admixture
the low emission factor of limestone (See Table 1) makes it Calcined clay is an acknowledged pozzolanic admixture in the
a viable choice to reduce emission potential in cementitious Indian standard; it is an ingredient of Portland Pozzolan Cement
materials by moderately and judiciously increasing filler as per IS 1489 Part 2 [25] . However, the potential utilization of
substitution level [14,15] . In addition to the curb on emissions, calcined clay has been undermined due to surplus availability
the energy demand of the binder and concretes for calcined of fly ash from thermal power plants in the last few decades
clay-limestone combination were found to be reduced by and lack of knowledge on the quality of clay to be used for
about 20% and 10% respectively with respect to OPC [16,17] . In such applications. Studies have shown that 1:1 kaolinite clays
current practice, most international standards allow only 5% possess excellent pozzolanic behaviour after dehydroxylation
[6]
limestone as performance enhancers in ordinary portland by thermal activation . Calcined clay with only 40% kaolinite
cement (OPC). Interaction from slow dissolution of calcite content was found sufficient to produce strengths similar to
brings in a whole range of new opportunities for utilization of plain portland cement by 7 days of curing [26] . The presence of
limestone, which requires only grinding, without any energy- reactive aluminates in calcined clay makes it an ideal constituent
intensive calcination process. Limestone is a readily available for co-substitution with calcite-rich materials such as limestone/
resource near clinkerization units. Hence, it does not require dolomite/ marble stone [27,28] .
extensive transportation as slag and fly ash to supplement
cement production houses. However, the slow uptake in Indian Limestone Calcined Clay Cement (LC ) binder makes use of
3
standards and industry acts as a barrier to limestone as a binder these two widely available resources and their interaction with
component. This necessitates extensive research studies to clinker compounds and between calcined clay-limestone. A
develop the state of art knowledge on the subject of such typical limestone calcined clay cement is composed of portland
composite cementitious systems, which can elucidate the areas clinker, calcined clay (>40%), limestone and gypsum, as shown in
of benefits and address the concerns for suitable adoption. Figure 2.

32 The IndIan ConCreTe Journal | FeBruarY 2020