Challenges of Low-Clinker Cement

In recent years, cement industry has been facing new challenges related, above all, to the need to reduce carbon emissions. Cement production accounts for about 8% of carbon emissions worldwide, mainly caused by the decarbonation of calcareous minerals used to produce clinker and by the large quantity of fuels necessary to reach the high burning temperatures. For each ton of clinker produced, approximately 0.85 tons of CO2 are emitted into the atmosphere.

To reduce the carbon footprint, cement industry is implementing a variety of new strategies. These include reduction of the clinker-to-cement ratio (the clinker factor), use of cleaner fuels (e.g., natural gas and hydrogen), utilization of alternative raw materials, carbon capture, usage and storage, renewable energy, local sourcing and green transports. Among these actions, the most effective one is to reduce the clinker factor. Currently, in North America the average clinker factor is around 0.85. To significantly reduce carbon emissions, it is estimated that the factor will have to be lowered at least to 0.75.

ASTM C595 for blended cements

ASTM C595 defines four types of blended cements with a lower clinker factor by integrating limestone, slag, pozzolan or some combinations of them. They are Type IL, Type IS, Type IP and Type IT cements, respectively. Type IS cement does not fall into the scope of this discussion, since slag is expensive and seldom used in producing cement. With Type IL cement, the clinker factor can only be reduced to a limited extent, prevailingly 8% to 12% and no more than 15% as specified by the standard. On the other hand, Type IP and IT cements allow much higher replacement(s) for clinker, enabling a higher reduction of the clinker factor, thanks to the pozzolanic reactions that contribute to stronger and more durable concrete in the long term. Thus, these two types of cements are of the highest potential for cement industry to explore toward sustainable cement production.

Challenges from Type IP and IT cements

Pozzolans can be categorized into “artificial” and natural pozzolans. The former mainly refers to fly ash and silica fume, while common natural pozzolans include pumice, basalt and diatomaceous earth, among others. Pozzolanic reaction is much slower than cement hydration at the early ages. To compensate for the strength loss caused, the cement having a decent amount of pozzolan is required to be ground to a higher fineness. In the meantime, natural pozzolans usually have a layered and/or porous microstructure that tends to absorb large amount of liquids during mixing. Due to these intrinsic characteristics of pozzolans, concrete with Type IP or IT cements frequently encounters higher water demand, loss of workability, and loss of the effectiveness of the superplasticizers.

Innovative MCH C-C cement additives

To mitigate these issues related to the blended cements, MAPEI has developed a new range of cement additives called MCH C-C (from Cement to Concrete). These additives have the peculiarity of, besides improving the grindability and mechanical performance of cement, enhancing workability both in mortar and concrete applications. Below are two examples that prove the effectiveness:

Type IT cement with MCH C-C 1001
The first example is a Type IT cement from a cement plant trial. Two cement samples were collected, one with their current grinding aid (baseline) and the other one with MAPEI’s MCH C-C 1001. The two samples have a similar Blaine value, at around 6300 cm²/g. and normal consistency of 27.7 and 26.6, respectively, corresponding to a 4% reduction with MCH C-C 1001. The initial and final set time are similar between the two cements.

Fig. 1 shows the strength of the standard mortar according to ASTM C109 tested at a fixed w/c ratio of 0.485. From the figure, the cube strength was increased by 10% to 15% at all the ages. With this same w/c ratio, the mortar flow was increased from 101 to 117 by using MCH C-C 1001. If the cement was tested at similar flow (110±5), greater strength increase is expected. All these cement and mortar testing were implemented by the plant lab on site.

 

It is well known that sometimes performance of concrete is different from that obtained from mortars. For this reason, the two cement samples were also tested in concrete, based on the commonly used mix design of 600 lbs./yd3 of cement. With the same amount of superplasticizer, the w/c ratio was reduced from 0.445 for concrete with baseline cement to 0.429 for the one with cement ground with MCH C-C 1001, while keeping the slump of the two mixes in the 4" to 5" (10 to 12.5 cm) range. Again, there is no significant difference in concrete set time, and the strength was increased greatly, by 10% to 15% as shown in Fig. 2. For the cement recipe, testing results of mortar and concrete correspond well to each other.

 

Based on all the testing results above, the addition of MCH C-C has allowed enhancements of the cement performance in both mortar and concrete. These enhancements can be taken advantage of to reduce ulteriorly the clinker factor or reduce the fineness for higher mill throughput.

Type IP(35) cement with MCH C-C 1001
The second example is a Type IP(35) cement from a cement plant trial as well. This was a pioneering cement with 35% integration of calcined clay, approaching the 40% limit set by ASTM C595 for Type IP. The purpose of the trial was to explore the feasibility of the calcined clay in cement production. Similarly, two cement samples were collected, one with straight glycol as the grinding aid, and the other one with MCH C-C 1001 at 2,000 ppm by weight of cement. The Blaine value of the two cements was about 7200 cm2/g, resulting in a higher normal consistency than usual: 32.0 and 31.4 for the baseline cement and cement with MCH C-C 1001, respectively. The water demand is only reduced by 2% from this testing. But in concrete, the water reduction was much higher, around 10% from w/c ratio of 0.64 to 0.57 while maintaining the slump of both mixes at 3" (7.5 cm). The concrete mix design was based on 517 lbs./yd3 of cement (ASTM C465) with 45% absolute volume of fine aggregate in the total aggregate. The compressive strength (Fig. 3) of the concrete is low across the board; however, the strength enhancement with MCH C-C 1001 is still clearly seen at all the ages, from 200 to 300 psi (1.38 to 2.07 MPa) at 1 day to 800 to 900 psi (5.52 to 6.21 MPa) at 7 and 28 days.

Apparently, the trial was just a starting point for a commercial Type IP cement. There are a lot of adjustments needed to bring up the cement quality. For example, limestone can be used to replace part of the calcined clay, thus reducing the water demand, because limestone is not as absorptive. With the benefits that MCH C-C can bring, any adjustments made will be more effective and the cement producer can reach the goal of a quality cement faster.

MCH C-C: A sustainable technical solution

These examples demonstrate that MCH C-C, the new range of cement additives developed by MAPEI, can help cement producers and users in the most difficult cases, thanks to the combination of numerous benefits: Increased grinding efficiency, improved workability and enhanced mechanical strength. Especially, they can be used in the case of a high substitution rate of clinker with materials that, under normal conditions, would lower the quality of the cements, making them very difficult to use.

MCH C-C additives make it possible to solve these problems and represent an answer to the challenges that the cement industry will face in the coming decades. This innovative technology provides promising paths to produce cements with a lower clinker content, lower production costs, lower carbon emissions and a higher quality, whose effects will be noticed in the applications of both mortars and concrete.

The expert’s opinion

In 2000 MAPEI introduced the “Cement Additives” line, products dedicated to cement production with the vision of helping the construction industry become a carbon-neutral industry starting from its main raw material: cement. These additives are meant to be used during the cement production process, to support the cement industry on the roadmap to produce modern cements with reduced carbon emissions. When MAPEI first approached this market, only traditional grinding aids were available – additives that are mainly formulated to solve agglomeration problems within cement mills and reduce electrical energy consumption. Since then, a number of innovations have been brought to the market by MAPEI Research & Development laboratories, evolving from the original grinding aids to the most advanced MCH C-C additives, a true “bridge” between cement and sustainable and durable concrete.

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