Unless Until the cement replacement material is pozzolanic we do not expect any reaction leading to expansion of product and that to so early. Problem seems to be over vibration leading to paste coming at the top of mould and giving the impression of expanding volume. Carbon fillers are inert and being finner they add to blocking pores. If early acceleration of cement hydration is any sign it should also happen without adding the filler.

Need more details regarding property of carbon filler and its pozzolanic property if any.

High suface area,higher than cement may cosume water quantity , cosume more if it reacts cosume less if not reacts ,all depends on filler property

Isothermal (or semi-adiabatic) calorimetry would be a good way to quantify the potential acceleration, if any.

Please give a clarification to everyone. Did the fresh product still shrink or expand? If the surface was shrinking, then due to the large specific surface of the filler, more water could be required. Or just with fine inert fillers a thin layer of the concrete surface quickly dries out. If the product still had an expansion, then this filler is most likely chemically active.

Hi Viktor,

Thank you for your response. The carbon filler is prepared from biomass. Our studies have shown that it is not chemically reactive; however, it is a good idea to perform chemical tests on the filler because the properties depend on the feedstock and other processing parameters.

The fresh cement product did expand within 10-15 mins of casting. From appearance, the surface was wet enough and therefore, possibility of drying or excessive shrinkage can be eliminated for now. Moreover, water-cement ratio is 0.45 which is not low. I redid the casting in small batches of materials (3-4 kg each) because I suspect generation of localized heat because this product does accelerate hydration. So far the mix seems okay. Will know more after conducting the required tests.

Thank you for the clarification. If the carbon material is prepared from biomass, and phosphorus is present as impurities, it is possible that the reaction with calcium hydroxide occurs and gas is released. This leads to an increase in volume.

3Ca(OH)2 + 6H2O + 2P4 = 3Ca(PH2O2)2 + 2PH3↑ (2);

It seems that, you should control the expansion owing to water absorption, reactivity, measuring changes in state variables (calorimetry), and specify it is crystalline or amorphous solid. these specifications will give you better understanding of expansion of the filler

You can refer to SAP in cement paste, may be you could consider the water absorption property of your carbon-based filler.

I suggest that you check the Physical and chemical properties of the filter material, for example, the presence of aluminate oxide can lead to the expansion of the mixture.

Good luck