High energy mixing is the most effective accelerator for HVFA concrete

I’ve been conducting experiments to determine a way to speed up the hardening of High-Volume Fly-Ash Concrete (HVFA). In this concrete 50% of total cementitious materials were replaced by fly ash (Class F).

I compared the following variants:

- Concrete without fly ash (100% Portlandcement type 1), conventional mixing

- Same concrete, high-energy mixing

- Concrete with 50% fly ash, conventional mixing

- Same concrete, high-energy mixing (HEM)

The ratio of water/ cementitious materials and aggregates remained the same in all the variants.

There was no improvement in workability in concrete with fly ash over one without (using conventional mixing). Therefore, I did not lower the water level.

However, in the case when high-energy mixing was used, workability remarkably improved. Naturally, a decrease in water level can only improve the results given here.

My tables show 2 times increase in strength after the first 24 hours, and up to 30% over the next 28, 60 and 90 days, when high-energy mixing is used.

The recommended method involves not all of concrete mix, but only its mortar component, partly or fully. The consumption of energy – 14,13 Kwh per cu.Y (18.5 Kwh per cu. m) or 45 Kwh per 1 ton of cementitious materials. These results I compared with method of additional cement grinding characterized by the same level of energy consumption.

Taking these results into consideration, I conclude that high-energy mixing will make it possible for HVFA concrete to be used in situations requiring accelerated increase of concrete strength, e.g. repair of roads and bridge desks.

Therefore the problem of slow hardening in “green concrete” may be solved with the high-energy mixing.

The results in strength of high-energy mixed mortar component of concrete in comparison with conventional mixing

Water cementitious materials ratio = 0.38;

Cementitious materials Sand ratio = 0.50

Portlandcement Type1 “LEHIGH Co”, Sand for All Purpose (“Quikrete Co”).

VARIANT	24 hours		3 days		28days		60 days		60 days
	psi	%%	psi	%%	psi	%%	psi	%%	psi	%%
1.Cement 100% Conven. mixing	2633	100	5396	100	7665	100	8073	100	8437	100
2.Cement 100% HEM	4438	168	7167	133	9556	125	10333	128	10104	120
3.Cement 50% FA 50% Conv. Mixing	933	35.4	2775	51.4	4646	61	6083	75	5417	64
4.Cement- 50% FA-50%, HEM	2167	82	4339	80.4	6396	83.4	6427	80	7250	86

The results in strength of concrete

The mix proportion in kg per Cu. M (lb per Cu. Y.)

Cementitious materials 424 (714.7),

Sand 850 (1432),

Coarse Aggregate

Marble chips (New England Silica, Inc) 1092 (1840)

Water 162 (273), HRWRA 7-10 L/Cu.m (5.3-7.6 L/Cu.Y.)

VARIANT	3 days		28 days
	psi	%%	psi	%%
1.Cement 100% Conv. mixing	4458	100	5709	100
2.Cement 50% FA-50%, conv.mixing	1863	42	3043	53
3.Cement 50% FA-50%, HEM	3185	71	5048	88

Comparison HEM and additional Cement grinding*

(Energy consumption is 45 Kwt-h per 1 ton of cement in both methods)

VARIANT	Effect of Strength increase, %%
	28 hours	3 days	28 days
1.Fine Ground Cement	22.5	13.9	5.6
2.Cement 100%, HEM	68	33	25
3.Cement 50%, FA-50%, HEM	132	56	37.6

*Fine Ground Cement in Concrete – Properties and Prospects /L. Lindstrom, B. Westerberg,

ACI Materials Journal, v.100,No.5, Sept.-Oct. 2003, p. 398-406/

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