Portland cement – limestone – silica fume systems in concrete

The use of Portland cement containing fine ground limestone is a common practice in European countries. On the other hand, the incorporation of supplementary cementing materials, such as industrials by-products (fly ash and silica fume), is sometimes recommended in order to solve both economical and environmental issues related to cement production. The purpose of this investigation was to establish, from our own experiments, whether there is a synergistic action when a ternary system of Portland cement – limestone – silica fume is used in mortar or concrete. Standard laboratory tests both heat of hydration and mechanical strength development were performed on pastes and mortars made with 70 % (by weight) of Portland cement, type CEM II/B-S (supplied by Dalmacijacement, K. Sucurac, Croatia), and 30 wt. % of cement replacement materials consisted of various combination of fine ground limestone (supplied by Konstruktor, Split, Croatia) and silica fume (supplied by Elkem, Norway). In addition, to these ternary systems, binary blends, such as Portland cement and fine ground limestone (LF), and Portland cement and silica fume (SF), along with 100 % Portland cement mortars, were investigated for comparison. The water-to-cement (w/c) ratio varied from 0.5 to 0.65 by increasing the replacement material addition, but the samples having been prepared to have the same flow table consistency, formed the pats of 122-130 mm in diameter. It is expected that the results and conclusions obtained here on mortars will be transferable to concrete. The results have shown that both silica fume and limestone can act as inert and active participants. Ratio of their effect changes depending both on their individual content and on the age of hydration. It was found that LF replacement of cement (up to15 wt. %) caused significant strength losses while SF replacement of cement (up to 15 wt. %) produced greater strength increases related to the control Portland cement mortar. Blending SF and LF (SF/LF ratio = 1) simultaneously with cement brought significant the compressive strength improvements after 28 days compared to the control Portland cement mortar. This opens up the possibility of partially replacing Portland cement by industrial by-products combined with non-processed microfillers. This concurs with strict requirements upon the cement production (environmental protection, resource utilisation and energy saving), without compromising the fundamental characteristics of the cementitious binder.