Influence of paper mill ashes on the properties of self-compacting concrete

Abstract Self compacting concrete (SCC) is characterized by a high workability that does not require any vibration to place it. Moreover, SCC must be stable during the placement and before setting in order to guarantee homogeneity and durability. The above performances can be obtained: 1) by decreasing the amount and size of the coarse aggregate; 2) by using a high volume of fines; 3) by the combined use of superplasticizer and viscosity modifying agent (VMA) in order to obtain high fluidity as well as high stability of the cement paste. Excessive amount of cement, dangerous for hydration heat, creep and shrinkage, can be avoided by using mineral additions. Some fine additions showing high specific surface area, such as silica fume or colloidal silica, are effective both as filler and viscosity improving agent. In this paper, SCCs containing ashes coming from the combustion of paper mill sludge as fine mineral addition were studied. In particular, workability and bleeding capacity of fresh mixtures, as well as compressive strength at different curing times of the hardened concrete, were examined. The paper mill ash (PMA) was used with two different rates and finenesses. When used without any preliminary grinding, no improvement in the concrete performance was observed since the high porosity penalizes the compressive strength and the moderate fineness (maximum size is about 240 µm, as received) does not improve the stability of the concrete mixture. The use of PMA seems to be effective for obtaining stable mixtures and avoiding VMA addition only after grinding, in order to significantly increase its fineness.