Production of belite rich clinkers from pulp and paper industry waste streams in laboratory and full scale

The pulp and paper industry generates large amounts of Ca-rich waste streams. In the current study these waste streams were transformed to reactive belite rich clinkers by low temperature clinkering. Additionally, the... [ view full abstract ]

The pulp and paper industry generates large amounts of Ca-rich waste streams. In the current study these waste streams were transformed to reactive belite rich clinkers by low temperature clinkering. Additionally, the clinkers were further activated by mechanical and chemical treatments after furnace. Belite clinkers and modified belite clinkers are a type of environmentally friendly clinker. This results from significantly lower clinkering temperatures compared to temperatures required in portland clinker. This study was performed in three steps. In the first step theoretical modelling was done to account for possible formation of harmful substances from the waste materials in the high temperatures of the clinkering process. Theoretical amounts of Belite, Y’eelimite and related cementious structures were calculated based on the raw material chemical and mineralogical composition. Various raw material compositions were tried varying specific chemical indexes between the different elements (Ca-S-Si). Techniques used for these were Bogue and modified Bogue calculations as well as thermodynamical phase-formation modelling with FactSaqe^TM – software. In the second step the raw materials were clinkered in laboratory furnace ranging from temperatures from 900 – 1200C. In the third step a full-scale production trial was launched. The full-scale clinkering was done in a rotary kiln. The amount of clinker produced was 300 t. The elemental composition analysis of raw materials was performed by means of SEM/EDX but also the changes in the crystalline structure were followed by means of XRD. The potential of the clinkers was determined based on their 14-, 28-, and 90-day compressive strengths. The effect of grinding was measured by dry sieving and measuring the obtained blaine surfaces. The fresh properties of produced cement (consistency) was followed using the vicat method (EN 196-3). The hydration of the cementious phases was also observed with semi-adiabatic calorimetry according to EN 196-9. The main crystalline phases identified were, belite, y’eelimite, gehlenite and wollastonite. Also, significant amount of XRD amorphous material was identified. The compressive strengths were tested according to standard EN 196-1. The obtained strength levels for the produced clinkers were between 10 – 16 MPa (90D). Supplementary grinding in a high intensity pin mill and simultaneous activation by means of different chemical activators (alkali and sulphate salts) provided the additional effect of enhancing the early strength development from 5 MPa to 10 – 12 MPa (14D) and up to 20 MPa for late strenght development (90D).