Composition and microstructure of CaCO3·MgO-MgSO4-H2O cement paste

The composition and microstructure of the dolomite-derived CaCO 3 ·MgO-MgSO 4 -H 2 O cement paste are characterized by interlaced framework units of “porous CaCO 3 skeletons rooted by 517 phase crystals”, forming a homogeneous framework matrix containing a high proportion of capillary pores. The interlaced framework units provide the primary source of strength of the cement paste, enabling high performance and superior environmental sustainability. • CaCO 3 ·MgO derived from dolomite is a green clinker. • MgO nanospheres are uniformly dispersed in the porous CaCO 3 skeleton. • A porous CaCO 3 skeleton rooted by 5 Mg(OH) 2 ·MgSO 4 ·7H 2 O crystals forms the framework. • The cement shows higher strength and lower CO 2 intensity than MgO-MgSO 4 -H 2 O cement. MgO-based cement (MBC) is regarded as a low-CO 2 alternative to Portland cement (PC), yet its advantages are challenged by high emissions from MgCO 3 calcination and the limited distribution of magnesite. Herein, a green and globally available CaCO 3 ·MgO-based cement (CMBC) derived from abundant dolomite was proposed, using the CaCO 3 ·MgO (CM)-MgSO 4 -H 2 O (CMOS) system as a representative case. The composition and microstructure of CMOS cement paste were systematically investigated, with MgO-MgSO 4 -H 2 O (MOS) and MgO + limestone powder-MgSO 4 -H 2 O (MCOS) systems as references. CM particles consisted of MgO nanospheres uniformly dispersed on the nano- to micro-scale pore walls of a micron-scale CaCO 3 skeleton. This promoted the formation of framework units described as “porous CaCO 3 skeletons rooted by 517 phase crystals.” The intergrowth and aggregation of these units produced a homogeneous framework matrix dominated by capillary pores (10 < D < 100 nm), unlike the large-pore structure (D > 100 nm) in MOS and MCOS pastes. Consequently, compressive strength of CMOS paste was governed by reinforced framework units rather than 517 crystals, leading to a 56.9% increase at 28 d compared with MOS paste. Moreover, CMOS cement qualified as a CaCO 3 -clinker-based green cement, and reduced carbon emissions by 22.9% and 50.9% per unit volume and strength.