Durability of Wood-Cement Composites with Modified Composition by Limestone and Stabilised Spruce Chips

Abstract

Limestone (LS) and stabilised secondary spruce chips (SCs) utilisation in wood-cement composites is still an unexplored area. Therefore, the main objective of the research presented here is the assessment of the long-term behaviour of cement-bonded particleboards (CBPs) modified by LS and SCs. Cement (CE) was replaced by 10% of LS, and spruce chips by 7% of SCs. The test specimens were stored in a laboratory and exterior environment (Middle Europe) for up to 2 years. The density, strength, and modulus of elasticity were evaluated after 28 days, and then in 6-month periods. The hygroscopicity was analysed separately. The mineralogical composition and microstructure were analysed due to possible LS participation during hydration. SC synergic behaviour in CBPs was also studied. After 2 years, the microstructure of the CBP was more compact, and denser. Strong carbonatation contributes to the improvement of CBP properties. The products of carbonatation were present in both the matrix and wood chips. The hydration of the matrix was almost finished. LS has a positive effect on the matrix microstructure development. LS acts both as an active component participating in the formation of the cement matrix structure and as an inert microfiller, synergic with hydration products. SCs have a positive effect on the hygroscopic behaviour of CBPs and slightly negative effect on the tensile strength.

Keywords: by-product; cuttings; limestone; long-term durability; mechanical properties; microstructure; particleboard; secondary spruce chips; stabilisation; wood–cement composite.

Figure 1

Production scheme of CIDEM Hranice, a.s., particleboards: 1—spilling; 2—preparation of mixture; 3—layering of boards; 4—pressing; 5—drying; 6—formatting; 7—storage; and 8—transport (highlighted cuttings by-product from formatting particleboards) [30].

Figure 2

Particle size and distribution: (a) cement CEM II/A-S 42.5 R and limestone VMV15-F; (b) primary spruce chips and secondary stabilised chips 0.5–2 mm.

Figure 3

Detail of structure (Keyence VHX-950F optical microscope): (a) secondary chips 0.5–1 mm; (b) secondary chips 1–2 mm.

Figure 4

Detail of structure (Tescan MIRA3 XMU scanning electron microscope): (a) finely ground limestone; (b) finely ground limestone; (c) secondary chips 0.5–1 mm; (d) secondary chips 1–2 mm.

Figure 5

Production of cement-bonded particleboards—CIDEM Hranice, a.s.: (a) layered mixture on a steel pad; (b) layering of individual steel plates with the mixture over each other prior to pressing.

Figure 6

Production of cement-bonded particleboards CIDEM Hranice, a.s.: board after pressing and thermal treatment, before formatting and grinding.

Figure 7

Exposure of test specimens in real climatic conditions over the course of 2 years (Central Europe, Czech Republic): (a) summer 2022; (b) winter 2023/2024.

Figure 8

Exposure of test specimens in real climatic conditions over the course of 2 years: (a) progression of average daily temperatures; (b) progressions of average daily air humidity.

Figure 9

Mechanical parameters of tested CBP: (a) density; (b) bending strength (exposure: /R—laboratory environment and /E—real climatic conditions).

Figure 10

Mechanical parameters of tested CBP: (a) modulus of elasticity in bending; (b) tensile strength perpendicular to the plane of the board (exposure: /R—laboratory environment and /E—real climatic conditions).

Figure 11

Sorption isotherms with hysteresis effect of tested CBP: (a) linear change; (b) thickness (lateral) change; (c) mass change; and (d) volume change (all specimens were stored under laboratory conditions).

Figure 12

Phase composition of reference wood–cement composite CBP-R—standard industry mixture of company CIDEM Hranice, a.s. (R28, R365, and R730—laboratory environment after 28 days, 365 days, and 730 days, E365 and E730—real climate of Czechia after 365 days and 730 days; E—ettringite, P—portlandite, C—calcite, and L—larnite).

Figure 13

Phase composition of modified wood–cement composite CBP-L—industry mixture modified by limestone (R28, R365, and R730—laboratory environment after 28 days, 365 days, and 730 days, E365 and E730—real climate of Czechia after 365 days and 730 days; E—ettringite, P—portlandite, C—calcite, and L—larnite).

Figure 14

Phase composition of modified wood–cement composite CBP-R—industry mixture modified by limestone and secondary chips (R28, R365, and R730—laboratory environment after 28 days, 365 days, and 730 days, E365 and E730—real climate of Czechia after 365 days and 730 days; E—ettringite, P—portlandite, C—calcite, and L—larnite).

Figure 15

Detail of structure (Keyence VHX-950F optical microscope)—reference boards CBP-R after 730 days ageing in laboratory environment: (a) sidewall view, yellow arrow—direction of the board pressing; (b) face-side view.

Figure 16

Detail of structure (Keyence VHX-950F optical microscope)—reference boards CBP-R after 730 days ageing in external climatic environment: (a) sidewall view, yellow arrow—direction of the board pressing; (b) face-side view.

Figure 17

Detail of structure (Keyence VHX-950F optical microscope)—reference boards CBP-L after 730 days ageing in laboratory environment: (a) sidewall view, yellow arrow—direction of the board pressing; (b) face-side view.

Figure 18

Detail of structure (Keyence VHX-950F optical microscope)—reference boards CBP-L after 730 days ageing in external climatic environment: (a) sidewall view, yellow arrow—direction of the board pressing; (b) face-side view.

Figure 19

Detail of structure (Tescan MIRA3 XMU scanning electron microscope)—reference boards CBP-R after 730 days ageing in laboratory environment: (a) cement matrix, IZT of matrix and chip; (b) detail of compact matrix—CASH phases with sulphur (yellow-highlighted); (c) matrix CASH phases with sulphur (yellow-highlighted) and portlandite (orange-highlighted); and (d) EDX spectrum of portlandite area—orange point “×” (see Figure 19c).

Figure 20

Detail of structure (Tescan MIRA3 XMU scanning electron microscope)—reference boards CBP-R after 730 days exposed to real climatic conditions: (a) cement matrix, IZT of matrix and chip, and hydration products in spruce chip (green-highlighted); (b) portlandite (orange-highlighted); (c) detail of calcite (carbonation product); and (d) EDX spectrum of calcite—yellow point “×” (see Figure 20c).

Figure 21

Detail of structure (Tescan MIRA3 XMU scanning electron microscope)—modified boards CBP-L after 730 days ageing in laboratory environment: (a) cement matrix, IZT of matrix and chip; (b) detail of LS grain in matrix (yellow-highlighted) and portlandite (orange-highlighted); (c) ITZ of matrix and LS grain in detail; and (d) EDX spectrum of LS grain—yellow point “×” (see Figure 21b).

Figure 22

Detail of structure (Tescan MIRA3 XMU scanning electron microscope)—modified boards CBP-L after 730 days ageing in laboratory environment: (a) cement matrix, IZT of matrix and chip; (b) detail of LS grain in matrix (yellow-highlighted); (c) carbonation products and LS grain (yellow-highlighted); and (d) detail of carbonation products within spruce chip.